Energy and Climate Change Committee - Minutes of EvidenceHC 60

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Oral Evidence

Taken before the Environmental Audit Committee

on Wednesday 12 June 2013

Members present:

Joan Walley (Chair)

Martin Caton

Katy Clark

Mark Lazarowicz

Caroline Lucas

Dr Matthew Offord

Mr Mark Spencer

Dr Alan Whitehead

Simon Wright


Examination of Witness

Witness: Aubrey Meyer, Global Commons Institute, gave evidence.

Q19 Chair: Aubrey, it is a great pleasure to welcome you before this particular Environmental Audit Select Committee. We know that there was a previous opportunity for you to give evidence to our predecessor Committee. It might be helpful to say at the outset, both to you and possibly to future witnesses who will be giving evidence this afternoon, that we are aiming to allocate something like 35 minutes to each different set of witnesses before us, so it is going to be very tight for time.

It is such a complicated area, such a complicated brief, so if you can try to explain your answers in plain English it will make it easier for us, particularly those of us who have had the opportunity to have direct presentations from you in the past. Our starting point is your concept of contraction and convergence. The starting point for us in the current inquiry that we are doing is whether or not you feel that the Climate Change Act targets as we have them are set in accordance with the principles of contraction and convergence.

Aubrey Meyer: As before, the answer is yes and no, and people’s opinions vary. As you will remember, in the previous inquiry Adair Turner took a direct question from you on this point and it is all on film, I am happy to say. His answer to your point was they did not call it contraction and convergence in the Climate Act because, if I remember the phrase, it became so "emotional", whatever that meant. But what did come out was that it was very strong support for what GCI had said and done with C&C.

So, in respect of the Act, is it C&C or not? In principle, yes. In practice, no, in the sense that the targets that are derived from the UK Climate Act are insufficiently precautionary. We are being too generous with the amount of carbon we can assume we can safely burn into the future, both nationally and internationally.

Q20 Chair: That leads me to my second question, which is whether or not the current Act that we have is sufficient in terms of the rate of change and the way in which the UK emissions are contracting. In your view, is the Climate Change Act that we have sufficient to meet the changes that we are seeing?

Aubrey Meyer: I want to answer encouragingly because in the UK there is a considerable effort involving DECC and other facets of Government to get something like a credible policy on climate going. In that sense it is an effort worth recognising.

However, in respect of setting this exemplary contraction and convergence principle for the world at the UN, the problem is the approach that was taken in the Climate Act, both in respect of contraction and concentrations, which for me is the issue for today, but also in respect of the second dimension, which is contraction and convergence-how you internationally allocate a safe contraction and concentration budget. For some reason, which is completely beyond me, they chose to prescribe that the convergence year would be 2050. In that sense, to go to the UN to say to all our international partners it will be done thus and what the convergence rate in the Climate Act numbers will be by 2050, when to be honest the straight numbers show you 80% of the budget has been used up by 2050. So it is a little rich to say to all the people who feel they are the innocent parties with the hugest opportunity costs, "You get equality at the end of term," in spite of the fact, and this is crucial, that in July of 2009-the Act existed but we had not yet gone to COP-15-the Chinese Government published an extensive paper that quite explicitly said, "We accept the contraction and convergence principle on the understanding that it does not prescribe the rate of convergence and from our point of view it is quite capable of having instant convergence and trading away the difference between permits and actual emissions, and we will start from there." The fact that we went to Copenhagen and prescribed 2050 and then came home and blamed the Chinese because they wrecked the negotiations was a little bit of a diplomatic faux pas. I am not sure whether we have recovered from that yet.

Q21 Dr Whitehead: In your submission you have raised a lot of concerns about the fact that a number of climate feedback effects are not included in the Met Office’s modelling that was used to determine both the Climate Change Act and carbon budgets. Could you take us through what feedbacks you think are missing?

Aubrey Meyer: Yes. As some of you will remember from the previous inquiry, the subject of omitted feedbacks came up yet again. I made a point of putting in the evidence that GCI had been making that point from roughly 1996 onwards, so it is not as if we were new to the issue. The response from the modelling community was, "Of course these feedback effects are there, but they are so full of uncertainties and complexities and difficulties about modelling them that we do not really know how to do it yet, so we will leave them out until we do." That persisted all the way through to 2008 when the Climate Change Act was created and then -and this is the shocking bit-they have persisted all the way through until now, in spite of what you will be told later on by UKMO members present today and probably listening right now, that they have made some progress on this. I can assure you that the RCP scenarios that are now in the IPCC AR5, due out in a year, still basically assert this mostly secondary feedback effect-free prognosis of what is coming.

My generic point to the Committee is to me the most important point, which is why I put it up on page 1 of the analysis. What we absolutely have to recognise is that there is a distinction that needs to be made between budget emissions, which we obviously, at least theoretically, can control and feedback emissions, which we obviously cannot. The difficulty is that, while we may be able to control human emissions because we somehow do what is necessary with carbon management, by the time you have instigated a process of warming that leads to what is already, for example, going on in the Arctic and in the contiguous area of permafrost around it, starting a melting process that is causing methane and especially carbon dioxide to be released in potentially vast amounts, there is nothing on God’s earth that we can do about that.

The coming on of this may be gradual and slow, and that is what these curves, which I hope you all have copies of, are designed to draw attention to. We are not claiming any more certainty for this than what the UK Met Office is claiming for the certainty with these enormous error bars on what is in the Climate Act. What we are saying is generically you must distinguish between budget emissions, which are policy emissions, and feedback emissions, which are consequential emissions about which you can do nothing, and because of that pursue a precautionary route to policy development. Precautionary in this context means not running risks we do not have to run, not incurring costs that we can avoid, and playing safe rather than sorry.

Q22 Dr Whitehead: You make that generic distinction, but nevertheless there will be at least some connection between the two in that the rate at which feedback mechanisms start to bite is a function to some extent of the extent to which those budgets over which we have some control begin to slow the process while they are in play.

Have you modelled in what you have put forward in these charts how that relationship might work? Or are you assuming, contrary to what you state has been deemed just an omission on the grounds of uncertainty of a number of feedback mechanisms, that these mechanisms either happen or they do not, and if they do happen they are outside any effects that those matters that are in our control might have on them? That is a rather more complicated question than I hope the answer will be.

Aubrey Meyer: Firstly, in response to your point about a list of omitted feedback mechanisms, let me tell you straight away, with appreciation to my colleagues in the Met Office, after the last inquiry they listed extensively on their website the extent of the feedbacks that were omitted. So it is methane from Arctic melt and permafrost melt; CO2 from Arctic and permafrost melt; it is water vapour that increases; it is loss of positive albedo; the gain of negative albedo as the ice melts. There are also some things that they have left out, like the new terror, which is that as the oceans warm the carbon-hungry organisms are subject to some kind of viral attack now, which is apparently a major attack on the carbon sink in the ocean. If you are asking me to put values on this, no, I can’t. If you ask me, do I know somebody who has done this, the answer is, no, I don’t. All I know is it is more and more talked about.

The point is for all the knowns that we know, we have a bunch of unknowns that we know we do not know, and then there is Caspar Weinberger saying there are all the unknown unknowns. This is not going away. The one absolutely key thing that is the sum of all the emissions, both human and non-human, is the Mauna Loa concentration curve, which is going up exponentially. The exponent is 2.1% per annum for the extent of the increase-important point. That is a very considerable rate of increase on this absolutely key, rock-solid piece of evidence, which has gone on for the best part of the last 40 years. That is the thing that we have to cause to come down if we want to control the temperature. To take up your question about consequential effects, if you want to stop feedback effects you have to slow the temperature. You can’t go straight to the feedback effects and say, "You should stop, we are dealing with the temperature somewhere else." The only thing we can do is to try to keep the temperature down. Primarily, the only convincing thing we can do to keep the temperature down is to keep our human budget emissions down because that may, if we are lucky, keep concentrations coming down, and there is no other way to configure that. There is no kind of packed lunch in the form of carbon sequestration waiting to come on in 2070 that is suddenly going to gobble it all up. It ain’t going to happen.

Q23 Dr Whitehead: I appreciate that, but the issue in terms of what you might think about in terms of policy formation in relation to keeping the temperature down, not as a precautionary principle but within that, and what you do about acting on a precautionary principle, itself has a number of ranges of precaution within it, depending on what material you have at your hand to decide at what point and in what form and what way that precautionary principle should be adopted. As you say, there are known knowns, unknown knowns, and unknown unknowns in this process. What you are suggesting is that in terms of, you might say, policy formation so far, unknown knowns and unknown unknowns are being excluded on the grounds that we did not quite know them all.

Aubrey Meyer: That goes on, that is for sure.

Dr Whitehead: Nevertheless that does not really help anybody very much in terms of trying to put in place-on the basis, presumably, of best science and best likelihood estimation-what those represent. Indeed, that is what you have tried to do to some considerable extent in these particular charts. Where does that leave us in terms of inclusion of at least known knowns and unknown knowns in the process and to what extent does that guide how one adopts the precautionary principle? That can range from stop all carbon emissions now to a slope much faster down the politically picked date of 2050, or indeed negative emissions post-2050 and so on, so even within a range of guidance scenarios that might emerge from that.

Aubrey Meyer: I hope I am going to respond to this because I understand the difficulty of describing it. The chains of causation are complex and uncertain.

The basis of precautionary policy in any circumstances is, do not run risks you do not have to run. The practical policy application of this, my dear friends, after all these years, is to unambiguously make the point that, whatever are the arguments for digging the carbon out of the ground and burning it off to the atmosphere, the cheaper option is to leave it in the ground and not do that and incur the long-term costs and consequence of doing that. That is the simple kind of brute, boy scout position on it all.

In terms of the management of all these unknowns, in a way C&C was conceived from the word go precisely to overcome the difficulties of the UN, which was that people quickly retreated into camps of saying, "We are not to blame; it is you in the United States or in Australia or in Britain. We have not had our turn yet. It is our turn now." The US, as you will remember, clearly took the position of saying, "This is not US warming; it is global warming and unless you are all in there is no deal." Basically, they are still saying that and here we are 20 years later. So C&C basically said, "Here is everybody in. That is a safe contraction and concentration amount." If you look at the back of this there is a model that gives you an online opportunity to literally manipulate this online so you can see not only the size of budgets and concentrations, but "in relation to our climate at-". But to say to everybody, "Let’s be a little Lincolneseque about this. Let’s, for reasons of practicality, just split this down the middle and say we will meet on equality," not because it is necessarily fair, but because it is rational and it is doable and it might overcome this perishingly stupid argument about blaming the other side-as you all probably may know, there has been a mixed take on all of that. The Climate Act was supposedly one of the more positive takes on that.

But the real thing here is that once you have the principle in play, you have to bring it to bear at rates that achieve enough soon enough to make it worth it, rather than, as your ex-member Colin Challen used to say, just achieving the heroic failure of doing too little too late when you might as well not have bothered to do anything and you have sadly played into the hands of all the contrarians who say you are trying to solve a problem that does not exist. In other words, whatever happens, there is nothing you can do about it whether you felt you should or not and we will all end up going in this hand basket to wherever it is going next, regardless of what anybody says or does. C&C, and therefore the Act, if it means anything, is aimed at generating a model of co-operation that is precautionary and doing enough soon enough rather than less than that, and critically then also enjoying the benefit of realising what it is like and what is possible as a result of being committed to that. It is not necessarily all bad news. It may mean that you have to fly less. It does not necessarily mean you have to bath less.

But the key thing is, the UNFCCC objective is a concentrations limit. They may not have put a value on it, but it has a value and it is a value above which we do not go. It is a bit like a speed limit or an alcohol limit. Limits are all over the place in our policy-rich environment. From there-an important point, this-it is not just a concentration of CO2 or whatever it might be, which is a volumetric figure; it is a tonnage. Those concentrations convert to tonnes and they are the same tonnes that we are digging up and burning and emitting and depositing in the atmosphere. You can do some fairly high-resolution measurements between the tonnes of emissions and the tonnes of concentrations. That is what the CBAT will do. You can begin to get a much clearer fix, not only for yourselves and your colleagues, but all our international colleagues, on what it really takes to come to terms beyond the uncertainties of the science and the waiting for evidence-based policy to emerge over the next 50 years as to how serious the feedbacks might be when you might well find that in some predictions the Arctic has already completely gone in summer by the end of next year. The rate of collapse is exponential. It has been pretty clearly demonstrated now. The idea that we have to sit around and wait for the next 50 years to find out if that happened and if we could measure it is completely daft.

Q24 Dr Whitehead: I do not think there is any suggestion, or certainly I am not making any suggestion, that one sit around and wait for irrefutable evidence of a succession of feedbacks that may or may not be taking place to the extent that we think they are or may be taking place at a much accelerated rate. However, as you say, unless some kind of handle at some stage is placed now on what those feedbacks look like, whether or not we have perfect information-

Aubrey Meyer: Yes. I completely agree with that.

Dr Whitehead: Then we will be in a position of saying we know those are happening but we can’t quantify them with any degree of success and therefore that is it, there is nothing we can do about it, so you do end up by default in the contrarians’ camp. Alternatively, one says that we can deal with the things we can deal with and we can’t deal with the things that we can’t deal with, so let’s hope the things we are dealing with have an effect.

Aubrey Meyer: It is a wing and a prayer, isn’t it?

Dr Whitehead: My central question is, how can we get a relationship between those two processes, bearing in mind operating the precautionary principle, which does give us some kind of practical guidance as to what policy then looks like? I think that is the central question.

Aubrey Meyer: Alan, that is perfect. That really is the question that hangs over this entire inquiry and effort, and that is exactly why this carbon budget analysis tool-this CBAT tool-exists. It is precisely to say, as Nicholas Stern, I was very interested to see, said a month ago to the IMF, "Just because these feedback effects that have been omitted are difficult to model does not mean you can put a value of zero in." No sensible person is going to do that. They even got this message from UNEP at the end of last year. This is the UN telling itself at the highest level, supposedly.

Q25 Chair: I am going to interrupt. I would like to try to concentrate on the very essence of Dr Whitehead’s question there, because I am very conscious of time.

Aubrey Meyer: Okay. This is where I am disappointed we can’t use the CBAT model1 on screen, but you will see straightaway the essential difference to note, first of all, is the generic one. The way the UK Climate Act has dealt with feedbacks makes it impossible to distinguish between what are budget and what are feedback emissions. This is a catastrophe. It creates a policy nightmare for anybody. The second thing is it wholly fails to recognise that feedback emissions will accelerate. They will not decelerate. They will accelerate in a way where we can do nothing about them-the strongest possible reason for arguing the precautionary case.

The CBAT model as an answer to how you integrate these things is the best possible answer I have managed to come up with in 20 years of trying desperately hard to communicate precisely this point and to take it out of what you could call the black box of economic and climate policy modelling into some kind of transparent process where you can literally read as you manipulate the thing on screen what the consequences are of the budget choices that you make.

Q26 Dr Whitehead: I appreciate we do not have enough time to do this particular knotty problem justice at all, but perhaps I could just give you a very brief example-albedo effect. That is very much a by-product of anthropogenic warming activity and has a feedback effect in a number of different ways, one of which, for example, would be the extent to which, say, we knowingly import large amounts of black carbon into the Arctic area, which quite possibly increases the albedo effect. That is an unknown known that may be a separate feedback effect from us, but nevertheless something that we are very much potentially doing a lot to accelerate as a result of very possible decisions that we might make in the very near future. It is the relationship between those two that I am particularly interested in trying to establish.

Aubrey Meyer: Let’s be clear about this. The fact that the ice, or some of it, is still there constitutes what you might call a negative forcing in that it reflects heat back. If you lose the ice, you not only lose the positive albedo; you absorb a lot of heat and mask a lot of the heat in the melting of the ice, but as that happens two things then are knock-on consequences. The first is you create a huge dark area of either ocean or newly revealed permafrost landmass that have negative albedo and in both instances the warming that occurs accelerates the release of gas-in the case of the ocean from clathrates, and in the case of the permafrost melted area from CO2 from biological activity and so on in the soil. The issue about black carbon being there if it goes there would only serve to accelerate that process because it is in effect a negative albedo. So there is no good news there.

Q27 Dr Whitehead: You might say, for example, the precautionary principle is, do not sail ships across the Arctic because they are the best possible thing you can think of to deposit black carbon straight on the ice as quickly as possible.

Aubrey Meyer: As a contrarian might argue.

Dr Whitehead: What I am trying to grasp at, in terms of those feedback effects, is their relationship with anthropogenic warming in the first instance and their further relationship with policy progression within that precautionary principle.

Aubrey Meyer: I just want to be clear. Are you arguing that the ships go in there emitting black carbon-

Chair: I do not think we want to concentrate on the ships.

Dr Whitehead: I was just using that as an example. I would personally think as a precautionary principle, do not send ships across the Arctic as the ice melts for the reason, among other things, of accelerating a feedback effect that we know about already but may well be additionally accelerated by our own efforts at that point. That is what I am trying to establish in terms of feedback

Aubrey Meyer: Alan, please, I invite you and your colleagues to be as radical as that on every single issue that relates to a choice about whether you release carbon or not. We are in real difficulties. This is not something we can fine tune at this point. We know enough to act in a precautionary manner and that if we do not we may well lose the day.

Q28 Caroline Lucas: Following on from Alan, I guess, and notwithstanding the known unknowns and unknown unknowns, is it possible to put some figures on the degree to which the current Climate Change Act targets would need to be changed and the carbon budgets tightened in order to reflect the projections that you are making? I appreciate that within that there will be a balance of risks that may or may not be acceptable. If at the moment we have an 80% cut by 2050 as a target, under your scenario what would that begin to look like? What could it begin to look like?

Aubrey Meyer: Half the size of the budget, not 395. It was originally 480 but now what is left is 395, take it down to somewhere like 200. James Hansen already told you, for example, at this very hearing, that the Climate Act target of 2 degrees is not safe, 1.5. He argues for 350, in other words bringing concentrations down to the value they were when we began this process. In 1992 they were 353.

On the point that you are raising, I struggled a lot with how to try to configure this in the policy language that has come to exist as a result of the Climate Change Act being created the way it was created. I finally came to the technique, if you will, on pages 28 and 26. You have the evidence and can look at those graphics. What I was trying to do here was not just to say GCI says it should be faster and so on, but to say in relation to what is going on, and the way what is going on has been argued, here is a way of coming in effect to the same conclusion that you came to with the Climate Act, only a way which you failed to lay emphasis on.

You can see it in the graphic at a stroke on page 26. Do not think of this as some horrific, repellent science graphic. This is a very simple piece of arithmetic. What you see there basically is the Climate Act budget divided between the LDCs, the less-developed countries, and DCs, the developed countries, with their convergence rate, the top line, by 2050. What you see above that is the full range of uncertainty published by the UKMO and the Climate Change Committee between their so-called 90-percentile and 10-percentile concentration outcomes, which we convert to a weight. We then take that weight-those are stock weights; they accumulate from the atmosphere-back into flow equivalent weights around that budget. In other words the budget could be bigger or could be smaller. So you have that technique.

If you go to page 24, it is the same thing again. It is probably easier to grasp there. The way they set it up was that the median was most probable, that concentrations were going to fall at that rate. I do not accept that, but I know that is what they have said and they are very determined to hold that position. So, holding to that position, we recalibrate the budget to be the weight equivalent of all those risk levels in the uncertainty on concentrations delivering that median concentration outcome. On page 23 that gives you a budget range that goes from the upper 473 Gt right down to the lowest, which is 204 Gt. That hits zero emissions by 2050, roughly speaking. That is exactly what we argued to you in the previous inquiry in 2009 and that is exactly, or as near as dammit, what James Hansen and Bill McKibben and all these people who take this issue of seriousness much more seriously than perhaps we do are saying as well, at least that.

To answer your question, should the Climate Act be used to deliver us more serious targets, if you do that kind of an analysis you can say to our dear colleagues in the Met Office, "In fact you may not have realised it, but you were saying this." It is worth putting to them and pointing out to them also that at the median case value where they asserted a full budget with that one falling concentration path that you see, they extraordinarily-or was it the Climate Change Committee or was it both of them, as I am unclear as to who pecked whom in this order?-gave an only 44% likelihood of achieving this magical 2 degrees. That is not exactly a high confidence level even if you have a high confidence in the goddamn model.

So, Caroline, it is a good question. That is the best way I could configure answering it. But it basically puts the whole approach that the Climate Change Act represents on trial itself because the primary thing is that it makes no sense from a policy point of view to mix up budget emissions and feedback emissions when nobody has a clue which is which with a range of uncertainty-I kid you not, look on page 22-that tells you that if you have the lowest concentration outcome, the entirety of that budget is returned to the sinks. If you have the highest concentration outcome, nearly 100% of it is retained in the atmosphere. The true answer is somewhere in the middle, called median. Do you really trust that? Especially when they also concede that all these major feedback effects, including permafrost melt and all the rest of it, are left out of the model and therefore have the value zero. I do not trust that. I do not know anybody who does. I am aware of the very determined effort to persist with this approach by the UKMO. I am very aware of the fact that they have cloned this all the way through into IPCC AR5 drafts and we will be landed with this for the next five or six years when the Arctic completely goes pop. If they want to do that, that is their choice. If you want to be guided by that, that is your choice. If you do not, that is also your choice. But the key question is, why are they doing this and who is telling them to do this? That is the real question.

Q29 Caroline Lucas: Thank you. I have one more very specific question, which is to do with the Met Office projection that carbon dioxide concentrations in the atmosphere will reduce after emissions peak.

Aubrey Meyer: There has been so much heat about this. To put it bluntly, just person to person, it is nonsense, but they insist that it is not. I think what is an indication of the extent of uncertainty that attaches to the UKMO judgment relates to page 20 in this evidence, and I think this is really important for this inquiry. Unbeknownst to me, after the previous engagement like this in 2009, the UKMO submitted a memo to the Committee addressed to me. Unfortunately nobody told me about it, so I only found out about it a month ago. What it said was, "Mr Meyer is wrong. We did include the coupled carbon cycle modelling into the runs from the Climate Act." I thought, "Well, that is news to me." I challenged the issue of 100% sink efficiency by 2050 because I did not think that was credible, but I did not know that bit. You then went to look and see what they actually did and that is what you can see on page 20. They split the difference, roughly, between the coupled and uncoupled model. Okay, it is an uncertain area, but the complete novelty that they introduced, contradicting 20 years of reporting by the IPCC from 1990 through to 2007, was the sudden introduction, because of this carbon fertilisation effect being included, of the fact that concentrations did not rise, which is what the C4MIP group they quoted said they did. They magically-in fact, the model was called Magic-reduced, and there you can see it. In other words, the sinks absorbed more than we emit within 40 years, if you follow that budget.

What I am saying to you now is that I do not believe that result for the simple reason that all the feedback effects they list as omitted themselves would guarantee that that would not be the case. That is a very mild way of stating what my real extent of anxiety is about all of this. So when Nicholas Stern rightly called for a new approach to the modelling, that is a primary area in which this new approach needs to be developed and I hope you will really press the Government with emboldened recommendations to respond on that point.

Q30 Simon Wright: I have a couple of questions about the international political context of your work. As we know, we should have binding climate change targets agreed by 2015 for 2020 onwards. I wonder if there are any thresholds or tipping points that you would point to on the near horizon that would mean an international agreement would be too late.

Aubrey Meyer: Good question. What can I say? I can’t give you an answer of a higher degree of certainty about that really serious matter than anybody alive because the blunt fact is that none of us knows. There is the possibility that we have already traversed so many tipping points, invisibly-these are unseen unknowns that you go over-that it may already be too late and therefore all this prognosis of having an international binding agreement by 2020 that somehow causes all this to go away is a pipedream in which case, as I say, the contrarians have won. The only way to respond to that situation is a bit like the way a parent would respond if their child had decided to experiment with crossing the motorway, aged five. You snatch it back and say, "Don’t go there. It is a certainty that you will get run over." A precautionary approach is to realise the danger, to make an assessment of the likelihood of getting across to the other side without getting run over, realise the odds are against you, and to play safe and stay on the pavement. That is a simple, everyday kind of parental guidance model.

In respect of the international agreement, the most poisonous effect in all of this has been from the word go-God knows I have been there 25 years now-this politics of blame, "It’s not me, your honour. It’s them what done it!" I can tell you, tragically in response to the UK Government and friends having fronted C&C at COP-15 in 2009 with the prescription of C&C 2050-it has been christened now C&C 2050, not for my pains-the Chinese were so incensed about it, you have seen a plethora of academic literature coming from the Chinese Academy of Social Sciences and so on, saying, "Convergence, even immediate convergence is not enough. We want accumulated per capita convergence. You had it all in the past. We must have it all in the future. Just get out of the way." This is not international consensus building, I tell you. The international programme to accommodate this from the radical side in the West is to develop a concept you may have heard of called Greenhouse Development Rights, which essentially says that it should all be income based. This sounds a little daring, let’s say. C&C is not income based; it is entirely emissions based. It says-get your head around this one if you can; I can’t-that the US must accept negative emissions entitlements by the year 2025, and that is the US and its friends. "This is better than C&C because it is fairer." I said to them, "I have only one question on this international sort of bonding exercise you are encouraging. Do you have any support on Capitol Hill for this approach?" The answer quite literally has been, "No, and we don’t need it because they are aliens. Aliens inhabit the Senate at this time." I take a deep breath and say, "Well, good luck".

Chair: I think we want precise questions. We are running out of time.

Q31 Simon Wright: Very quickly, we are moving towards an agreement that may or may not include some of the feedback effects that you have referred to. What do you think the effect would be of including uncertain feedbacks on the likelihood of an international agreement? Would including more stringent targets alongside uncertain feedbacks make international agreement less likely?

Aubrey Meyer: I know no analogy to give a comforting answer to this, but it is a bit like telling the doctor not to tell a patient how sick it is because it will make it miserable or make it less likely that it will live cheerfully. It is a real headache. As far as the IPCC is concerned, Bob Watson said in San Francisco as ex-chair last year, "We have made a point in the IPCC of erring on the side of caution and conservatism because if we didn’t we would be shredded by the contrarians." And you think you can’t play tactical games and footsie-footsie with contrarians under the table about this. Just be candid and be honest about what you know and about what you do not know, and do not claim to have the God’s truth and the exclusive right to hold the microphone.

Q32 Chair: I think that is the end of our allocated time. Obviously, it is a very complex issue. Thank you for coming along this afternoon and also for the evidence that you submitted.

Aubrey Meyer: Thank you for everything you have done for the last at least 20 years.

Examination of Witnesses

Witnesses: Professor Andrew Shepherd, University of Leeds, and Professor Myles Allen, University of Oxford, gave evidence.

Q33 Chair: I welcome you both to our session this afternoon. I think you realise that our time is limited and we are going to be constrained by votes in the House as well.

Before we get into the detailed questioning from my colleague Dr Offord, may I say to you, Professor Shepherd, we were very grateful for the evidence and help that we received with our Arctic report in respect of your late colleague, Professor Laxon, and we wish to express our condolences there? Dr Offord is going to commence the questioning.

Dr Offord: I would like to ask Professor Allen a question about global temperatures. Recently some people have dismissed climate change because of the flatlining of global temperatures. Temperatures have not risen as fast as we previously predicted, or had been predicted by climate models, even though we know that emissions do continue to rise year on year. <?oasys [np ?>What do you make of such claims?

Professor Allen: The assertion that temperatures have not risen as fast as predicted is simply wrong. There is a range of predictions made, and in fact the temperatures of the past decade have been pretty much exactly as was predicted for this decade back in the 1990s. It was an astonishingly accurate prediction.

On the issue about what the latest data imply for the future, there have been a number of papers published recently that suggest that the highest responses of the current climate models-the models right at the extreme top of the range of behaviour we are getting at the moment-look less likely, but the bulk of models are still, within the range of uncertainty, consistent with the observations. As I stressed in my little information note that I provided you with, there is no reason to see what is happening at the moment as a reason to abandon mitigation.

Chair: I have just realised that it was going to be Martin Caton who was going to ask this series of questions. I do apologise. I will revert to Mr Caton.

Q34 Martin Caton: What I was going to ask about was the results of the CryoSat-2 research. Can you tell us something about that?

Professor Shepherd: As you heard, the evidence that was submitted before was by my colleague Seymour Laxon and my other colleague, Katharine Giles, both of whom died in accidents this year. We are grateful for the work that they have done. I run the centre that they were part of. Our centre is an outfit that looks at satellite observations, as you have said, and CryoSat is the most recent mission we have.

When Seymour talked last year, we only had two years of data and so we did not have a great deal of information to give you. Since then, we have published some studies using the data and we have found that the instrument is working very well in the way that we expected it to work. It is now detecting 10 times as many sea ice floes as the previous mission. The sea ice cover in the Arctic is not a single entity. It is lots of different icebergs, if you like, and it is important for us to be able to detect every single iceberg. What we see from the data, when we compare it to the previous missions, is that the sea ice in the Arctic has decreased in thickness as well as in area over the past 10 or 20 years. The changes in thickness are just as important as the change in area, so the total reduction in volume of sea ice is somewhere between 20 and 40 cubic kilometres per year over the past decade. Some of that is due to reduction in area and some to thickness.

We also see some interesting things about the inter-annual variability, which leads us to believe that looking at one year’s worth of data-and most climate scientists know this in any case-is not a good way to analyse climatic variability because we have episodic events, sometimes associated with weather, which drive inter-annual variation. So the sea ice minimum from last year, for example, was largely driven by a very intense storm event that destroyed some of the sea ice pack. That is what most people understand to be the case. When we look at the CryoSat data, we see that in fact that storm not only destroyed the sea ice pack; it compressed it together and the sea ice pack that remained was in fact at the end of last summer thicker than the previous year. So it is more complicated than people have believed. However, in the following winter-this last winter-there was a reduction in the sea ice thickness as well. We see that from our latest results. That sets a different story for this year because the sea ice pack going into this summer is thinner than it was at the beginning of last year. So, all else being equal, we would expect to see another significant increase in the amount of sea ice melting this year.

Q35 Martin Caton: Do we understand fully the reasons for the record Arctic melt?

Professor Shepherd: Of last year? It is a combination of the long-term climatic changes that the Arctic is experiencing, and that is priming the sea ice pack to be more vulnerable. The sea ice has been thinning for a few decades as well-the satellite data show us that-at least for the past 20 years. That makes it more vulnerable to disintegration through melting, and that it is a fairly long-term process, but also to these episodic storm activities. That is what happened last year. It was a combination of the thinning, which primed the ice to be more prone to destruction, and the actual event that destroyed it. All things being equal, that could happen in the future.

Q36 Martin Caton: When we conducted our Arctic inquiry last year, we asked several witnesses when they predicted that we would see ice-free summers in the Arctic and we got a range of responses then from 2015 to about 2030. I notice Mr Meyer today has come to next year. Do you have an answer to that?

Professor Shepherd: There are two ways to predict changes in the future. One of them is to extrapolate the observations that we have right now. It is not a very sophisticated method. Those are the predictions that remove the summertime ice pack in the coming years. If you look at the climate models-notwithstanding the criticism they have just received, and I am sure that they will be defended in a moment’s time-around half of them predict that the Arctic region will be sea ice free by 2060. It is fair to say that there are omitted processes in those models that lead the median or mean predictions of those models to overestimate the extent of the sea ice pack. So the observations still fall outside confidence limits of those predictions and you might expect the Arctic region to be sea ice free some time shorter than 2060. I would say within two decades would be a more realistic prediction.

Q37 Martin Caton: Is your CryoSat data helping to harden up on that?

Professor Shepherd: Absolutely. Our centre works on collecting observations, but also incorporating or developing new physics-based models of how the sea ice pack interacts with the atmosphere and the ocean. The performance of those models needs to be evaluated first before you can build them into a climate model, and we do that. Then we build them into climate models and we have a very fruitful collaboration with the Met Office to improve the physics within those models. It is not a binary situation. It is a fluid situation. We are aware of the deficiencies, but we do strive to improve the models that we have.

Q38 Martin Caton: From what you began by saying about annual variations-I suspect I might already know the answer to this-do you have any idea what is the outlook for this year’s summer melt season?

Professor Shepherd: I am not a meteorologist. There are several meteorologists in the room. They perhaps will not make intermediate climate predictions, but what we saw from last year is that the destruction of a significant part of the sea ice pack, as I have said, was due to an anomalous weather event. It is true to say that its vulnerability was associated with how thin the ice was at that time. People have looked at the state of conditions just from temperature changes-that is the heat exchange between the atmosphere and the ocean-and they believe that last year would have been a record minimum in any event, even had the destruction of sea ice through weather not occurred last year. When we look at the data that we have right now, the sea ice is thinner at the start of the season than it was last year, so that would lead you to believe, all things being equal, that the sea ice might be thinner even still at the end of this summer than it was last year.

Q39 Martin Caton: How do you see research on sea ice volume progressing?

Professor Shepherd: Absolutely the key problem to be solved is the way that the sea ice interacts with the ocean. We believe that the ocean delivers more heat and causes more melting than climate models currently allow. That is well understood and people are working to try to improve their models in that regard. But also the way that the reduction in the sea ice cover alters the ocean surface properties-for example, the drag on the atmosphere that in turn affect atmospheric conditions and patterns of atmospheric circulation that include weather changes-is something that is important for us to look at in the future as well. So the way that the ice interacts with the ocean and the atmosphere needs to be studied in more detail. You would think that this is something that is commonly known, but in fact a sea ice floe might be something the size of this room and of course you can’t expect to incorporate that into a general circulation model that might have a resolution of 25 kilometres. So we have to parameterise these processes and we are moving forwards and improving those parameterisations.

Chair: Thank you. I will now turn to Dr Offord. I would also thank Professor Allen for the further piece of evidence that was submitted to our Committee today.

Q40 Dr Offord: Professor Allen, I understand that your work shows that the climate is less sensitive to carbon emissions than some people have asserted. Bearing that in mind, do you feel that there is any case for a change in our emissions target?

Professor Allen: The short answer is no. It is important not to oversell the adjustments to the climate system properties that have come out from looking carefully at the most recent decade. In summary, what we are seeing is a 25% to 30% revision in the upper limit, with no change in the lower limit, of the so-called transient climate response. That is the warming we would expect at about the time we reach double CO2. An even more useful number for this Committee is a new concept that some of you may recall my talking about the last time I spoke. I can’t remember who was there, but I talked about the total carbon budget in terms of what was underground before we started burning it and putting it in the atmosphere. It has emerged that there is a very simple relationship between the total amount of carbon we dump in the atmosphere and the temperatures we get to. When we first pointed this out in 2009 we were giving 2 degrees per trillion tonnes as a ballpark central estimate. More recent work has revised that down a bit. We would now say a range of central estimates would be 1 to 2 degrees per trillion tonnes. Given that we had 4 trillion tonnes available, that is plenty of carbon to take temperatures up to whatever temperature you might wish to contemplate or not wish to contemplate, as the case may be. So the idea that these revisions make mitigation unnecessary is fanciful.

Q41 Dr Offord: Apart from climate sensitivity, are there any other factors that are affecting global temperature rises?

Professor Allen: Clearly, we have natural variability going on in the record and probably contributing to the discrepancy between the model simulations and observed temperatures over the past decade, along with a contribution from drivers of climate that may have been left out of these runs. You should bear in mind that the runs that we are doing now for the Intergovernmental Panel on Climate Change were in effect designed back in 2005, using projections of what, for example, atmospheric aerosol loadings might have been. And if those projections are different from reality, and that will be affecting that discrepancy. So other things are affecting climate, but it is very important to understand that in the big picture what drives peak temperatures is cumulative emissions of CO2, so this number of 1 to 2 degrees per trillion tonnes of carbon is firming up. It is interesting that when we published the paper saying it was about 1 to 2 degrees per trillion tonnes of carbon-that is, recently, when we gave this update-a lot of the contrarians came out and said, "Well, this is what we were saying all along." If everybody can agree on that, then we can perhaps move forward, because that is a perfectly reasonable working number to consider the budget and it does not, as I stress, imply that we can relax in mitigation efforts.

Q42 Dr Offord: Would simpler climate change models, based particularly on temperature trends rather than emissions, better reflect your observations?

Professor Allen: It would always be foolish to extrapolate. I don’t know what a climate model based on a temperature trend would look like unless it was just fitting a straight line to 10 years of data, but I am sure you would not want to fit a straight line to the last six months of data and say that we will be cooking by Christmas. It makes no sense to just extrapolate temperature trends. However, we should seize on simple relationships where we can find them because it is very easy to make this topic very complicated. In the little information note, I did point out there is a really simple way of thinking about the budget in that figure 1. If we want to keep the budget below a trillion tonnes, and emissions are currently going up at roughly the rate I am showing, a couple of per cent per year, then we need to decrease from now on, on average, by 2.5% per year. If we discover that the response is slightly less high than previously thought, so in fact if we only get 1.5 degrees of warming per trillion tonnes instead of 2 degrees of warming per trillion tonnes, then it means we have to reduce emissions on average by 1.5%, or 1.4%, per year instead of 2.5% per year. But given that emissions are currently going up, depending on exactly which period you choose, of the order of 2% per year, the difference between needing to reduce at 1.25% per year and needing to reduce at 2.5% per year is really neither here nor there as far as climate policy is concerned. They need to come down. That is the bottom line.

Q43 Dr Offord: I also believe that you suggested a new way of tackling emissions by using carbon sequestration to ensure the safe limits on cumulative emissions are not breached. Can you explain to me particularly what the pros and cons of that approach are?

Professor Allen: It is not a new technology or fix in that way. It is a new approach to climate policy and it is particularly motivated here by trying to make the setting of climate policy less vulnerable to new science. One of the things we have discovered to our cost as the scientists, in the reactions to this recent paper about revising down the estimates of the response, lots of people jump on this and say, "We’ve got another 10 years so we don’t need to worry about this." So it is any excuse to do nothing. If you are setting climate policy entirely in terms of these short-term budgets, then whenever you get a bit of good news from the scientists it can be spun as, "Okay, we don’t need to do anything for the next commitment period." On the other hand, if you ever get a piece of bad news from the scientists, of course you can’t go back and say we should have done more in the last commitment period. So you end up, as Professor Meyer was suggesting, that it looks like the only sensible climate policy is the sort of extreme precautionary one where you take the most pessimistic possible view in order to ensure that you never get anything other than good news coming in from the scientists. That, however, makes you vulnerable to the fact that that outer limit on the response is the most uncertain part of our science. Trying to quantify all those low-probability, high-impact events that Professor Meyer was talking about is the hardest part of our science.

So what I am suggesting is that a much more robust way of framing policy would be to explicitly set a policy in place that can be adjusted to the emerging evidence. Short-term budgets are essentially assuming that future politicians will have extraordinary nerve to tighten them as necessary as the new science comes in. A much more robust approach, we are suggesting, would be a regulatory one where you essentially impose a regulation on the fossil fuel industry to begin to sequester carbon now, anticipating that they might have to up the rate at which they sequester carbon in, say, 2030, when temperatures reach 1.5 degrees, if they do reach 1.5 degrees by then, and we discover that we need to raise our game. So the bottom line I am suggesting is that the short-term budget approach, which we tend to be fixated on both within the UK and internationally, is very difficult to adapt to emerging science. Although economists say regulatory approaches are less optimal, it might in fact be much easier to adjust to the emerging science.

Q44 Caroline Lucas: What technical barriers might there be to doing that tomorrow or within a short time?

Professor Allen: There are no insuperable immediate technical barriers. It is a bit like peak oil. We just do not know how much it is going to cost, and I do not think we are going to find out how much it is going to cost by offering to pay for it, which is essentially what we are doing at the moment by paying for demonstrator plants. The only way you are going to find out what carbon capture and storage costs is by making it compulsory for industry to do it. They will almost certainly then discover that it is a lot cheaper than they are claiming it is at the moment.

Q45 Caroline Lucas: At the moment, other people are saying that the technology could be 15 to 20 years away.

Professor Allen: Let me give you an example. A few years ago Chevron and others discovered a gas reserve off Australia that was mixed with a very large amount of CO2. They were told by the Australian Government, "You can only pump that gas if you bury the CO2," and as somebody within the industry put it to me, "We thought about it for about five minutes and we said yes." Mandatory sequestration works. It is easy. You just implement it. Anybody who sells carbon should be required to be burying CO2. It is very simple.

Q46 Caroline Lucas: Is it happening there now?

Professor Allen: This plant is being built right now and as of next year will be the largest sequestration plant in the world. It required no carbon price. It was just they were told to do it and they did it. This industry can do stuff.

Q47 Dr Whitehead: Could I return to the vexed issue of feedbacks? We hear and we have heard, as you mentioned, the idea that when you roll in together the known unknowns, unknowns and so on, take a precautionary principle at the bottom of that rolling up and, as you say, then the only good news after that is good news. However, there are, I consider, a range of different probabilities within that. There are some absolutely catastrophic unlikely events, as you have mentioned. There are some feedback mechanisms that we do not know as much as we should know about, but nevertheless have a fair degree of understanding about, and there is also a fair degree of likelihood to them. What sort of position is the research in terms of that sort of gradation of feedbacks?

Professor Allen: There are clearly feedbacks that we do not know about and feedbacks that we do know about that are not included in the current climate models. It is important to stress that, for example in the latest IPCC assessments, a lot of these carbon cycle feedbacks that were discussed earlier may not have been taken into account in the explicit RCP-driven simulations of the large climate models, but they were taken into account in the sort of budget analysis that was done afterwards to try to work out what the implied carbon budget was for these different scenarios. Revealingly, when you take these feedbacks into account, or when you take into account the feedbacks that we know about, many of these scenarios suggest that a 2 degree future requires significant negative anthropogenic emissions for much of the second half of this century. Obviously negative anthropogenic emissions means capturing CO2 from the air by some mechanism and burying it underground, or getting it out of the active carbon cycle somehow.

Q48 Dr Whitehead: So, geo-engineering, effectively?

Professor Allen: That is a red flag for many people-

Dr Whitehead: That is why I mentioned it.

Professor Allen: If you call any negative emission scenario a geo-engineering scenario, this would be a form of geo-engineering. It is the carbon dioxide removal end of geo-engineering.

Q49 Dr Whitehead: If you are talking, for example, regulatory sequestration as a result of carbon production, then the logic of the next phase is regulatory capture without carbon production.

Professor Allen: The key point is that if we are facing a future where we might need substantial levels of negative emissions, we need to be developing the technology and the expertise and the experience now. Above all, we need the experience in which reservoirs are going to leak, because some of them will. We need to be building that experience up now. We can’t leave it until the 2040s before we start to deploy large-scale carbon capture and we certainly can’t leave it until the 2060s, when we start to try to do it in a negative emissions mode where it is all too little too late, it is fantastically expensive and it is also the most dangerous way of doing it.

Q50 Dr Whitehead: But what that scenario means, even what we talk about in terms of individual carbon backpacks under conversion scenarios, for example, is that those must continue reducing over a considerable period of time after the magic date of 2050 and it needs to be modelled, presumably.

Professor Allen: On how you would implement this, we would argue that the right point to manage the carbon budget is as the carbon comes out of the ground. We have the rather curious arrangement that carbon comes into Europe through a handful of pipes and holes in the ground and yet we try to regulate the carbon budget of Europe through millions and millions of exhaust pipes. It does not make a lot of sense. Upstream is what works and the way a mandatory sequestration regime would work is that you would make it a requirement that if you want to dig up fossil carbon and if you want to sell it, you have to prove that carbon dioxide is being buried at the appropriate rate. It need not be the same carbon dioxide, of course. You can pay somebody else to do it. But that would be the way forward.

The key point is that what this approach would mean is that we would be capturing the cheap carbon now-the stuff that would otherwise be burnt and vented to the atmosphere in coal-fired power stations-rather than venting it into the atmosphere and then capturing it much more expensively later. So, while CO2 is still very readily available, let’s just get on with it.

Q51 Dr Whitehead: Isn’t there a missing third of this? I fully appreciate what you are saying, that you institute a regulatory regime that has a regulatory gas tap of 1 to 12 so that you could turn up the anti-gas tap, depending on the nature of the emerging science as that relates to feedbacks and refining of the level of carbon emissions and so on. However, there is this element that I find difficult to quantify-and I don’t know the extent to which emerging science is doing that-and that is how human action deals one way or another with the intensity of feedbacks once one knows what the intensity of that feedback is likely to be. I did give the example earlier of albedo effect and black carbon, and there is a whole range of other examples that one can give in terms of mitigation of particular effects feeding into the overall effect by particular policy instruments. Is that something that you-

Professor Allen: Yes. The key thing to focus on here is the fact that different effects operate on different time scales and fast effects are relatively easy to respond to because you realise that it is having an effect, you stop it, effect solved. The real challenge we have are the cumulative effects, particularly CO2. You put a tonne of CO2 into the atmosphere and its impact will persist for millennia. So that is the irreversible process and that is why we need to be readying ourselves for the possibility that by 2100 we may well be pumping CO2 back out of the atmosphere. We need to be prepared for that eventuality. I am not suggesting that it is necessarily going to happen, but that is one of the eventualities we need to be working towards. Carbon capture is such an obvious part of the century time scale view of this whole phenomenon and it bemuses me why it is not a higher priority-I am not a carbon capture specialist at all; I am just a climate modeller, but I step back and I look at the problem as a whole and see all this carbon sitting underground, most of it in India and China, and everybody having every intention of using it. Without this technology it is going to end up in the atmosphere, so why are we not going all out to make sure that technology is available to our children when they need it?

Q52 Dr Whitehead: Or don’t use it in the first place, presumably?

Professor Allen: If this Committee is confident that we can ensure that India’s coal is not used by the citizens of India in 2080-

Dr Whitehead: Technically.

Professor Allen: I take your point.

Q53 Dr Whitehead: Yes, any more than we are likely to be confident that all that use of coal will then be sequestered at the same time.

Professor Allen: I would place greater confidence in our ability and the ability of the fossil fuel industry as a whole to address the problem than I would in our ability to tie the hands of the Indians of 2080 and tell them not to touch their coal. That is a relative statement.

Q54 Caroline Lucas: We have been talking a lot about the science of climate change and I wanted to talk more about the politics and the psychology of it, in a sense. We have the extraordinary situation that, as the seriousness of climate change becomes ever more evident, we also have the growth of climate scepticism or whatever we want to call it. The media clearly is not playing a helpful role. Professor Allen, you had a really interesting article in the Mail on Sunday a couple of weeks ago. I do not know if you saw how it was presented, but there was a nice big strapline across the top saying something like "Green Eco Con" and the sense of what you were saying was utterly distorted by everything else that was around that double-page spread, it seemed to me at least, in the Mail on Sunday.

Professor Allen: I hope the members of this Committee will appreciate that I wrote the words not the layout.

Q55 Caroline Lucas: I was not for a second suggesting that you were responsible for the way in which it was presented, but that to me encapsulates what I am trying to talk about, which is that on one hand I want scientists to speak out more because there is this chasm between what is the scientific consensus, largely, and public perception. But then when scientists do try to speak out more, like your article, with which I was in agreement, you then have the undermining of that, in my view at least, by the media. My question is, from your perspective, what is the best way to try to close that chasm?

Professor Allen: I know-as I commented in an earlier article, David Rose is a neighbour of mine and we frequently have conversations about this-there are certain people who see climate change as an evil plot by the European Commission to take over their lives and therefore reject it regardless of the science. For the record, I would not necessarily put David Rose into that category, but I suspect many of his readers would probably fit into the category. I think the problem is the way that the whole issue has been framed to people. If you tell people that the only solution to this problem is a wholesale reinvention of their lifestyle, then people who do not necessarily want their lifestyle reinvented for them find they have no option but just to reject the whole issue entirely.

If it was seen as bickering over the level of regulation in a large and not particularly popular industry, we could be in a different place to the place we are in now. We could be arguing about how much carbon Shell should be sequestering next year. I put it to you that if that was the argument, people’s views on it would be very different from what they are now and we would be well on our way to solving the actual problem. So that is why I am putting it to you. I have to make this pitch really hard because I know a lot of economists say regulation is wrong-it is economically suboptimal-but if this conversation was simply about the level of regulation of a particular industry, an industry that is the most profitable industry in history ever, and that would include illegal industries as well, it would be a very different conversation. So I think the difficulty is the way we have allowed the problem to be framed as essentially a lifestyle discussion rather than essentially a technical problem about how one particular industry deals with its waste.

Caroline Lucas: I would love us to have more time, but I am sure that we do not, so thank you.

Chair: Professor Shepherd and Professor Allen, thank you both for your time this afternoon. Thank you very much indeed.

Examination of Witnesses

Witnesses: Professor Julia Slingo, Chief Scientist, Met Office, and Dr Jason Lowe, Head of Knowledge Integration and Mitigation Advice, Met Office, gave evidence.

Q56 Chair: I start by welcoming you back to our Committee, Professor Slingo, and also thanking Dr Lowe for coming along as well. I think that you sat in on the previous two sessions. I am sure there will be an opportunity for you to respond to some of the comments that have been made. Without going there at this stage, I want to take up where Caroline Lucas left off with our previous witness and ask about public perceptions and the way in which there is a sort of mismatch between accurate reports and inaccurate reporting and what the Met Office has done to put that right. But, then, isn’t there a deeper issue about how does the public understand this issue, given the conversions of scientific evidence altogether?

Professor Slingo: I think this is at the heart of a lot of the difficulties we have. The science is incredibly complex. We have to find ways to make that science accessible without diminishing both the integrity of that science and the importance of the science message. I think what we have learned is that it is easy to dumb this down, and I think that a lot of that has come perhaps from the way in which climate change has been couched over the last 20 years in terms of global temperature values, so we are still <?oasys [np ?>talking about a 2-degree target and of course most people don’t understand what that means. It does not sound terribly challenging for them. I think what we need to talk about is what that means at the regional and even local level, and it is this dangerous for whom, when and where. So a lot of what I think we have to do now is to personalise the message. We have to use the science that we know and understand to help personalise that message. Often I think we get scared about putting complex science out. My experience has always been, since coming to the Met Office, that the more that we put our science out, albeit in language that the non-specialist can understand, the greater the appreciation of what our job as scientists is in this whole debate, but also it is a question of an educational role.

I also think very strongly that we need to put it into context. We should not be talking about the science so much in abstract terms. Last week, we launched the Climate Service UK with Ed Davey, the Secretary of State, and that is about making the science relevant, seeing how science can be used to the benefit of society to tackle what undoubtedly will be challenges as we go forward. It is a process, a dialogue, that we need to set up between us, the generators of the science and the evidence, and the users of that evidence and that science who are making decisions about their private lives, about their businesses, about Government policy. I see the establishment of the Climate Service UK as pretty much of a landmark thing that the UK has done to help create a space in which that dialogue can develop and, I hope, thrive.

Q57 Chair: You talked in your opening remarks about the integrity of the science. I think that there is a sense that there is not the same integrity in terms of the press reporting. Do you see that just as a natural response, a natural scepticism, or do you think there is anything more deep-seated that is causing this cynicism to be there?

Professor Slingo: That is a difficult question. It is a value judgment. I think that in some areas of the press there are undoubtedly opinions-I will not call it vested interests-that are formed not on the basis of the best science, but because they believe that presumably it will sell more papers, which ultimately of course is a driver for many parts of the media.

Q58 Chair: There is consensus among the scientists, isn’t there?

Professor Slingo: Absolutely, and yet I think we all see that balance is not given fairly between the scientific evidence, the weight of the evidence, and the consensus among the scientific community, and indeed many people who are not climate scientists but appreciate the scientific arguments, and the few who would seek to deny that evidence. I think it comes back always for us to how we communicate that science clearly, make it relevant, put it in context. It is very challenging. I can’t think of a science where that challenge is any greater because virtually everybody knows about climate change and everybody is concerned about it. It is perhaps something that we really do have to spend much more effort on. Perhaps Jason would like to comment on this.

Dr Lowe: On the general point of trying to communicate climate science, we found in the AVOID programme we did not just address the climate science but we also looked at the impacts and at the technology associated with mitigation, for instance. Certainly in my experience, what people are tending to respond to is what the future will be like for them, of which what the weather will be like is one component, an important component, but it is about describing the entire lifestyle.

The other thing I would add is, from the perspective of a working scientist, almost by our very approach we try to be impartial and we are trying to take on board new evidence as it comes. It can be quite challenging sometimes if, for instance, we are dealing with a very polarised view from either climate advocates or climate sceptics.

Q59 Chair: Just going back to what you were saying about communicating the science, in terms of your own work, how frequently do you review what is happening and how do you report that to the Committee on Climate Change, to Government Ministers? Is that reporting procedure you have of the work that you do an opportunity as well for you to get the message across about what is happening?

Professor Slingo: Yes. There is no doubt that we know that climate science moves forward and often at pace. Of course, the Met Office Hadley Centre provides almost continual updates to our major stakeholders, DECC and DEFRA, and of course to the Government Chief Scientist, on policy-relevant advances in climate science.

Q60 Chair: How frequent is that?

Professor Slingo: It can be weekly, and often if a new paper comes out in the literature, which they have seen, they will ask us to comment on it. For the Climate Change Committee itself, we certainly engage in the process of updating the carbon budgets and our evidence is sought as a formal part of that. We are just beginning that process again to update our best understanding-not just of the science we produce in the Met Office Hadley Centre, but of all the international science, all the evidence that is available from modelling and observations-to frame what we would consider to be the best scientific advice we can provide.

Q61 Chair: How does that relate to the IPCC?

Professor Slingo: That process occurs every two years and of course we are a major player in the IPCC. At the fifth assessment report we have a large number of contributing authors and we have probably contributed more model simulations than virtually any other group in the world, so we take the IPCC process very seriously. That being said, it is in a sense a consensus view of the world scientific community and because of that it can only happen every six or seven years because of the nature of the demands and of course taking account of the pace of the science. I think it is really important that between those assessments we provide updated evidence. The science is continually moving on and I think Professor Allen made the point that the simulations that we produced for the fifth assessment report were set in motion eight years ago, the model development for that maybe a decade ago, so it is really important that we communicate the latest modelling results and science to Government on a continuous basis.

Dr Lowe: Can I add to that on the Committee on Climate Change? If we take the dates of the fourth assessment of 2007, we worked very closely with the Committee on Climate Change in 2008, going into 2009, and what we were doing there were the actual simulations that had been well documented, but also reviewing the wider literature with them. We worked with them again very closely and formally ahead of the fourth budget period and provided a comprehensive review of our understanding of climate impacts and also thresholds and feedbacks in the climate system. We have started very recently-this month-an assessment of the climate budgets based on the new science that we have seen over the last one or two years. The time scale of that is designed so it will deliver after the fifth assessment comes out so we can also take that view into account. In between those we have regular informal contacts with members of the secretariat.

Q62 Dr Offord: Your recent decadal forecast predicts lower global temperature rises than you previously thought. What prompted that change?

Professor Slingo: I think we need to be clear what the decadal forecast is trying to do. It is what we call a climate prediction from the current state of the climate system. It is telling us much more about natural variability in the climate system and very little that relates to climate change per se. I think the one you are alluding to is the one we published as part of our research outputs to DECC and DEFRA at the end of last year in which we suggested that in the next five years we were not expecting a very significant rise in global temperatures. That forecast was made from the current state of the oceans and atmosphere at the end of last year, so it is really looking at the variability in the climate system-things like El Nino, things like the Atlantic Multidecadal Oscillation, and how they will play out in the next five years. That forecast was very clear that the current state of particularly, the oceans would continue to control global temperatures over the next five years in this particular way. It says virtually nothing about climate change. I think again that was an example of the work being misrepresented in the media to try to make a story out of something that we believe is a very important part of the science that we need to do. I did produce a science report that explained the whole process of decadal prediction and how we should interpret these forecasts. We took it as an opportunity to put our science out there and to show the various elements that come into play when you look at near-term climate change. It is this intersection between anthropogenic global warming and natural climate variability in the system that makes predicting the next five or 10 years or so really very challenging.

Q63 Dr Offord: I am sure that we all agree there are a number of factors that affect global temperature rises, particularly in response to issues such as greenhouse gases, including climate sensitivity. How much do we know about climate sensitivity and is this included in climactic models?

Professor Slingo: As we have already heard from Professor Allen, we can use observations to assess climate sensitivity. As far as the climate models are concerned it is an emergent property, not the climate model. It is a result from all the complex interactions of the physical processes that go on in the climate system, from very fine scales to very large scales, from short time scales to long time scales. It is, as I say, an emergent property. It is really important to understand that in no way are we putting in the answer for what we think the climate sensitivity is. It is also important that we do use observations in some ways to check whether our climate models are within sensible bounds. Even with the latest updates to what we think the transient climate response is or the equilibrium climate sensitivity, using the last decade of observations, certainly the model we use and many of the leading models around the world are still within the range of those estimates.

Q64 Dr Offord: Thank you. Professor Hansen told us back in May that the greenhouse gas effect meant that more energy is entering the atmosphere than is leaving it and he said that was creating a pipeline of stored-up warming. He warned that once the pipeline is added to existing warming we would be getting close to the boundaries of dangerous climate change, even if emissions are stopped now. Do you agree with his assessment on that?

Professor Slingo: Yes. The lifetime of carbon dioxide is so long in the atmosphere and it is really important that we look at this as a cumulative problem. We should not look at what our emissions are in any particular year. We know that the accumulation of carbon in the atmosphere and the response of various parts of the climate system, particularly the oceans, are quite slow. Therefore, even if we stopped emitting today, we are committed to a certain level of warming out for the next century or so. Those changes are slow and the way in which the system comes back into equilibrium from the perturbed state that Jim Hansen describes takes so long that we will end up at a different equilibrium state that will be warmer than where we started, all other things being equal. Jason, do you want to add to that?

Dr Lowe: If I could say on the first bit, yes, we agree with Professor Hansen’s view that there is a top of the atmosphere imbalance, for the reasons that Professor Slingo said. There are two other relevant points to mention. One of them is associated with this idea of climate sensitivity and just what processes are included. When I have consulted the literature that Professor Hansen has published, he highlights that, on top of what we have traditionally talked of here as the climate sensitivity, these very vast processes, there is an additional set of earth system processes that may lead to a higher value of sensitivity but on very long millennia time scales-for instance, associated with changes in the ice sheets.

The other thing I would add is in terms of the target long-term goal that different people advocate. I think from the Met Office point of view we would recognise that the science can provide guidance on the level of impacts, give or take uncertainty, as the climate changes. We would probably also agree that we are learning more about potential thresholds and additional earth system feedbacks, but I don’t think we would advocate one particular target such as 1.5, as Professor Hansen often puts forward.2

Q65 Dr Offord: My final question, perhaps slightly cheekily, is, are you saying that now your forecasts are saying that we won’t have any hot summers in the next 10 years or so?

Professor Slingo: Have our forecasts said that? I don’t believe they have.

Q66 Martin Caton: Aubrey Meyer said that the Met Office claimed to include all feedback effects in its projections on global emissions when it had not. Would you like to respond to that assertion?

Professor Slingo: Yes. It is absolutely untrue. We have, as I think has already been made clear for the fifth assessment report, entered the fifth assessment with a full earth system model that includes feedbacks associated with the terrestrial carbon cycle. It includes dynamic vegetation, so this is the long-term changes in forests and shrub land and so on, ocean bio-geochemistry and interactive atmospheric chemistry. So, to say that we don’t include them is absolutely wrong.

What we are very cautious about is not to introduce processes-feedbacks that we know are either not well constrained by observation so that we can evaluate what is coming out or where the science is not mature enough to be able to put something in. Again, I come back to the integrity of the science. We could put an awful lot of these known unknowns into the model, but how would we know that the results we were getting out were trustworthy? That doesn’t mean we don’t look at them, and I think he raised the question of permafrost. It is correct that at the moment we do not have a model of permafrost included in the earth system model and that is because we know that there are still massive uncertainties in basically defining the depth of carbon concentrations that are in permafrost and those sorts of thing.

Nevertheless, we have worked with the international community to see what the range of likely outcomes would be, and the example I will give is by the end of the century under a high emissions scenario. That is the RCP8.5. We can estimate it is somewhere between 50 and 270 petagrams of carbon could be released and between 2 and, I think it is, 59 teragrams of methane. From that you can show that the additional warming that you would get from those emissions would lie between 0.08 and 0.36 degrees C. That at least is now beginning to give us a bound, so for him to say that we don’t think about these things-we do.

Q67 Chair: Wasn’t he saying all feedbacks? Wasn’t that the issue?

Professor Slingo: Yes, and I think we also know that. The one he did not mention is wetlands. We know wetlands are a massive natural source of methane in the current climate and we are looking at that. None of us would deny that some of these feedbacks are potentially large, and our overall view is that together they represent a positive feedback on the system. In other words, they will lead to additional burdens of carbon in the atmosphere. We are trying, as knowledge develops and as observations improve, working across the range of science disciplines that you have to engage with, but we need to find ways to at least set the lower and upper bounds of what we think will be the likely scale of these feedbacks, and that is what we have done with the permafrost.

I should also say that we are just starting a major programme of research and development with the Natural Environment Research Council to build the UK’s earth system model for the future, which we hope will enter the sixth assessment report. That is an activity that is going to draw on the best science that exists among our terrestrial ecologists, our oceanographers, our ocean bio-geochemists. We are going to work with people who deal with the cryosphere, the ice and so forth. That is the way we can move forward and gradually develop an understanding at least of what the range of risks we face are. We are a long way from being quantitative, but I think we know far more about the potential bounds of some of these feedbacks than Mr Meyer suggested.

Dr Lowe: Just to add something very specific here to do with how we have provided information to the Committee over time. We have seen what we provide is a package of information that consists of quantitative information from model simulations, but also information on our interpretation of the literature. That goes from what we also provide to a whole range of stakeholders including DECC and DEFRA. What we are seeing over time is more of those things that we were just starting to understand in the literature perhaps in 2007 and 2008. We are getting to the point now where we can start to make the first quantitative estimates. That is what the new model, the HadGEM2 ES model, is letting us do. What we are doing in the most recent piece of work specifically with the Committee on Climate Change, that I mentioned earlier, is to ask the question, "Given the newer quantitative understanding, what does that specifically mean for the global carbon budgets?" I think it is important to recognise that it is this whole package of information. It is about understanding the literature as well as building it, as understanding improves, into the model.

Q68 Martin Caton: Is the objective to reach a stage at some point where you can quantify all these different feedback effects in your climate models, or is that unrealistic?

Professor Slingo: I think it is, what do you mean by "all"? One of the other things that as scientists we have to make a judgment on is what the priorities are, because when you start to do the thought processes of all the potential feedbacks that can go on in the system and include those all in a model it would make it massively complex. Part of our job, and with our scientific colleagues, is to say, "What are the big things we absolutely have to cover off?"

For example, we have had a good look at the carbon cycle. We know where there are gaps in knowledge, but I think we have a bit of an understanding about the carbon cycle and its time scales of adjustment, but then you say one of the limitations on the carbon cycle will surely be the nitrogen cycle. We don’t include the nitrogen cycle, but we have assessed potentially what those limitations would be and in our view they are big enough that we have to include the nitrogen cycle going forward.

It is an evolutionary process, but it is one where I believe very strongly that we have to do it in a very structured way. We have to prioritise it in terms of what we believe are the magnitudes. We also have to prioritise them in the sense of the level of knowledge that we currently have, and in the sophistication of the observations and the comprehensiveness of the observations. If we can’t constrain the model and evaluate the model against observations, I think we are going into fairyland, and I am very reluctant that we ever go into fairyland. It has to be strongly rooted.

Q69 Martin Caton: Another criticism that Aubrey Meyer made was that you overestimated how fast CO2 in the atmosphere will reduce, because he says carbon sinks will not absorb as much as you predicted. Would you like to comment on that?

Professor Slingo: I will let Jason talk on that one.

Dr Lowe: I believe what is being referred to here is if we look at a time series of the median output of the atmospheric CO2 concentration, what we will see in the scenarios such as the 2016-R-4L is that the CO2 concentration is peaking partway through the century. It is actually peaking mid-century and then after the peak it is declining quite slowly. The temperature does not decline, it is levelling out. The point of argument HadGEM2 ES-certainly my understanding of it-is that the peaking of CO2 and the decline begins before the emissions have fallen to zero, the man-made emissions. So, we are still putting man-made emissions into the atmosphere, but we appear to be in a situation where the CO2 concentration is falling.

You see that in a whole range of earth system models of different complexities from different groups. What is happening is that during the period you increase the concentration-the period we are going through in reality today and will go through over the near-term decades-the concentration of CO2 in the changing climate actually alters the sinks. As we start to reduce the emissions, we have changed the sinks from the present-day value, and we have changed them in such a way that they are able to take up sufficient carbon that the CO2 concentration is able to peak and come down the other side. We could provide this as supplementary evidence later, but the literature backs this up from a whole range of different model structures.

Q70 Martin Caton: Given the direction of debate in some circles creating an alleged dichotomy between growth and tackling climate change, is there any pressure on you as Met Office scientists to be more cautious than you would otherwise be on what is needed to avoid dangerous climate change?

Professor Slingo: Certainly not. Our role in the Met Office is to provide the best underpinning science, honestly, transparently, openly. We place very highly the peer review process. Last year, for example, the Hadley Centre published 149 papers in the leading journals, with seven in Nature. For us it has to be absolutely about the integrity of the science. If we don’t do that, I think I am failing as a scientist. This is deeply rooted in the way we do research and we sometimes do have to say things that perhaps people don’t want to hear. But at the end of the day it is all about the truth and our best understanding of what the science is telling us-not just our science, but obviously our peers around the world.

Q71 Dr Whitehead: Do you consider legal action from the point of view of the Met Office? If you are described as "the crazy climate change obsession that has made the Met Office a menace", that appears to be libellous, does it not?

Professor Slingo: It is a very difficult area for us, and of course I get personal attacks that for many people would be considered libellous. In a sense, what would be gained by it, is my view? At the end of the day, we have to carry on doing the job that we have been asked to do, which is to provide the best scientific evidence to support what decisions Government have to make, and increasingly what wider society will have to make. My job as Chief Scientist is just to ensure the integrity of that process. If people want to throw bricks at me then, yes, it makes me angry. It actually makes me very, very sad because science is about the search for truth and it is about being honest, and these people are not being honest. I don’t think that it would further the cause of putting out the truth of the research we do to take legal action. That is where I am. Jason, did you want to comment? You are obviously the butt of this sort of thing too.

Dr Lowe: Personally, it makes me more determined to be impartial in the science that I do. I know that when I sit down and do a piece of research I am as impartial as I possibly can be. The same goes for the scientists around me who work in my team. In some ways it is disappointing that not everyone is impartial, but that is the world we live in, isn’t it?

Q72 Dr Whitehead: I think I probably agree with you, but I thought maybe I should ask. When you came to see us recently you talked about the Arctic and how climate change is affecting the Arctic. In terms of emerging science, has there been any process of looking at the view that you expressed when you came to see us previously and has your view changed at all-for example, in terms of the likely date for an ice-free Arctic sea in the summer?

Professor Slingo: I came and spoke to you just over a year ago, I believe, on that. At that time, I said we were waiting for the CryoSat-2 data to confirm particularly the volume loss, because very much the critical aspect of what is happening in the Arctic is the thickness of the ice and what that means for the ice cover loss. We have heard from Professor Shepherd today there are fantastic results coming out of CryoSat-2. It is an important landmark. What I said at the time-that I did not believe that the Arctic would be ice-free next year-still stands from all the scientific evidence that we have seen. I think that the best evidence coming out now from the fifth assessment report, using all the streams of evidence, not just the model simulations, is that the most likely date is around 2030, which is what I believe I said last year.

There is no doubt that as each year goes by we see increasing evidence of the fragility of the system, and therefore it is really important that we draw together all the best scientists to work on this problem, and that is very much what I have been working on in the last few weeks. Indeed, certainly going through in the next few months, we need to pull together the polar science community, particularly after the loss of Seymour Laxon and Katharine Giles, which is devastating to the community. I do not think overall a radical change in the science, but of course I was clear then that this is a very complex system. It involves the ocean, the atmosphere, the sea ice, natural variability, and, increasingly, weather variability and how the winds blow across the Arctic in summer. That hasn’t changed, and again our push is to understand that system as best we possibly can and improve the processes in our models.

One of the very exciting developments for us in the Met Office is that we have made some very significant advances in our seasonal forecasting model and we are now using that to forecast the progress of the Arctic through the seasons. It is a research tool, but what it does is allow you to immediately confront the model with observations of what is actually happening and what you are actually trying to forecast. That is proving to be a very powerful way in which we can unpick the processes and gaps in our model versus the observations. So I think we can make significant process in this in the next few years and I am confident that we will be able to be more certain about the future fate of the Arctic in the next two or three years.

Q73 Dr Whitehead: In terms of the CryoSat-2 results, are you incorporating that in terms of what we have been talking about this afternoon: positive feedback particularly relating to albedo and the suggestion that, as you have mentioned, the persistent change in early summer wind patterns and the extent to which that raises a question of whether a tipping point has already been reached for those sort of effects?

Professor Slingo: Yes, absolutely. These are the big issues there. The CryoSat-2 data have given us a benchmark around the ice thickness that helps us to see where the issues are in our sea ice model. We also know that the Arctic Ocean is warming up and we do some very detailed budget analysis in the fully coupled model to understand the role of both bottom melt, because the ocean is gradually warming up as it acquires more heat from the fact that we lose ice in summer, and the way in which the winds and the atmosphere are driving the top surface melt or disintegration of the ice.

What we are seeing from CryoSat, and as was emphasised by Professor Shepherd, is that it is a very fine-scale, complex system and, as modellers, we are still trying to model this, and we have to because of the computational constraints of relatively coarse resolutions. One of the things that has been a real push for us in the last few years is to develop prototype much higher resolution models to try to understand where the gaps are in our lower resolution models that we have to use for long-term prediction. That whole process of using observations model theory is how we have to go forward. It is not one thing alone that drives progress in this field.

Q74 Mark Lazarowicz: Some scientists have found that increased ice loss has occurred because of a persistent change of early summer Arctic wind patterns. Is this evidence of a tipping point having been reached?

Professor Slingo: No. I think that is probably a fair comment. There is no doubt that in the last few years we have had some quite perturbed wind patterns around the northern hemisphere that may be associated with a very persistent cold phase of the equatorial Pacific, the so-called La Nina-type setup. We don’t know whether that is unusual, and certainly the Arctic cyclone last summer that broke up so much of the ice was unusual, but not unprecedented. I think what we are seeing is a system that is gradually becoming more fragile. As the ice thins-and I have talked about it can be forced from the top and warmed from below as well-it is more susceptible to varying wind patterns. I think that is what is going on, rather than that the winds themselves have changed.

Q75 Mark Lazarowicz: I will move on to another question that again raises the issue of whether there are longer-term conclusions that can be reached or whether it is simply an event that we will just see how things go in the longer term. The melting of the Greenland ice sheet last year set new records. Have you drawn any conclusions from that, particularly in terms of supporting trends or in relation to your assessment of the risk to sea levels?

Professor Slingo: You have to be very careful there because that was about the appearance of summer melt pools across the surface of Greenland. Throughout the seasonal cycle we have to look at the complete budget of the Greenland ice sheet. In terms of sea level, there has just been a major paper led by a colleague from Reading University on the various contributions to sea level rise over the last century. Looking through that, the conflicting evidence for the contribution from Greenland just tells you where we are, which is this is really hard. One reconstruction shows a mass loss that is equivalent to 0.2 mm per annum rise in sea level and another one says that there is a mass gain equivalent to -0.3 mm per annum. This is where we are, and we have to look at the system throughout the seasonal cycle, both through the winter gain and the summer loss. Jason, I don’t know whether you want to comment on this.

Dr Lowe: In some ways you probably had the perfect person to ask in one of your previous people. For instance, work published by Professor Shepherd trying to reconcile the estimates of ice sheet mass balance has certainly been a step forward and tells us a lot more about the contributions from the different ice sheet sources over the past couple of decades, but even when we look to the latest generation of climate models and climate projections, one of the weaknesses is it is still very difficult to model the ice sheets at a high enough spatial resolution and include all those processes such as how ice moves in outlet glaciers and the feedbacks associated with the termination point of those outlet glaciers. When we look at projections of sea level rise from the Greenland component, and other components as well, we can still expect to see quite wide error bars or uncertainty ranges for this IPCC assessment at least, I would expect.

Q76 Mark Lazarowicz: Finally, on a different point, I understand the Met Office is convening experts from around the world to discuss the rate of summer melting in the Arctic at some stage in the near future. Is that the case and can you tell us anything about that initiative?

Professor Slingo: One of the things that is getting increasingly prominent in the media and debate around scientific circles is whether the changes going on in the Arctic are influencing northern hemisphere weather patterns and producing essentially more extreme weather. There are some papers that purport to detect a change in northern hemisphere waves, the big gyrations, the blocking weather patterns that we saw recently when we had a bit of fine weather-whether they are becoming more locked and stationary, stuck in a particular place, or whether larger amplitude. The whole notion that climate change could manifest itself quite subtly through what we call climate disruption is something that we need to spend more time looking at. In other words, it is taking us from the global down to the regional. There is a lot we still do not understand about what might be the impact of the warming Arctic on our weather. Recent work that we have done at the Met Office suggests that it certainly contributes to the likelihood of having a colder winter, but we have not yet found any detectable signal for summer. That does not mean that there is nothing there yet, but I think we do need to come together and say, "How much do we know? Where are the gaps? How do we detect that there is a change in our northern hemisphere weather patterns, and if we can detect a change what do we attribute it to?" There are lots of factors, but the Arctic could be one of them and we need to be getting to grips with that.

Chair: Thank you very much indeed. We will be very interested in any outcomes you get from that meeting of experts to look at that. It has been a lengthier session than usual, so I thank you and the previous witnesses and the members of the Committee for this lengthy session on this most important issue of carbon budgets. Thank you very much indeed for your attendance this afternoon.

[1] Note by witness: The CBAT model is at http://www.gci. o

[2] The Met Office notes that Professor Hansen has discussed a range of global average climate targets in journal publications. His recent paper ( Hansen et al 2013 , Environ. Res. Lett. 8 011006 doi:10.1088/1748-9326/8/1/011006 ) includes information on the warming for the earlier “Alternative Scenario” by Hansen and Sato, noting that with a climate sensitivity of 3°C, the peak warming is estimated to be around 1.5°C for this scenario. This paper also discusses the potential need for lower temperature limits such as 1°C and an initial atmospheric CO2 concentration target of 350ppm.


Prepared 3rd October 2013