Examination of Witnesses (Questions 113-153)
PROFESSOR JOHN
MITCHELL OBE AND
DR JASON
LOWE
23 JUNE 2009
Q113 Chairman: Good morning. Thank
you for coming in. Also, I think it is very helpful that you have
heard the previous evidence as well. As we go through the points
we want to discuss you may wish to comment on what Professor Anderson
has said. Could I start with a general question? Do you think
that the targets and budgets which the Government are now presenting
are actually consistent with limiting the risk of dangerous climate
change?
Professor Mitchell: If I can go
back a bit, I think one of the big issues is uncertainty in climate
sensitivity. One can specify what the emissions are and work out
what the concentrations are, but the uncertainty comes when one
tries to translate that into a temperature target, and I think,
in taking the approach that we have recently, rather than having
single values but trying to look at a probability distribution
of what that sensitivity is, is a major step forward. It is the
first attempt at this and the science may change, but I think
it makes a lot more sense than what people have tried to do in
the past, taking a single scenario.
Q114 Chairman: Using that approach.
Professor Mitchell: I certainly
approve of a probabilistic approach.
Q115 Chairman: Let us try and look
at that now. If we take the scenarios about a range of concentrations
and what that means in terms of global average temperature rise,
do you think at the moment, when we are talking about aiming for
a 50% risk of exceeding two degrees centigrade, that is consistent
with the level of concentrations we are likely to achieve given
the emissions pathway?
Professor Mitchell: The whole
point of having probability distribution is to allow you to look
at risk, so it is more a policy issue what level of risk you take,
but from the science point of view what we are trying to do is
quantify that level of risk to the degree we can given our current
understanding of climate, and I think we are satisfied, given
the understanding we have at present, that we have specified those
levels as well as we can.
Dr Lowe: I would like to add something
to that. The risk estimate comes from our uncertainty in this
particular quantity of climate sensitivity, but there are different
estimates of the uncertainty, so, if you like, there is uncertainty
on the uncertainty. What has been done in the Climate Change Committee
work is that they have taken the 50:50 value from one particular
climate sensitivity distribution and the particular distribution
they have chosen is the higher distribution. So they have actually
taken a precautionary approach. I prefer to think of the 50:50
as choosing a value from the centre of the distribution, i.e.
half the models are above it, half are below it. So it is operating
in a region where we have more faith in those models but it has
this precautionary point of view that we have taken, this particular
estimate of uncertainty.
Q116 Chairman: We are getting into
a lot of unknown unknowns! If we wanted to significantly reduce
the risk of exceeding two degrees, which is said to be 50:50—that
is the present level—does that imply aiming for a much lower
concentration of greenhouse gas in the atmosphere?
Professor Mitchell: Yes, that
is the whole ethos behind the approach, that it allows you to
say, "Well, we are going to take a 50:50 chance", or,
"We think, if we are going to take a precautionary principle,
we are going to cover a much lower risk of exceeding that value.
Q117 Chairman: Okay. So that is the
scientific, reasonably confident conclusion. Is it possible to
quantify how much effort we are going to have to make if we want
to get that 50% risk of exceeding two degrees to increase.
Professor Mitchell: To reduce
the uncertain uncertainty?
Q118 Chairman: Well, yes. Suppose
we were to say we only want 10% risk of exceeding two degrees?
Can we then translate that into a scientific objective as well
in terms of greenhouse gas concentration?
Professor Mitchell: I think, in
general, as one goes to lower levels of risk, just from a statistical
point of view, it becomes more difficult to pinpoint that accurately.
I think we actually have looked at 1% risk, but the uncertainty
in the tail of the distribution is much greater than in the centre,
and that is one reason for emphasising the centre. In the same
way, in the recent adaptation scenarios that have been released,
we have not gone beyond 10%, because you are starting to get larger
uncertainties once you get down to that level of risk.
Q119 Chairman: Looking at the issue
of carbon budgeting, which is what is actually scheduled as the
aim of this inquiry, we have got budgets set as far as 2022 now
and in a couple of years' time the next period up to 2027 will
also be set. That is 17, 18 years. Do we need to consider setting
budgets any further in advance than that, or is that sufficient?
Professor Mitchell: I think, from
a science point of view, if you know where you want to get to,
then you probably want to look out further so that you can check
that what you are doing in the short term is consistent with what
you may need to do in the longer term. If you are just setting
targets over the next 15 years but you are not sure where you
are going after that, then I think you are leaving open the case
where you may come into a situation where you cannot achieve your
longer-term aim. The second comment I would make is that, of course,
science does change, and one does need to be aware of being able
to update the science as appropriate, and I think that needs to
be incorporated in whatever legislation you are putting in place.
Dr Lowe: It is probably worth
adding there is a halfway house where, yes, it would be great
to specify the entire trajectory of emissions, and that is what
we do with the modelling work, but a halfway house is that you
specify the emissions for some time into the future, perhaps up
to 2030 or 2050, but with that you also have a cumulative total,
so a much longer-term time horizon as well, because of the long
response times in the system.
Q120 Chairman: One of the powerful
points made earlier was that we are investing in infrastructure
whose lifespan will go well beyond 2027. When we build a new aircraft
now the expectation is that it will be used for probably 30 years.
The same might be true of a power station when we have got no
date for carbon capture and storage. So we are making decisions
which will directly affect emissions well into the 2030s, which
would seem to strengthen the case for having budgets which go
with that.
Professor Mitchell: That is certainly
the case for the longer-term infrastructure as well.
Q121 Colin Challen: I wonder if you
have a view on what kinds of developments in the science might
take place which would trigger, perhaps, a review of the budgets
that the Climate Change Committee has set and whether there has
been anything in recent months which, in your view, could actually
count as such an important change: because, I think, correct me
if I am wrong, we have been told that the Independent Climate
Change Committee has based the bulk of its science on the IPCC
Fourth Assessment of the science which is a few years old and
has been through a peer review process, and so on.
Professor Mitchell: I think it
could put things in perspective. I have been involved in this
science for about 35 years and in 1978 one of the National Academy
of Science committees came out with a range of about one and a
half to four and a half degrees for doubling CO2, and that has
not changed that much over that period. There has been a lot of
oscillation within it. The second comment I would make is that
science progresses regularly; so trying to predict surprises can
be difficult. Having said that, I think the assessment made in
2007 was a good and solid assessment. I think the recent meeting
in Copenhagen tended to emphasise some of the more speculative
aspects of climate change, including the ice sheets and perhaps
methane clathrates, and those probably are issues for the longer
term and, of course, for mitigation in this committee, but I think
one always has a problem as a scientist between giving what is
the well-established view but being aware of possible surprises
in the future. There were a couple of issues that came up in Copenhagen.
One was the rate of rise of sea level—have we underestimated
the rate of melting ice sheets—the other is probably what
happens to methane which is locked in the tundra, and, again,
that is something which is not going to be an issue until you
get to the larger increases in temperature.
Q122 Colin Challen: You heard the
evidence from our previous witness (which, I have to say, I tend
to agree with) that, like engineers, we should perhaps over engineer
our structure. If we are going to build a bridge, you try and
build it, these days, to withstand totally unlikely events so
that it is going to last and do the job. Should not science also
be doing that, given that we are discovering all the time and
the discoveries tend to go in the wrong direction? I am thinking
about things like ocean acidification, and so on, much better
understood now than maybe a few years ago, but a lot more to learn,
and that applies to a great many of these areas. So should we
actually not just say we will fix ourselves on a central band
or the more optimistic opinion but to over engineer?
Professor Mitchell: There is a
cost that comes with that, and, I think, again coming back to
looking at the more probability/risk-based approach, that is where
the science is going. Take something like the Thames Barrier.
You are between two extremes. You do not want to under engineer
that and then have a catastrophic flooding event; on the other
hand you do not want to over engineering it and spend a huge amount
of money for a risk that is very small; and that will depend on
the particular case that you are looking at. So the level of risk
is different for different applications, and that is why we have
taken a probabilistic approach. In terms of taking a precautionary
principle, I think that is very much a political decision and
the role as a scientist is to provide the evidence which supports
that in the most faithful way possible.
Q123 Colin Challen: I was at the
Copenhagen Conference earlier this year. The impression I got
was that the error band, if you like, on the models has been consistently
optimistic and that the empirical data that is now coming in,
in droves, points to a worse picture. Do you think that the Climate
Change Committee has the flexibility to handle this and to make
recommendations in a timely fashion?
Professor Mitchell: Could I clarify
what you mean by "optimistic"?
Q124 Colin Challen: In terms of?
Professor Mitchell: The modelling.
Q125 Colin Challen: If we look at
the ice sheet data that has been observed, the trend there has
been significantly worse than was predicted. This is going back
just a few years, but, all the same, you can see the trend is
below the worst model, if you like. I am not a scientist; I am
just trying to express the graphs that I have seen based on the
empirical data versus the models.
Professor Mitchell: I think one
has to be careful looking at observations, because they include
both the longer-term trends due to greenhouse gases but also short-term
variability. In your example, I am not sure whether you are referring
to land ice or sea ice.
Q126 Colin Challen: Sea ice.
Professor Mitchell: Certainly
there has been a very marked decrease in sea ice. We know year
to year and over a period of several years that can vary a lot,
and we have to be careful we do not base policy on what turns
out to be a short-term natural event which exaggerates the rate
of climate change, but, similarly, not to underestimate the effects
due to short-term effects which reduce it. So part of the science
is to try and clarify whether that is due to natural variability,
and that is one of the things that the probabilistic approach
can take into account, because we can look at the observations,
we can look at natural variability, we can factor that into the
estimates that we make for the future. Again, I think it emphasises
the importance of going from single model estimates to looking
at what the range of variability is, what the sources of uncertainty
are, and it also leads you then into how you much focus efforts
to reduce those uncertainties.
Dr Lowe: Can I add to that? With
sea ice in particular, yes, there has been a lot of attention
as to whether the models can actually reproduce the recent rapid
declines. What we found in a version of our own model is that,
when you include natural variability and you put that on top of
the climate signal, then you can get year to year variations as
large as some of the recent ones we have seen. Can I just bring
in land ice, because there was so much focus on that at Copenhagen?
The emphasis there was on evidence that suggested further acceleration
of the contribution of land ice to sea level rise, but emerging
in the literature there are other counter-arguments. For instance,
there was a talk at the AGU late last year that presented evidence
of a slow down of some of the outlet glaciers and some modelling
work was published in Nature Geoscience on that. So, as
part of taking a balanced view, we look at both the studies that
are suggesting acceleration and the studies that suggest deceleration.
Q127 Colin Challen: Just to be clear,
would you say that the evidence generally points to the models
being pretty much correct within their range of uncertainties?
Professor Mitchell: I am not aware
of anything which shows a large disagreement. The UKCP scenarios
were produced taking a wide range of models but then looking at
observational constraints, how well they simulated certain aspects
of the present climate, how well they simulated recent trends,
to weed out those models which were less credible than the others,
and certainly the results we got from that are very consistent
with IPCC 2007. There is a dilemma, in that do you, when some
new science comes in, latch on to it immediately with meetings
like Copenhagen, or do you allow a longer period to assess the
science, to weed out things which have not been thought through
properly? It is a real dilemma. One of the issues with the IPCC
is that it is such a long process that it can leave things out
i.e. new results that have appeared in the last two or three years.
Q128 Colin Challen: Should the Climate
Change Committee that we have set up (and I think the UK is recognised
as being a leader in climate change science) have a shorter timescale
for reviewing these things than relying on the IPCC's four-year
timescale?
Professor Mitchell: I think, in
terms of the Climate Change Committee, it is obviously sensible
to take into account the latest information. I think they would
need to do so in terms of the background of the IPCC assessments
but looking carefully at any changes from that, perhaps investigating
that and, coming back to Professor Anderson's comment, looking
at the science and making sure actually it does stand up to scrutiny
and it is not just a single paper which is based on perhaps some
short-term evidence.
Dr Lowe: We also have a new project
that is funded by DECC and Defra called AVOID. It used to be Avoiding
Dangerous Climate Change but it has been shortened. The entire
purpose of that project is to make sure that the mitigation science
pulls through to government. So it is in the process of producing
a set of scenarios that build on those of the Climate Change Committee,
and it is not just a single institute study, it involves the Met
Office, the Grantham Institute, the Tyndall Centre, the Walker
Institute. So the idea is that we pull through this science on
a more rapid basis, and we do have regular contact with staff
from the Climate Change Committee.
Q129 Joan Walley: I am not a scientist,
so I am getting a little bit confused with all of this modelling
and the way in which the modelling is shaping the policy that
comes out of it. Previously we had Lord Turner, in his evidence
to the committee[5],
saying that climate models incorporate carbon cycle feedbacks,
and then it turns out that there is a distinction between feedbacks.
Then there is the concern about the Global Commons Institute saying
that you have got coupled and uncoupled models. I am just wondering
if you can explain to me, in layman's language, the way in which
the Climate Change Committee has taken its evidence in this coupling,
whether or not it has taken all the concerns on board that it
should have done and whether or not there is not a sort of faster
race where not the whole thing is based on what is actually happening.
Professor Mitchell: In terms of
the modelling, I think the first thing to make clear is that it
is based on physical, biological and chemical processes which
we understand to a greater or lesser extent. So it is not like
economic modelling, where you have various empirical models, it
is actually based on the laws of physics. In terms of what you
include in the model, the earlier models did not include the carbon
cycle. Those processes have now been added, so that the carbon
concentration depends on how the biosphere changes, how the ocean
carbon cycle changes in terms of temperature, circulation and
so forth. In adding feedbacks, we do not explicitly add a feedback:
we will add the processes that we understand to be important,
and, when those work together, the feedback will come from that,
so we do not prescribe feedbacks specifically. When we say things
are coupled, it means that all those processes are combined together
and work together, rather than running one model and then running
another model. I am trying to remember the question.
Q130 Joan Walley: The concern that
I have is that, in evidence that we have had from the Global Commons
Institute from Aubrey Mayer, he has pointed out that the IPCC
has specifically said the omission feedbacks from models was an
issue and that the real question is whether or not you have coupled
or uncoupled feedbacks. Is that something which you have taken
into account?
Professor Mitchell: The models
will take into account all the feedbacks we are aware of that
we think are important, then we can quantify that we understand,
and to that extent the Climate Change Committee has obviously
done that. Science being science, we uncover new feedbacks and
there is a delay in being able to incorporate those in the complex
models. One can use simple models to get, if you like, a fast-track
estimate of what the effect would be, but one would have to refer
to the more complex models to make sure that when you add that
additional feedback you are actually taking into account all the
processes that are important.
Q131 Joan Walley: Two things on that.
The first thing is that Aubrey Meyer said that the models used
by the Committee on Climate Change were uncoupled. Therefore,
his recommendation was that, because they were uncoupled, they
were not suitable. Would you agree with that?
Dr Lowe: I am going to take that
one. I had a look at the submission from the Global Commons Institute
last night and the figure I think you refer to comes from IPCC
in chapter 10 and, in this context, "uncoupled" refers
to whether temperature feeds back onto the carbon cycle, so where
the temperature and rainfall can affect how trees take up carbon,
and it has a very particular meaning. For the curve in question,
basically you run the model without this effect of climate feedback
on to trees and the biosphere and you get one number, you run
it again with this effect, the coupled version, you get a different
number and, if you have got the same emissions going in, the coupled
version leads to typically a higher concentration because you
are increasing the emissions that come back from the biosphere.
The runs that the Climate Change Committee used to include those
feedbacks, so in that definition they were described as coupled.
The precise values we use to work out the magnitude of the coupling
comes from elsewhere in IPCC and from a study referred to as a
C4MIP study, which to date is the most comprehensive analysis
of that particular type of feedback onto the carbon cycle.
Q132 Joan Walley: In your written
evidence that you have given to us[6]
(and, as I say, I am not a scientist, so this is all very difficult
for me) you say that the models that were used by the Committee
on Climate Change were suitable, but you also go on to say that
you would call for further simulations using the Earth System
Model. Does that mean that you are not accepting that the data
and the assumptions that were taken by the Climate Change Committee
were adequate and sufficient, or are you saying that far more
needs to be done on this modelling, taking into account coupled
and uncoupled versions? If so, can you say why this is necessary,
and what would be the costs of doing that and what would be the
benefits of doing that work?
Professor Mitchell: I think one
of the reasons is that the Earth System Models, which take into
account all these feedbacks—they take into account the weather,
the oceans, the forests, the effect of the carbon cycle on the
oceans—are expensive to run. So what we do is take those
models and we run them over a number of scenarios and then we
can use that to produce simple models, much in the way that Professor
Anderson has used—taking the global models and simplifying
them. You can then do a lot of investigations very cheaply.
Q133 Joan Walley: I thought our earlier
witness this morning, Professor Anderson, was warning us, that
because, for example, food, deforestation, aviation and shipping
had not been taken fully into account, that was going to open
up massive short-comings in the way in which the whole premise
of what was going forward was taking place.
Professor Mitchell: I think that
refers to how you control emissions. In the modelling, we take
the human induced emissions, and those are prescribed. So I think
that was referring more to how you reduce emissions rather than,
given an emissions scenario, how then you include that in the
model.
Q134 Joan Walley: But the modelling
has, somehow or another, to be connected to where the emissions
are, does it not, at some stage?
Professor Mitchell: That is more
the socio-economic modelling, which the Met Office is not involved
with. We will start with a set of emissions which will be tied
to some kind of socio-economic scenario, make certain assumptions
about aviation, and so forth. As far as the atmosphere is concerned,
it does not really matter where the carbon comes from, it gets
well mixed, and hence it is not relevant to the science of working
out what the climate impacts are given a set of emissions.
Q135 Joan Walley: So why is it necessary
to do this Earth System Model?
Professor Mitchell: To know what
the effect of carbon dioxide is on climate, taking into account
all the different interactions between the atmosphere, the ocean,
how the carbon cycle itself responds both to changes in climate
and to changes in carbon dioxide, because it will respond to the
induced changes of carbon dioxide. So it is to make sure you have
got a holistic picture of the whole system of climate and the
carbon cycle. To do that properly you need a full three-dimensional
model, and, as I say, that is too expensive to run a lot of scenarios.
Q136 Joan Walley: How do you mean
too expensive?
Professor Mitchell: In terms of
computer time. These things are enormously expensive in terms
of computer time, and that is probably one of the main limitations.
Q137 Joan Walley: Are you saying
there is not the capacity to actually do this work?
Professor Mitchell: There is not
the capacity to do, in detail, all scenarios.
Q138 Joan Walley: If it is needed,
why can we not be doing it?
Professor Mitchell: To a first
approximation you can take the complex model and look at the results
and simplify those to get broad relationships between emissions,
temperature and carbon dioxide, and that is what we have done.
When we look at those results, we will then come back and check
any key results with the global model, but we cannot explore the
whole range.
Dr Lowe: The type of models we
have used, the simple models, they are simple Earth System Models
and they are good at reproducing some of the features of the more
complex three-dimensional models that John refers to. So they
are good at producing global average temperature, and we have
tested them by comparing them with the more complex models over
a range of different scenarios. At the moment there are fewer
simulations of the type of scenarios we are talking about here,
with very strong mitigation. I think part of the suggestion is
that it would be nice to do some further testing with this particular
type of scenario. As it happens, we have now done some of that
very recently and we find a simple model does have some skill
even for that type of scenario. The second point, though, is that
the complex models do give you something extra. They tell to you
what is happening regionally so you can actually go down and look
at the process within the model regionally and say, "Is that
realistic?"; whereas with the simple global model you can
only look on a global average; you are averaging out some of the
information. Also, because the complex model it has this more
elaborate way of representing processes, if there are any surprises
within the system, any local more rapid changes, then you will
see them within the three-dimensional model. So we would not run
this more elaborate model for the 729 model variants that we ran
for every scenario that was used in the Climate Change Committee,
what we would suggest doing is maybe picking one, two or three
of those across the temperature range, almost as a check, to see
what is happening regionally.
Q139 Joan Walley: Is that something
you are going to be doing automatically or is that something which
somebody somewhere needs to be showing leadership on and pushing
for?
Dr Lowe: That is something the
AVOID project has in mind doing.
Q140 Joan Walley: So it is doing
it anyway.
Dr Lowe: We are doing it anyway.
Q141 Joan Walley: To the extent that
you need it to be done. It is doing it fully, 100%?
Dr Lowe: It is doing a subset
of three-dimensional model runs. I think it is a matter of trying
to debate how many you would really want to do.
Q142 Joan Walley: How many would
you really want? How many would it be nice to have and how many
is it necessary to have?
Professor Mitchell: It depends
on the degree to which you want to take out your results. The
assumption is that the simple model does a reasonably good job,
but, as Jason has alluded, there may be certain regimes where,
either locally or even globally, you do see some marked responses,
some extreme responses, which are unexpected. So it is a question
of how certain you want to be of those results.
Q143 Joan Walley: So that I am clear,
is that actually incorporated in this AVOID project that you referred
to that, is it, Defra are doing?
Dr Lowe: Defra and DECC are funding
this.
Q144 Joan Walley: I just need to
know, yes or no, are they covering it in full or does some pressure
need to come from somewhere to make sure that it happens?
Dr Lowe: Some of it has already
happened (some of it has now been done); some more of it is planned
on the work plan, on the timeline of AVOID already.
Q145 Joan Walley: Is that exactly
what you want to happen, or are you asking for more than is currently
funded or possible?
Professor Mitchell: I think there
is always room for improvement. It is not just on the litigation
side but also on the adaptation side. In terms of the, modelling,
as Jason alluded to, changes regionally in the carbon cycle add
up to the global total. Therefore, the more accurately you can
do regional climate change, the more accurately you can look at
the carbon budgets as well as the climate change to which we are
adapting. One of the issues that we have had with the UKCP scenarios
is that we are aware that, for example, the modelling of storm
tracks, which are particularly important for climate, is poor
in models. So one of the things we would like to do is to do that
better. For that we need high resolution, for that we need more
computing, and that is something that the Met Office is already
engaged in working towards. The limitations, perhaps not so for
mitigation, except in terms of the scope of things you can cover,
but for adaptation being able to model in more detail is important.
Q146 Mr Chaytor: To what extent do
you think the Committee on Climate Change has taken on board the
scientific evidence and translated it directly into appropriate
policy recommendations, or do you think the committee is being
too pragmatic in terms of only recommending what it judges to
be politically feasible?
Professor Mitchell: What we have
done at the Climate Change Committee is they have said these are
the scenarios we would like you to look at. We have run those
scenarios to see what the effect on climate is and what the effect
on the carbon cycle is. Jason probably has had more contact with
that. That is work he has been involved in. I have also had contact
on the adaptation side, where I have explained what we have done
for the UKCP scenarios, so I know they are listening to what the
science is and taking it into account. In terms of mitigation
scenarios, you have had direct contact with the Climate Change
Committee.
Dr Lowe: Yes. This has been quite
a long process. We have had numerous discussions on the various
uncertainties. So several of the topics like feedbacks that have
come up today, there have been numerous discussions behind those.
These, again, have not just involved one or two people, they have
involved multiple experts coming in. In that way I think there
has been a fairly good examination of the available science, that
is the available science that is coming through AR4, but also
the post AR4 science. For instance, there were staff from the
committee at the Copenhagen Climate Change Conference to see what
new was coming out of that. That seems pretty current in terms
of pulling in new information.
Q147 Mr Chaytor: But in terms of
the committee's reports, the committee's recommendations over
targets and budgets, do you think that what we now have in respect
of targets and budgets accurately reflects the significance of
the scientific recommendations, or do you think there is some
mediation and some compromise there?
Dr Lowe: When I look in a report
I can see how the budget numbers trace through to the climate
simulations that we ran. I am not sure if that answers your question
directly.
Q148 Mr Chaytor: It answers it indirectly.
You mentioned the uncertainties. There are uncertainties over
CO2 emissions but also uncertainties over non CO2 emissions. Could
you say a bit more about that?
Dr Lowe: Yes and no. No, in the
sense that for us those emissions are the input, if you like,
and then we combine the uncertainty on those scenarios with the
climate modelling uncertainty. Yes, in the sense that in both
the Climate Change Committee work, and now extending that in the
on-going projects, we run a range of different scenarios. These
have a range of different CO2 and non CO2 gases. One particular
uncertainty, in the form of atmospheric aerosols, we have looked
at in a lot of detail to see how that moves the temperature probability
results around, for instance, so it is in there.
Q149 Mr Chaytor: In terms of what
is missing, a lot of the science has progressed rapidly in recent
years, and who knows what new insights are going to be developed
shortly, but what are the most important missing areas of data
now? What knowledge do you need most urgently to increase the
level of certainty about your predictions?
Professor Mitchell: There is a
lot of uncertainty but of the two main areas, one is probably
cloud climate feedbacks. Clouds can either cool the climate, because
they reflect the solar radiation back to space, but they also
have a very strong greenhouse effect. So very small changes in
cloudiness can have quite an effect on the earth's budget and,
of course, with a warmer and moister atmosphere, it changes the
distribution of clouds and models struggle to agree on what those
changes are: so that is one of the biggest sources of uncertainty.
We have been looking at different models to understand the key
processes in determining that uncertainty, what observations we
have to make to increase the physical understanding, to reduce
the uncertainty, particularly through things like satellite, through
aircraft measurements, and so forth. So that is the one big area.
The other area is the carbon cycle, which is relatively new in
terms of our system modelling. Jason mentioned the C4MIP, which
is a carbon cycle climate change inter-comparison project. Again,
looking at the models, trying to understand why they differ, then
relating that to our understanding of the real system and making
the measurements that we need to improve our modelling of it.
In terms of the carbon cycle, because it is newer, I think in
some ways there is more ground for progress. We know from weather-forecasting
the problem of improving cloud simulations is very difficult,
but, on the other hand, I have been involved in this 20 or 30
years and we have not reduced cloud uncertainty, but I think we
are now getting to the stage where people really are concentrating
on the processes rather than just running new simulations for
scenarios, and I think there is a need for science to concentrate
on that if we are not going to go on with this level of uncertainty.
Q150 Mr Chaytor: Are we dealing here
with things on such a gigantic scale and over such a long time-frame
that scientists really will have to accept that there will always
be massive uncertainties? If we are trying to make scientific
assessments to inform public policy in 50 years' time, has this
ever been done before? Can you think of analogies of previous
projections over such a long period of time?
Professor Mitchell: Not over that
period of time, but I think you are right. What we tend to do
is prioritise those things where we know we can make a difference
quickly, but, on the other hand, if we do not start soon looking
at some of these long-term uncertainties, we certainly will not
reduce them. To some extent it is an act of faith. With science
being science there are some things which we will be able to develop
and some things which we will not, but we certainly need to maintain
that effort, and I think there is a danger, if we do not do that,
we could be five, 10 years down the road and find we actually
cannot say more than we can at present. So it is maintaining the
longer-term research to reduce those uncertainties at the same
time as making specific efforts to answer the sort of questions
that you are asking today.
Q151 Chairman: Your memo to us[7]
suggested that the estimate that the Committee on Climate Change
have made on the probability of staying below two degrees centigrade
represented a precautionary approach. Can you explain exactly
what you mean by the phrase "precautionary approach"
in this context?
Dr Lowe: That is a much easier
one to do with a diagram actually, so we may need to supply that
afterwards, but I will have a go first. It comes back to this
point that one of the key uncertainties is climate sensitivity,
and there are several different estimates of that measure of uncertainty.
The Hadley Centre produces one, other climate institutes produce
another. There are of the order of 15 of these now, but maybe
more, because more are cropping up, and these uncertainty estimates
are made in differing ways. If you were to sit down, the simplest
thing would be to say, if we are interested in relating the stabilisation
concentration of CO2 that gives a 50% chance of going over two
degrees, we would come out with a different number for that CO2
concentration depending on which of those uncertainty distributions
we go for, and what we find is that the Murphy et al distribution
that we used in this work tends to give, if you like, the lower
chance of staying below two, or it suggests that you need a lower
concentration than some of the other versions. Again, this would
be much easier with a diagram. I think perhaps a diagram with
a couple of arrows may clear up the point very simply.
Q152 Chairman: In that case we will
wait for the diagram. There is quite a significant difference
between the Committee on Climate Change's work and the work done
by Professor Anderson. How do you explain that?
Dr Lowe: Firstly, it is a very
different method. We are starting with the emissions: from that
we are working forward, calculating the concentration of greenhouse
gases, and from that we are calculating the temperature rise.
Professor Anderson is working the other way: he is looking at
an existing model study that has levelled out CO2 concentrations
at 450 ppm, CO2 only. That has given him, if you like, a lump
of carbon in total. He has then said, okay, if this is our allowable
lump that leads to 450, some of that is used up already with what
has come to the present day, some of that will be used up with
non CO2 gases and what we have got left we will divide up over
the years with a particular shape. So it is working backwards
from the target. One thing it assumes is that you can take this
lump of CO2, this cumulative CO2 amount, and apply it as a cumulative
CO2 equivalent; so you can include the other gases in that. We
are not as convinced that you can do it in that way, and so to
test that Professor Anderson has been kind enough to supply his
emissions, and one thing we have been doing very recently is running
them forward through the method we use. When we do that we find,
if we take a particular case which peaked in 2015, I believe,
and run that forward, because of our more precautionary climate
sensitivity value, that gives a chance of exceeding two degrees
of the order of 65% rather than the 50:50. So it is worse, because
we are now running it with our precautionary estimate, but then,
when we put in aerosols, we find that pulls the probability down
again. So it pulls it down from 65% to a little under 40%. I think
the main point there is that it is a different set of assumptions
but it is moving the numbers around in terms of probability by
several per cent. What I found looking at additional studies—because
published recently there was also work by Meinshausen et al, by
Miles Allen et al and by Martin Parry et al—is that the
studies tend to become quite close in terms of the temperature
level they approach within point two, point three degrees, but
they disagree more on the probability numbers, and in some ways
that suggests that the central estimate of the 50:50 temperature
is actually a more robust measure to use when comparing different
techniques. We would be more than happy to sit down and take the
inter-comparison with Professor Anderson further and really tease
out what the difference is between the two studies.
Q153 Chairman: I am sure that will
be interesting. Are there any other questions? No. Is there any
other burning issue that we should have raised with you but we
have failed to do so?
Professor Mitchell: No, I do not
think so, thank you.
Chairman: No. Then thank you very much
for your time. We are grateful to you.
5 Carbon Budget: Minutes of Evidence, Tuesday
4 February 2009, HC 234, published 26 March 2009. Back
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