Intergovernmental Panel on Climate Change Fifth Assessment Report: Review of Working Group I contribution - Energy and Climate Change Contents

3  Scientific conclusions of Working Group I contribution to the Fifth Assessment Report

Overall impression

34. A very large number of the respondents to our inquiry, most representing eminent scientific establishments, praised the WGI contribution to AR5 as being an authoritative, comprehensive and robust analysis of the past, present and likely future states of the climate.[71] Many highlighted the consistency of conclusions with previous reports and growing evidence base.[72] Professor Sir Brian Hoskins, Director of the Grantham Institute for Climate Change at Imperial College London and a former IPCC author, told us:

    After more years of research and observations, there is not a huge change in the conclusions of the AR5 report and the science compared with the previous reports. […] The main thing is there is a continued and stronger message that we can see the changes in the climate system that are consistent with the increase in greenhouse gases […]. Understandings have increased and models have become more complex, but still the same sorts of conclusions ride through as were there in the previous reports.[73]

Dr Stott, head of the Climate Monitoring and Attribution team at the Met Office and IPCC lead author, suggested that the WGI contribution to AR5 supported the case for tackling climate change:

    You will see further evidence to support the conclusion that warming is unequivocal and then we have more evidence from right across the climate system of the effects of human influence on the observed changes. That is not just in temperatures of the atmosphere. It is also in temperatures of the ocean, reductions in snow and ice, rising sea level, and changes in some climate extremes. We have more evidence that strengthens that assessment on human influence on climate and then we also have new evidence around the future, the conclusion that continued emissions of greenhouse gases will cause further warming and further changes in the climate system. Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions.[74]

There were, however, a number of scientific conclusions reported by the IPCC that have received criticism from both inside and outside the scientific community. This chapter will examine those criticisms in more detail.

Causes of climate change

35. The science of establishing the causes of climate change is known as "attribution" and is dealt with explicitly in Chapter 10 of the WGI contribution to AR5. The IPCC concluded that it is "extremely likely" that more than half of the observed increase in global mean surface temperature (GMST) from 1951 to 2010 was caused by human influence, primarily through increases in greenhouse gases.[75], The resulting impact of this human-caused surface warming on the atmosphere, oceans (heat content, sea-level, acidification, salinity etc.), global water cycle, global ice-systems and extreme weather events is discussed in the WGI contribution to AR5. The phrase "extremely likely" has a 95% level of certainty associated with it; an increase from the 90% level of certainty attached to a similar statement found in AR4 (which was released in 2007).[76]

36. The Grantham Institute for Climate Change at Imperial College London stated that there is "no real question that the climate is changing and that a significant part of these changes is due to human greenhouse gas emissions".[77] We have found no credible scientific source that disputes the claim that human activity has an influence on the climate. There are, however, a small number of scientists who dispute the extent to which human influence is the dominant factor in recently observed climate changes.[78] Richard Lindzen, Alfred P. Sloan Professor of Meteorology at Massachusetts Institute of Technology (MIT) and a former IPCC lead author, for example, did not agree with the IPCC's conclusion, claiming that "the attribution of most of the small warming to man is faulty and inconsistent".[79] Professor Lindzen and others allege that the IPCC overlooked natural factors such as internal variability and the influence of the sun (through cosmic rays) as considerable contributors in the explanation of the warming trends (see paragraph 39).[80] Additionally, some have queried the scientific justification for assigning a 95% confidence level to the claims about the human-influence on global warming since 1951, and for the IPCC attribution arguments in general.[81]

37. The IPCC process for attributing the causes of climate change was well-received by the majority of the scientific community. In this process, the relative size, timing and spatial pattern of phenomena which might influence whether the earth warms or cools (known as climate forcing and including, for example, greenhouse gas emissions) is matched against observed climate change in a method known as "fingerprinting".[82] This was explained in detail by Dr Shuckburgh, Head of the Open Oceans research group at the British Antarctic Survey and Fellow of the Royal Meteorological Society:

    There are particular patterns that one anticipates, being the pattern of change that you would expect from increasing greenhouse gases. That is a pattern of the change over time and the change in space. The change in space is both the geographical pattern of the change and the change with altitude as you go up through the atmosphere.

    There are particular patterns that one can identify that would be characteristic of the changes to the climate system you would anticipate from increasing greenhouse gases and that you would anticipate from natural fluctuations. For example, the most well-known natural fluctuation, the El Niño phenomenon, is associated with particular temperature changes, particularly in the Pacific Ocean. There is a particular pattern that one identifies with the El Niño phenomenon. If you look to the pattern of temperature change geographically, which one would anticipate from increasing greenhouse gases, it is different.

    You identify those patterns—those are the fingerprints—and then you can look to the observations of what has occurred over the recent time history and disentangle how much of a contribution has come from one pattern and how much of a contribution has come from another pattern. That is the scientific basis for formulating those statements.[83]

38. In establishing the clear pattern of human-influence, the importance of ruling-out of the so-called "null hypothesis"[84] was also emphasised, particularly by Dr Peter Stott:

    We, of course, checked out this null hypothesis; that we could explain the observed changes purely by these types of internally generated variability […] It is fully factored into the assessment where we conclude that the dominant cause of the warming since the mid-20th century is anthropogenic and not these natural factors.[85]

    […] Our null hypothesis starting point is that we can explain it just by internal natural processes and we can rule that out at very high confidence levels.[86]

The importance of the time-scale was made very clear when distinguishing between human influences and natural variability, as stressed by Professor Sir Brian Hoskins:

    [Climate change] is not all natural and it is not all anthropogenic. It is a mixture. What we have on all timescales is a mixture and this makes it difficult to understand and a very interesting scientific problem, but on a day-to-day basis it is dominated by natural variability […] It is disentangling these two, but with the very strong theoretical basis and the observed warming over a 100-year period giving some agreement we are seeing what we think is the fingerprint of our anthropogenic warming on that century timescale. When you get down to the decadal timescale, then we see a lot of the natural variability tending to dominate.[87]

39. In addition to internal variability and changes in the brightness of the Sun as experienced by the Earth (Total Solar Irradiance), the IPCC also considers the plausibility of cosmic rays playing a role in influencing global cloud cover (and thus surface temperatures). The IPCC concluded that the link between cosmic rays and cloud properties did "not prove to be robust on the regional or global scale" despite the fact that "there has been [a] hypothesis for decades".[88] Professor Lindzen pointed out that this field is subject to ongoing experiments.[89]

40. The slight increase in confidence in the statement of attribution (95% in AR5 compared to 90% in a similar statement in AR4) was made, in part, on the basis of the ability of the IPCC to draw upon a "larger body of refereed literature".[90] Professor Brigitte Nerlich and Dr Luke Colins from the University of Nottingham were critical that there was minimal elaboration of how the 95% figure was achieved, despite the recommendations of the InterAcademy Council.[91] Dr Stott, however, provided the justification for this increased confidence:

    Previously in the AR4 there was a lot of variability apparently in the observational records, which we have now understood was an artefact of a different bias in those measurements. That is one example of where we have improved our confidence, but it is not just in terms of ocean temperatures. It is also in terms of other factors such as the retreating ice and snow. We have developed our understanding of the water cycle. We have developed our understanding of extremes. When you look into our chapter 10, you will see we have a table at the back there with 33 rows where we have itemised all the evidence, both the evidence where we have multiple lines of evidence and also where we have remaining uncertainties. We have a greater wealth of evidence now than we had, with better understanding, with more observations and also with the improved models as well.[92]

41. There has been some confusion about the IPCC's use of (un)certainty language.[93] Professor Brigitte Nerlich and Dr Luke Colins explored the language used to convey the IPCC's conclusions in the media. They argued that conveying scientific information, including information about levels of (un)certainty, is difficult because scientists and the general public understand this word differently.[94] James Painter, Head of the Journalism Fellowship Programme at the Reuters Institute of Journalism drew a comparison between the IPCC "95% certain" attribution statement and the "95% certain" statement made by scientists concerning the link between smoking and lung cancer.[95] Professor Brigitte Nerlich and Dr Luke Colins argued, however, that analogies could be misleading.[96]

42. The WGI contribution to AR5 presents the most compelling evidence to date that many of the changes to the climate recorded in the latter half of the 20th Century were driven by post-industrial human activity. We have not found convincing evidence that challenges the IPCC's conclusion in this matter. There is increased confidence in the IPCC projections that, with rising greenhouse gas concentrations, we will continue to see warming (and the changes to the climate associated with warming) in this century and beyond.

Sensitivity of the climate to carbon dioxide

43. In order to ascertain the potential impact of human-influence on the climate system, it is crucial to understand how sensitive the surface temperature of the Earth is to changes in the concentration of carbon dioxide in the atmosphere. The IPCC reports on two important parameters that reflect this relationship: the longer-term Equilibrium Climate Sensitivity (ECS),[97] and the shorter-term Transient Climate Response (TCR).[98] The two are related, but differ slightly in their definitions so that the TCR is generally lower than the ECS. They can be calculated from combinations of model simulations, observations and references to the historical climate record. On ECS, the WGI contribution to AR5 concluded:

    Equilibrium climate sensitivity is likely in the range 1.5°C to 4.5°C (high confidence), extremely unlikely less than 1°C (high confidence), and very unlikely greater than 6°C (medium confidence). The lower temperature limit of the assessed likely range is thus less than the 2°C in the AR4, but the upper limit is the same. This assessment reflects improved understanding, the extended temperature record in the atmosphere and ocean, and new estimates of radiative forcing. […] No best estimate for equilibrium climate sensitivity can now be given because of a lack of agreement on values across assessed lines of evidence and studies.[99]

On TCR, the WGI contribution to AR5 concluded:

    The transient climate response is likely in the range of 1.0°C to 2.5°C (high confidence) and extremely unlikely greater than 3°C.[100]

The assessed ranges for these values have remained very similar with each successive IPCC Assessment Report, prompting Professor Sir Brian Hoskins to comment that:[101]

    I think one of the amazing things about the IPCC is how the equilibrium climate sensitivity range has stayed the same over the years. That would seem we have not been doing any science to try and refine this, but during that period those started with an atmosphere-only model with a fixed ocean underneath. Then gradually the system has become the whole depth of the ocean involved, the vegetation on the land and the ice. The system has become so much more complex and interactive that is modelled and uncertainty ranges could well increase as you do that. The triumph has been that most of them have stayed about the same.[102]

44. Most, but not all, of the discussions on the IPCC treatment of sensitivity has related to the ECS. Professor Lindzen and Nicholas Lewis, a climate researcher and mathematician, both argued that the figure in WGI contribution to AR5 is too high. The IPCC's slight increase in uncertainty of the figure was also criticised.[103] However, Professor Myles Allen, Professor of Geosystem Science at the University of Oxford and former IPCC Lead Author, who took a leading role in the IPCC discussions of ECS urged that the focus should be on the TCR which, he argued, was a better indicator of the warming expected over the coming century.[104] This point was repeated by a number of others, including Mr Lewis, who told us that "for policy purposes, the transient climate response, which is approximately over a 70-year period during which you have forcing concentrations rising, is more policy relevant".[105] The TCR is much better known than ECS; there is a smaller intrinsic uncertainty associated with it and, as the IPCC demonstrates, a much better agreement between models and observational estimates.[106] Although efforts to better understand and constrain ECS will continue, Professor Allen told us that:

    Everybody talks about it because everybody else talks about it, but nobody can quite remember why we are talking about it in the first place.[107]

45. In narrowing the possible range of estimates for both ECS and TCR from observational measurements, the statistical methodology of the IPCC was bought into question by Mr Lewis, who explained to us that:

    The bulk of the studies estimating [ECS and TCR] use a subjective Bayesian method, not all of them. Mine uses an objective method. One or two of them use non-Bayesian methods. The non-Bayesian methods and the objective Bayesian methods give very much the same answer. The subjective ones, basically the answer you get depends on the subjective assumptions you have fed in. [108]

From this Mr Lewis argued that the range of sensitivities considered by the IPCC was too high. However, the IPCC gave consideration to Mr Lewis' argument and concluded that:

    Bayesian methods to constrain ECS or TCR are sensitive to the assumed prior distribution. They can in principle yield narrower estimates […] but there is no consensus on how this should be done robustly.[109]

The view of the IPCC was strongly reinforced by Professor Allen, who also disagreed with Mr Lewis' analysis.[110] The arguments concerning statistical methodology are likely to continue both in the academic literature and future assessments of climate sensitivity. For the time being, it is important to realise that these arguments only affect one of a number of methodologies for calculating sensitivity, all of which have been taken into account by the IPCC to produce their likely ranges of ECS and TCR.[111]

46. We also examined claims that uncertainties in other areas of the IPCC report (such as those surrounding the role of aerosols and natural variability) could lead to overestimation of climate sensitivity.[112] Professor Allen demonstrated that these uncertainties had been accounted for by the IPCC through reference to a simple model.[113] Mr Lewis disagreed and provided evidence to suggest that uncertainties could lead to temperature projections that were below those of the IPCC.[114] Professor Allen responded:

    These multimodel ranges are only considered "likely" by IPCC, so the fact that it is possible to get below them by assuming a maximally optimistic scenario should not be that surprising.[115]

A number of witnesses also pointed out that there were unknown aspects of the climate that might lead to increasing sensitivities as the planet warms.[116]

47. Some argued that, after the cut-off date for submitting academic papers to be considered as part of the WGI contribution to AR5, some results had been released that argued for higher sensitivities, and some for lower.[117] Professor Allen reflected on this and claimed that "there is no clear pattern of results after the deadline pointing towards lower or higher sensitivities than results published before the deadline".[118] The debates around climate sensitivity demonstrate the diligence of the IPCC, as Professor Allen went on to illustrate:

    I was part of the internal discussion group that made the decision to lower the lower band on climate sensitivity, which made a lot of noise at the time. I have had a lot of feedback from colleagues in the scientific community about whether or not that conclusion was justified. Papers have come out since the IPCC report that point to substantially higher values for the climate sensitivity and people invariably send me emails saying, "See", when these papers come out. That is the process at work. Scientists continue to check what other scientists have done and obviously high profile statements, such as those made in the IPCC summaries, are checked even more carefully than others. They get a lot of scrutiny.[119]

48. The WGI contribution to AR5 has considered the full range of both Equilibrium Climate Sensitivity and Transient Climate Response and given the best assessment possible within the constraints of the evidence available at the time. It does not appear that a consistent pattern for higher or lower sensitivities than that stated in the WGI contribution to AR5 has emerged since its publication.

The "hiatus"

49. The observed global mean surface temperature (GMST) reported by the IPCC for the last 15 years shows a much smaller increase than over the past 30 to 60 years, despite the decade of the 2000s being the warmest in the instrumental record.[120] Depending on which data are used, the temperature rise from 1998-2012 is estimated to be around one-third to one-half of the trend from 1951-2012; this slowdown in warming has become known as the "hiatus" or "pause".[121] A number of respondents argued that the WGI contribution to AR5 did not adequately address this issue.[122] For example, Professor Pierre Darriulat, former Research Director of CERN and currently Professor of Physics at VATLY in Hanoi in Vietnam, said that:

    It is undeniable that the pause has come as a surprise in a context where anthropogenic C02 emissions keep increasing. It has obvious implications on factors that are not properly taken into account in the climate models. As such, it deserves a very critical study aiming at a proper evaluation of the uncertainties attached to predictions. This is what should be expected from a serious scientific approach.[123]

The WGI contribution to AR5 explicitly addressed the issue of the hiatus. It concluded that it "is attributable in roughly equal measure to a cooling contribution from internal variability [such as changing patterns in the El Niño-Southern Oscillation] and a reduced trend in external forcing [through volcanic aerosols and a reduction in solar activity]".[124] It also concluded that "barring a major volcanic eruption, most 15-year GMST trends in the near-term future will be larger than during 1998-2012".[125] On this, an IPCC Review Editor commented:

    The issues of a "lack of warming" over the last 10-15 years was raised by several reviewers. I think the way this has been handled via Box 9.2 [where the IPCC explicitly addresses the hiatus] is outstanding. However, so far as I can tell, Box 9.2 has not been subject to external review as a consequence of timing. This is worrisome.[126]

50. Subsequent evidence has confirmed that a number of witnesses supported the conclusions of the IPCC. For example, Dr Stott told us that, "the assessment that was made in the AR5 has been borne out by further evidence we have had since" including the contribution from internal variability such as the Pacific decadal oscillation which could play a role in the redistribution of heat with in the oceans.[127] Dr Stott's explanation of internal variability and increased ocean heat uptake was re-iterated by several other witnesses.[128] Additionally, we were referred, by a number of witnesses, to the body of evidence released since the publication of the WGI contribution to AR5 corroborating the IPCC conclusions on the hiatus.[129] Not all of the individual pieces of evidence published since the WGI contribution to AR5 are consistent with the IPCC explanation of the hiatus, but this is an area of on-going research that will require long-term, robust data before firm conclusions on the hiatus can be made.[130]

51. Regardless of the discussed explanations for the hiatus, arguably the most frequent criticism of the IPCC was its failure to predict the occurrence of the hiatus in any of the earlier Assessment Reports. The IPCC acknowledged that only 3% out of 251 climate models managed to predict the hiatus as defined above, although each Assessment Report since 1990 has stated that warming will be non-linear and subject to influence by natural variability.[131] More recently, global climate models have factored in periods of reduced warming, temperature stasis and cooling, though they were not designed to be able to predict the timing of these events precisely.[132] As Professor MacKay, Chief Scientific Adviser to DECC, explained:

    Many of the model runs do show slowdowns that last as long as the latest slowdown. They happen at random times, and it would have been an amazing coincidence if any of those simulations had happened to precisely nail [this hiatus].[133]

We note that the hiatus contributed in part to the IPCC decision to revise short-term model projections and slightly lower the estimates for equilibrium climate sensitivity.[134] According to Professor Lindzen, as the hiatus continues, it will lead to further downward revisions of estimates of climate sensitivity.[135]

52. Amongst the debate surrounding the hiatus, one point that was frequently re-emphasised is that the pause in GMST must be considered in the context of the long-term global energy budget (which includes all gains of incoming energy and all losses of outgoing energy).[136] In this regard, Professor Sir Mark Walport, Government Chief Scientific Adviser, explained that:

    The hiatus is the change in the atmospheric temperatures. There is absolutely no hiatus in the totality of the global energy inventory, which is going up relentlessly. […] Well over 90% of the heat uptake is in the oceans, and that is continuing.[137]

This point reflects the fact that short term changes in GMST are not good metrics for discussing overall warming,[138] thus the WGI contribution to AR5 uses a variety of different measurements to reach its conclusions.[139] When asked whether Ministers had taken account of the hiatus the Minister for Climate Change, Rt Hon Gregory Barker, told us;

    There is a whole barrel of different evidence that informs public debate and underpins our approach. As Minister, I rely principally not on the story of the day but on the ongoing scientific advice […]. I think if you are making public policy one has to rely on the qualified experts for the long-term advice that they give you rather than be guided by whatever happens to feature on a tabloid headline.[140]

53. Periods of hiatus are consistent with earlier IPCC assessments that non-linear warming of the climate is to be expected and that forced climate changes always take place against a background of natural variability. The current period of hiatus does not undermine the core conclusions of the WGI contribution to AR5 when put in the context of the overall, long-term global energy budget. Despite the hiatus, the first decade of the 2000s was the warmest in the instrumental record and overall warming is expected to continue in the coming decades.

Climate models

54. The results of climate models dominate the second half of the WGI contribution to AR5. They are used to simulate a variety of near- and long-term global and regional climatic behaviours. The results of the simulations are then used in the projection of future climate changes and in the attribution of the cause of observed climate changes.[141] Climate models have continued to become increasingly sophisticated and capable, now factoring in a greater level of complexity than ever before.[142]

55. Critics have called into question a number of issues surrounding climate models, particularly relating to short-term projections for global mean surface temperature (GMST).[143] Notably, less successful areas of the short-term regional model outputs are used to bring into question the ability of the IPCC to predict climate changes over the long-term and global scales. For example, Professor Lindzen considered that:

    You can add complexity to a model and that has happened a lot, but it has not helped [the IPCC] do major things with ocean processes that are still known to be important. We like to think of progress as kind of linear. You spend more money, you make it bigger and get it better, but there has been no discernible increase in scale that has been associated with the advances. There are claims that regional climate has improved, but there are still notable errors in describing today's regional climate with current models, so that naturally gives one pause with predictions […] If they can't get today's distribution of regional climate right, why would they be reliable for the future?[144]

56. Under closer scrutiny, many of the criticisms of model projections appear unfounded. Firstly, generalised statements concerning their reliability do not reflect the complexity and diversity of the models. As Professor MacKay illustrated:

    There are many models. There are complex, general-circulation models; there are simpler energy-balance models. One of the things the IPCC did was to evaluate the models that are used, and this evaluation looked at their abilities to simulate mean climate, historical events in climate change, variability on multiple timescales and regional modes of variability. There is a mixed scorecard. There are some things that are modelled well but some things such as regional variability are not yet well captured by the models. They are improving in many of these areas. As the computers become more powerful and the models are able to represent the earth's system at higher resolution, the accuracy of many of these features in the system is improving.[145]

This point was also emphasised by Professor Allen:

    Yes, the IPCC does consider the reliability of climate models and finds they are much more reliable for some variables (like global temperature) than others (like local precipitation). If the question is posed as "are the models reliable in all respects" the answer is trivially "no".[146]

57. Secondly, many of the criticisms brought to our attention gave a distorted impression of the models' performance by selectively emphasising areas in which the models have behaved less well (particularly across the hiatus period, see paragraph 49).[147] We note that the WGI contribution to AR5 devoted an entire chapter to the evaluation of climate models and demonstrated that:[148]

i)  Models generally have a good record of replicating GMST, and other climate observations, dating back to the first IPCC report in 1990;[149]

ii)  Models have improved, and continue to improve, in terms of their agreement both with themselves and with observed climate data;[150]

iii)  The increasing levels of complexity within models, though introducing additional uncertainties, have helped to improve climate projections;[151]

iv)  The models are not perfect, their limitations are recognised by the IPCC authors and poorer performing areas are highlighted and discussed at length.[152] The increasing use of performance metrics, which judge models against their ability to reproduce observable climate data, has helped to safeguard against undue weight being given to poorer quality results.[153]

58. The process through which model results are interpreted and used to generate projections for future climate shows an appropriate level of critical scepticism by the IPCC authors. The output of the climate simulations are subject to scrutiny and expert judgement and the results are tested against the sensitivity towards their initial conditions and key uncertainties.[154] This fact was emphasised repeatedly, including by Professor Allen, who said:

    The key point is there is an enormous amount of judgment in running climate models. There is nothing sacrosanct about ranges that come out of the CMIP5[155] ensemble. We rely on a whole range of lines of evidences, including observations and these models, to provide these projections and those are the projections that are elevated to the Summary for Policymakers.

    Just to pick up on the notion, "You no longer rely on the models", there are certain things for which I would never rely on these models and there are other things that I would. That is where the expert judgment comes in. You have to look at a model and ask yourself, "Is it appropriate to the question that I am asking with it?" If you wanted me to use one of these models to predict whether there is a white Christmas in 2030, it wouldn't work, but that doesn't mean that the model is useless for giving us the big picture estimates of what warming we should expect over the coming century. There is no mechanical process for taking a set of model results and turning a handle and getting a set of answers out.[156]

59. It is widely acknowledged that decadal forecasts and the difficulties and uncertainties associated with short-term forecasting should not be confounded with the longer-term projections and "broad direction of travel" in respect of overall warming.[157] The Grantham Institute for Climate Change at Imperial College London emphasised the requirement to contrast the differences between the types of projections:

    We have less confidence in projected near-term changes which are likely to remain heavily influenced by internal variability […] Nevertheless there is no evidence of a mechanism that would significantly reduce the intensity of the projected climate change by 2100.

    If you do the assessment of the models against a whole raft of climate indicators over those longer-term time scales that you need to do to assess their fitness for purpose for the longer-term changes […] we see that the models do a good job of representing the larger time-scale changes.[158]

It further criticised a pre-occupation with observations and projections made on the decadal time-scale:

    This emphasis on decadal temperature changes is, we believe, ill-advised. In the context of climate change, relatively short-term indices of the state of the climate system have little relevance. It is quite conceivable for there to be a future decade in which the Earth's surface temperature cools even with continued climate change.[159]

60. We were pleased to see that officials at DECC had a good understanding of both the strengths and weaknesses of climate models. David Warrilow, Head of Climate Science and International Evidence at DECC, highlighted that models are not trying to project; they are trying to simulate a process:

    When you come down to projection on the near term, like one to two decades, we are into a very different set of problems and I do not think there is an easy answer to doing that. It is something that the scientific community is now working on because they are conscious that there is an interest in trying to project what would happen in the near term, but it is almost analogous for weather forecasting. Weather forecasting is good for up to four or five days, probably, but if you try to look forward two to three weeks, the chaos of the system makes it much more difficult to predict. Even predicting a season ahead is quite difficult, but if you want to predict a decade you are trying to predict a level of chaos that is very difficult to predict. I think the IPCC has done the best that it can with the information available, but I do not think people in the IPCC would say that this is the final answer to the problem.[160]

61. Crucially, when considering climate change it is important to recognise that the models represent one of multiple lines of evidence. Professor Sir Mark Walport made it clear that:

    It simply is not just models. There are an awful lot of observations here as well. The models are part of the story and the models look into the future, but they also use the past to see whether they have modelled accurately. One of the interesting things about the fifth report is how little changed it is from the fourth report and the third report. What is happening is that the uncertainty is gradually reducing. The very short report on the physical science base is absolutely clear: warming of the climate system is unequivocal. Each of the last three decades has been successively warmer. The rate of sea-level rise has been larger. Human influence on the climate system is clear. That is the conclusion of the report. I think that is enough for policymakers to start making decisions.[161]

62. Finally, we have found that the WGI contribution to AR5 plays a pivotal role in encouraging the development of more complex computer models, as pointed out by Professor Corinne Le Quéré, Director of the Tyndall Centre for Climate Change Research and an IPCC lead author, and her colleague Professor Andrew Watkinson, Professor in Environmental Sciences, both based at the University of East Anglia:

    The IPCC process provides a major incentive to co-ordinate modelling activities, without which it would be more difficult to make intermodal comparisons and assess the reliability of climate models […]

    One of the major advantages of the current system is that it provides a focus for modelling activities in WGI, which are essential to review the projections of climate change. Furthermore, the assessment process provides the only forum where all the components of the climate system can be looked at (e.g. the atmosphere, ocean, ice, land, biogeochemical cycles), and consistency across different streams of research can be analysed.[162]

It is essential that these vast, modelling infrastructures are being used as effectively as possible and that consideration is given to their future direction. Professor Allen questioned whether modelling capacity was being used to answer the right questions. He stressed the need to develop high resolution, regional, real-time attribution systems:

    The most obvious and important area in which further effort is required to reduce uncertainty is the implications of these global changes for regional climate and extreme weather events. It is shameful, in my view, that we are still unable as a community to quantify the role of human influence on high-impact events like Typhoon Haiyan. The IPCC process may inadvertently have contributed to this situation through its focus (mandated by the commissioning governments) on long-term projections of global climate change, which are necessarily reliant on coarse-resolution models.[163]

63. A number of witnesses highlighted a requirement to maintain and improve modelling hardware, for example the Met Office Hadley Centre, a UK-based institute for climate modelling, reported to us that "along with the need to run large ensemble simulations, there is a clear need for supercomputing capacity and infrastructure".[164] In this regard, it is vital that modelling research institutes receive the support they require if key uncertainties about the climate are to be resolved. DECC responded to this point stating:

    Just looking at the Hadley Centre, we have that regularly reviewed and the last review indicated that we were getting very good value for money from it; that the spend was not dissimilar to what was being spent in other modelling centres around the world and that it was at an appropriate level. The only real question was whether we would be able to maintain our position vis-à-vis computing power and that is something that is now being actively pursued by BIS. I think we are looking at the availability of computing power, which is one of the constraints on getting better predictions.[165]

The Minister told us:

    I think we definitely punch above our weight in terms of the reputation and quality of the science that is produced by those centres and it is a huge tribute to the people that work there.[166]

64. The models used in the IPCC's Assessment Reports have a successful history of simulating past climate and their future projection of substantial warming over the next century in all but the most aggressive mitigation scenarios is well founded and overwhelmingly clear.

65. In the light of the WGI contribution to AR5, the Government should commission a strategic review of UK modelling facilities to discern how current computing capacity could be used more effectively to reduce remaining uncertainties. The review should highlight areas of potential national and international collaboration between modelling centres and any funding shortfalls that need to be met.

71   Ian Strangeways (IPC 022), Dr Ruth Dixon (IPC 023), Department of Energy and Climate Change (IPC 025), Met Office (IPC 026), Royal Meteorological Society (IPC 029), Grantham Institute for Climate Change (IPC 032), Royal Society (IPC 034), University of Reading (IPC 035), Natural Environment Research Council (IPC 036), Myles Allen (IPC 037), Corinne Le Quéré and Andrew Watkinson (IPC 050), Grantham Research Institute on Climate Change and the Environment (IPC 051) Back

72   Grantham Institute for Climate Change (IPC 032), University of Reading (IPC 035), Corinne Le Quéré and Andrew Watkinson (IPC 050) Back

73   Q1 [Professor Hoskins] Back

74   Q1 [Dr Stott] Back

75   IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis (2013), p869, The majority of the warming contribution from man-made greenhouse gases is from carbon dioxide, but the IPCC also make clear that other species (such as methane and nitrous oxide) are of considerable significance. Back

76   IPCC Working Group I Contribution to AR4, Summary for Policymakers, Climate Change 2007: The Physical Science Basis (2007), p10, A discussion of how the WGI contribution to AR5 treats (un)certainty can be found on pages 138-142 of the IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis (2013). Back

77   Grantham Institute for Climate Change (IPC 032) Back

78   Roger A. Pielke Sr (IPC 011), Friends of Science Society (IPC 015), Dr A Neil Hutton (IPC 028), Professor Richard Lindzen (IPC 047), Judith Curry (IPC 052) Back

79   Professor Richard Lindzen (IPC 047) Back

80   Q92 [Professor Lindzen], Dr Norman Page (IPC 058), Dr A Neil Hutton (IPC 028) Back

81   Q87 [Professor Lindzen], Q91 [Professor Lindzen], Christopher Walter (IPC 005), Nongovernmental International Panel on Climate Change (IPC 042) Back

82   Q9 [Dr Stott] Back

83   Q129 [Dr Shuckburgh] Back

84   The null hypothesis stipulates that the observed climate changes would have occurred with no human intervention through internal variability. Back

85   Q9 [Dr Stott] Back

86   Q27 [Dr Stott] Back

87   Q10 [Professor Hoskins] Back

88   Q92 [Professor Lindzen], IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis (2013), p886, Box 10.2 Back

89   Q92 [Professor Lindzen], Richard Lindzen (IPC 068) Back

90   Q1 [Dr Stott], University of Reading (IPC 035) Back

91   Brigitte Nerlich (IPC 031), InterAcademy Council, Climate Change Assessments: Review of the Process and Procedures of the IPCC (August 2010)  Back

92   Q27 [Dr Stott] Back

93   Brigitte Nerlich (IPC 031), James Painter (IPC 044) Back

94   Brigitte Nerlich (IPC 031) Back

95   Q164 [Mr Painter] Back

96   Brigitte Nerlich (IPC 031) Back

97   Equilibrium Climate Sensitivity refers to the temperature change to the surface of the Earth observed after a doubling of carbon dioxide followed by enough time to allow all the different aspects of the climate to reach equilibrium (a process that may take hundreds of years). Back

98   Transient Climate Response refers to the warming to the surface of the Earth observed after a doubling of carbon dioxide over a 70 year period. Back

99   IPCC Working Group I Contribution to AR5, Summary for Policymakers, Climate Change 2013: The Physical Science Basis (2013), p16 Back

100   As above Back

101   Q222 [Professor MacKay] Back

102   Q24 [Professor Hoskins] Back

103   Q24 [Professor Allen], Nicholas Lewis (IPC 017), Professor Richard Lindzen (IPC 047) Back

104   Qq4-5 [Professor Allen], Q24 [Professor Allen], Myles Allen (IPC 078) Back

105   Q82 [Mr Lewis] Back

106   Q5 [Professor Allen] Back

107   Q5 [Professor Allen] Back

108   Q69 [Mr Lewis], Q83 [Mr Lewis], Nicholas Lewis (IPC 017), Nicholas Lewis (IPC 070)  Back

109   IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis (2013), p82-85 Back

110   Q24 [Professor Allen], Myles Allen (IPC 073), Myles Allen (IPC 078) Back

111   IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis (2013), p82-85 Back

112   Q68 [Mr Lewis], Q81 [Professor Lindzen, Mr Lewis], Professor Richard Lindzen (IPC 047)  Back

113   Q3 [Professor Allen], Myles Allen (IPC 073) Back

114   Nicholas Lewis (IPC 070) Back

115   Myles Allen (IPC 078) Back

116   Grantham Institute for Climate Change (IPC 032), Myles Allen (IPC 078) Back

117   Barry Brill (IPC 018), Myles Allen (IPC 037) Back

118   Q6 [Professor Allen] Back

119   Q4 [Professor Allen] Back

120   IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis (2013), p769 Back

121   IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis (2013), p192-194 Back

122   Christopher Walter (IPC 005), John McLean (IPC 016), Barry Brill (IPC 018), Nongovernmental International Panel on Climate Change (IPC 042), Professor Pierre Darriulat (IPC 049), Judith Curry (IPC 052) Back

123   Professor Pierre Darriulat (IPC 049) Back

124   IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis (2013), p769-772 Back

125   As above Back

126   IPCC Working Group I Contribution to AR5, Reports of the Review Editors, Climate Change 2013: The Physical Science Basis (2013) Back

127   Q47 [Dr Stott] Back

128   Q78 [Mr Lewis], Q198 [Professor MacKay], Q208 [Professor MacKay], Department of Energy and Climate Change (IPC 025), Met Office (IPC 026), Grantham Institute for Climate Change (IPC 032) Back

129   Met Office (IPC 026), Grantham Institute for Climate Change (IPC 032), Natural Environment Research Council (IPC 036), Corinne Le Quéré and Andrew Watkinson (IPC 050), Grantham Research Institute on Climate Change and the Environment (IPC 051), WWF (IPC 054), Royal Meteorological Society (IPC 074) Back

130   Q80 [Mr Lewis], Q208 [Professor MacKay], Friends of Science Society (IPC 015), Royal Meteorological Society (IPC 029), University of Reading (IPC 035), Natural Environment Research Council (IPC 036) Back

131   John McLean (IPC 016), IPCC Working Group I Contribution to AR5, Summary for Policymakers, Climate Change 2013: The Physical Science Basis (2013) p15, IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis (2013), p769-772, Ed Hawkins, Tamsin Edwards and Doug McNeall, Nature Climate Change, Vol. 4, March 2014, 154-156 Back

132   Q15 [Dr Stott], Q47 [Professor Hoskins, Dr Stott], Q198 [Professor MacKay] Back

133   Q206 [Professor MacKay] Back

134   Myles Allen (IPC 037) Back

135   Q78 [Professor Lindzen] Back

136   Q118 [Dr Shuckburgh], Q123 [Dr Shuckburgh], Q198 [Professor Mackey, Professor Walport], Q208 [Professor Walport], Q223 [Professor Mackey, Professor Walport], Grantham Research Institute on Climate Change and the Environment (IPC 051) Back

137   Q198 [Professor Walport] Back

138   Roger A. Pielke Sr (IPC 011) Back

139   IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis (2013), p38 Back

140   Q239 [Mr Barker] Back

141   IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis (2013), p867-1136 Back

142   IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis (2013), p741-866 Back

143   Christopher Walter (IPC 005), Roger A. Pielke Sr (IPC 011), Friends of Science Society (IPC 015), Alan Gadian (IPC 020), Dr Norman Page (IPC 058), Ian Strangeways (IPC 022), Dr A Neil Hutton (IPC 028), Nongovernmental International Panel on Climate Change (IPC 042), Professor Richard Lindzen (IPC 047) Back

144   Qq76-77 [Professor Lindzen] Back

145   Q194 [Professor Mackay] Back

146   Myles Allen (IPC 037) Back

147   Christopher Walter (IPC 005), Friends of Science Society (IPC 015), John Christy (IPC 055) Back

148   IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis (2013), p741-866 Back

149   IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis (2013), p64-65 Back

150   IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis (2013), p824-825 Back

151   Q7 [Dr Stott], Met Office (IPC 026), IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis (2013), p748-753 Back

152   IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis (2013), p815-816, p1013-1015 Back

153   IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis (2013), p753-759, p809-810, p821-828 Back

154   Q12 [Professor Allen, Dr Stott], Q123 [Dr Shuckburgh], Q195 [Professor MacKay], Q207 [Professor Mackay], Grantham Institute for Climate Change (IPC 032), Myles Allen (IPC 073), Royal Meteorological Society (IPC 074), Met Office (IPC 076), IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis (2013), p959-961, p1009-1012 Back

155   CMIP5 stands for Coupled Model Intercomparison Project Phase 5 and represents a set of coordinated climate model experiments.  Back

156   Q13 [Professor Allen] Back

157   Q3 [Professor Allen, Professor Hoskins, Dr Stott],14 [Professor Allen], Qq195-196 [Professor MacKay, Professor Walport], Q199 [Professor Walport], Met Office (IPC 026), IPCC Working Group I Contribution to AR5, Climate Change 2013: The Physical Science Basis (2013), p964-965 Back

158   Grantham Institute for Climate Change (IPC 032) Back

159   As above Back

160   Q237 [Mr Warrilow] Back

161   Q203 [Professor Walport] Back

162   Corinne Le Quéré and Andrew Watkinson (IPC 050), University of Reading (IPC 035) Back

163   Myles Allen (IPC 037) Back

164   Q34 [Dr Stott], Met Office (IPC 026), Grantham Institute for Climate Change (IPC 032) Back

165   Q253 [Mr Warrilow] Back

166   Q253 [Mr Barker] Back

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Prepared 29 July 2014