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Scrutiny of IPCC report “Climate Change 2013: The Physical Science Basis”

During the House of Lords Energy Bill Debate on 28 October 2013, Viscount Ridley announced that there were nine ways in which the recent IPCC report (AR5) was less alarming and less certain than the previous IPCC report in 2007 (AR4).

This document examines these nine claims using the relevant text from the two reports. Six are found to be incorrect – meaning either that there is no supporting evidence from the text of AR5 relative to AR4, or that AR5 explicitly states that the issues are less relevant to long-term climate projections. Important caveats apply to the remaining three claims.

AR5 presents many advances in knowledge since AR4. The overall message, however, remains remarkably similar to that in AR4 and all previous IPCC assessments stretching back to 1990. It confirms the reality and risks of human-induced global warming. For example the reports states:

  • Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850.
  • Observational and model studies of temperature change, climate feedbacks and changes in the Earth’s energy budget together provide confidence in the magnitude of global warming in response to past and future forcing.
  • Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system. Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions.
  • Projected climate change… is similar to AR4 in both patterns and magnitude, after accounting for scenario differences.

The CCC has analysed in detail the implications of AR5 for the UK’s emissions targets. We found no major change that would support a weakening of ambition, and continue to recommend that the legislated carbon budgets and 2050 target are an appropriate minimum level of action.


The nine claims

The text of Viscount Ridley’s speech as recorded in Hansard is given below. For each claim, statements from the AR4 and AR5 reports are provided (from the Summary for Policymakers in each case, unless otherwise stated) along with a brief conclusion.


Claim 1

“Let me give nine separate examples of ways in which the Intergovernmental Panel on Climate Change has retreated to a slightly less alarming and less certain position than six years ago.

“First, it acknowledges the pause or standstill in temperature for the first time.”

What AR4 stated: “Eleven of the last twelve years (1995–2006) rank among the 12 warmest years in the instrumental record of global surface temperature (since 1850)… The linear warming trend over the last 50 years… is nearly twice that for the last 100 years.”

What AR5 states: “Each of the last three decades has been successively warmer at the Earth’s surface than any preceding decade since 1850… Due to natural variability, trends based on short records are very sensitive to the beginning and end dates and do not in general reflect long-term climate trends. As one example, the rate of warming over the past 15 years… is smaller than the rate calculated since 1951.”

Conclusion: IPCC has always showed and discussed charts of up-to-date global annual average temperature records. In 2007, at the time of the IPCC’s last assessment, discussion of a pause since 1998 would have been irrelevant as this is much too short a period to measure any meaningful climate trend. In the latest assessment, it notes that the trend since 1998 has been lower, but still cautions against interpreting this as being significant in terms of climate.


Claim 2

“Secondly, it acknowledges for the first time since its report in 1990 that the medieval warm period was at least as warm as today on a global level and therefore that today’s temperatures are not unprecedented in the last thousand years.”

What AR4 stated: “Average Northern Hemisphere temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years and likely the highest in at least the past 1,300 years.”

What AR5 states: “Reconstructions show, with high confidence, multidecadal periods during the Medieval Climate Anomaly… that were in some regions as warm as in the late 20th century. These regional warm periods did not occur as coherently across regions as the warming in the late 20th century.” And from the Paleoclimate chapter, “For average annual Northern Hemisphere temperatures, the period 1983–2012 was very likely the warmest 30-year period of the last 800 years (high confidence) and likely the warmest 30-year period of the last 1400 years (medium confidence). This… is consistent with AR4”

Conclusion: Neither AR4 nor AR5 make statements about global temperature in the Medieval Period, only the Northern Hemisphere where there is enough data. It is clear on reading the full AR5 report that the authors have not found any change here – the last few decades are likely the warmest in at least the last 1,300 years (i.e. including Medieval times).


Claim 3

“Thirdly, it acknowledges for the first time that Antarctic sea ice is slowly expanding not retreating, which was not predicted by its models.”

What AR4 stated: “Antarctic sea ice extent continues to show interannual variability and localised changes but no statistically significant average trends.”

What AR5 states: “It is very likely that the annual mean Antarctic sea ice extent increased at a rate in the range of 1.2 to 1.8% per decade… between 1979 and 2012. There is high confidence that there are strong regional differences in this annual rate, with extent increasing in some regions and decreasing in others… Most models simulate a small downward trend in Antarctic sea ice extent, albeit with large inter-model spread, in contrast to the small upward trend in observations.”

Conclusion: The previous IPCC assessment did not state that Antarctic sea ice is retreating, but that it is variable with no significant trend. Now that we have a long enough record of observations, the IPCC has noted a small upward trend. It is true to say models on average predict a downward trend (although not all do); the IPCC chapters on Cryosphere and Model Evaluation discuss in detail the possible reasons for the small increase and the limitations of the models.


Claim 4

“Fourthly, it acknowledges that 111 of its 114 models overstated warming in the last 15 years.”

What AR4 stated: “Since IPCC’s first report in 1990, assessed projections have suggested global average temperature increases between about 0.15°C and 0.3°C per decade for 1990 to 2005. This can now be compared with observed values of about 0.2°C per decade, strengthening confidence in near-term projections.”

What AR5 states: “The long-term climate model simulations show a trend in global-mean surface temperature from 1951 to 2012 that agrees with the observed trend (very high confidence). There are, however, differences between simulated and observed trends over periods as short as 10 to 15 years (e.g. 1998 to 2012).” From the Technical Summary: “an analysis of the full suite of [model] simulations… reveals that 111 out of 114 realisations show a [temperature] trend over 1998–2012 that is higher than the [observational record]… whereas during the 15-year period ending in 1998, it lies above 93 out of 114 modelled trends.”

Conclusion: As the “pause” was not a topic of debate in 2007 the IPCC did not address it, although AR4 did make a statement about the recent 15-year trend being consistent with past IPCC predictions. This time AR5 notes that nearly all models run too hot over the period since 1998 but also notes that the models generally underestimated the warming in the preceding 15-year period. Over periods of 50 years or more – the timescale in which we expect the signal from greenhouse gases to be clearer – the models show good agreement with observations.


Claim 5

“Fifthly, it acknowledges that the range of equilibrium climate sensitivity is lower than it was six years ago.”

What AR4 stated: “It is likely to be in the range 2°C to 4.5°C with a best estimate of about 3°C, and is very unlikely to be less than 1.5°C. Values substantially higher than 4.5°C cannot be excluded, but agreement of models with observations is not as good for those values.”

What AR5 states: “[It] 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).”

Conclusion: The IPCC has lowered the bottom of the likely range, but the top end of the likely range remains the same. It is interesting to note that IPCC Assessments one to three (1990-2001) gave a range of 1.5-4.5°C, as did the Charney Report to the US Government in 1979. So the conclusions of IPCC AR5 lie in line with long-standing climate advice (albeit supported by far more evidence and thus greater confidence).


Claim 6

“The sixth thing that the IPCC acknowledges is that transient climate response is lower.”

What AR4 stated: From the Technical Summary “it is very likely to be greater than 1°C and very unlikely to be greater than 3°C”.

What AR5 states: “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.”

Conclusion: Caution is required in comparing these two statements as they are not directly comparable. It does however appear that high values, especially above 3°C, are now considered less likely.


Claim 7

“Seventhly, it acknowledges that sea-level rise, which is definitely happening, is lower than some authorities, such as Professor Rahmstorf, have tried to persuade us that it is.”

What AR4 stated: AR4 gave a range for 21st Century sea level rise of 0.18-0.59m, based on SRES scenarios, noting that “ice flow from Greenland and Antarctica… could increase or decrease in the future. For example, if this contribution were to grow linearly with global average temperature change, the upper ranges of sea level rise… would increase by 0.1 to 0.2 m. Larger values cannot be excluded.”

What AR5 states: AR5 gives a likely range across RCP scenarios of 0.26-0.82m, which includes estimates of future ice flow from Greenland and Antarctica. From the Sea Level Change chapter: “For scenario SRES A1B, which was assessed in the AR4, the likely range… in the AR5 is… 0.40-0.75m… compared with the AR4 projection of 0.21-0.48m for the same scenario and period.”

Conclusion: A full reading of AR5 makes it clear that projections of future sea level are now higher, not lower, than AR4. Caution is required in comparing the sea level ranges due to use of different scenarios, but it is clear that the IPCC is now more confident that ice loss from Greenland and Antarctica will add to the rise. In any case, the studies with higher projections by Rahmstorf and colleagues were not part of IPCC AR4 conclusions at all, because they have all been published since.


Claim 8

“Eighthly, it says, using the words ‘very unlikely’, which it specifically defines in statistical terms, that a collapse of the Gulf Stream is very unlikely, that a collapse of the west Antarctic or Greenland ice sheet is very unlikely and that an explosion of methane from clathrates on the ocean floor is very unlikely.”


The Gulf Stream

What AR4 stated: “It is very unlikely that the [ocean circulation which drives the Gulf Stream] will undergo a large abrupt transition during the 21st century. Longer-term changes… cannot be assessed with confidence.

What AR5 states: “It is very unlikely that [it] will undergo an abrupt transition or collapse in the 21st century for the scenarios considered. There is low confidence… However, a collapse beyond the 21st century for large sustained warming cannot be excluded.”

Conclusion: The IPCC reports are aligned on this issue. There has been no reduction in projected risk. The assessment that Gulf Stream collapse is “very unlikely” applies only to this century, and does not exclude the possibility of later collapse.
The Antarctic Ice Sheet

What AR4 stated: “Current global model studies project that the Antarctic Ice Sheet will remain too cold for widespread surface melting and is expected to gain in mass due to increased snowfall. However, net loss of ice mass could occur if dynamical ice discharge dominates the ice sheet mass balance.” And from the Global Projections chapter, “Owing to limited understanding of the relevant ice flow processes, there is presently no consensus on the long-term future of the ice sheet.”

What AR5 states: “Abrupt and irreversible ice loss from a potential instability of marine-based sectors of the Antarctic Ice Sheet in response to climate forcing is possible, but current evidence and understanding is insufficient to make a quantitative assessment.” In the Long-Term Projections chapter “[It is] exceptionally unlikely that either Greenland or West Antarctic Ice sheets will suffer near-complete disintegration” before 2100.

Conclusion: AR4 does not contain a comparable statement about the likelihood of ice sheet disintegration by 2100 specifically. But the other statements in AR4 and AR5 do not suggest a reduction in risk. Both assessments highlight the uncertainty, but it is AR5 that mentions the possibility of abrupt Antarctic Ice Sheet loss, not AR4.
The Greenland Ice Sheet

What the AR4 stated: “Current models suggest that… the surface mass balance becomes negative at a global average warming (relative to pre-industrial values) in excess of 1.9°C to 4.6°C. If a negative surface mass balance were sustained for millennia, that would lead to virtually complete elimination of the Greenland Ice Sheet”

What AR5 states: “There is high confidence that sustained warming greater than some threshold would lead to the near-complete loss of the Greenland ice sheet over a millennium or more, causing a global mean sea level rise of up to 7 m. Current estimates indicate that the threshold is greater than about 1°C (low confidence) but less than about 4°C (medium confidence) global mean warming with respect to pre-industrial.”

Conclusions: There appears to be higher confidence in AR5 that long-term collapse of the Greenland Ice Sheet is a real likelihood under further warming, and a lower range for the threshold temperature beyond which this might commence.
Methane clathrates

What the AR4 stated: the Climate Coupling chapter mentions in passing that “[clathrate] decomposition represents an important positive CH4 feedback to be considered in global warming scenarios on longer time scales [i.e. 1,000 to 100,000 years].”

What AR5 states: the Long-Term Projections chapter states that “initial estimates of the 21st century positive feedback from methane clathrate destabilization are small but not insignificant… Nevertheless, on multi-millennial timescales, the positive feedback… is potentially larger”. It is “Very unlikely that methane from clathrates will undergo catastrophic release” before 2100.

Conclusion: Again there is no comparable statement in AR4 regarding the likelihood of catastrophic 21st Century release. Both assessments consider clathrates to be more important over longer timescales. If anything, there is more evidence since AR4 that at least some release is possible before 2100.


Claim 9

“Ninthly, it says that it has low confidence in a number of tipping points that were previously thought to be possible concerns: the collapse of tropical forests, of boreal forests and of the monsoon, an explosion of greenhouse gases from the Arctic permafrost and an increase in megadroughts. It says that it has low confidence in these things. These are major retreats.”

Assessments of confidence in these changes is a new feature for AR5. There are no similar confidence statements in AR4 by which to judge directly whether or not there have been “major retreats”. Nevertheless we compare the AR5 statements with relevant parts of the text in AR4 below.
Tropical forest collapse

What AR4 stated: the Global Projections chapter contains a sentence that “A climate model simulation into the future shows… a continuous reduction in the forest of Amazonia… While evolving continuously over the 21st century, such a change and ultimate disappearance could be irreversible, although this result could be model dependent.”

What AR5 states: the Long-Term Projections chapter states that “Uncertainty concerning the existence of critical thresholds in the Amazonian and other tropical rainforests purely driven by climate change therefore remains high, and so the possibility of a critical threshold being crossed… cannot be ruled out.” There is “Low confidence in projections of the collapse of large areas of tropical forest.”

Conclusion: AR4 only mentions a result from a single climate model. This suggests strongly that confidence in Amazon forest collapse was no higher then than it is now.


Boreal forest collapse

What AR4 stated: Very little is said about boreal forest change, however the Climate Couplings chapter suggests that boreal forests are predicted to increase, not collapse, with future warming.

What AR5 states: the Long-Term Projections chapter contains a sentence that “Evidence from field observations and biogeochemical modelling make it scientifically conceivable that regions of the boreal forest could tip into a different vegetation state under climate warming, but uncertainties on the likelihood of this occurring are very high”. There is “Low confidence in projections of the collapse of large areas of boreal forest.”

Conclusion: There is no reference to boreal forest collapse in AR4, only AR5. Rather than a decrease in confidence, this suggests the possibility of a new risk that was not appreciated before.


Monsoon collapse

What AR4 stated: the Global Projections chapter concludes that “An increase in precipitation is projected in the Asian monsoon… However, the uncertain role of aerosols… complicates the nature of future projections.”

What AR5 states: From the Long-Term Projections chapter “it is unlikely that an abrupt transition to the dry summer monsoon regime will be triggered in the 21st century.” There is “Low confidence in projections of a collapse in monsoon circulations.”

Conclusion: Similar to the discussion of boreal forests, there is no reference to a risk of collapse in AR4, only AR5. This does not suggest the risks are reduced.
Permafrost emissions

What AR4 stated: Very little is said about the potential for carbon release from melting permafrost, although the Climate Couplings chapter states “Observations indicate substantial increases in [methane] released from northern peatlands that are experiencing permafrost melt” while the Global Projections chapter notes that model projections are yet to account for this process.

What AR5 states: the Long-Term Projections chapter notes that it is “Possible that permafrost will become a net source of atmospheric greenhouse gases (low confidence).” The Biogeochemical Cycles chapter states “under sustained Arctic warming, modelling studies and expert judgments indicate with medium agreement that a potential combined release totalling up to 200 PgC as carbon dioxide equivalent could occur by the year 2100.”

Conclusion: Since AR4 there has been progress in quantifying future emissions from permafrost. All studies show a net increase in emissions, and therefore warming. As a result AR5 includes, for the first time, a quantitative projection. While this has low confidence attached to it, it clearly represents a higher level of confidence in permafrost emissions relative to AR4.



What AR4 stated: the Global Projections chapter concludes that “There is a tendency for drying of the mid-continental areas during summer, indicating a greater risk of droughts in those regions.”

What AR5 states: the Long-Term Projections chapter states that “AR4 climate model projections and [AR5 climate model projections] both suggest widespread drying and drought across most of southwestern North America and many other subtropical regions by the mid to late 21st century”. There is “Low confidence in projections of changes in the frequency and duration of megadroughts.”

Conclusion: Despite the low confidence in predicting the details, projections show an overall drying for many regions. AR5 clearly points to consistency with AR4 on this issue.


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