Review of the Draft Climate Science Special Report (2017) / Chapter Skim
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III. Comments on Each Chapter of the Draft Climate Science Special Report
III. Comments on Each Chapter of the Draft Climate Science Special Report
Pages 13-56
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From page 13... ...
The introduction to extremes in Section 1.2.4 provides an appropriate discussion of trends in extremes, but the lack of consideration of extreme event attribution is a missed opportunity. Second, the long section on the hiatus in Box 1.1 of the draft CSSR gives that event much more prominence than is warranted.
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From page 14... ...
The chapter makes the legitimate point that global mean temperature is better known after 1900 than before, but an earlier period can be safely used. Specific Review Comments Related to the Statement of Task Does the chapter accurately reflect the scientific literature?
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From page 15... ...
(Very high confidence) In the major uncertainties provided in the traceable accounts for Key Finding 4, the text should emphasize the uncertainty in the magnitude of climate feedbacks.
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From page 16... ...
Sections that need strengthening include those on precipitation, extreme events, and land processes. Integration of ocean heat content into the discussion of SLR and quantifying the changes and trends described in these sections would also benefit the chapter.
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From page 17... ...
A succinct discussion of how climate sensitivity differs from transient climate forcing would also be helpful. Mention of the sources of uncertainties illustrated in the relevant figures in the draft CSSR, such as climate sensitivity, future GHG emissions, and ocean heat uptake, could also be useful.
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From page 18... ...
As written, some of the evidence base is listed in the "uncertainties" section of the traceable accounts instead of in the "description of evidence base" section. Revising the traceable accounts to clarify the evidence vs.
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From page 19... ...
Recommended changes to structure None beyond those previously described. III.3 CHAPTER 3: DETECTION AND ATTRIBUTION OF CLIMATE CHANGE Summary This chapter is intended to convey the message that the observed changes in global climate since the mid-20th century are detectable and largely attributable to human influences, which is an important point that is referenced in other parts of the draft CSSR.
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From page 20... ...
Some recommended citations are provided in the next section. • The chapter could also benefit from some emphasis on the importance of this detection and attribution science for determining whether human influence on climate variables (and on individual extreme events or classes of extreme events)
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From page 21... ...
The Committee suggests that the chapter would be strengthened by adding a key finding that highlights advances in the detection and attribution of features of climate change that go beyond simple global mean surface temperature. For example, "The science of event attribution is rapidly advancing with the understanding of the mechanisms that produce extreme events and the development of methods that are used for event attribution" (paraphrased from NASEM, 2016a)
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From page 22... ...
3 to CMIP5. The discussion of the difference between the IPCC Special Report on Emissions Scenarios (SRES)
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From page 23... ...
It is unclear what value there is in including discussion of the World Climate Research Programme COordinated Regional climate Downscaling Experiment (CORDEX) in the draft CSSR.
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From page 24... ...
However, more evidence is needed in the traceable account for the statement that economic growth has begun to decouple from fossil fuel combustion. Key Finding 4: Combining output from global climate models and dynamical and statistical downscaling models using advanced averaging, weighting, and pattern scaling approaches can result in more relevant and robust future projections.
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From page 25... ...
As mentioned several times in the draft CSSR (e.g,. page 158, lines 18-19)
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From page 26... ...
Section 4.3.3 focuses on Empirical Statistical Downscaling Model (ESDM) , but results do not figure prominently in the draft CSSR.
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From page 27... ...
Finally, it is not clear why the "Low" confidence box is checked in the traceable accounts. Key Finding 3: Increasing temperatures and atmospheric specific humidity are already having important influences on extremes (high confidence)
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From page 28... ...
Chapter 6 is generally well written and flows nicely, but could be improved by expanded discussion of extreme heat, the influence of the Dust Bowl on the observed record, and other topics detailed here. The Committee has the following concerns about the treatment of extreme events in this chapter: • The extreme metrics were often difficult to understand, especially the definition of warm and cold "spells." How brief are "brief periods"?
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From page 29... ...
Sea surface temperatures are barely discussed in the chapter and are not mentioned in Key Finding 1, so it is odd the topic is included in the traceable account, and while the data sources are given, no details are provided to show how the main conclusions are reached. Key Finding 2: Accompanying the rise is average temperatures, there have been -- as is to be expected -- increases in extreme temperature events in most parts of the United States.
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From page 30... ...
Also, the description of evidence base provided for this key finding should include a discussion of how extreme temperatures during the Dust Bowl years have impacted relative changes in extreme temperatures over the recent period. The role of this event needs to be discussed in key findings (perhaps given its own key finding, or a discussion box)
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From page 31... ...
nighttime low temperatures, that are consistent with the recommendations in Chapter II about extreme events. Recommended changes to structure As part of the restructuring recommended for Chapter 3, some of the attribution information could be moved to Section 6.2.
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From page 32... ...
Generally, the traceable accounts require the inclusion of more details about the science supporting the key findings and references to the literature. As written, there is not enough information to follow the line of evidence that underpins the findings.
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From page 33... ...
A table showing changes for three to four definitions of extreme precipitation would be helpful, or a strong justification for selecting only the 2-day 5-year event for the bar charts in Figure 7.7 and 1-day 20-year event for the map in Figure 7.8. A figure illustrating changes in snow cover extent could also be useful to this chapter and Figure 7.5 could be moved to Appendix B in the draft CSSR.
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From page 34... ...
Key Finding 1: Recent droughts and associated heat waves have reached record intensity in some regions of the United States, but, by geographical scale and duration, the Dust Bowl era of the 1930s remains the benchmark drought and extreme heat event in the historical record. (Very high confidence)
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From page 35... ...
The magnitude of projected snowpack decreases in Key Finding 4 may be understated. The draft CSSR could reasonably use words like "substantial," as virtually all projections show large decreases in snowpack by mid-century.
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From page 36... ...
Specific Review Comments Related to the Statement of Task Does the chapter accurately reflect the scientific literature? Are there any critical content areas missing from the chapter?
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From page 37... ...
See previous comments for recommendations to improve likelihood/confidence statements associated with the key findings. Are statistical methods applied appropriately?
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From page 38... ...
If an effective figure pertaining to the North Atlantic can be found, that might be more useful. In Figure 9.2 the only results of any apparent statistical significance pertain to the western Pacific region and thus are of relatively limited interest for this U.S.-focused draft CSSR.
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From page 39... ...
Mechanisms of variability are important when it comes to improving understanding, but given the draft CSSR is intended to inform NCA4 description of impacts, it is important to quantify as well as possible variability on all time scales, irrespective of physical cause. Finally, it would also be beneficial to indicate that floods due to storm surge are covered in Chapter 12, Sea Level Rise.
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From page 40... ...
• None of the chapter's key findings appear in the ES. Specific Review Comments Related to the Statement of Task Does the chapter accurately reflect the scientific literature?
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From page 41... ...
One cannot conclude that the evidence supports the finding. Key Finding 4: Surface temperatures are often higher in urban areas than in surrounding rural areas, for a number of reasons including the concentrated release of heat from buildings, vehicles, and industry.
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From page 42... ...
. For these reasons, this topic has both importance and policy urgency, and a thorough treatment in the draft CSSR is important.
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From page 43... ...
. Observed sea and land ice loss across the Arctic is occurring faster than climate models predict (very high confidence)
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From page 44... ...
Finally, it is important to be sure that GHG emissions from thawing permafrost are considered consistently throughout the draft CSSR. In particular, the discussion of remaining allowable emissions consistent with meeting the 2oC goal (ES, page 27, lines 17-24)
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From page 45... ...
In a further departure from previous assessments, the new, individual sea level scenarios are placed in context with published probabilistic projections of future sea level following standard RCP emissions scenarios (e.g., Kopp et al., 2014, 2016)
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From page 46... ...
Are there any critical content areas missing from the chapter? The Committee thinks that the chapter accurately reflects the current scientific literature on this topic, although the discussion of drastic Antarctic ice-sheet retreat could be broadened by comparing the recent results of DeConto and Pollard (2016)
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From page 47... ...
Given the importance of long-duration winter storms on East Coast flooding in particular, the Committee recommends considering whether this key finding should be extended to include a comment on extratropical cyclones, in addition to Hurricanes. The traceable accounts reflect the current state of science, and confidence levels are appropriate.
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From page 48... ...
In particular, the role of the oceans in storing heat, and the link between changes in ocean heat content and changes in sea surface temperature could be discussed. In addition, the
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From page 49... ...
The Committee recommends that this chapter be revised so as to improve consistency across the draft CSSR. Specific Review Comments Related to the Statement of Task Does the chapter accurately reflect the scientific literature?
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From page 50... ...
Under a low future scenario (RCP2.6) , global average ocean acidity is projected to increase by 35% and oxygen projected to decrease by 2% by 2100.
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From page 51... ...
The chapter's key findings largely miss the opportunity to make what could be the chapter's central point: a consequence of the essentially permanent nature of warming from CO2 is that stabilization of CO2 at any given concentration can only be achieved if CO2 emissions fall to zero or become negative, to compensate for the remaining emissions of other GHGs and land-use change. Stabilizing warming at 1.5°C or 2°C requires emissions to fall to zero within a few decades, and even stabilizing warming at 3°C or 4°C requires zero emissions a few decades after that.
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From page 52... ...
It is not really useful to discuss the lag between emissions reductions and concentration reductions because the CO2 problem is essentially one of cumulative emissions, such that delaying action in the near term makes it more difficult to solve the problem in the longer term. Key Finding 2: Limiting the global-mean temperature increase to 3.6°F (2°C)
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From page 53... ...
reductions after 2030 would be required to achieve even the lesser goal of significantly reducing the likelihood of a global mean temperature increase greater than 7.2°F (4°C)
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From page 54... ...
The treatment of climate intervention would be clearer with an increased emphasis on the fact that climate intervention strategies are much less well known than climate change and that a reasonable foundation for decisions will require a big expansion of technology development as well as knowledge, especially in the area of governance and political dimensions. III.15 CHAPTER 15: POTENTIAL SURPRISES: COMPOUND EXTREMES AND TIPPING ELEMENTS Summary The Committee found this chapter to be a welcome addition to the discussion of climate science and recommends it be expanded.
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From page 55... ...
Specific Review Comments Related to the Statement of Task Does the chapter accurately reflect the scientific literature? Are there any critical content areas missing from the chapter?
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From page 56... ...
well quantified, they do not include all of the processes that can contribute to positive feedbacks, correlation of extremes, and abrupt and/or irreversible changes. For this reason, future changes outside the range projected by climate models cannot be ruled out (very high confidence)
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