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«Major points: Recent data and research supports the importance of natural climate variability and calls into question the conclusion that humans are ...»

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Hearing on

“The President’s U.N. Climate Pledge”

15 April 2015

Judith A. Curry

Georgia Institute of Technology


Major points:

Recent data and research supports the importance of natural climate variability and calls into question the

conclusion that humans are the dominant cause of recent climate change:

The hiatus in global warming since 1998 • Reduced estimates of the sensitivity of climate to carbon dioxide • Climate models predict much more warming than has been observed in the early 21st century •

We have made some questionable choices in defining the problem of climate change and its solution:

The definition of ‘dangerous’ climate change is ambiguous, and hypothesized catastrophic tipping points • are regarded as very or extremely unlikely in the 21st century.

Efforts to link dangerous impacts of extreme weather events to human-caused warming are misleading and • unsupported by evidence.

Climate change is a ‘wicked problem’ and ill-suited to a ‘command and control’ solution • It has been estimated that the U.S. INDC of 28% emissions reduction will prevent 0.03oC in warming by • 2100.

The inadequacies of current policies based on the Precautionary Principle are leaving the real societal consequences of climate change and extreme weather events (whether caused by humans or natural variability)

largely unaddressed:

We should expand the frameworks for thinking about climate policy and provide policy makers with a • wider choice of options in addressing the risks from climate change.

Pragmatic solutions based on efforts to accelerate energy innovation, build resilience to extreme weather, • and pursue no regrets pollution reduction measures have justifications independent of their benefits for climate mitigation and adaptation.




Hearing on “The President’s U.N. Climate Pledge” 15 April 2015 Judith A. Curry Georgia Institute of Technology curryja@eas.gatech.edu I thank the Chairman and the Committee for the opportunity to offer testimony today on ‘The President’s U.N. Climate Pledge.’ I am Professor and former Chair of the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology. As a climate scientist, I have devoted 30 years to conducting research on a variety of topics including climate feedback processes in the Arctic, the role of clouds and aerosols in the climate system, and the impact of climate change on the characteristics of tropical cyclones. As president of Climate Forecast Applications Network LLC, I have been working with decision makers on climate impact assessments, assessing and developing climate adaptation strategies, and developing subseasonal climate forecasting strategies to support adaptive management and tactical adaptation.

I am increasingly concerned that both the climate change problem and its solution have been vastly oversimplified.1 My research on understanding the dynamics of uncertainty at the climate science-policy interface has led me to question whether these dynamics are operating in a manner that is healthy for either the science or the policy process.2 As a result, I am concerned that the U.S. Intended Nationally Determined Contribution (INDC) to the United Nations Framework Convention on Climate Change (UNFCCC) will do essentially nothing to change the climate, and the U.S. and other nations will remain vulnerable to climate surprises and extreme weather events.

My testimony focuses on the following issues of central relevance to the U.S. INDP:

• Weakening case for dangerous human-caused climate change

• The climate change response challenge

• Expanding the policy options for responding to climate change A weakening case for dangerous anthropogenic climate change Scientists agree that surface temperatures have increased since 1880, humans are adding carbon dioxide to the atmosphere, and carbon dioxide and other greenhouse gases have a warming effect on the planet.

However there is considerable disagreement about the most consequential issues:

• Whether the warming since 1950 has been dominated by human causes

• How much the planet will warm in the 21st century

• Whether warming is ‘dangerous’ The central issue in the climate change debate is the extent to which the recent (and future) warming is caused by human-caused greenhouse gas emissions versus natural climate variability – variations from 1 Curry, JA and Webster PJ 2011: Climate science and the uncertainty monster. Bull Amer Meteorol. Soc., 92, 1667-1682.

http://journals.ametsoc.org/doi/pdf/10.1175/2011BAMS3139.1 2 Judith Curry, Statement to the Subcommittee on Environment of the U.S. House of Representatives Hearing on Policy Relevant Climate Science in Context, 25 April 2013. https://curryja.files.wordpress.com/2013/04/curry-testimony-2013-il.pdf the sun, volcanic eruptions, and large-scale ocean circulations. My 2014 testimony before the Senate Environmental and Public Works Committee3 argued that the 2013 report from the Intergovermental Panel on Climate Change (IPCC AR5 WG I)4 weakened the case for dangerous anthropogenic climate change relative to the IPCC AR4 published in 2007. A summary is presented here of recent data and research that supports the importance of natural climate variability and calls into question the IPCC’s conclusion that humans are the dominant cause of recent climate change. The policy relevance of this issue is that if humans are not the dominant cause of climate change, then attempts to modify the climate through reducing greenhouse gas emissions will have little impact on future climate change.

Hiatus in global warming

The IPCC AR5 notes a slowdown in surface warming since 1998:

“[T]he rate of warming over the past 15 years (1998–2012) [is] 0.05 [–0.05 to +0.15] °C per decade which is smaller than the rate calculated since 1951 [of] 0.12 [0.08 to 0.14] °C per decade.”

This figure shows the recent global temperatures through 2014 from several different global data sets5:

The media touted 2014 as the ‘warmest year’ in the historical record; however, given the uncertainties in the analyses, 2014 was in a statistical tie with 2010 and 2005. The UK dataset HadCRU, with perhaps a more realistic assessment of uncertainties, found 2014 to rank among the top 10 warmest years, all of which are since 1998. While the recent decade is the warmest in history, the ties for warmest year further reflect a plateau in the warming.

So we have no significant temperature increase since 1998, which has been a period with 25% of the total human CO2 emissions. This hiatus in warming is at odds with the 2007 IPCC AR4 report, which expected warming to increase at a rate of 0.2 °C per decade in the early 21st century.

Numerous recent research papers have highlighted the importance of natural variability associated with circulations in the Atlantic and Pacific Oceans, which is now believed to be the dominant cause of the hiatus. If the recent warming hiatus is caused by natural variability, then this raises the question as to what extent the warming between 1975 and 1998 can also be explained by natural climate variability.

3 Judith Curry, Statement to the Senate Committee on Environment and Public Works 25 April 2014 http://www.epw.senate.gov/public/index.cfm?FuseAction=Files.View&FileStore_id=07472bb4-3eeb-42da-a49d-964165860275 4 IPCC reports can be obtained at http://www.ipcc.ch 5 Figure courtesy of Robert Rohde of the Berkeley Earth Surface Temperature team Sea ice The IPCC AR5 acknowledges the strong role of natural variability in determining sea ice variability and

change on multidecadal time scales. Nevertheless, the IPCC AR5 concluded:

“[I]t is very likely that the Arctic sea ice cover will continue to shrink and thin all year round • during the 21st century. It is also likely that the Arctic Ocean will become nearly ice-free in September before the middle of the century (medium confidence).”

Below are satellite observations of sea ice variability through 2014.6

In 2013 and 2014, Arctic sea ice recovered from its summertime minima during the period 2007-2012.

Notably, Arctic sea ice volume (a metric that combines both horizontal extent and ice thickness) shows a continuing increase since 20127. During 2014, Antarctic sea ice set a wintertime maximum record.

A recent paper by Swart et al.8 emphasized that internal climate variability can mask or enhance humaninduced sea-ice loss on timescales ranging from years to decades or even a century. A new paper by Zhang9 clarifies the natural fluctuations that influence Arctic sea ice loss – heat transported by the Atlantic and Pacific, and wind patterns over the Arctic that drive sea ice out from the central Arctic, where it melts in the North Atlantic. In particular, the recent cooling in the high latitudes of the North Atlantic is associated with the current recovery of the sea ice in the Atlantic sector.

Clearly, there is a lot going on with respect to variability in Arctic and Antarctic sea ice that cannot be explained directly or even indirectly by warming from human-caused greenhouse gases. Climate models do not simulate correctly the ocean heat transport and its variations. Scientists do not agree on the explanation for the increasing Antarctic sea ice extent, and the key issue as to whether human-caused warming is the dominant cause of the recent Arctic sea ice loss remains unresolved.

6 http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/seaice.anomaly.arctic.png http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/seaice.anomaly.antarctic.png 7 http://psc.apl.washington.edu/wordpress/wpcontent/uploads/schweiger/ice_volume/BPIOMASIceVolumeAnomalyCurrentV2.1.png 8

Swart et al 2015 Influence of internal variability on Arctic sea-ice trends, Nature climate Change, 5, Pages: 86–89 DOI:

doi:10.1038/nclimate2483 9 Zhang, R. 2015. Mechanisms for low-frequency variability of summer Arctic sea ice extent, Proceedings of the National Academy of Sciences, doi:10.1073/pnas.1422296112 Sensitivity Human-caused warming depends not only on increases in greenhouse gases but also on how ‘sensitive’ the climate is to these increases. Climate sensitivity is defined as the global surface warming that occurs when the concentration of carbon dioxide in the atmosphere doubles. If climate sensitivity is high, then we can expect substantial warming in the coming century as emissions continue to increase. If climate sensitivity is low, then future warming will be substantially lower.

The most relevant definition of climate sensitivity is the actual change of surface temperature in 70 years if carbon-dioxide concentrations double, called the ‘transient climate response’. The IPCC AR4 (2007) concluded that the transient climate response is very likely larger than 1°C and very unlikely greater than 3°C. The IPCC AR5 (2013) concluded that the transient climate response is likely [17-83%] in the range of 1 to 2.5°C.

Last year, Nicholas Lewis and I published a paper10 that found transient climate response to have a likely range of 1.05-1.80°C. Using an observation-based energy balance approach, our calculations used the same data for the effects on the Earth’s energy balance of changes in greenhouse gases, aerosols and other drivers of climate change given by the IPCC AR5. Our range for the transient climate response is much narrower, with far lower upper limits, than reported by the IPCC AR5.

Recent research suggests even lower values of the transient climate response. The greatest uncertainty in these estimates is accounting for the effects of small aerosol particles in the atmosphere, which have a cooling effect on the climate (partially counteracting the greenhouse warming). A new paper by Stevens11 constrains the impact of aerosols on climate to be significantly smaller than assumed in the AR5.

Nicholas Lewis has re-run the calculations using aerosol impact estimates in line with this paper. The likely range for the transient climate response is 1.05 to 1.45°C. By contrast, most climate model estimates of transient climate response are higher than 1.8°C. Research continues to assess the methods used to estimate climate sensitivity. However, the reduced estimates of aerosol cooling lead inescapably to reductions in the estimated upper bound of climate sensitivity.

Are climate models running too ‘hot’?

These new climate sensitivity estimates, combined with the slowdown or ‘hiatus’ in global warming since 1998, add to the growing evidence that climate models are running too ‘hot.’ The near-term temperature projections of the climate models are shown below, compared with observations of global temperatures through 2014.12 The observed global temperatures, particularly since 2011, are below or just at the bottom bound of the 5-95% envelope of the CMIP5 climate model simulations. Overall, the trend in the model simulations is substantially larger than the observed trend over the past 15 years.

Note the hatched red area, this seems to be a concession to the hiatus. The IPCC cites ‘expert judgment’ as the rationale for lowering the projections (indicated by the red hatching), to account for the apparent oversensitivity of the models.

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