by Judith Curry

U.S. and European Union envoys are seeking more clarity from the United Nations on a slowdown in global warming that climate skeptics have cited as a reason not to “panic” about environmental changes, leaked documents show.

Bloomberg has an article entitled Global Warming Slowdown Sought in UN Climate Report. Excerpts:

Government envoys from around the world will debate the final wording of the summary at an IPCC meeting that starts in Stockholm on Sept. 23. That document, formally the Summary for Policymakers, is designed to be used by ministers working to devise by 2015 a global treaty to curb climate change.

The current version of the summary needs more information about the hiatus, according to the EU and the U.S.

“The recent slowing of the temperature trend is currently a key issue, yet it has not been adequately addressed in the SPM,” the EU said, according to an official paper that includes all governmental comments on the draft report. The U.S. comment suggested “adding information on recent hiatus in global mean air temperature trend.”

The draft report includes possible reasons for the slowing rate, including natural variability, volcanic eruptions and a drop in solar energy reaching the Earth.

“Much of the information is present but it requires a lot of effort on the part of the reader to piece it all together,” the 28-nation EU said in the comments document.

The U.S. requested clarity on the implications of the data, commenting “this is an example of providing a bunch of numbers, then leave them up in the air without a concrete conclusion.”

Norway, Denmark and China requested information on the role oceans have played in the slowdown. China cited three scientific papers, including a study in the journal Geophysical Research Letters in May that found deep ocean waters below 700 meters (2,300 feet) haveabsorbed more heat since 1999.

A separate study in the journal Nature Aug. 28 linked the hiatus to a cooling of surface waters in the eastern Pacific. The cut-off date for papers to be considered in the UN report was March 15.

The UN World Meteorological Organization defines climate as the average weather over a 30-year period, and scientists say the 15-year slowdown isn’t long enough to mark a trend. Hungary and Germany, both EU members, cited this as a reason to delete any reference to the hiatus in the summary, while Japan questioned the purpose of using a 15-year average.

“A 15-years period of observation is not sufficient to give a qualified analysis of the global mean surface temperature trend in an assessment of climate change,” Germany said. It also said the use of the word “hiatus” is “strongly misleading” because “there is not a pause or interruption, but a decrease in the warming trend.”

Bob Ward’s take

Some quotes from Bob Ward in the Bloomberg article:

Including more information on the hiatus will help officials counter arguments that the slowing pace of global warming in recent years is a sign that the long-term trend may be discounted, according to Bob Ward, policy director at the Grantham Research Institute on Climate Change and the Environment at the London School of Economics.

“In the public debate, there are people who are using the slowdown to say global warming is less of a problem than thought,” Ward said in an interview yesterday. “It has to be fully explained in the summary.”

Addressing the hiatus is important because skeptics of man’s influence on warming the planet have seized on the slowing pace temperature increase as evidence that scientists have exaggerated the impact of manmade greenhouse gases. That supports their assertion that there’s less need for expensive policies to curb carbon emissions from factories, vehicles and deforestation.

“Some people have suggested that the slowdown means that climate sensitivity is lower,” said Ward from the Grantham Institute.

JC’s take

Well, I’ve looked at the leaked AR5 material and I am holding off judgment until I see what they come up with in the final Summary for Policy Makers. But the ‘sausage making’ in all this is rather mind boggling.

It is of course absolutely essential for IPCC’s credibility to handle the issue of the pause in a comprehensive way, including implications of this for detection, attribution, and sensitivity. The pause is currently the predominant issue in the public debate on climate change, and has been for arguably over a year.

Some policy makers may want this issue addressed so that they can effectively counter ‘denier’ claims; others may be more suspicious of the IPCC and want to see the IPCC justify its conclusions and confidence levels in view of the pause.

As an example to the IPCC of what tackling the pause looks like, see the evidence and arguments presented in my recent Congressional Testimony:

Relevant excerpts from my testimony (see original for references and figures):

Figure 1 shows a long-term increasing trend, and particularly during the last 25 years of the 20th century. However, since 1998 there has been no statistically significant increase in global surface temperature. While many engaged in the public discourse on this topic dismiss the significance of a hiatus in increasing global temperatures because of expected variations associated with natural variability, analyses of climate model simulations find very unlikely a plateau or period of cooling that extends beyond 17 years in the presence of human-induced global warming.

James Hansen has recently written: “The five-year mean global temperature has been flat for the last decade.” Hansen interprets this as “a combination of natural variability and a slow down in the growth rate of net climate forcing.” Hansen then suggests that “global temperature will rise significantly in the next few years as the tropics moves inevitably to the next El Nino phase.” Others have suggested that the pause could last up to two decades or even longer, owing to the transition to the cool phase of the Pacific Decadal Oscillation that is associated with a predominance of La Nina (cool) events.

The fifth phase of the Coupled Model Intercomparison Project (CMIP5) has produced a multi-model dataset that includes long-term simulations of twentieth-century climate and projections for the twenty-first century and beyond, as well as an entirely new suite of initialized decadal predictions focusing on recent decades and the future to year 2035. While providing the underlying basis for the forthcoming IPCC AR5, the CMIP5 model output has been made freely available to researchers through a distributed data archive. An analysis provided by Ed Hawkins at the University of Reading compares the global average surface temperatures from the HadCRUT4 dataset with 20 models from the CMIP5 simulations (Figure 3).

The comparison in Figure 3 shows that observations particularly since 2005 are on the low end of the envelope that contains 90% of the climate model simulations. Extrapolation of the current flat trend would place the observations outside of the 90% envelope within a few years. While the observations remain within the substantial range of the climate model simulations, the trend in the model simulations is substantially larger than the observed trend over the past 15 years.

When considering possible physical reasons for the plateau since 1998, it is instructive to consider the previous mid-century plateau in global average surface temperature (Figure 1). The IPCC AR4 explained this previous plateau in the following way: “the cooling effects of sulphate aerosols may account for some of the lack of observational warming between 1950 and 1970, despite increasing greenhouse gas concentrations.” And “variations in the Atlantic Multi-decadal Oscillation could account for up to 0.2oC peak-to-trough variability in NH mean decadal temperature.”

Recent research on the impact of aerosols on radiative forcing of the climate has demonstrated that the overall cooling from aerosols is less than previously thought owing to a larger role for black carbon aerosols that have a net warming effect on climate.

With regards to multi-decadal natural internal variability, previous IPCC reports consider this issue primarily in context of detection of an anthropogenic warming signal above the background ‘noise’ of natural variability. The IPCC’s attribution of the late 20th century warming has focused on external radiative forcing, and no explicit estimate of the contribution of natural internal variability to the warming was made. A recent paper by Tung and Zhou suggests that the anthropogenic global warming trends might have been overestimated by a factor of two in the second half of the 20th century. They argue that a natural multidecadal oscillation of an average period of 70 years with significant amplitude of 0.3–0.4°C is superimposed on the secular warming trend, which accounts for 40% of the observed warming since the mid-20th century. Tung and Zhou identify this oscillation with the Atlantic Multidecadal Oscillation (AMO), although recent research suggests a more complex multidecadal signal propagating through a network of synchronized climate indices. Tung and Zhou argue that not taking the AMO into account in predictions of future warming under various forcing scenarios may run the risk of over-estimating the warming for the next two to three decades, when the AMO is likely in its down phase.

The recent research on natural internal variability and black carbon aerosols, combined with ongoing plateau in global average surface temperature, suggests that the AR4 estimates of climate sensitivity to doubling CO2 may be too high, with implications for the attribution of late 20th century warming and projections of 21st century warming. The IPCC AR4 conclusion on climate sensitivity is stated as:

“The equilibrium climate sensitivity. . . is likely to be in the range 2oC to 4.5oC with a best estimate of about 3oC and is very unlikely to be less than 1.5oC. Values higher than 4.5oC cannot be excluded. .”

This estimate of equilibrium climate sensitivity is not easily reconciled with recent forcing estimates and observational data. There is increasing support for values of climate sensitivity around or below 2oC. The meta-uncertainty of these estimates remains high owing to inadequacies in the methods used to determine sensitivity from observations and models. If the climate models are running too ‘hot’ in terms of predicting climate sensitivity that is too high, what are the possible problems with the models that might contribute to this? While the direct forcing from greenhouse gases is well understood, possible problems are associated with the magnitudes of the water vapor feedback and the cloud feedback. The cloud-radiative feedback is one of the most uncertain elements of climate models; even the sign is uncertain, although most climate models produce a positive cloud-radiative feedback (warming effect).

The key conclusion of the IPCC AR4 is: “Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.”

So what is the evidence for, and against, a dominant role in the climate since the mid-20th century of increasing human-induced greenhouse gas concentrations, and what are the major uncertainties? Below is my summary interpretation of the available evidence. Evidence for: Long-term trend of increasing surface temperatures, for more than a century.

Theoretical support for warming as greenhouse gas concentration increases.

Long-term trend of increasing ocean heat content[2], although the trend for the past 10 years has been small in the upper 700 m of the ocean[3].

Decline in Arctic sea ice since 1979, with record autumn minimum in 2012.

Sea level rise since 1961, although multi-decadal variability and confounding factors such as coastal land use and geologic process hamper interpretation of these data.[4]

Results from climate model simulations. Evidence against: No significant increase in globally averaged temperature for the past 15 years.

Lack of a consistent and convincing attribution argument for the warming from 1910-1940 and the plateau from the 1940s to the 1970s.

Growing realization that multidecadal natural internal variability is of higher amplitude than previously accounted for in IPCC attribution analyses. There are major uncertainties in many of the key observational data sets, particularly prior to 1980. There are also major uncertainties in climate models, particularly with regards to the treatments of clouds, solar indirect effects and the coupled multidecadal oscillations between the ocean and atmosphere. Further, there are meta-uncertainties regarding the methods used to make arguments about attribution of climate change and determine sensitivity to increasing greenhouse gases. And finally, climate models are apparently incapable of simulating emergent phenomena such as abrupt climate change. In light of these uncertainties, what can we say about the future climate of the 21st century? Most scientists anticipate a decrease in solar forcing in the coming decades, but noting the absence of understanding the solar indirect effects on climate, this is not expected to dominate climate change in the 21st century. If the climate shifts hypothesis is correct, then the current flat trend in global surface temperatures may continue for another decade or two, with a resumption of warming at some point during mid-century. The amount of warming from greenhouse gases depends both on the amount of greenhouse gases that are emitted as well as the climate sensitivity to the greenhouse gases, both of which are associated with substantial uncertainties. It looks to me like the national and international policy makers are expecting a serious treatment of the pause issue, I have shown them one way to approach the issue of the pause in an integrated way. In spite of Michael Mann’s tweeted response to my testimony ‘typical denier talking points,’ the issues I raise are not easily dismissed, and my ideas are out there in the public domain and at least some politicians are paying attention to my arguments. And the recent Nature article on the central Pacific control on global climate adds fuel to my arguments. If anyone can refute my arguments, I would be most interested in seeing this.