globalwarmingquestionCredit: Image191: DreamstimeA major new study published in Nature Geoscience reports that future global warming is likely to be significantly less than many climate model projections have suggested. The authors cannot be characterized by opponents as climate change "deniers." Using recent data from the continued slowdown in global temperature increases, the researchers estimated new equilibrium climate sensitivity and transient climate response numbers.

As Phys.org explains:

The sensitivity of our planet to a doubling of the concentration can be expressed using two different measures. One measure, the transient climate response, describes the immediate, short-term warming. This figure is the one that really matters to policy makers. The other measure, the equilibrium climate sensitivity, describes the long-term commitment once the climate system has come into balance with the enhanced level of greenhouse gases.

The new Nature Geoscience study found:

The most likely value of equilibrium climate sensitivity based on the energy budget of the most recent decade is 2.0 °C, with a 5–95% confidence interval of 1.2–3.9 °C. ...

The best estimate of TCR based on observations of the most recent decade is 1.3 °C (0.9–2.0 °C).

The U.N.'s Intergovernmental Panel on Climate Change has consistently held to a higher estimate of climate sensitivity of between 2 to 4.5 degrees Celsius with a best estimate of 3 degrees. In other words, the new best estimate  reported in Nature Geoscience is a full degree lower than the IPCC's.

The new study is something of a triumph for statistician Nic Lewis (who is a co-author) whose methods to estimate climate sensitivity were used in this new study. In a Journal of Climate study earlier this year Lewis calculated climate sensitivity at between 1 and 3 degrees Celsius. For those interested in delving deeper into the numbers, see Lewis' post at Watts Up With That.

Given this lower estimate, the popular science magazine New Scientist characterizes the new findings as "a second chance to save the climate" by which it means that the most draconian efforts to reduce greenhouse gas emissions from burning fossil fuels will likely not be necessary to keep the eventual average in increase in global temperature under the 2 degrees Celsius threshold. 

With regard to the transient temperature increase, Rational Optimist (and Reason contributor) Matt Ridley, is, well, somewhat more optimistic. He observes:

The most likely estimate is 1.3C. Even if we reach doubled carbon dioxide in just 50 years, we can expect the world to be about two-thirds of a degree warmer than it is now, maybe a bit more if other greenhouse gases increase too. That is to say, up until my teenage children reach retirement age, they will have experienced further warming at about the same rate as I have experienced since I was at school.

At this rate, it will be the last decades of this century before global warming does net harm. As the economist Bjørn Lomborg recently summarised the economic consensus: “Economic models show that the overall impact of a moderate warming (1-2C) will be beneficial [so] global warming is a net benefit now and will likely stay so till about 2070.”

The new calculations are signficantly below earlier figures reported by the U.N.'s Intergovernmental Panel on Climate Change. As Climatologist James Annan from the JAMSTEC Yokohama Institute for Earth Sciences in Yokohama, Japan notes:

The results are described in rather strange terms, considering what they have actually presented. They argue that the new result for sensitivity "is in agreement with earlier estimates, within the limits of uncertainty". But of course none of the published estimates are inconsistent with each other in the sense of having non-overlapping uncertainty ranges - no-one credible has excluded a value of about 2.5C, that I am aware of. The contrasting claim that the analysis of transient response gives a qualitatively different outcome (being somewhat lower than both the previous IPCC assessment, and the range obtained from GCMs) is just weird, since both their ECS and TCR results are markedly lower than the IPCC and GCM ranges.

This looks like a pretty unreasonable attempt to spin the result as nothing new for sensitivity, when it is clearly something very new indeed from these authors, and implies a marked lowering of the IPCC "likely" range. Although the paper does not explicitly mention it, the "likely" range for equilibrium climate sensitivity using the full 40y of data seems to be about 1.3-3C (reading off the graph by eye, the lower end may be off a bit due to the nonlinear scale). So although the analysis does depend on a few approximations and simplifications, it's hard to see how they could continue to defend the 2-4.5C range.

It will indeed be interesting to see if the IPCC continues "to defend the 2-4.5C range."