Last week the Research Council of Norway announced the results of a new assessment of the climate system’s “sensitivity” taking into account the leveling off of global temperatures during the decade from 2000 to 2010. The study projects that a doubling of atmospheric carbon dioxide (CO2) concentrations over pre-industrial levels will increase global temperatures by between 1.2°C and 2.9°C, with 1.9°C being the most likely outcome. That is considerably cooler than the UN IPCC Fourth Assessment Report (AR4) estimate of 2°C to 4.5°C, with 3°C as the most probable outcome.
Climate sensitivity is an estimate of how much warming results from a given increase in CO2 concentrations. Estimates typically project the amount of warming from a doubling of CO2 concentrations over the pre-industrial (year 1750) level of 280 parts per million (ppm). At the current rate of increase (about 2 ppm/yr), a doubling to 560 ppm is expected by mid-century.
Climate alarm depends on several gloomy assumptions — about how fast emissions will increase, how fast atmospheric concentrations will rise, how much global temperatures will rise, how warming will affect ice sheet dynamics and sea-level rise, how warming will affect weather patterns, how the latter will affect agriculture and other economic activities, and how all climate change impacts will affect public health and welfare. But the chief assumption is the range of projected warming from a doubling of CO2 concentrations — the sensitivity estimate.
When the reseachers at the Center for International Climate and Environmental Research – Oslo (CICERO) applied their computer “model and statistics to analyse temperature readings from the air and ocean for the period ending in 2000, they found that climate sensitivity to a doubling of atmospheric CO2 concentration will most likely be 3.7°C, which is somewhat higher than the IPCC prognosis.” However, “when they entered temperatures and other data from the decade 2000-2010 into the model, climate sensitivity was greatly reduced to a ‘mere’ 1.9°C.”
Referring to the IPCC AR4 warming forecasts, project manager Terje Berntsen, a geoscience professor at the University of Oslo, commented: “The Earth’s mean temperature rose sharply during the 1990s. This may have caused us to overestimate climate sensitivity.”
No single study can make a dent on the self-anointed “scientific consensus.” But the Norwegian study is one among several recent studies that call into question the IPCC sensitivity assumptions. Cato Institute climatologist Patrick Michaels recently summarized a partial list of such studies in Forbes magazine:
Richard Lindzen gives a range of 0.6 to 1.0 C (Asia-Pacific Journal of Atmospheric Sciences, 2011); Andreas Schmittner, 1.4 to 2.8 C (Science, 2011); James Annan, using two techniques, 1.2 to 3.6 C and 1.3 to 4.2 C (Climatic Change, 2011); J.H. van Hateren, 1.5 to 2.5 C (Climate Dynamics, 2012); Michael Ring, 1.5 to 2.0 C (Atmospheric and Climate Sciences, 2012); and Julia Hargreaves, including cooling from dust, 0.2 to 4.0 C and 0.8 to 3.6 C (Geophysical Research Letters, 2012). Each of these has lower and higher limits below those of the Intergovernmental Panel on Climate Change.
In Addendum: Climate Change Impacts in the United States (pp. 26-28), Michaels and his colleague Chip Knappenberger discuss those studies in greater detail and also illustrate with two graphs how the IPCC AR4 warming projections should be adjusted in light of more recent climate sensitivity research. Note that the ‘long, fat tail’ of high-end warming projections in AR4 is absent from projections based on more recent science.
TOP: A collection of probability estimates of the climate sensitivity as presented in the IPCC AR4. The horizontal bars represent the 5 to 95 percent ranges, and the dots are the median estimate. BOTTOM: A collection of post-IPCC AR4 probability estimates of the climate sensitivity showing a lower mean and more constrained estimates of the uncertainty. The arrows below the graphic indicate the 5 to 95 percent conﬁdence bounds for each estimate along with the mean (vertical line) where available.
Michaels comments: “People are beginning, cautiously, to dial back 21st century warming because there has been none. Because dreaded sea-level rise is also proportional, those estimates are going to have to come down, too.”
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Update (Jan. 29, 2013). I noticed yesterday (but neglected to mention) that there is no link to the Bernsten team’s sensitivity study in the Research Council of Norway’s press release. Now I know why. The ever-vigilant Anthony Watts reports that the study has not been peer reviewed. The press release should have mentioned this; it didn’t. Indeed, it is shoddy to issue press releases about studies that have not passed peer review and have not been accepted for publication by a reputable journal. Bloggers too should abstain from commenting on studies they have not read with their own eyes. I have always followed that rule — until yesterday. Apologies. Never again.