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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

What the science says...

Hansen's 1988 results are evidence that the actual climate sensitivity is about 3°C for a doubling of atmospheric CO2.

In 1988, James Hansen projected future temperature trends using 3 different human greenhouse gas emissions scenarios identified as A, B, and C. Scenario A assumed continued accelerating greenhouse gas growth. Scenario B assumed a slowing and eventually constant rate of growth, and Scenario C assumed a rapid decline in greenhouse gas emissions around the year 2000 (Hansen 1988). As shown in Figure 1, the actual increase in global surface temperatures has been less than Scenario B, which is the closest to reality.

Figure 1: Global surface temperature computed for scenarios A, B, and C, compared with observational data

As climate scientist John Christy noted, "this demonstrates that the old NASA [global climate model] was considerably more sensitive to GHGs than is the real atmosphere." Unfortunately, Dr. Christy decided not to investigate why the NASA climate model was too sensitive, or what that tells us. There are two main reasons for Hansen's warming overestimates:

Scenario B, which was the closest to reality, slightly overestimated how much the atmospheric concentrations of greenhouse gases would increase (particularly methane and chlorofluorocarbons [CFCs]).
Hansen's climate model had a rather high climate sensitivity parameter. Climate sensitivity describes how sensitive the global climate is to a change in the amount of energy reaching the Earth's surface and lower atmosphere (a.k.a. a radiative forcing).

If we take into account the slightly lower atmospheric greenhouse gas increases and compare the observed versus projected global temperature warming rates, as shown in the Advanced version of this rebuttal, we find that in order to accurately predict the global warming of the past 22 years, Hansen's climate model would have needed a climate sensitivity of about 3.4°C for a doubling of atmospheric CO₂. This is within the likely range of climate sensitivity values listed as 2-4.5°C by the IPCC for a doubling of CO2, and even a bit higher than the most likely value currently widely accepted as 3°C.

In short, the main reason Hansen's 1988 warming projections were too high is that he used a climate model with a high climate sensitivity, and his results are actually evidence that the true climate sensitivity parameter is within the range accepted by the IPCC.

Updated on 2010-09-26 by dana1981.

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