The United Nations Intergovernmental Panel on Climate Change (IPCC) is in the midst of finishing its Fifth Assessment Report (AR5) on the topic. Based on a series of content leaks, it seems as if the AR5 has so much internal inconsistency that releasing it in its current form will be a major fiasco.
The central issue of climate change science is the earth’s equilibrium climate sensitivity (ECS)—that is, how much the earth’s average surface temperature will increase as a result of a doubling the atmospheric carbon dioxide concentration. New and mutually consistent re-assessments of this important parameter are appearing in the scientific literature faster than the slow and arduous IPCC assessment process can digest them (presuming it even wants to—given that they are making the current AR5 look pretty bad).
Further, even if the IPCC is able to do an adequate job of assimilating this evolving and quite convincing science, the vast majority of the rest of the IPCC’s report will also have to be changed as it is highly dependent on the magnitude of the climate sensitivity.
By now, though, it’s too late in the game (the final report is due out in early 2014)—the cows have all left the IPCC’s barn on these subjects and it’s too late to round them all up and rebrand them.
The First Order Draft (FOD) of the IPCC’s AR5 was leaked/made public back in December 2012. In the FOD, the IPCC recognized the significance of the earth climate sensitivity, calling it:
[T]he single most important measure of climate response because the response of many other climate variables to an increase in CO2 scales with the increase in global-mean surface temperature.
The IPCC went on to assess the current scientific knowledge as to the magnitude of the climate sensitivity this way:
Despite considerable advances in climate models and in understanding and quantifying climate feedbacks, the assessed literature still supports the conclusion from AR4 [the IPCC’s Fourth Assessment Report published in 2007] that climate sensitivity islikely in the range 2–4.5°C, and very likely above 1.5°C. The most likely value remains near 3°C. An ECS greater than about 6–7°C isvery unlikely, based on combination of multiple lines of evidence. [emphasis in original]
The IPCC’s “mostly likely value” in the FOD is 3.0°C. This is interesting in and of itself because according to the same First Order Draft the average climate sensitivity produced by the climate models used by the IPCC is 3.4°C, which is already some 13% higher than the IPCC’s 3.0 degrees.
If the IPCC thinks there was something systematically wrong with the models it was using, then it should just come out and say it. The fact that the IPCC (currently) will not do so does not exactly inspire confidence in its ability to stand up and criticize. In reality, the average climate sensitivity of acollection of results published within the past 2-3 years suggests that the value is closer to 2.0°C, and recent observations of the pace of global temperature rise hint at a value even a bit lower than that.
The IPCC has come under growing pressure from the scientific community—especially members of the commission directly involved in climate sensitivity research—to reflect these new, lower estimates.
There are some indications, though, that the IPCC times may be a–changing.
According to The Economist, which claims to have seen the most recent draft of the IPCC’s AR5, the IPCC’s assessment of climate sensitivity has now been altered. Here is how The Economist describes what is in the new IPCC draft concerning the equilibrium climate sensitivity:
Both the 2007 IPCC report and a previous draft of the new assessment reflected earlier views on the matter by saying that the standard measure of climate sensitivity (the likely rise in equilibrium temperature in response to a doubling of CO2 concentration) was between 2°C and 4.5°C, with 3°C the most probable figure. In the new draft, the lower end of the range has been reduced to 1.5°C and the “most likely” figure has been scrapped. That seems to reflect a growing sense that climate sensitivity may have been overestimated in the past and that the science is too uncertain to justify a single estimate of future rises.
If this is true, the combination of the IPCC lowering the low end of the range of possible climate sensitivity values and the IPCC deciding not to provide its assessment of the “most likely” value strongly suggests that the average climate model sensitivity of 3.4°C is even further removed from the science on the topic and less justifiable. Just how far removed the climate models really are is kept under wraps by the IPCC by its no longer supplying a “most likely” value. If the “most likely” value were to be assessed at 2.5°C, then the climate model average would be 36% too high compared to the science. If the “most likely” value were to be 2.0°C, then the model average climate sensitivity would be some 70% too high.
Because, as the IPCC admits, the change in many other climate variables “scales with the increase in global-mean surface temperature,” the climate impacts derived from the model projections (which essentially is what the IPCC is all about) are also (substantially) too high.
The IPCC has three options:
- Round-file the entire AR5 as it now stands and start again.
- Release the current AR5 with a statement that indicates that all the climate change and impacts described within are likely overestimated by around 50%, or
- Do nothing and mislead policymakers and the rest of the world.
We’re betting on door number 3.