Later this month, a long-awaited event that last happened in 2007 will recur. Like a returning comet, it will be taken to portend ominous happenings. I refer to the Intergovernmental Panel on Climate Change’s (IPCC) “fifth assessment report,” part of which will be published on Sept. 27.
There have already been leaks from this 31-page document, which summarizes 1,914 pages of scientific discussion, but thanks to a senior climate scientist, I have had a glimpse of the key prediction at the heart of the document. The big news is that, for the first time since these reports started coming out in 1990, the new one dials back the alarm. It states that the temperature rise we can expect as a result of man-made emissions of carbon dioxide is lower than the IPPC thought in 2007.
Admittedly, the change is small, and because of changing definitions, it is not easy to compare the two reports, but retreat it is. It is significant because it points to the very real possibility that, over the next several generations, the overall effect of climate change will be positive for humankind and the planet.
Specifically, the draft report says that “equilibrium climate sensitivity” (ECS)—eventual warming induced by a doubling of carbon dioxide in the atmosphere, which takes hundreds of years to occur—is “extremely likely” to be above 1 degree Celsius (1.8 degrees Fahrenheit), “likely” to be above 1.5 degrees Celsius (2.4 degrees Fahrenheit) and “very likely” to be below 6 degrees Celsius (10.8 Fahrenheit). In 2007, the IPPC said it was “likely” to be above 2 degrees Celsius and “very likely” to be above 1.5 degrees, with no upper limit. Since “extremely” and “very” have specific and different statistical meanings here, comparison is difficult.
Still, the downward movement since 2007 is clear, especially at the bottom of the “likely” range. The most probable value (3 degrees Celsius last time) is for some reason not stated this time.
A more immediately relevant measure of likely warming has also come down: “transient climate response” (TCR)—the actual temperature change expected from a doubling of carbon dioxide about 70 years from now, without the delayed effects that come in the next century. The new report will say that this change is “likely” to be 1 to 2.5 degrees Celsius and “extremely unlikely” to be greater than 3 degrees. This again is lower than when last estimated in 2007 (“very likely” warming of 1 to 3 degrees Celsius, based on models, or 1 to 3.5 degrees, based on observational studies).
