IPCC Revises Climate Sensitivity

What do changes to the temperature increase ranges mean?

Hussein Shihab/Wikimedia Commons

A leaked draft copy of the Intergovernmental Panel on Climate Change's fifth assessment report (AR5) surfaced earlier this summer and triggered a small tempest among climate bloggers, scientists and skeptics over revelations that a key metric, called the "Equilibrium Climate Sensitivity" (ECS), had been revised downward. The ECS's lower threshold had been extended by half a degree -- from 2 degrees Celsius (3.6 degrees Fahrenheit) to 1.5 C (2.7 F) -- indicating that a lower range of warming now fell within the IPCC's range of "likely" possibilities.

Climate skeptics pounced on this change as proof that earlier estimates had been overblown, while some climate bloggers questioned whether the IPCC had lowballed its estimates to avoid confrontation with skeptics. According to a sample of scientists contacted by ClimateWire, however, the revised ETS does not much alter the picture of overall planetary warming or how humanity needs to respond to it.

"That change is pretty minor when you work through its implications for policy," said Gavin Schmidt, a climate modeler at the NASA Goddard Institute for Space Studies. The upper limit of the range remains the same in both reports -- 4.5 C (8.1 F) -- he noted, while most mainstream scientists put their "best guess" for climate sensitivity somewhere in the middle of the range, between 2.5 and 3.5 C.

Dropping the lower threshold of the "likely" range by half a degree is unlikely to cause world governments to scrap their climate action plans, he said.

In fact, the lower limit is not so much a departure from as it is a return to previous estimates.

"Interestingly, that range of 1.5 to 4.5 [C] was what was there in the first three [IPCC] assessments," said Venkatachalam Ramaswamy, director of the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory and a co-author of the IPPC's AR5 "Summary for Policymakers." "In the Fourth Assessment, it narrowed to 2 to 4.5 [C], and now it's gone back to 1.5 and 4.5."

The change, Ramaswamy said, is based on new observations casting doubt on earlier model simulations and reflects a slight increase in uncertainty, rather than a new, concrete target for warming.

What the ECS left out
While the IPCC synthesis may strive for the scientific "middle ground," there are plenty of individual scientists who argue for a higher, or lower, range of values.

A group of scientists from a cross-section of scientific institutions, including Ramaswamy and Columbia University Earth Institute's James Hansen, published a paper earlier this summer arguing for a broader climate sensitivity metric. While the ECS factors in such "fast" feedback effects as changes in water vapor -- water itself is a greenhouse gas, and saturates warm air better than cold -- they argued that slow feedbacks, such as changes in ice sheets and vegetation, should also be considered.

"I think part of the reason slow feedbacks weren't included [in previous IPCC reports] was that they were assumed to be too slow to be relevant to human-induced climate change," said Michael Previdi, lead author of the report. The values derived by the ECS represent a stabilization of temperatures, and when systems like ocean carbon sinks are added to the mix, stabilization can take millennia. But ignoring long-term feedback cycles could mean the ECS misses important developments in the near-term, he said.

"The melting of continental ice sheets is expected to have an effect on the climate in this century," he said.

Factoring in slow feedbacks from ice and vegetation changes would generate a significantly higher ECS, likely in the 4 to 6 C (7.2 to 10.8 F) range, the paper notes. The range would be pushed even higher if climate-greenhouse gas feedbacks from land and ocean carbon sinks were also included.

Of course, these higher values would have to be set in the context of a longer time scale, meaning their full effects wouldn't be felt for thousands of years.

A more immediately useful metric included in the IPCC report is probably the transient climate response, which describes the response of climate systems to gradual increases in carbon dioxide and can be applied to a particular moment or period of time.

Ultimately, said Gavin, the ECS is best thought of as a mental exercise -- a way of estimating a range of outcomes without predicting them directly.

"It's like a measure of IQ, only applied to models," he said. "Knowing the IQ of a model isn't going to predict how it'll do on a test, or whether or not it'd flunk a job interview. But it still has a connection."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500

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