Several IPCC Models Showing 5°C Climate Sensitivity

This week's Science magazine is reporting that several (at least eight) climate models being used as input to the IPCC's 6th Assessment Report have an exceptionally high climate sensitivity of about 5°C.

(Climate sensitivity is how much the planet's surface will warm for a doubling of CO2 in the atmosphere, after incoming and outgoing energy are again in balance. It's currently thought to be in the 2 - 4.5°C range, and calculations have stayed in that range for almost three decades now.)

5°C (9°F) would, of course, be significant warming and a significant change from current scientific thinking.

The article, by Paul Voosen, says modelers are working to understand which of their recent refinements are responsible for the surge. Modelers note it's still too early to know for sure, and that it's out-of-line with sensitivity estimates taken from past periods of climate change.

For example, the latest model from the GFDL in Princeton...

...incorporated a host of improvements in their next-generation model. It mimics the ocean in fine enough detail to directly simulate eddies, honing its representation of heat-carrying currents like the Gulf Stream. Its rendering of the El Niño cycle, the periodic warming of the equatorial Pacific Ocean, looks “dead on,” says Michael Winton, a GFDL oceanographer who helped lead the model's development. But for some reason, the world warms up faster with these improvements. Why? “We're kind of mystified,” Winton says. Right now, he says, the model's equilibrium sensitivity looks to be 5°C.

Models from ETH Zurich, CCCM in Canada, GFDL and NCAR are all running hot. CMIP6, the Climate Model Intercomparison Model, where models all run the same scenarios and compare results, may sort out the issues, but it's running late, impacting deadlines for the first drafts of the 6AR.

And this is interesting:

In assessing how fast climate may change, the next IPCC report probably won't lean as heavily on models as past reports did, says Thorsten Mauritsen, a climate scientist at Stockholm University and an IPCC author. It will look to other evidence as well, in particular a large study in preparation that will use ancient climates and observations of recent climate change to constrain sensitivity. IPCC is also not likely to give projections from all the models equal weight, Fyfe adds, instead weighing results by each model's credibility.

The IPCC 6AR is scheduled to come out in 2021.