New Paper: Possible Much Higher Climate Sensitivity
October 26, 2020
Don’t have the paper or much evaluation yet.
New paper describes a possible cloud feedback that enhances warming.
Conventional wisdom is that every few years there are papers that come up with much lower, or much higher, values for climate sensitivity, and yet the pendulum always swings back to around 3 degrees C.
UPDATE: Andrew Dessler responds:
Abstract
The equilibrium climate sensitivity of Earth is defined as the global mean surface air temperature increase that follows a doubling of atmospheric carbon dioxide. For decades, global climate models have predicted it as between approximately 2 and 4.5 °C. However, a large subset of models participating in the 6th Coupled Model Intercomparison Project predict values exceeding 5 °C. The difference has been attributed to the radiative effects of clouds, which are better captured in these models, but the underlying physical mechanism and thus how realistic such high climate sensitivities are remain unclear. Here we analyse Community Earth System Model simulations and find that, as the climate warms, the progressive reduction of ice content in clouds relative to liquid leads to increased reflectivity and a negative feedback that restrains climate warming, in particular over the Southern Ocean. However, once the clouds are predominantly liquid, this negative feedback vanishes. Thereafter, other positive cloud feedback mechanisms dominate, leading to a transition to a high-sensitivity climate state. Although the exact timing and magnitude of the transition may be model dependent, our findings suggest that the state dependence of the cloud-phase feedbacks is a crucial factor in the evolution of Earth’s climate sensitivity with warming.
So far, climate models have been pretty accurate.
I believe the new paper suggests that a new mechanism kicks in at higher temps. There is some paleo support for that, in that during the Paleo-Eocene Thermal maximum, it might have gotten hotter than we can explain with current models.
Climate Denial Crock of the Week 
October 26, 2020 at 8:41 pm
There’s a mistake in the proving part for the 8 degrees of feedback though. It applies a climate sensitivity of 1.2 degrees / (w/m**2) per CMIP6 models but since the CMIP6 models must have some cloud feedback in them it’s incorporating part of itself twice (itself as a forcing and some part of itself as a feedback also). That isn’t in the modeling part that produces the large reduction in tropical ocean cumulus cloud cover due to downwelling above them at 1,200 – 1,300 ppmv CO2, it’s in the final part that says the ~7.6 degrees (from memory) is about right using the CMIP6 1.2 degrees / (w/m**2)
October 26, 2020 at 11:09 pm
Excellent video from Yale Climate Connections Greenman studios – Mann does plain speaking, we are talking about temperatures to be experienced mid century and uncertainty is not our friend.
Svante Arrhenius, in the 19th century estimated doubling would cause between 5 to 6 °C (9.0 to 10.8 °F), later reduced the estimate to around 4 °C using brain and paper only – not a bad estimate, we should have listened and started earlier.
At these temperature levels a lot of people will be put through hell, this is a global average estimate, it will be much hotter in some parts.
WE rely on organisations like ESA, JMA, NASA and NOAA, for accurate data and vital policy making.
Certainly not a time to appoint Heartland Sympathetic chiefs and managers. Come on Earth we can and must do better than this.
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“Climate scientists are bracing for the potential disruption of NOAA’s climate work with the appointment of two prominent climate science deniers and a former campaign official for President Donald Trump to top agency positions this fall.”
https://insideclimatenews.org/news/23102020/noaa-climate-denial-david-legates
October 26, 2020 at 11:13 pm
“While this year will be memorable for many reasons, it is now more likely than not that 2020 will also be the warmest year for the Earth’s surface since reliable records began in the mid-1800s. ”
https://www.carbonbrief.org/state-of-the-climate-2020-on-course-to-be-warmest-year-on-record
October 26, 2020 at 11:16 pm
“Carbon can be released from coastline sources much more easily than previously thought.”
https://www.eurekalert.org/pub_releases/2020-10/uota-cpm102320.php
October 27, 2020 at 10:33 am
The present transient climate response (TCR) climate sensitivity based on changes since 1750 AD is 2.81 degrees based on IPCC AR5 GHGs/aerosols & Global Mean Surface Temperature (GMST) & latest energy imbalance slope (I think that one is 2011-2018 but I haven’t read it yet). An obvious increaser would be the Arctic region snow/ice reduction albedo reduction. Suppose GHGs & aerosols were held steady where they are from now to decades past a century then would Arctic region snow/ice reduction continue at a pace fast enough to increase TCR from the present 2.81 degrees (as the present 440,000-gigawatt heater drops to zero by surface/air warming) is the question. I’m thinking that probably it would somewhat.
October 27, 2020 at 10:37 am
So if it warmed +0.65 degrees under that hypothetical situation then TCR is 2.81 degrees if IPCC AR5 GHGs/aerosols estimates are accurate, but if it warmed to equilibrium > +0.65 degrees then TCR is correspondingly more, and if it warmed to equilibrium < +0.65 degrees then TCR is less.