I'm not going to say anything about this research because I've not read the paper, but it looks important. If someone out there writes something up I'll put a link here.

Here's the deal. Climate sensitivity is, very oversimplified, how much the surface of the planet heats up as we add CO2 and other greenhouse gasses to the atmosphere. More specifically, equilibrium climate sensitivity is the number of degrees C the atmosphere at face height and the sea surface heat up with a doubling of CO2 from pre-industrial levels.

If our atmosphere had just nitrogen and CO2 and that's it, the number would be fairly low, about 1.2 degrees C. But live would not exist here because there would be no water, so we would not be having this conversation. The fact that we are having this conversations suggests the existence of water vapor, which cranks up sensitivity quite a bit, because more CO2 means more heat means more water vapor. That is just one of a number of "positive" (read not good) feedbacks on climate sensitivity.

I've noted before that if you offer a group of informed climate scientist the chance to guess a single number for climate sensitivity, using the Free Beer method, is something like 3.0. Certainly not less than 2.0. But it could just possibly be much higher, like 6. The chances of climate sensitivity being 6 are small, and if it turned out to be, then we are truly Doomed. But here's the thing. The upper range of possible values for this important number is what is sometimes called a "fat tail." The chances are low, but not so low they can be ignored.

Here's a picture of a fat tail.

Even a value of 4 or 5 would be bad, and the chances are not vanishingly small that this would be the value.

So, about the latest research.

Title: Long-term cloud change imprinted in seasonal cloud variation: More evidence of high climate sensitivity

Authors: Chengxing Zhai, Jonathan H. Jiang, Hui Su

Abstract: The large spread of model equilibrium climate sensitivity (ECS) is mainly caused by the differences in the simulated marine boundary layer cloud (MBLC) radiative feedback. We examine the variations of MBLC fraction in response to the changes of sea surface temperature (SST) at seasonal and centennial time scales for 27 climate models that participated in the Coupled Model Intercomparison Project phase 3 and phase 5. We find that the intermodel spread in the seasonal variation of MBLC fraction with SST is strongly correlated with the intermodel spread in the centennial MBLC fraction change per degree of SST warming and that both are well correlated with ECS. Seven models that are consistent with the observed seasonal variation of MBLC fraction with SST at a rate −1.28 ± 0.56%/K all have ECS higher than the multimodel mean of 3.3 K yielding an ensemble-mean ECS of 3.9 K and a standard deviation of 0.45 K.

Potential meaning: Ruh roh.

These results are not particularly unexpected. But one would hope that more research would show a lower number, because we really don't want this to be a higher number.

See also: Future warming likely to be on high side of climate projections, analysis finds, which covers A Less Cloudy Future: The Role of Subtropical Subsidence in Climate Sensitivity, by John Fasullo and Kevin Trenberth. Science, 9 November 2012:

An observable constraint on climate sensitivity, based on variations in mid-tropospheric relative humidity (RH) and their impact on clouds, is proposed. We show that the tropics and subtropics are linked by teleconnections that induce seasonal RH variations that relate strongly to albedo (via clouds), and that this covariability is mimicked in a warming climate. A present-day analog for future trends is thus identified whereby the intensity of subtropical dry zones in models associated with the boreal monsoon is strongly linked to projected cloud trends, reflected solar radiation, and model sensitivity. Many models, particularly those with low climate sensitivity, fail to adequately resolve these teleconnections and hence are identifiably biased. Improving model fidelity in matching observed variations provides a viable path forward for better predicting future climate.

See also: A bit more sensitive, which discusses "Spread in model climate sensitivity traced to atmospheric convective mixing" by Stgeven Sherwood, Sandrine Bony, and Jean-Louis Dufrense, in Nature, January 2 2014.

Equilibrium climate sensitivity refers to the ultimate change in global mean temperature in response to a change in external forcing. Despite decades of research attempting to narrow uncertainties, equilibrium climate sensitivity estimates from climate models still span roughly 1.5 to 5 degrees Celsius for a doubling of atmospheric carbon dioxide concentration, precluding accurate projections of future climate. The spread arises largely from differences in the feedback from low clouds, for reasons not yet understood. Here we show that differences in the simulated strength of convective mixing between the lower and middle tropical troposphere explain about half of the variance in climate sensitivity estimated by 43 climate models. The apparent mechanism is that such mixing dehydrates the low-cloud layer at a rate that increases as the climate warms, and this rate of increase depends on the initial mixing strength, linking the mixing to cloud feedback. The mixing inferred from observations appears to be sufficiently strong to imply a climate sensitivity of more than 3 degrees for a doubling of carbon dioxide. This is significantly higher than the currently accepted lower bound of 1.5 degrees, thereby constraining model projections towards relatively severe future warming.

See also: Overlooked evidence - global warming may proceed faster than expected