What must first be noted is that the lead author, Steve Sherwood, is from the Climate Change Research Centre, University of New South Wales and is a colleague of Chris Turney – the global warming cheer-leader currently stuck in the Antarctic ice. The paper is largely unfounded speculation – no evidence or measurements in sight – but speculation alarmist enough for Nature to publish it. The paper – according to the Nature Editor
offers an explanation for the long-standing uncertainty in predictions of global warming derived from climate models. Uncertainties in predicted climate sensitivity — the magnitude of global warming due to an external influence — range from 1.5° C to 5° C for a doubling of atmospheric CO2. It has been assumed that uncertainties in cloud simulations are at the root of the model disparities, and here Steven Sherwood et al. examine the output of 43 climate models and demonstrate that about half of the total uncertainty in climate sensitivity can be traced to the varying treatment of mixing between the lower and middle troposphere — and mostly in the tropics. When constrained by observations, the authors’ modelling suggests that climate sensitivity is likely to exceed 3° C rather than the currently estimated lower limit of 1.5° C, thereby constraining model projections towards more severe future warming.
Clouds are not well understood it seems but they are the answer!
The time-scale for their predictions – till 2100 is sufficiently far away that nothing can be confirmed or denied.
Presumably Sherwood was one of those advising the pilgrims trapped in the Antarctic.
Spread in model climate sensitivity traced to atmospheric convective mixing, Steven C. Sherwood, Sandrine Bony & Jean-Louis Dufresne, Nature 505, 37–42, doi:10.1038/nature12829
Abstract: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.
It all smacks of post-rationalisation.
Garbage In Garbage Out.
The k2p blog 