"The modelled response of low clouds does not appear to be dominated by a single feedback mechanism, but rather the net effect of several potentially competing mechanisms as elucidated in LES and GCM sensitivity studies (e.g., Zhang and Bretherton, 2008; Blossey et al., 2013; Bretherton et al., 2013). Starting with some proposed negative feedback mechanisms, it has been argued that in a warmer climate, low clouds will be: (i) horizontally more extensive, because changes in the lapse rate of temperature also modify the lower tropospheric stability (Miller, 1997); (ii) optically thicker, because adiabatic ascent is accompanied by a larger condensation rate (Somerville and Remer, 1984); and (iii) vertically more extensive, in response to a weakening of the tropical overturning circulation (Caldwell and Bretherton, 2009)." - AR5 draft pg 7-20
The authors base the claim upon their computer model which allegedly overturns the prior 'settled science' on clouds and thereby proclaims the globe will warm 4C by 2100.
"They report in Nature that updraughts of water vapour can rise 15 kms to form high clouds that produce heavy rains, or the vapour can rise just a few kilometers before coming back to the surface without forming rain clouds. When this happens the process actually reduces the overall cloud cover because it dessicates the clouds above: it draws away water vapour from the higher regions in a process called convective mixing. [see New paper finds IPCC climate models don't realistically simulate convection and thus convective mixing]
Prior posts on clouds and water vapor as negative feedbacks which cool the planet
Climate models in the past have tended to predict high cloud formation that damps warming. [No - models have predicted the opposite: that high clouds increase the 'greenhouse' effect and increase warming] What Sherwood and his colleagues have done is demonstrate that the world may not work like that."
Warming climate may cut cloud cover