Hypotheses on cloud response to global warming?
Recent studies suggest several cloud feedback mechanisms which appear to be consistent among GCMs and also supported by other lines of study. The above figure provides a summary of the mechanisms expected to operate in a warmer climate: 1) high clouds are expected to rise in altitude and thereby exert a stronger greenhouse effect 2) Middle and high-level cloud cover tends to decrease 3) Storm tracks shift poleward, drying the subtropics and moistening the high latitudes, which tends to cause positive feedback by shifting cloud cover to latitudes that receive less sunshine 4) Most GCMs predict decreases in low cloud amount, especially in the subtropics 5) Over middle and high latitudes, GCMs suggest that warming-induced transitions from ice to water clouds may result in more opaque clouds.
Despite these robust cloud responses, large uncertainties remain. In particular, it is not clear to what extent the reduction of high cloud cover may compensate the positive feedback of rising high cloud altitude. The response of cloud water, its phase transition and radiative effects are sensitive to representations of cloud precipitation microphysics, which remains highly uncertain and difficult to constrain with observations. Further, the mechanisms causing reductions in low cloud amount in GCMs may differ significantly among models and lack a well-accepted theoretical basis. Cloud process models suggest a variety of potentially opposing response mechanisms that may account for the current spread of GCM feedbacks. More research is needed to narrow down cloud feedback uncertainties as well identifying possible emergent constraints.
Figure: from IPCC AR5 report (Chapter 7, Fig. 11): Robust cloud responses to greenhouse warming.Anticipated changes are shown by arrows, with red indicating robust positive feedback contributions and grey indicating feedback contributions which are small and/or highly uncertain.