Clouds in a Warmer Climate: Friend or Foe?

Introduction Past and Current Research, and Difficulties. The role of clouds in modulating the earth's climate is complex. Clouds can shield the earth's surface from incoming solar radiation, helping to cool the earth like the relief we feel beneath the shade of a tree. The clouds having the most impact are those that have smaller droplets, and are closer to the earth's surface, and especially over the oceans. The emission of radiation by the earth, like that on a cold, clear night, is limited by the presence of higher-altitude clouds, warming the earth. The current best estimate is that the net effect of clouds on the radiation balance of the earth is a cooling effect- approximately 0.7 W [m.sup. -2]. This figure is a median value of the estimates, and there is still a significant amount of uncertainty (Forster et al., 2007). In this sense, though, clouds can be thought of as "friend", helping to offset the warming climate. These two effects upon the radiation balance of the earth have dominated the literature related to clouds and climate change. The radiative roles of aerosol acting within clouds, the so-called "first indirect effect" that represents enhanced reflecting of solar radiation back to space via decreased droplet size within the clouds resulting from more aerosol, the "second indirect effect" that describes how additional aerosol can slow precipitation formation and thus increase cloud longevity and the amount of solar radiation reflected back to space, and the "semi-direct effect" of soot aerosol within cloud droplets warming the cloud and decreasing its lifetime and the amount of solar radiation reflected back to space, have all been under intense investigation (Forster et al., 2007). Quantification of these effects is paramount to understanding how much the earth will warm, globally and regionally (Stephens 2005). And yet, progress has been slow. The latest IPCC assessment states "The response of cloud cover to increasing greenhouse gases currently represents the largest uncertainty in model predictions of climate sensitivity . " (Trenberth et al., 2007) and furthermore, "In response to global warming, the cooling effect of clouds on climate might be enhanced or weakened . " (Randall et al., 2007). Why is the influence of clouds so difficult to determine? The reasons are multifold. From a cloud microphysicist's view, significant details of precipitation processes in clouds are still unknown. Current theory cannot explain how warm, shallow clouds form rain as quickly as they do, nor can it explain the large quantities of ice observed in deeper clouds. General principles of precipitation formation are understood, but fundamental knowledge essential for prediction of precipitation has remained elusive and thus confounds estimates not only of precipitation but also of cloud characteristics important for their influence on the radiation budget such as particle size, cloud areal coverage, and longevity. Although hypotheses abound regarding the details of precipitation processes, past and present capabilities in experimental, observational, and numerical methods fall short of providing enough information to test these hypotheses sufficiently. No method, experimental, observational or numerical, can address all the scales of the problem simultaneously: the aerosol that nucleate cloud droplets are on the order of tenths of micrometers, and the extent of clouds and their accompanying motions that must be considered can be as large as hundreds of kilometers (Fig. 1). Different methods can study a subset of the scales involved while neglecting effects outside of that range, but merging the results with those at other scales remains an ongoing challenge. An Additional Area of Inquiry: Precipitation Efficiency. From calculations based on observations over the last half-century, clouds precipitate one meter of rainfall annually over the globe (e.g. Wallace and Hobbs, 2006). Different regions receive more or less than this amount, determined not only by large-scale and regional weather patterns, but also the local aerosol and thermodynamic characteristics of the environment. …