[1] Among the quantities that govern projections of future climate, few are as consequential—or as stubbornly uncertain—as the effect of aerosols on clouds. [2] Aerosols, the minute particles suspended in the atmosphere, serve as the seeds upon which cloud droplets condense; increase their number, and a cloud will tend to form more numerous but smaller droplets, rendering it brighter and more reflective. [3] Because this brightening returns sunlight to space, aerosols have exerted a cooling influence that has partially masked the warming caused by greenhouse gases. [4] The difficulty is that the magnitude of this masking is poorly constrained, and the uncertainty propagates directly into estimates of how sensitive the climate is to carbon dioxide.
[5] The reasoning runs as follows. [6] The warming observed to date reflects the net of two opposing forces: the heating from greenhouse gases and the cooling from aerosols. [7] If the aerosol cooling has been large, then the greenhouse heating it offsets must also be large, implying a highly sensitive climate; if the cooling has been small, a less sensitive climate suffices to explain the same observations. [8] The temperature record, in other words, cannot by itself pin down sensitivity without an independent measure of the aerosol effect. [9] Early estimates leaned heavily on global climate models, but these encode the very cloud processes in dispute. [10] More recent work has turned to natural experiments—the transient dimming after large volcanic eruptions, the bright tracks left by ships in otherwise clean marine air, and lulls in industrial output—to isolate the aerosol signal directly. [11] The results have tended to favor a weaker aerosol effect than the models assumed, nudging estimates of climate sensitivity neither toward the reassuring low end nor the alarming high, but toward a narrower band in between.
