One of the great uncertainties in projecting global warming is accounting for the effects of small particles in EartlYs atmosphere. Progress is nonetheless being made with this fiendishly complex problem. Generally accepted climate projec-tions for the year 2100, compared with today, predict a global average temperature increase ranging from around 5 8°C to a more benign, but still worrisome, 1 4 °C. Which of these futmes awaits us depends, in part, on aerosols—tiny par tides in the atmosphere —and their effect on clouds Ackennan et al (page 1014 of this issue) conclude that the lole of these particles in incieasi ng the water content of clouds, and so cloud reflectivity, is smaller than previously thought.
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