After dithering for decades, governments finally seem to be paying serious attention to the problem of global climate change. Late last year, at the Paris climate conference, they adopted a major new agreement to limit global warming, beginning a process to strengthen commitments to reduce greenhouse gas emissions over time. For many observers, the promises of the Paris conference offer too little, too late, because emissions are high and still rising and because there will be major disruptions to the climate even if countries meet their emissions-reduction pledges. Nevertheless, it had been 18 years since the world’s governments left a major climate summit with an agreement in hand, so just getting to yes in Paris has offered climate diplomacy fresh credibility.
Until now, governments have focused on limiting the greenhouse gases that cause global warming and its attendant hazards, such as rising sea levels and stronger storms. But there is more to climate change than higher temperatures. Many of the activities that cause greenhouse gas emissions—burning coal for power, diesel for transport, and wood for cooking, for example—also yield ultra-small particles known as aerosols, which blanket vast areas in a haze that blocks and scatters sunlight. By reducing the solar energy that reaches the earth’s surface, aerosols reduce evaporation and slow the water cycle that governs where, when, and how much rain falls.
For years, climate scientists have believed that a warmer world would be wetter, because higher temperatures hasten evaporation and increase rainfall. But even when these higher temperatures are accounted for, a world dimmed by aerosols will in fact be drier in many places—including some areas, such as the Sahel and other regions in sub-Saharan Africa, that have long suffered from drought because they rely on rainfall to sustain subsistence agriculture. According to many of the most reliable models, such as those produced by the National Center for Atmospheric Research and Princeton University’s Geophysical Fluid Dynamics Laboratory, China, North America, and South Asia are
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