Climate scientists at the Harvard School of Engineering and Applied Sciences (SEAS) have discovered that particulate pollution in the late 20th century created a \"warming hole\" over the eastern United States -- that is, a cold patch where the effects of global warming were temporarily obscured. While greenhouse gases like carbon dioxide and methane warm Earth\'s surface, tiny particles in the air can have the reverse effect on regional scales. \"What we\'ve shown is that particulate pollution over the eastern United States has delayed the warming that we would expect to see from increasing greenhouse gases,\" says lead author Eric Leibensperger (Ph.D. \'11), who completed the work as a graduate student in applied physics at SEAS. \"For the sake of protecting human health and reducing acid rain, we\'ve now cut the emissions that lead to particulate pollution,\" he adds, \"but these cuts have caused the greenhouse warming in this region to ramp up to match the global trend.\" At this point, most of the \"catch-up\" warming has already occurred. The findings, published in the journal Atmospheric Chemistry and Physics, present a more complete picture of the processes that affect regional climate change. The work also carries significant implications for the future climate of industrial nations, like China, that have not yet implemented air quality regulations to the same extent as the United States.