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Critics pounced. The flaws in Mann's reconstruction were proof that climate data were unreliable, they claimed. And the so- called "hockey stick controversy" was born. With the credibility of climate data at stake, Shindell decided to weigh in. With Mann as one of his co- authors, he ran his own model, which included the impact of atmospheric chemistry. It confirmed that the reduced solar output of the 17the century, combined with chemical feedback in the atmosphere— ozone — caused major regional climate changes but not a big overall change in global patterns. Europe and parts of North America got colder, but other areas, including Africa and Australia, showed no major cooldown. "This is why Mann's large -scale reconstructions showed only slight global changes," Shindell says. "It was a major finding, not only to validate Mann's work and the agreement between climate data and models in general, but also to show that atmospheric chemistry played a much larger role than previously thought in affecting climate change, and that regional changes could be large even if global change was slight." The success of the paper, which has since been cited in nearly 570 other peer- reviewed studies, spurred Shindell to turn his sights to an even bigger challenge. "The question I wanted to answer next was: Why do some regions change in one way, while others don't? That was not well understood at all, but it was clearly crucial," he says. IIN III II " "'I "'" III;;; G IIR " "'I "'" E111.1.) II II III III° °III To unearth the answer, Shindell began to study tropospheric chemistry and the interactions of all SLCPs, not just ozone. The more he discovered about the uneven distribution of SLCPs in the troposphere, their uneven contributions to anthropogenic forcing, and how they interact with longer -lived greenhouse gases like carbon dioxide, the more certain he grew that it was neither logical nor efficient to segregate climate change and air pollution as separate problems. "Through my work with UNEP, the U.S. Climate Change Science Program and other initiatives, I was coming into contact with medical and agricultural researchers and economists who were studying the broader health impacts of air pollutants," he says. "It became clear that we were not dealing with global warming or air pollution, it was global warming and air pollution. They were directly related and we had to attack them as one." Working with these experts from other fields, Shindell expanded the focus of the assessment report he was chairing for UNEP. "We quantified health impacts, we quantified crop yield impacts and we quantified climate impacts. It was like preparing a menu ready - made for policymakers," he says. "We showed that we had 16 measures through which we could demonstrate that there were multiple benefits of reducing short -lived climate pollutants." UNEP published the assessment report in 2011 and founded the Coalition for Climate and Clear Air the following year to achieve the objectives Shindell and his colleagues had set forth. By 2013, the I PCC had shifted its focus as well.