He studies climate change, air quality, and links between science and policy. He has been an author on >250 peer-reviewed publications, received awards from Scientific American, NASA, the NSF and the EPA, and is a fellow of AGU and AAAS.
He has testified on climate issues before both houses of the US Congress (at the request of both parties), developed a climate change course with the American Museum of Natural History, and made numerous media appearances as part of his outreach efforts. He chaired the 2011 UNEP/WMO Integrated Assessment of Black Carbon and Tropospheric Ozone, and was a Coordinating Lead Author on the 2013 Fifth Assessment Report of the IPCC and on the 2018 IPCC Special Report on 1.5°C. He also chairs the Scientific Advisory Panel to the Climate and Clean Air Coalition of nations and organizations.