Mernild is one of the world's leading climate scientists in the fields of glaciology and hydrology, specializing in the impacts of climate change in the Arctic and on the cryosphere, especially the ice sheets (glacier mass balance) and water levels. He has contributed to a number of international scientific reports, including the annual Arctic Report Card from National Oceanic and Atmospheric Administration (NOAA) and the "Melting Snow and Ice: A Call for Action" report, which Vice President Al Gore presented at the United Nations Climate Change Conference in Copenhagen in 2015.