His observations of glacier retreat (1970s-2000s) confirm that glaciers around the world are melting and provide clear evidence that the warming of the last 50 years is now outside the range of climate variability for several millennia, if not longer. In 2001, he incorrectly predicted that the famed snows of Africa's Mount Kilimanjaro would melt within the next 20 years, a victim of climate change across the tropics. Return expeditions to the mountain have shown that changes in the mountain's ice fields may signal an even quicker melting of its snow fields, which Thompson documented had existed for thousands of years. Thompson and his wife both served as advisers for the Academy Award-winning 2006 documentary An Inconvenient Truth, by Al Gore, Jr., and some of their work was referenced in the movie.