an open letter published on the eve of Earth Day 2021, the group of former presidents and government officials, scientists, novelists and activists urge leaders around the globe "to act now to avoid a climate catastrophe by stopping the expansion of oil, gas and coal.""Climate change is threatening hundreds of millions of lives, livelihoods across every continent and is putting thousands of species at risk," the letter states. "The burning of fossil fuels – coal, oil, and gas – is by far the major contributor to climate change."Greenhouse gases trap heat and make the planet warmer, and human activities are responsible for almost all of the increase in greenhouse gases in the atmosphere over the last 150 years, according to the U.S..