" In the 1990s, before shale, gas never accounted for more than 17 percent of generation. But, with the arrival of shale, gas was highly competitive on price, and environmental opposition had made it virtually impossible to build a new coal-fired plant in the United States. As late as 2007, coal generated half of U.S. electricity. By 2019, it was down to 24 percent, and natural gas had risen to 38 percent. That was the main reason why U.S. carbon dioxide (CO2) emissions dropped down to the levels of the early 1990s, despite a doubling in the U.S. economy. "

― Daniel Yergin , The New Map: Energy, Climate, and the Clash of Nations