Mr. Trump, on the other hand, has urged Europeans to buy more U.S. oil and gas. The United States is already the world’s leading exporter of natural gas and the biggest producer of oil, and the Trump administration is encouraging new development. Mr. Trump has also signed executive orders to expand the burning and mining of coal, the dirtiest fossil fuel. The moment was all the more remarkable because the United States is responsible for the largest share of global emissions since the Industrial Revolution. Mr. Trump assailed environmentalists for wanting to “kill all the cows,” a claim for which there is no evidence.