Back in the United States, the National Weather Service has declared an excessive heat warning for vast swaths of the Midwest and East Coast, with warnings that high humidity and triple-digit temperatures could combine to bring heat-related illnesses and deaths. In Washington, D.C., the heat index could reach 115 degrees this weekend, when some 290 million American are likely to experience temperatures above 90 degrees. This comes as the National Oceanic and Atmospheric Administration reported that worldwide surface temperatures last month were the highest ever recorded for the month of June—a finding consistent with global heating caused by human activity.