The amount of planet-warming carbon dioxide in the Earth’s atmosphere has hit a new record, as humanity struggles to rein in emissions of greenhouse gasses from burning fossil fuels. The new record comes as tens of millions of people are grappling with extreme weather in the United States. Much of the western U.S. is experiencing the first major heat wave of the year, which is driving temperatures 20 to 30 degrees Fahrenheit hotter than what is normal for June. In the Southwest, temperatures are lingering well above 100 degrees. Such extreme, prolonged heat is directly related to human-caused climate change,...