A massive heat dome settled above the eastern half of the United States over the Fourth of July weekend, bringing triple-digit temperatures, disrupting travel and prompting emergency measures for millions of people. At least 25 people died in New Jersey due to extreme heat and humidity, and more than 185 million people — over half of U.S. residents — were under heat alerts over the weekend. This follows a record-shattering European heat wave that’s been blamed for thousands of deaths across Spain, France and Germany. Climate scientists say the burning of fossil fuels is the primary cause of global warming and the reason why heat waves are becoming longer, hotter and more frequent. “Global warming is accelerating past thresholds that we used to consider unacceptable,” says climate journalist David Wallace-Wells. “There are a lot of questions about why we haven’t prepared adequately for these heat waves,” says Wallace-Wells, but he argues that it is still possible to retrofit the planet to mitigate the climate catastrophe.