Even if countries take moderate action on climate change, by the end of this century, Phoenix is expected to have an extra month of days above 95 degrees Fahrenheit, while Washington, D.C., is expected to have another three weeks of these sweltering days, as the Climate Impact Lab and New York Times reported. A new study suggests that even days that are an average of 90 degrees Fahrenheit, or 32 Celsius, might have long-term, negative impacts on developing fetuses. The stress of the hot weather might show up as reduced human capital once those fetuses reach adulthood. Maya Rossin-Slater, a health-policy professor at Stanford University, said she and her team wanted to understand the long-term consequences of climate change on people. For the study, published today in the Proceedings of the National Academy of Sciences, she and other researchers looked at data on births, weather, and earnings in half the states in the United States. For a given county, on a given day, they measured how many days above 90 degrees a child born that day would have experienced during gestation and during their first year of life. They then compared that person’s salary as an adult to someone born in that same county on that same day in other years. It turned out fetuses and infants exposed to a single extra 90-plus degree day made $30 less a year, on average, or $430 less over the course of their entire lifetimes. Right now, the average American only experiences one such day a year. (This study looked at the average temperature throughout the entire day, not the highest temperature that day.) By the end of the century, there will be about 43 such days a year. In addition to birthday and county, the researchers also controlled for gender and race. Rossin-Slater said it is unlikely the difference in earnings could be explained by something other than heat. “It’s really hard to figure out what else it could be. They set up a really good study design,” said Kathryn Grace, a professor of geography, environment, and society at the University of Minnesota, who was not involved in the study. What’s more, the study used data from the 1970s, when more and more people were installing air