On July 17, 1997, a day when thermometers in the nation’s capital broke 100 degrees Fahrenheit, the chair of the White House Council of Economic Advisers delivered a speech to the Senate about an issue of mounting concern. 

“Although a great many scientists believe that global climate change is already underway, the more serious potential damages associated with increasing concentrations of greenhouse gases are not predicted to occur for decades,” said Janet Yellen, then a 51-year-old economist who’d previously served on the Federal Reserve’s board of governors. “This means that the benefits of climate protection are very difficult to quantify.”