Climate economists refer to it as “the most important number you’ve never heard of." The social cost of carbon (SCC) tries to capture the cost of an additional ton of carbon-dioxide pollution in a single number—around $47 in present dollars. Using it, more than $1 trillion worth of benefits have been calculated in economic-impact assessments that accompany environmental regulations. But now that the Environmental Protection Agency is headed by Scott Pruitt, a climate-change sceptic who is friendly with fossil-fuel firms, the maths is likely to be redone. Lab co-director Michael Greenstone is quoted in this report.