In 2006, a 700-page report landed on desks in London and quickly reached ministries from Beijing to Brasília. Its message was simple: ignoring the climate would cost more than fixing it. For many mainstream economists of the time, this was close to heresy. Their models treated the climate crisis as a minor blip that growing economies could absorb.

Glaciers and ice caps carried no value in those accounts, a standing forest added to GDP only once it had been felled, and a livable climate did not deserve a line on the spreadsheet. The author of the report, Nicholas Stern, was a former World Bank chief economist then advising the British prime minister, Gordon Brown. What made his review so disruptive was that it did not abandon the profession's tools. It used standard methods that other economists could follow, and it reached three conclusions that were radical at the time: inaction would cost more than action, climate change was a market failure, and economic growth could continue while greenhouse gas emissions fell.

The argument turned on a single technical choice, the discount rate. Economists normally value future profits and losses below present ones, a habit that makes sense when auditing a company's optimistic forecasts. Applied to the climate, however, it quietly shrinks the worth of people not yet born. A harm that arrives in 2100 almost vanishes from the calculation, so protecting a child's future looks uneconomic.