I think I had never heard of William Nordhaus until he was awarded the Nobel Prize in Economics in 2018. The award was bestowed in recognition of his work modeling the costs and benefits of acting on climate change through limiting emissions. At the time, I worked with climate economists, who remarked (like others) that it was surprising the award had not been at least co-granted to Martin Weitzman. Nordhaus, I later learned, was similarly surprised.

Like Nordhaus, Weitzman argued that the economics of climate change must be guided by the science. But he disagreed with Nordhaus’s approach to discounting the future, and instead referenced ideas like risk aversion and the precautionary principle to conclude it would be worth it to spend a lot of money right now to prevent the worst from possibly happening. Weitzman pointed out tremendous uncertainty in the forecasts of climate impacts, including tipping points, large error bars, and unknown unknowns. In economics-speak, he characterized this as enormous “downside risk,” including a potentially small—but fundamentally unknown—chance of total human annihilation.