The study relies on earlier research by Marshall Burke, an economist at Stanford. In that earlier work, he had found that when temperatures were hotter than average (for any reason) , economic growth slowed in poor countries but accelerated in rich countries. That’s because the world’s richest countries are by and large already in cooler latitudes, while poor countries are disproportionately concentrated around the Equator, where even a slight increase in temperature can be devastating to crop production, human health and labor productivity.

For this latest study, Dr. Burke, along with Noah Diffenbaugh, a climate scientist, looked at more than 20 climate models to estimate how much countries have warmed since 1960 specifically because of climate change. Then, they estimated what each country’s economic performance could have been without such a temperature rise.

Most of the world’s poor countries are poorer today than they would have been had those emissions not altered the climate, while many rich countries, especially in the northern belt of the Northern Hemisphere, are richer than they would have been, the study found.

Between 1961 and 2000, climate change dampened per capita incomes in the world’s poorest countries by between 17 percent and 30 percent. Among the countries hardest hit were also some of the largest. India, the world’s second most populous country, would have been 30 percent richer without climate change, the study concluded. For Nigeria, the most populous country in Africa, that figure was 29 percent.