Scorching heat waves have gripped the world in recent weeks from the Pacific Northwest to Northern Europe and, most tragically, Japan, where more than 100 mostly elderly people have died. The usual caveat applies: no single event can be specifically tied to climate change. Nonetheless, it offers an unsettling preview of what may be in store for the coming century.

Just how much should the world worry? Optimists often note that most countries will be richer by the end of this century, and societies are adaptable, both of which ought to reduce the harm from a warming climate. But an exhaustive new study focusing only on heat-related damage reaches a sobering conclusion: by the year 2099, even with economic growth and adaptation, 1.5 million more people will die each year around the world because of increased heat. By comparison, 1.25 million people died in 2013 in all traffic accidents world-wide.

Moreover, adaptation extracts an economic toll, from installing more air conditioning to curbing outdoor activity. The study calculates that such efforts effectively more than double the cost of climate change-induced heat.

The conclusion comes from a team of economists, scientists, computer modelers and other experts working under the aegis of the Climate Impact Lab, a think tank based at the University of Chicago. Their study—the lead authors of which are Michael Greenstone of the University of Chicago, Solomon Hsiang of the University of California at Berkeley, Trevor Houser of the Rhodium Group and Robert Kopp of Rutgers University—is being published Wednesday by the nonpartisan Becker Friedman Institute at the University of Chicago.

While climate change can have many repercussions from rising sea levels, changed crop yields, drought, migration and potentially civil unrest, this study considers only heat. Excessive heat can lead to brain and kidney damage and cardiovascular stress, especially for those over 64, which is why Japan’s recent experience is relevant. Japan is an aging society and the rest of the world will get steadily older over the coming century.