Climate change is making giant heat waves crawl slower across the globe and they are baking more people for a longer time with higher temperatures over larger areas, a new study finds.

Since 1979, global heat waves are moving 20% more slowly — meaning more people stay hot longer — and they are happening 67% more often, according to a study in Friday’s Science Advances. The study found the highest temperatures in the heat waves are warmer than 40 years ago and the area under a heat dome is larger.

Studies have shown heat waves worsening before, but this one is more comprehensive and concentrates heavily on not just temperature and area, but how long the high heat lasts and how it travels across continents, said study co-authors and climate scientists Wei Zhang of Utah State University and Gabriel Lau of Princeton University.

From 1979 to 1983, global heat waves would last eight days on average, but by 2016 to 2020 that was up to 12 days, the study said.

Eurasia was especially hit harder with longer lasting heat waves, the study said. Heat waves slowed down most in Africa, while North America and Australia saw the biggest increases in overall magnitude, which measures temperature and area, according to the study.

“This paper sends a clear warning that climate change makes heat waves yet more dangerous in more ways than one,” said Lawrence Berkeley National Lab climate scientist Michael Wehner, who wasn’t part of the research.