Earth’s largest hot desert, the Sahara, is getting bigger, a new study finds. It is advancing south into more tropical terrain in Sudan and Chad, turning green vegetation dry and soil once used for farming into barren ground in areas that can least afford to lose it.

Yet it is not just the spread of the Sahara that is frightening, researchers say. It’s the timing: It is happening during the African summer, when there is usually more rain. But the precipitation has dried up, allowing the boundaries of the desert to expand.

“If you have a hurricane come suddenly, it gets all the attention from the government and communities galvanize,” said Sumant Nigam, a professor of atmospheric and oceanic science at the University of Maryland and the senior author of the study. “The desert advance over a long period might capture many countries unawares. It’s not announced like a hurricane. It’s sort of creeping up on you.”

The study was published Thursday in the Journal of Climate. The authors said that while their research focused only on the Sahara, it suggests that climate changes also could be causing other hot deserts to expand – with potentially harsh economic and human consequences.

Deserts form in subtropical regions because of a global weather circulation called the Hadley cell. Warm air rises in the tropics near the equator, producing rain and thunderstorms. When the air hits the top of the atmosphere, it spreads north and south toward the poles. It does not sink back down until it is over the subtropics, but as it does, the air warms and dries out, creating deserts that are nearly devoid of rain.

“Climate change is likely to widen the Hadley circulation, causing northward advance of the subtropical deserts,” Nigam said.

Nigam and the study’s lead researcher, Natalie Thomas, a doctoral candidate at the University of Maryland, used data from the Global Precipitation Climatology Center to arrive at their finding.