The Sahara, the world’s largest desert, stretching more than 3.5 million square miles, has grown by 10 percent over the last century due to a combination of natural climate variations and global warming, according to new research published in the Journal of Climate.

Most of the expansion has happened to the south, during historically rainy summer months, with the Sahara’s borders edging closer to agricultural areas in Sudan, Chad, and Mauritania. Northward expansion of the Sahara has happened largely during the winter months, the study found.

“The annual perspective is not very informative for water resource planning, for agricultural development,” study co-author Sumant Nigam, a climate scientist at the University of Maryland, told E&E News. The most important information is what’s happening during the “agriculturally sensitive summer season,” he said.

The growth of the Sahara, particularly toward the south, has been influenced by the Atlantic Multidecadal Oscillation, a natural climate cycle that changes the Atlantic Ocean from warm to cold phases every 60 to 80 years and can impact rainfall patterns across much of Africa. But global warming may also be altering atmospheric circulation patterns, such as the Hadley cell, which moves air from the Equator to the subtropics, drying it out as it goes and creating many subtropical deserts, including the Sahara.

The new study is one of the first to look at changes to the Sahara over the past century by examining precipitation data. This is in contrast to previous research that studied the desert’s boundaries over shorter, decades-long periods, relying on satellite images and vegetation patterns.

“Surely there is an important anthropogenic influence there, but it is also being met with natural cycles of climate variability that add and subtract in different periods,” Nigam said. “Understanding both is important for both attribution and prediction.”