The Sahara Desert—which takes up about 3.6 million square miles of northern Africa—is growing ever larger, signaling daunting news for people living in the Sahel border region who stand to lose valuable arable land to the expanding desert.

The boundaries of the world's largest hot desert, already around the size of China or the continental U.S, have grown roughly 10 percent since 1920 due to natural climate cycles as well as man-made climate change, according to a new study by National Science Foundation (NSF)-funded scientists at the University of Maryland.

"The trends in Africa of hot summers getting hotter and rainy seasons drying out are linked with factors that include increasing greenhouse gases and aerosols in the atmosphere," said Ming Cai, a program director in NSF's Division of Atmospheric and Geospace Sciences.

As a summary of the new study pointed out, as the Sahara expands, the Sahel retreats—putting the region's fragile savanna ecosystems and human societies under threat.

"These trends have a devastating effect on the lives of African people, who depend on agriculture-based economies," Cai noted.

For the study, published this week in the Journal of Climate, researchers analyzed annual rainfall data recorded throughout Africa from 1920 to 2013. Deserts are defined as places that receive less than four inches of rain per year. After analyzing the rainfall data, the researchers determined that many areas in the Sahara now fall under this threshold.

"It is shown that the Sahara Desert has expanded significantly over the twentieth century, by 11 percent-18 percent depending on the season, and by 10 percent when defined using annual rainfall," the study states.

The researchers only studied the Sahara, but the results suggested that other deserts could be expanding as well.

"Our results are specific to the Sahara, but they likely have implications for the world's other deserts," said Sumant Nigam, an atmospheric and ocean scientist at the University of Maryland, and the senior author of the study.

According to the research team, natural climate cycles such as the Atlantic Multidecadal Oscillation and the Pacific Decadal Oscillation have primarily driven the Sahara's rapid expansion, but these forces account for just two-thirds of the Sahara's total expansion.

The remaining one-third can be attributed to human-caused climate change. Nigam explained that deserts usually form in the subtropics because of what's called Hadley circulation, through which air rises at the equator and descends in the subtropics. Climate change, the researchers warn, could widen that circulation and cause subtropical deserts to inch north.

"Climate change is likely to widen this Hadley circulation, causing the northward advance of subtropical deserts," said Nigam, adding that at the same time, "the southward creep of the Sahara suggests that additional mechanisms are at work."

The African continent is one of the most vulnerable to climate change, yet its inhabitants are the least responsible for it. Truly, one of the great injustices of our time.

The researchers suggested that longer climate records that extend across several climate cycles are needed to reach definitive conclusions.

"Our priority was to document long-term trends in rainfall and temperature in the Sahara," said Natalie Thomas, a researcher at the University of Maryland and lead author of the paper. "Our next step will be to look at what's driving these trends, for the Sahara and elsewhere."