The Sahara Desert Is Growing. Here's What That Means

By Mindy Weisberger, Senior Writer | March 29, 2018
The Sahara — the world’s biggest hot desert — is getting even bigger. In fact, it is currently about 10 percent larger than it was nearly a century ago, and scientists suggest that climate change is partly responsible.

In a new study, researchers examined rainfall data gathered across Africa, consulting records dating back to 1920 and noting how changing conditions affected regions around the boundaries of the great desert.

They discovered that while some natural climate cycles could partly explain reduced rainfall and desert expansion southward, human-driven climate change is also playing a part. And if climate change continues unchecked, the Sahara's slow growth will likely continue, the study authors reported. [The Sahara: Facts, Climate and Animals of the Desert]

 Previously, scientists had explored the Sahara's expansion by examining satellite data dating back to the 1980s. This study, which was supported by the U.S. National Science Foundation, is the first to analyze long-term trends in rainfall and surface air temperature over a timescale of nearly an entire century, the study's lead author, Natalie Thomas, a doctoral candidate in atmospheric and oceanic science at the University of Maryland, told Live Science.

Deserts are defined as places on Earth that receive less than 10 inches (25 centimeters) of rainfall per year, according to the U.S. Geological Survey (USGS). With a surface area of about 3.6 million square miles (9.4 million square kilometers), the Sahara is the third-largest desert in the world. Only the cold deserts are bigger: icy Antarctica's frozen desert spans about 5.5 million square miles (14.2 million square km), and the Arctic desert covers around 5.4 million square miles (13.98 million square km), USGS reported.

"A strong expansion"
The study authors originally set out to examine seasonal cycles of temperature and rainfall across Africa, consulting data spanning 1920 to 2013. But their attention was quickly drawn to trends of decreasing precipitation in the Sahel, a semiarid region linking the Sahara to the savannas of Sudan. By looking at this more closely, they hoped to discover how rainfall trends might be linked to the Sahara's growth over time, according to Thomas.

To a certain extent, many deserts' boundaries expand and contract seasonally, as conditions fluctuate between wetter or drier. But the researchers found that there has been "a strong expansion" of the Sahara within the 20th century, Thomas said.

Depending on the season, the Sahara experienced growth of at least 11 percent, and it grew by as much as 18 percent during the driest summer months, according to data collected over roughly 100 years. Over the course of a century, it steadily expanded to become about 10 percent bigger than it was in 1920, the study authors reported.

Much of the Sahara's overall size increase can be explained by climate cycles driven by anomalies in sea-surface temperatures. These cyclical changes in turn affect surface temperatures and precipitation on land, and their impact can last for decades, according to the study.

Decades of drought
One such cycle, the Atlantic Multidecadal Oscillation (AMO), entered what is known as a "negative phase" — with cooler-than-average sea-surface temperatures — in the 1950s, bringing heat and dry conditions to the Sahel region and fueling a drought that lasted until the 1980s, Thomas said.

Using statistical methods, the scientists compensated for the effects of the AMO on average rainfall, and thereby calculated how much of the Sahara's growth could be explained by the dryness that the cycle's negative phase produces. They estimated that the AMO accounted for about two-thirds of the desert's expansion — but one-third of the Sahara's remaining growth was likely the result of climate change.

The researchers' findings point to changes that occur over decades rather than in a single year, and that makes it hard to predict exactly how the Sahara's continued growth could affect the wildlife and people near its changing borders. But as the places where humans grow food become increasingly drier, some areas could become more vulnerable to drought, bringing a greater risk of famine to the people who live there, Thomas said.

The findings were published online today (March 29, 2018) in the Journal of Climate.
Original article on Live Science
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