What Lies Ahead for the Sahara?

The Sahara remains the most sparsely inhabited place on the planet, with the exception of the frozen wastes of Antarctica. Most of the herders and traders who once made a living here have vanished, their cultures crushed by contact with outsiders and shifts in trade routes that no longer require goods to cross the harsh desert. Various mineral deposits and oil have spurred some economic activity, including iron, copper, manganese, uranium, phosphates, coal, oil, and natural gas. But most deposits are scattered, and the lack of water has prevented large-scale development of long-term settlements that sometimes outlive mining booms.

Mostly, those economic booms have left the desert even more harsh by using up the million-year-old water stored underground. The sharp drop in water table levels has been blamed for complex changes that have largely destroyed the ancient lifestyles of the desert people. For instance, the increasing aridity has increased the threat of locust plagues as the ravenous grasshopperlike insects have focused on the more concentrated green areas. As the dropping water tables dry up oases and rare springs, the people of the desert have grown increasingly concentrated, which has triggered other environmental changes.

As a result of those local changes and the warming of the planet in the past century, the Sahara has resumed its expansion after 2,000 years of relative stability. As a result, the Sahara may once again play a crucial role in shaping the climate of the whole planet because of its ability to generate massive dust storms.

Climate experts have measured a significant increase in average temperatures planetwide in the past century. Most climate scientists blame much of the increase on the significant increase in carbon dioxide in the atmosphere as a result of human pollution, including power plants and automobiles. Carbon dioxide does not block the long wavelengths of light entering the atmosphere from the sun, but it does absorb the shorter wavelength infrared radiation reflected by the sun-warmed rocks, soil, and plants on the surface. As a result, the increasing concentrations of carbon dioxide trap heat in the lower atmosphere, warming up the surface of the planet.

That has apparently caused an expansion of the desert and higher summer temperatures. That, in turn, pours extra energy into the wind currents that created the Sahara in the first place. The increasing wind has driven a significant increase in Sahara dust storms. Moreover, the windstorms themselves seem to decrease rainfall, which reduces plant life, increases temperatures, and therefore drives even more wind. This perfectly illustrates the way in which temperatures can create unexpected feedback loops that exaggerate the effect.

Increasing the dust in the air can reduce rainfall over a wide area, according to a recent NASA study published in The Proceedings of the National Academy of Sciences. The results surprised scientists, who thought that putting more dust particles into the air might increase rainfall by giving the water vapor in clouds particles to form around, which they thought would create more and bigger raindrops. In fact, the opposite seems to happen, according to a study that compared satellite measurements of clouds with rainfall measurements on the ground. Apparently when dust particles enter a cloud, very small water droplets form on the vastly increased number of particles. These abnormally small water droplets hit each other, break up, and then reform without growing large enough for gravity to overcome air resistance and make them fall out of the cloud. As a result, rain clouds seeded with dust particles hang onto their moisture much longer. Therefore, the clouds that drift over desert areas that generate big dust clouds more often move on without dropping their moisture. As a result, the dust created by the expansion of the desert drives away the rain, which in turn increases the rate of desertification. This tendency of dust to increase desertification could play a surprising and unexpected role in shaping the planet's climate in the face of global warming. Perhaps it even played a role in the rapid expansion of the Sahara Desert 6,000 years ago.

Already, the dust of the Sahara has been showing up in some unexpected places. Saharan dust blown up into the atmosphere has increased significantly in recent decades, for reasons scientists don't fully understand. Some studies show that dust kicked up into the atmosphere by storms in the Sahara can affect cloud formation across the north Atlantic, according to an analysis of satellite data. Other studies have shown that African dust blown out of the Sahara appears to trigger lethal outbreaks of meningitis, probably because the dust irritates people's nasal passages and that makes it easier for meningitis spores to enter their bloodstreams. Surprisingly, African dust makes up one-half of the breathable particles in the air over Miami, Florida. As a result, some drifting Saharan dust storms can actually trigger clean air alerts in Miami when they drift into town. In yet another surprising fallout from African dust, some doctors fear that the high iron content in African dust could cause more health and lung problems even in North America, according to a study published recently by the American Geophysical Union.

Already, this increase in wind, desertification, and dust storms could be affecting the climate, according to several alarming bits of evidence. Glaciologist Lonnie Thompson and a research team at Ohio State University have been comparing the air trapped in 6,000-year-old glaciers to today's atmosphere. This fossil air trapped in bubbles when the ice froze 6,000 years ago can reveal the composition of the atmosphere during the period stone tool–wielding nomads finally abandoned the Sahara as the desert expanded. Thompson combined the analysis of that fossil air with other clues, including tree-ring data from Ireland and England that provides a rainfall record going back 7,000 years. Those lines of evidence both suggested that the rapid expansion of the Sahara 6,000 years ago corresponded to a planetary drought. However, the speed of the changes did not quite match the scope of the changes noted in the past 100 years. Already, 10,000-year-old glaciers have been melting all over the planet, from the top of Africa's Mount Kilimanjaro to the high peaks of the Peruvian Andes.

Thompson's team concluded that the climate changed abruptly 5,200 years ago and the Sahara expanded by perhaps 50 percent in a matter of decades. He suggested that the speed of that change shows that although natural feedback loops may hold back climate change up to a point, those feedback loops also may greatly accelerate change once conditions go past a certain threshold. That is a worrisome finding in a time when carbon dioxide and temperature levels are rising measurably each year.