Dust from the Sahara desert can warm the atmosphere, increase the production of clouds, and prolong drought conditions. Now, researchers have found evidence that intensive farming is responsible for a significant portion of that dust. Experts are cautious, but the connection suggests that factoring in dust production could lead to better global climate models.
The world's biggest sandbox is also its biggest source of airborne dust. The Sahara region's famous sandstorms also carry aloft tiny grains of minerals, pollen, and soil, which westerly trade winds can transport for thousands of kilometers. For example, a weather station in Barbados, in the Caribbean Sea, has been collecting dust samples from the Sahara ever since it started operating in the 1960s.
Atmospheric scientists have long known that dust from the Sahara influences precipitation and cloud formation over Africa by providing the solid nuclei around which water droplets can form. They also have assumed that the onset of commercial agriculture in the region nearly 200 years ago probably increased the amount of dust swept into the air, because previous research in other regions had suggested such a connection. But there has been scant historical evidence for the effects of agriculture on dust production.
Now researchers have collected sediments from an area of the eastern Atlantic Ocean called the Mauritania Canyon, which is about 100 kilometers off the coast of western Africa and downwind from the Sahel region, the southern portion of the Sahara. The team used the sediments, which provide a regional climate record going back about 3200 years, to track cycles of precipitation and drought in the Sahel. The researchers could do this, they explain in tomorrow's issue of Nature, because dust grains differ substantially in chemistry and size from the grains of sediment deposited by river runoff. So, they could identify any changes in precipitation by the proportion of dust grains to sediment grains—more airborne dust means less precipitation, whereas more runoff grains mean more rain.
Based on the data from the samples and the sudden appearance in sediments from the 19th century of particles from maize, mullet, sorghum, and groundnuts, the researchers concluded that the onset of agriculture in the region led to a spike in airborne dust, eventually more than doubling its concentration in the Mauritania Canyon sediment. That means there is "a considerable human influence on dust generation starting with the advent of commercial agriculture in the Sahel," says marine geologist and lead author Stefan Mulitza of the University of Bremen in Germany. Therefore, he adds, it's important "to further understand the role of African dust in the climate system and incorporate [it] into climate models."
Atmospheric chemist Joseph Prospero of the University of Miami in Florida agrees that further studies are necessary and important. One reason: researchers need to confirm that the dust/grain size differential between the land surface and river runoff is consistent. They'll need to sample sediments from more locations to rule out any unique local effects. Also, says Prospero, how much impact human-generated dust has on climate remains "an open question."
It's a "very important" paper because it's the first to show a large increase in dust in North Africa over the past couple of centuries, says atmospheric scientist Natalie Mahowald of Cornell University. She agrees with Prospero that verifying the correlation between the size of the dust grains and their source is essential. "We won't know the truth until we have more observations, similar to the one they present here," she says.