NASA scientists find connection between fires and droughts in sub-Saharan Africa

Over the last several decades, the Northern Sub-Saharan African region has been hit by a number of severe droughts and the associated societal impacts, such as food shortages that result in deaths of both people and livestock. The Sahel zone, an area of semi-arid land just south of the Sahara Desert that stretches across the entire width of the continent, has suffered a large number of these droughts.

There are a number of potential causes of African droughts, some natural and some manmade. Past studies on the factors that influence droughts in the Sahel have tended to focus on land–atmosphere interactions and the influence of sea surface temperatures, such as the effects of the periodic temperature shift in the Atlantic Ocean known as the Atlantic Multi-decadal Oscillation.

Among the human activities that can drive drought conditions is overgrazing, which reduces the moist vegetation covering the soil, thereby impacting the soil’s ability to retain moisture. Moist soil is capable of contributing water vapor to the atmosphere and helping generate rainfall. In other words, replacing that moist soil with drier desert soil diminishes the overall capacity for rain in the region.

Another manmade contributor to Sahel droughts is the burning of biomass. Though the possible causal relationship between biomass burning and decreased rainfall in the Sahel has received little scrutiny, a study published in the journal Environmental Research Letters last September sheds some light on the relationship between the two.

“We wanted to look at the general impacts of burning on the whole spectrum of the water cycle,” Charles Ichoku, a senior scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland and the lead author of the study, said in a statement.

The Northern Sub-Saharan African region accounts for 20 to 25 percent of global carbon emissions from biomass burning, Ichoku and his co-authors note in the study. Herders often burn land to promote the growth of grass, while farmers set fires to convert the landscape into farmland and remove unwanted biomass after the harvest season. Just like overgrazing, these fires dry out the soil and disrupt the local hydrological system, including rainfall patterns.

“The role of biomass burning in this phenomenon is not obvious, especially because the dry biomass-burning season (November–April) is out of phase with the rainy season, which occurs mainly from May to October,” Ichoku and co-authors write. “However, a mixture of desert dust and smoke from biomass burning is known to contribute to high aerosol loads in the NSSA atmosphere.”

The reason this is important is because the small particles known as aerosols that are released into the air by the smoke from fires can reduce the likelihood of rainfall. Water vapor in the atmosphere condenses on certain types of aerosols called “cloud condensation nuclei” to form clouds. Once enough water vapor has accumulated, rain droplets begin to form — but if there are too many aerosols in the atmosphere, the water vapor can become too spread out, inhibiting the formation of rain droplets.

Ichoku and team used satellite records from 2001 to 2014 to analyze what impact fires had on soil moisture, precipitation, evapotranspiration, and other water cycle indicators. They found a clear pattern emerged: “There is a tendency for the net influence of fire to suppress precipitation in northern sub-Saharan Africa,” Ichoku said.

In years that had an exceptionally high amount of biomass burning during the dry season, for instance, they found that soil moisture, evaporation, and vegetation greenness decreased in the wet season that followed. All of these water cycle indicators help create rain.

Though these results show a correlation between fires and the factors that provoke droughts, as opposed to a causal relationship, the data Ichoku and team gathered will help scientists to improve climate models such that the relationship between biomass burning and its impacts on drought might one day be more fully understood.

The study also made some apparently paradoxical findings, such as the fact that when fires decreased by two to seven percent per year between 2006 and 2013, there was no proportional increase in precipitation. Ichoku says that one possible explanation for this is the type of land that was burned: He and his colleagues found that during that same timeframe, more forests and wetlands were converted to cropland than in previous years. Ironically, it may have been the Sahel’s droughts that lured people to areas with more water, where they set fires to create farmland — burning the very type of land that provides significant amounts of moisture to the atmosphere that can eventually become rain.

“The removal of vegetal cover through burning would likely increase water runoff when it rains, potentially reducing their water retention capacity and invariably the soil moisture,” Ichoku said. “The resulting farming would likely deplete rather than conserve the residual moisture, and in some cases, may even require irrigation. Therefore, such land cover conversions can potentially exacerbate the drought.”

CITATION

• Ichoku, C., Ellison, L. T., Willmot, K. E., Matsui, T., Dezfuli, A. K., Gatebe, C. K., … & Okonkwo, C. (2016). Biomass burning, land-cover change, and the hydrological cycle in Northern sub-Saharan Africa. Environmental Research Letters, 11(9), 095005. doi:10.1088/1748-9326/11/9/095005

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