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Africa is a source of uncertainty in carbon cycle calculations. By some estimates, the continent's landscapes emit 2.1 billion tons more carbon dioxide than they take up each year—about equal to 1.5 times the annual emissions from coal-fired power plants. But other estimates are almost the complete opposite, suggesting that the continent's copious plant matter takes up 2.0 billion more tons of carbon dioxide per year than it releases.

This uncertainty exists in part because the amount of carbon Africa takes up and emits varies greatly from year to year and partly because there is a dearth of available surface observations across the continent. Yun and colleagues investigated the reason for these fluctuations by applying a suite of atmospheric transport models to data from the Orbiting Carbon Observatory-2 (OCO-2), a satellite-borne instrument that tracks carbon dioxide emissions across Earth's surface. By filling a critical observational gap over Africa, the OCO-2 satellite has allowed researchers to examine the continent's carbon cycle in unprecedented detail.

Scientists previously suspected that temperature was the prime factor influencing plant growth and therefore carbon dioxide emissions. Instead, these researchers found that in Africa, moisture levels have a much bigger impact.

Different types of landscapes react to moisture quite differently, however. In shrublands and grasslands, plants take full advantage of water when it becomes available by increasing their mass with little energy expenditure. That reaction means that in wet years, shrublands and grasslands take up a lot of carbon and expel very little, substantially shifting the continent's carbon flux. In contrast, forests and savannas emit and take up about the same amount of carbon in wet conditions; their overall impact on the continent's carbon flux is therefore smaller.

These findings, in the journal Global Biogeochemical Cycles, suggest an explanation for the long-standing question of why Africa was such a weak carbon sink during the El Niño event of 2015–2016. The continent was unusually dry during that time, leading to stalled plant growth and carbon uptake.

Rainfall is expected to change in Africa in the coming decades. Overall, moisture availability is expected to increase in the north and decrease in the south, but precipitation will likely be patchy, leading to discrete bursts of plant growth. The researchers emphasize that the long-term operation of OCO-2 is essential for monitoring how African ecosystems respond to these shifting rainfall patterns. Taking moisture fluctuations into account could enable more accurate predictions of how the carbon cycle will respond to climate change.

This story is republished courtesy of Eos, hosted by the American Geophysical Union. Read the original story here.