Temperature Increases Threaten the Congo Basin Rainforest in the 21st Century More Than Precipitation Changes

The climate of western and central equatorial Africa - referred to here as the Congo Basin region - is one of the most convectively-active areas on the planet. It supports a large, diverse tropical forest and sequesters carbon in vast peat deposits. Despite its importance, the region is under-studied and poorly observed.

As is typical near the equator, seasonality in the Congo Basin region is defined by variations in precipitation not temperature. While the Congo Basin precipitation climatology is often described as being bimodal, classification studies do not agree with this characterization. The misunderstanding arises when inhomogeneous averaging regions that cross the equator are used. The equinoctial seasons produce the highest rainfall rates, but summers are also wet. Winter is the sole dry season.

A number of studies have analyzed precipitation trends in observations, but a clear signal has not emerged. GCMs do not represent the regional climate with high accuracy, and their projections disagree. Analyses of forest health document a decline and attempts by some species to migrate to higher, cooler environments. Tree growth in the tropical forest exhibits a strong negative correlation with temperature. Ambient temperatures in tropical rainforest ecosystems are close to the maximum for optimal metabolic function and a warming of 3.9°C is estimated as a threshold beyond which photosynthetic activity is severely compromised.

Here, convective-permitting (CP) model simulations with 3-km resolution are used to predict greenhouse gas-forced changes in the climate of the Congo Basin. CP modeling allows for the explicit treatment of convection within the governing equations, without the use of convective parameterization. It improves the simulation of Congo Basin rainfall and enhances confidence in the predicted precipitation changes under greenhouse gas forcing.

Simulated surface temperatures in the Congo Basin region forced by the SSP585 scenario increase by 4.0 - 5.4 K at the end of the century, depending on region and season. Increases in atmospheric water vapor and changes in clouds provide important feedbacks to the radiative forcing. No statistically significant changes in regional-scale precipitation occur, but convective storms intensify locally.

Since estimates based on optimal temperatures for photosynthesis suggest a 4 K warming causes a decline in tropical forests, these results suggest that temperature increases pose a greater threat to the health of the Congolese rainforest than precipitation changes in the 21^st century.