Examining the impact of short–lived climate forcers on temperature and precipitation over Eastern Africa

Abstract

Short-lived Climate Forcers (SLCFs) such as nitrogen dioxide (NO2), sulphur dioxide (SO2) and carbon monoxide (CO) influence the radiation budget and are also harmful air pollutants. Here, satellite observations, numerical modeling and deep learning were used to generate knowledge about these species over Eastern Africa. First, an examination of the observations revealed that NO2 and CO were mainly emitted from savanna fires. Cities and towns like Nairobi, Kampala, Dar es salaam, Mombasa and Juba were important hotspots of NO2 while Mount Nyiragongo was the dominant SO2 hotspot. Overall, there was no trend in NO2 while SO2 had a decreasing trend. CO had a decreasing trend at low levels (850 hPa to 500 hPa), and an increasing trend at the upper levels of the atmosphere (400 hPa to 1 hPa). This indicated that there was vertical ascent of CO. Importantly, despite the increase in anthropogenic activity from 2005 to 2020, gas abundances arising from natural emission sources were still dominant. Second, the Weather Research and Forecasting model coupled with chemistry (WRF-Chem) was applied to simulate the observed gas abundances during the peak periods. The simulations were then bias corrected using a deep convolutional autoencoder (WRF-DCA) and linear scaling (WRF-LS). The results showed that WRF-Chem successfully simulated the seasonal variation of the gas species but with some bias. WRF-DCA and WRF-LS both reduced the bias in NO2 and SO2 simulations. Overall WRF-DCA performed better than WRF-LS. Third, WRF-Chem was used to make idealized simulations to reveal how temperature and precipitation will respond to a doubling of NO2, SO2 and CO emissions. The results showed that only temperature and precipitation in the December to February season could significantly change. The response signal in the other three seasons could not be clearly discerned. Fourth, WRF was used to downscale the output of a global chemistry–climate model for the 2021 to 2055 period. The results revealed that SLCF mitigation is projected to cause an increase in annual mean surface temperature and precipitation over the majority of Eastern Africa. Only a few areas are expected to experience a decrease in annual mean precipitation. The global drive to mitigate SLCFs alone could cause additional climate change in Eastern Africa. Therefore, climate policy should aim at mitigating both SLCFs and greenhouse gases simultaneously.