Local fires reduce rainfall in the dry season in southern Africa. Soot particles, also known as black carbon, absorb solar radiation and make the atmosphere warmer and drier. This in turn affects precipitation.
From Centre for International Climate and Environmental Research (CICERO)
Emissions of particles, in particular soot, from local fires in southern Africa have increased due to human activities. Along with increasing global CO2 levels, these particles have led to a reduction in precipitation during the dry season in the region. The findings, published in a new Nature Communications paper by CICERO and the University of Leeds, illustrate that reducing or cleaning local fires may help to counteract reduced rainfall.
“Climate projections show that southern Africa, already prone to droughts, will get even less rain if greenhouse gas emissions continue unabated. Observations since the 1950s indicate that some precipitation reduction has already occurred”, says lead author of the article, Øivind Hodnebrog, researcher from CICERO (Center for International Climate and Environmental Research – Oslo).
The new paper published in Nature Communications, shows that these reductions in rainfall during the dry season are due to a combination of increases in global CO2 concentrations, and to increased fire emissions that take place within the southern Africa region itself.
“Soot particles, also known as black carbon, absorb solar radiation and make the atmosphere warmer and drier. This in turn affects precipitation. Vast amounts of such particles are emitted from fires during the dry season each year. These fires are both natural and a result of human activities, for instance associated with farming practices”, Hodnebrog continues.
The new study illustrates that, in addition to improving air quality, reducing fires and their associated particle emissions locally may be a useful way to mitigate reduced rainfall in the region.
“The soot particles typically stay in the atmosphere for only a few days, but can still lead to changes in precipitation locally. CO2 on the other hand, is a greenhouse gas that can remain in the atmosphere for hundreds of years after emission. Combatting the effects of increasing CO2 on precipitation therefore requires global efforts of emission reduction”, Hodnebrog concludes.
Observations indicate a precipitation decline over large parts of southern Africa since the 1950s. Concurrently, atmospheric concentrations of greenhouse gases and aerosols have increased due to anthropogenic activities. Here we show that local black carbon and organic carbon aerosol emissions from biomass burning activities are a main cause of the observed decline in southern African dry season precipitation over the last century. Near the main biomass burning regions, global and regional modelling indicates precipitation decreases of 20–30%, with large spatial variability. Increasing global CO2 concentrations further contribute to precipitation reductions, somewhat less in magnitude but covering a larger area. Whereas precipitation changes from increased CO2 are driven by large-scale circulation changes, the increase in biomass burning aerosols causes local drying of the atmosphere. This study illustrates that reducing local biomass burning aerosol emissions may be a useful way to mitigate reduced rainfall in the region.
Local biomass burning is a dominant cause of the observed precipitation reduction in southern Africa; Øivind Hodnebrog, Gunnar Myhre, Piers M. Forster, Jana Sillmann & Bjørn H. Samset;Nature Communications7,Article number:11236 doi:10.1038/ncomms11236
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