Over the past 30 years, the frequency of thunderstorms in some rapidly-growing African coastal cities has doubled, with a huge portion of this increase connected to the impact of deforestation on the local climate, a new study has discovered.
A late afternoon storm over a forest, Yamoussoukro, Ivory Coast (Image Credit: Cornelia Klein).
It is generally known that when vegetation is removed, rainwater runoff and the risk of mudslides increase, which happened in Sierra Leone’s capital city, Freetown, in August 2017, causing the deaths of 1,100 people.
However, a new study headed by the UK Centre for Ecology & Hydrology (UKCEH) has exposed that more recurrent storm activity in coastal areas is a second, formerly unrecognized, way in which deforestation can intensify flooding.
The study examined 30 years of satellite data in southern West Africa to determine how weather patterns had been changed due to deforestation, through variations in heating and moistening of the air.
The scientists discovered the removal of large portions of woodland had significantly worsened the effects of global warming in coastal areas of the region, which covers Sierra Leone, Guinea, Liberia, Ghana, Ivory Coast and Nigeria. Since 1991, in deforested areas, the occurrence of storms has doubled, while the increase in forested areas has been about 40%.
The deforested land had been converted for agricultural use and fuel for cooking to support neighboring growing populations but the storms and subsequent rainfall impact both urban and rural areas.
The study has been published in the journal PNAS.
The extent of increase in coastal storm activity is likely to vary in different regions, depending on the local climate, but we would expect deforestation to have a similar effect in other coastal deforested areas. Around 40 percent of the world’s population lives within 100km of the coast*, so increases in flash flooding causes disruption to millions of people’s lives. Our findings, therefore, provide a warning to fast-growing coastal cities across the world.
Chris Taylor, Professor and Study Lead, UKCEH
Scores of people live by the sea because of the food and economic advantages that it offers.
In Africa and South-East Asia, large portions of coastal tropical forests are being removed, global climate change is already affecting communities, and drainage and other infrastructure are mostly incapable of coping with huge flooding.
In Freetown especially, the residents are already facing numerous, damaging effects of climate change, including flash flooding from storms and extreme temperatures.
Deforestation is exacerbating the impacts of climate change in some of the least resilient cities on Earth, making it much harder for these communities to cope with extreme weather events.
Chris Taylor, Professor and Study Lead, UKCEH
Earlier research has connected deforestation with decreased regional rainfall in Amazonia. However, the ocean robustly impacts local weather patterns, and the new study is the first study into the influence of deforestation on storm events in coastal areas.
Local weather patterns are dominated by sea breezes and deforestation strengthens these winds that carry moisture inland, triggering more afternoon downpours.
Dr Cornelia Klein, Study Co-Author, UKCEH
The study was part of continuing research by UKCEH and partners into past, current and projected future climate change in West Africa, where flash flooding is progressively common during the rainy season. A research study in 2017, also headed by Professor Taylor, revealed that global warming was accountable for a tripling in the incidence of extreme Sahel storms in just 35 years.
The study received funding from the Natural Environment Research Council (NERC), part of UK Research and Innovation.
Sarah Webb, NERC’s Associate Director for International, says: “COP26 highlighted significant challenges in many parts of the world caused by climate change.
“Research funded by NERC is helping global communities make future decisions on sustainable land management, urban planning, and agricultural practices, as well as draw up emergency response plans. This is supporting them in adapting to, and mitigating, the effects of climate change, leading to greater resilience.”
Journal Reference:
Taylor, C. M., et al. (2022) “Late-stage” deforestation enhances storm trends in coastal West Africa. PNAS. doi.org/10.1073/pnas.2109285119.