Greater thunderstorm frequency arising from deforestation near several West African coastal cities is thought to further increase the risk of flash flooding, said the UK Centre for Ecology & Hydrology (UKCEH).
According to the research, the frequency of thunderstorms over some fast-growing African coastal cities has doubled over the past 30 years, with much of this increase linked to the impact of deforestation on the local climate.
While it is widely accepted that the removal of vegetation increases rainwater runoff and the risk of mudslides, UKCEH believes that more frequent storm activity in coastal areas is a second, previously unrecognized, way in which deforestation can increase flooding.
The study analyzed three decades of satellite data in southern West Africa to establish how weather patterns had been altered because of deforestation, through changes in heating and moistening of the atmosphere.
The researchers found the removal of large areas of woodland had greatly exacerbated the effects of global warming in coastal areas of the region, which includes Guinea, Sierra Leone, Liberia, Ivory Coast, Ghana and Nigeria. In deforested areas, the frequency of storms has doubled since 1991, while the increase in forested areas has been around 40%.
In these areas, deforested land had been turned over to use for agriculture and fuel for cooking to support nearby growing populations but the storms and resulting rainfall affect urban as well as rural areas.
Professor Chris Taylor of UKCEH, who led the study, published in the journal Proceedings of the National Academy of Sciences (PNAS), said, “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% 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.”
Taylor added, “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.”
Previous research has linked deforestation with reduced regional rainfall in Amazonia, however, the ocean strongly influences local weather patterns, and the new study is believed to be the first investigation into the impact of deforestation on storm activity in coastal areas.
Dr Cornelia Klein, research scientist at UKCEH and a co-author of the study, added, “Local weather patterns are dominated by sea breezes, and deforestation strengthens these winds that carry moisture inland, triggering more afternoon downpours.”
The study was part of ongoing research by UKCEH and partners into past, current and projected future climate change in West Africa, where flash flooding is increasingly common during the rainy season. A study in 2017, also led by Professor Taylor, showed global warming was responsible for a tripling in the frequency of extreme Sahel storms in just 35 years.
The research was funded by the Natural Environment Research Council (NERC), part of UK Research and Innovation.
Sarah Webb, NERC’s associate director for international, said, “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.”