Research led by the UK Centre for Ecology & Hydrology (UKCEH) has now revealed that more frequent storm activity in coastal areas is a second, previously unrecognized, way in which deforestation can increase flooding.
It is widely known that removal of vegetation increases rainwater runoff and the risk of mudslides, which happened in Sierra Leone’s capital, Freetown, in August 2017 when 1,100 people died. But, the frequency of thunderstorms in some fast-growing African coastal cities has also doubled over the past 30 years — with much of this increase linked to the impact of deforestation on the local climate — the study has found.
The study analysed three decades of satellite data in southern West Africa to establish how weather patterns had been altered as a result 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 only been around 40 percent.
The 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 now affects both urban and rural areas.
Professor Chris Taylor of UKCEH, who led the study published in the journal 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 per cent of the world’s population lives within 100km of the coast, so increases in flash flooding causes disruption to millions of people’s lives.,” continued Taylor. “Our findings therefore provide a warning to fast-growing coastal cities across the world.”
In Africa and South-East Asia, large areas of coastal tropical forest are being destroyed, global climate change is already having an impact on communities, and drainage and other infrastructure are often ill-equipped to cope with major flooding. In Freetown in particular, the residents are already suffering from multiple, damaging effects of climate change, including extreme temperatures and flash flooding from storms.
Previous research has linked deforestation with reduced regional rainfall in Amazonia. However, the ocean strongly influences local weather patterns, and this new study is the first investigation into the impact of deforestation on storm activity in coastal areas.
“Local weather patterns are dominated by sea breezes and deforestation strengthens these winds that carry moisture inland, triggering more afternoon downpours,” explained Dr Cornelia Klein of UKCEH, a co-author of the study.
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.
