We all know that forests are vital to fighting global climate change. But, we don’t often consider the impact they have on the climate at a local scale. Our research shows that Storms are more common in West Africa because of deforestation and thus increased the risk of disastrous flash flooding in the region’s coastal cities.
A large portion of west Africa was deforested during the 20th Century. This was due to international trade in goods such as cocoa and the displacement of people through conflict. The rapid growth of cities, especially along the coast, is also contributing to deforestation.
We can see how many pockets of forest have disappeared since the advent of satellites. To calculate trends in storm frequency, we analysed satellite images of clouds every thirty minutes since 1990.
Rain is a common occurrence in this tropical region. We weren’t looking for this sort of rain, though. These storms are less frequent, but can produce deluges of several inches of rain in an hour or so. This is enough to flood low lying areas and trigger torrents to drain particularly vulnerable catchments.
More storms – but where?
We found that although there was some a General increaseAll across the entire regionThis is due to global warming. There was a lot of patchiness in the trends and it was well mapped to areas where deforestation was taking place.
What is the impact of deforestation on rainfall? Deforested areas become hotter than the remaining forest in the daytime. This warming can trigger local breezes and these atmospheric circulations may kick off storms.
Storm trends are especially strong when deforestation occurs within just a few kilometres from the coast. The sea breeze has a strong effect on rainfall. This is why torrential afternoon downpours are a common occurrence as the breeze brings water over the land. Sea breeze circulation is caused by the contrast between cool ocean and warm land. Deforestation results in warmer land which increases the temperature difference, which intensifies sea breeze thunderstorms.
Cities at risk
We found that coastal areas with significant deforestation experienced afternoon storms twice as often as 30 years ago. However, regions with more stable soil cover saw 30-40% increases. The sharp increase in storminess around coastal deforestation hotspots is important because they coincide with many of the region’s cities, for example, Freetown and Monrovia, the capitals of Sierra Leone and Liberia, respectively.
In the region, cities are often exposed to flash flooding because of their poor drainage networks, particularly in informal settlements. It is well-known that tree removal increases storm runoff, but our results show that the likelihood of intense thunderstorms happening in the first instance increases if there is no forest management.
Freetown, which saw an increase in 2017. devastating flash flood triggering mudslidesWith the loss of over 1,000 lives, it is urgent to consider the effects of local changes in storm patterns and global warming.
Computer models used to project global climate change are unable to capture the changes in rainfall within a few kilometres from the coast. These conventional models lack spatial detail to show how deforestation or sea breezes affect storms at smaller scales. To give planners the information they need to design climate-resilient communities, we are now using higher resolution climate simulations. These models are capable of capturing key physical processes. Cutting-edge climate science.
The emerging picture of increased rainfall in a landscape that is predominantly deforested contrasts with what we know from processes in the Amazon rainforest where most of our research has been conducted. Atmospheric circulations play a significant role in the increase in rainfall when small patches of forest are removed. This is similar to what happens in west Africa. Amazonia’s deforested areas are becoming larger. Rainfall decreasesBecause of the drying effect of lower transpiration.
The ocean’s proximity is the key difference between these two regions. The ocean in west Africa acts as a source for moisture, so rainfall responses are dominated by changes of wind rather than humidity. This is not true for the Amazon rainforest, where the majority of deforestation occurs hundreds of kilometers inland. However, most deforestation hotspots across the tropics are relatively close to oceans, as in south-east Asia. Deforestation in large numbers will likely cause storms to respond in a similar way to the Amazonian decreases.
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