The deadly landslide in Freetown, Sierra Leone on Monday morning was undoubtedly partly the result of deforestation. During the rainy season, Sierra Leone is one of the wettest places on earth, statistically. And from that point of view, it is a surprise that such disasters don’t happen more often.
It looks as though the changing climate may make the future risk even greater as more frequent and more violent rainy season events such as this are a measurable consequence.
In tropical Africa, much of the seasonal rain comes from thunderstorms and often these cluster together, creating mesoscale convective systems (MCS). In simple terms, these are arcs of giant thunderstorms that can cover half a country.
Since the start of satellite observations, these have been observed and recorded with rather more reliability than is possible over the sparsely populated area of the Sahel. The systems grow and and move to the west, usually leaving West Africa trough Guinea or Sierra Leone. (Tellingly they are the start of most Atlantic hurricanes.)
You may be surprised to hear that African thunderstorms are among the most intense on earth. That finding came from a scientific paper published in 2006, a study using a worldwide database. A more recent investigation has found that the number of these storms has increased in sub-Saharan Africa, by a factor of three, since 1982.
That latter conclusion came from another scientific paper, published this year, discovering evidence that looks like a clear link to a warming climate. It is a well-established expectation that temperatures will rise, as a global average, and that local variations will be vastly different.
That leads to an increase in the energy available for hurricanes, thunderstorms, tornadoes and simple rain showers. We see and measure this change. However, in sub-Saharan Africa and particularly in the Sahel, the change has been more dramatic.
The Sahara Desert has heated up as predicted with a warming globe but the edge of the desert, the Sahel, during the wet season, has not. As a result, the temperature gradient from desert to tropics is now steeper. It is differences in temperature that cause weather – they create a reason for air to move.
And as you cannot change the laws of physics, the resulting thunderstorms, grouped as mesoscale convective systems, now have more vigorous construction. They also have the necessary shear in wind direction and strength within the thunderheads to create an efficient up- and downdraught circulation.
This increase in the intensity of these major storm systems looks like a result of climate change. The climate forecast sees the Sahara warming more which leads directly to a future of more intense rainfall events, especially in the Sahel. Resultant devastating flooding and landslides will become more common.