How climate change could erase the African savannah

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Wagdy Sawahel, Catherine Brahic and Christina Scott
14 December 2005 02:15

Africa's fabled grasslands could vanish due to climate change, causing huge changes to both the economy and the ecology of much of the continent, say researchers on the Science and Development Network website, SciDev.Net.

Trees and shrubs could take over some of Africa's savannahs if, as many predict, rainfall in the Sahel increases in the next 50 years as human use of fossil fuels triggers global warming.

Savannahs -- broad grasslands with scattered trees -- are both economically important and ecologically unique. The balance between tree cover and grassland is crucial, influencing both plant and livestock production, as well as ecological systems such as the water cycle.

Now a team of scientists led by an India-born, United States-based researcher has found that annual rainfall plays a key role in determining how the balance is maintained.

Using data from 854 sites across Africa, Mahesh Sankaran, of the Natural Resource Ecology Laboratory at Colorado State University, reports in the prestigious journal Nature this week that savannahs split into two types: stable and unstable.

Stable savannahs are those that receive less than 650mm of rainfall per year. Here, the amount of rain restricts the number of trees, allowing grasses to co-exist.

Unstable savannahs receive more than 650mm each year. The amount of tree cover in such savannahs is determined not by rainfall, but by disturbances such as fires and animals feeding.

Sankaran's data suggests that if rainfall increases across parts of Africa, as several climate change models predict, some savannahs would gradually be taken over by trees.

Changes in the tree-grass balance would have significant impacts on plant and livestock production, biodiversity, and the water and carbon cycles. The research effectively combines the two dominant schools of thought on how trees and grasses can co-exist in savannahs -- whether the balance is regulated by the availability of resources such as water, or by disturbances such as fire.

Both views are valid at different points in the rainfall cycle, Sankaran told SciDev.Net.
It is very important to understand what drives the savannahs in order to help manage them as climate conditions change, Sankaran said. But there is considerable dispute about the future of much of North Africa under climate change, underscoring the difficulties in assessing one of the most complex mechanisms on the planet.

Rising temperatures in the Sahara desert could actually be beneficial, reducing drought in the Sahel region immediately south of it, say researchers Reindert Haarsma and colleagues, of the Royal Netherlands Meteorological Institute. They say their study is the first to consider the roles of both land and sea-surface temperatures.

The Haarsma computer model suggests that if emissions of greenhouse gases are not reduced, higher temperatures over the Sahara would cause 25% to 50% extra daily rainfall in the Sahel by 2080 during the months from July to September.

Haarsma explained that the Sahara desert heats up faster than the oceans, creating lower atmospheric pressure above the sands. This in turn leads to more moisture moving in from the Atlantic to the Sahel.

The Sahel stretches from Senegal to Somalia and encompasses large parts of Mali, Mauritania and Niger, where drought -- together with locusts -- caused a major famine this year.

Earlier this year, US-based scientists published research that also suggested climate change could increase rainfall in the Sahel.

However, just recently a new computer model utilising additional information predicted that the drought-prone Sahel region faces "dramatic drying" during the next 50 years because of climate change from greenhouse gases.

The new results, published in the Proceedings of the National Academy of Sciences, are unusual. But researchers are taking the new predictions seriously as the computer model is among the best at simulating 20th-century climate -- a test of how well models can predict future trends.

Several research groups around the world are analysing the model, which has also been submitted to the Intergovernmental Panel on Climate Change, the consortium of scientists that researches the issue and advises the United Nations.

Isaac Held, of the US National Oceanic Atmospheric Administration, thinks the dramatically different results might be related to how the complex computer model simulates the behaviour of clouds.

While scientists are comfortable with the uncertainty over climate change, the ambiguity does have a distinctly negative political pay-off. Politicians such as US President George Bush have been arguing that until the scientific community has complete consensus on global warming, it would be unwise and premature to try to do anything to address the situation -- such
as signing the Kyoto Protocol. -- SciDev.Net

The authors write for the London-based global science news agency, the Science and Development Network, which carries free links to the original research as well as a free archive of reports and research on climate change in the developing world