Wet or dry? Sahel's uncertain future
Climate change in Africa's Sahel region challenges researchers and policymakers because the best models around predict opposite outcomes, reports Catherine Brahic.
Computer models of future climate often disagree about the scale of likely change but predictions for the Sahel are also contradictory about the direction of change and give policymakers little help in preparing for the future.
The semi-arid Sahel stretches across Africa just south of the Sahara. The region suffered severe drought during the second half of the 20th century, and the famine it caused is thought to have claimed hundreds of thousands of lives and affected millions more.
Scientists initially believed that the decline in rainfall was caused by overgrazing and people clearing vegetation to make way for more farming and herding. But since the mid-1980s, several computer models have suggested that changes in the surface temperature of the oceans have changed the dynamics of the West African monsoon and are therefore to blame.
This hypothesis has gained widespread support but there is still some disagreement. Different models point the finger at different oceans — some say the influence of the Indian Ocean is most important, others the difference between the North and South Atlantic.
Most scientists agree that the greenhouse gases and aerosols that human activities release into the atmosphere are partially to blame for changing ocean temperatures.
The question, then, is how this will affect future rainfall. Again, the answers depend on the models used.
Last year US-based researchers Martin Hoerling and James Hurrell looked at all of the most recent climate models, averaged them out, and came to the conclusion that the Sahel's recent fate would be reversed in the 21st century.
Global warming, they concluded, would bring much-needed rainfall to the region — one of the very few positive outcomes of greenhouse gas emissions (see Decades of drought predicted for southern Africa).
But in late 2005, Isaac Held of the US National Oceanic and Atmospheric Administration published the results of a new climate model that suggested that, far from becoming wetter, the Sahel faces a period of "dramatic drying" if greenhouse gas emissions are not checked (see Climate model refutes predictions of wetter Sahel).
Held's team readily admitted that their results, from a single model, should not be the basis for policy decisions. But one striking point meant the results could not be discarded. The model mimicked the region's recent climate more faithfully than any previous one had — an important measure of how reliable it is.
Why the differences?
Understanding why the models predict such widely divergent futures "is a scientific priority that requires really getting into the bowels of the models" says Alessandra Giannini, a climate expert at Columbia University in the United States.
"There must be something in the models' physics that is causing them to respond differently."
For instance, several researchers have pointed out that many of the models show cooler present-day sea temperatures near the Americas and warmer ones close to Africa when the reality is the other way around — suggesting that the models are flawed.
Hurrell attributes the difficulties in modelling future Sahel rainfall to the "multiple competing influence of [factors that have] comparable importance".
His research with Hoerling suggests that global sea-surface temperatures play a strong, and possibly dominating, role in determining how much rain falls in the Sahel — more so than, for instance, temperatures above Africa.
"But the problem gets more difficult as evidence points to the Atlantic, Indian and Pacific oceans all playing a role through different mechanisms," he says adding that different parts of the Sahel might be affected differently by the relative influence of each ocean.
To complicate matters, the relative importance of factors affecting the Sahel's climate is tipped in different directions by the different models.
And Giannini's colleague Michela Biasutti says that some of her research suggests the importance of Atlantic temperatures could shift in future. "It may be that changes in land temperatures or atmospheric stability become more important," she says.
|Women draw water from a well in a small village in Mali in
one of the worst hit regions during drought in the mid-1980s
Building up a reliable dataset
"One thing is clear," says Richard Washington, a climate scientist at Oxford University in the United Kingdom. "If we carry on with the models as they are, we are just going to get different answers all the time, so we need to fix their basics."
Part of the problem is simply that scientists have little data about the West African monsoon to build into the models.
The African Monsoon Multidisciplinary Analysis (AMMA) is a massive international experiment that seeks to remedy this by measuring all the factors affecting this complex weather system that brings seasonal rain to the Sahel.
More than 400 scientists from 25 countries are participating, using a wide range of instruments including aircraft, ships, radars and weather balloons that transmit data back to Earth from devices called radiosondes.
"The debate at the moment in the Sahel has hinged on models, but they are not a particularly sharp tool in that region," says Washington. AMMA will improve the models by measuring "everything that lives and breathes in Africa over the next few months, and hopefully over the next few years as well", he explains.
"At moment, there are only four radiosonde balloons across the whole Sahel. But with two million euros [US$2.5 million] going into radiosondes [through AMMA], we will have a much better resolution of the whole depth of the atmosphere in three dimensions — in four dimensions including time — so we will be able to measure the things that make rain there properly for the first time."
|Photo taken from an aeroplane collecting data for the AMMA
project over Senegal
|Credit: Jim McQuaid|
Scientific uncertainty breeds policy uncertainty and the situation in the Sahel is very much in doubt.
For example, if climate models were to predict a five-centimetre rise in sea level for the Netherlands but others predicted a ten-centimetre rise, policymakers there could play safe and commission large dykes. But in the Sahel it is not a simple case of scale and governments are left with little indication of whether to prepare for more or less rain.
The uncertainty over the Sahel's future could also influence how aid money is distributed, as development agencies understandably prefer to tackle known problems. Washington believes this could favour East Africa — where predictions of increased rainfall appear reasonably certain.
"It is important to get predictions for the Sahel right," he says. "Chad, Mali, Mauritania, Niger and so on are arguably the countries that most need aid. So long as the models disagree, they are going to be neglected."
"There is a clear need to help regions that risk suffering the effects of climate change," he says. "If we cannot say what that risk entails, even to the point of being unable to predict if a region will receive more rain or suffer from drought, as in the Sahel, then aid efforts may well fail in the next few decades. That would be a disaster — that would be back to square one. Sorting out climate prediction is therefore a clear priority."
Washington is not accusing aid agencies of mismanagement. Rather, he is pointing out the critical role that climate research plays in guiding policy. Policymakers, particularly those who try to integrate climate-change preparations into all areas of government planning, from energy to agriculture, rely on the fruits of research. But uncertain predictions will inevitably lead to uncertain — or absent — policies.