The Sahel in the 21st century: drier or wetter?

The latest Human Development Report 2006 published today by the United Nations Development Program (UNDP) shows that efforts to help developing nations adapt to the impacts of climate change are "woefully inadequate". Rich countries have focused on ways to reduce carbon emissions but have largely ignored helping poor nations cope with the consequences, it says. The observation that systems like the Kyoto Protocol's Clean Development Mechanism fail in serving the least developed countries, is substantiated by a set of 14 Sustainable Development Opinion Papers published recently by the International Institute for Environment and Development.

The UNDP report called Beyond Scarcity: Power, Poverty and the Global Water Crisis [*.doc], says climate change "now poses what may be an unparalleled threat to human development". It reiterates the warning that certain parts of Africa might experience greater droughts and extreme weather events due to climate change. Most notably, and controversially, it paints a grim outlook for the vast Sahel region, which is predicted to become drier, resulting in declines of up to 25% for cereal crop yields.

We want to focus on this projection because the African Monsoon and the Sahel - the vast region South of the Sahara, stretching over 10 African countries and feeding millions - have become somewhat of a symbol for climate prediction researchers. An overview of recent simulations shows that it is impossible to say whether this region will become wetter or drier. This uncertainty has led the UN to shift some of its attention away from climate change prevention towards a policy of climate change preparedness and adaptation (see the recent Climate Summit held in Nairobi).

The predictions on future rainfall in the Sahel differ widely, with most studies showing an increase in rainfall of up to 50%, while some show a decrease of up to 25%. This uncertainty is highly problematic for policy makers and aid agencies alike; it also affects projections about the bioenergy potential of the region. If the "wet" scenarios were to be correct, the Sahel would be propelled into becoming a zone with a huge potential for energy crops, simply because the region is so vast; were the "dry" scenarios to come true, planning would have to shift radically towards ensuring food security.

Below we present a chronological but by no means exhaustive overview of the most recent analyses, and some of the implications of their contradictory results:
bioenergy :: biofuels :: energy :: sustainability :: desertification :: climate change :: monsoon :: Africa :: Sahel ::

1. Hoerling and Hurrell, june 2005
Detection and Attribution of 20th Century Northern and Southern African Monsoon Change [*pdf] and abstract, published in the Journal of Climate.

James Hurrell of the US National Centre for Atmospheric Research and Martin Hoerling of the US National Oceanic and Atmospheric Administration published the most authoritative study to date, using 18 differen models. The results project countries in the Sahel to receive much more rain to 2050 due to temperature changes in the Atlantic ocean. The changes are only partially caused by greenhouse gas emissions.

The researchers drew their conclusions, which they presented at the annual conference of the American Geophysical Union on 24 May, after analysing 60 computer models that imitate global climate. Temperature changes in the Atlantic have already caused the increased rainfall observed over the last 5 years in the Sahel, the researchers suggest.

For much of 1950-2000, the southern Atlantic Ocean was warmer than the northern Atlantic. This, say the researchers, drew rain-bearing monsoon winds away from the Sahel, contributing to the very dry conditions there.

From the 1990s, however, the situation switched and the northern Atlantic became warmer than the southern ocean, partly because of higher levels of greenhouse gases in the atmosphere. As a result, say the authors, we are already witnessing more rainfall in the Sahel.


2. Haarsma et al, September 2005
Sahel rainfall variability and response to greenhouse warming [abstract], Geophysical Research Letters, Vol. 32, 2005

Reindert Haarsma and colleagues of the Royal Netherlands Meteorological Institute used a computer model to predict the effects of rising temperatures on rainfall over Africa. They say their study is the first to consider the roles of both land and sea surface temperatures.

The model suggests that if emissions of greenhouse gases are not reduced, higher temperatures over the Sahara would cause 1-2 millimetres of extra daily rainfall in the Sahel by 2080 during the months from July to September. This might seem small, but it is 25 to 50% more than fell in the 1980s. The increased rainfall strongly reduces the probability of prolonged droughts.

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

3. Hadley Centre for Climate Prediction and Research
The UK's Hadley Centre for Climate Prediction and Research modelled the recent evolution of global drought and made projections for the twenty-first century. It forecasts a wetter future for Central Africa, the Horn and East Africa and parts of coastal West Africa. Although higher rainfall could come equally in the destructive form of heavy inundations as well as beneficial rain, it raises the intriguing possibility of environmental refugees from a dehydrated Europe flooding into Africa by the mid-century.

4. Held et al, November 2005
Simulation of Sahel drought in the 20th and 21st centuries [abstract], Proceedings of the National Academy of Sciences

As SciDev.net notes, most models predict that the Sahel will get wetter between now and 2050, so the Held results, which are based one a single and new model, are unusual.

Lead author Isaac Held, of the US National Oceanic Atmospheric Administration, cautions that his team is not claiming to have a definitive answer.

They point out that there are uncertainties about how temperatures in the Atlantic and Indian Oceans, and in the Sahara desert, affect climate in the Sahel. Predictions of future climate should therefore be based on multiple models, they say. "Our model does a very good job on the 20th century and produces interesting results in the 21st century," says Held.

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.

Held says it is unclear why his team's model produces such a dramatically different result for the Sahel, but he thinks it might be related to how the model simulates the behaviour of clouds.


5. Chris Reij, Andrew Warren, Kjeld Rasmussen et al, Oct. 2006
Africa's deserts are in "spectacular" retreat [article to summarizing different studies], New Scientist, Oct. 2006

Different teams of scientists have observed a spectacular greening of vast parts of the Sahel and a retreat of the Southern Saharan Desert. This confirms the trend observed by most climate models that witnessed an increase in rainfall over the Sahel.

The southern Saharan desert is in retreat, making farming viable again in what were some of the most arid parts of Africa. Burkina Faso, one of the West African countries devastated by drought and advancing deserts 20 years ago, is growing so much greener than families who fled to wetter coastal regions are starting to go home.

New research confirming this remarkable environmental turnaround is to be presented to Burkina Faso's ministers and international aid agencies in November. And it is not just Burkina Faso. New Scientist has learned that a separate analysis of satellite images completed this summer reveals that dunes are retreating right across the Sahel region on the southern edge of the Sahara desert. Vegetation is ousting sand across a swathe of land stretching from Mauritania on the shores of the Atlantic to Eritrea 6000 kilometres away on the Red Sea coast.

Nor is it just a short-term trend. Analysts say the gradual greening has been happening since the mid-1980s, though has gone largely unnoticed. Only now is the evidence being pieced together.
Firewood and grassland

Aerial photographs taken in June show "quite spectacular regeneration of vegetation", in northern Burkina Faso, according to Chris Reij of the Free University, Amsterdam.

There are more trees for firewood and more grassland for livestock. And a survey among farmers shows a 70 per cent increase in yields of local cereals such as sorghum and millet in one province in recent years. The survey, which Reij is collating, was paid for by Dutch, German and American overseas aid agencies.

Meanwhile, Kjeld Rasmussen of the University of Copenhagen has been looking in detail at sand dunes in the same area. Once they seemed to be marching south. But since the 1980s, he says, there has been a "steady reduction in bare ground" with "vegetation cover, including bushes and trees, on the increase on the dunes".

A team of geographers from Britain, Sweden, Denmark has spent the summer re-examining archive satellite images taken across the Sahel. Andrew Warren of University College London told New Scientist that the unpublished analysis shows that "vegetation seems to have increased significantly" in the past 15 years, with major regrowth in southern Mauritania, northern Burkina Faso, north-western Niger, central Chad, much of Sudan and parts of Eritrea.

But there is confusion over why the Sahel is becoming green. Rasmussen believes the main reason is increased rainfall since the great droughts of the early 1970s and 1980s. But farmers have also been adopting better methods of keeping soil and water on their land.

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.


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."


Policy implications
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.