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Are the starving refugees wandering the plains of Africa this month victims of the greehouse effect? Is the drought in the Sahel, which has returned with a vengeance in the past year, a side effect of global warming? Is the desiccation of half a continent a temporary or permanent phenomenon?

Climatologists are beginning the 1990s as they spent the 1980s, looking at two dispiriting graphs. One shows a sequence of rising average world temperatures. This trend began in the early 1970s, continued through the 1980s, which saw five of the six warmest years ever recorded, and culminated in a new world record in 1990. The second graph shows rainfaill in the Sahel, the drought-prone region of Africa on the southern edge of the Sahara. In every year since 1967, rainfall has been less than the long-term average.

Within the drought era there have been bad years and very bad years. The devastating droughts of 1972 and 1973 were exceeded by those of 1983, 1984 and 1987. A return to near-normal rainfull in 1988, and, in some places, 1989 prompted sighs of relief. But the relief turned to tears when the figures for 1990 were released, revealing what the farmers of the region had known for months: 1990 was as dry as 1972 and 1973, and worse even than 1984 in parts of Ethiopia and Sudan.

Equally alarming for those seeking a cause for the failure of the rains is that the years when the Sahel is exceptionally dry coincide with the years in which there are record global temperatures. It happened in 1983, again in 1987 and now, once more, in 1990. The coincidence is not exact – for example, 1988 was relatively wet in the Sahel and was also the hottest year ever.

Nonetheless, the trends are ominous. As the scientific consensus grows that the current global warming is probably a consequence of the greenhouse effect, the suspicion that the Sahel drought has a similar cause gains ground. Jean Palutik from the Climatic Research Unit at the University of East Anglia says: ‘As the drought persists from year to year, more and more climatologists are coming round to the view that this is true.’

The matter urgently needs resolution, not least because African governments are unsure whether to assume that wetter times will come or to plan for permanent drought. But the issue is of global importance, too. It may be that global warming will reveal itself not in gradual temperature rises but in huge and abrupt changes in climate in particular parts of the world.

Rainfall data for the Sahel region are always hard to assess. Rain gauges are few and far between. The graphs showing rainfall trends rely on 30 or so gauges, some of which may be unreliable. And the rain itself is patchy. Last August, as northern Ethiopia fried, there were intense storms in parts of southwestern Ethiopia and in Suadan, which flooded 400,000 people.

Nonetheless, data collected by the US government’s Climate Analysis Center show a clear pattern in 1990 for the Sahel ‘wet season’ (May to September). Across the west of the region – in Mauritania, Senegal, Burkina Faso and Mali – rainfall was three-quarters of normal, and it was less than half the usual in eastern Sudan and northern Ethiopia, a zone that also experienced severe drought in 1989. In Eritrea, the war-torn northeast of Ethiopia, rainfall was about 40 per cent of normal. Temperatures in the eastern Sahel were unusually hot often 4 °C above the normal of around 40 °C.

The Climate Analysis Center reported in its weekly bulletin at the end of October that ‘a lack of significant rain across most of central Chad, Sudan and northern Ethiopia, along with unseasonably hot weather, has caused widespread and severe agricultural damage, especially in east-central Sudan’. That was the moment when aid agencies first sounded the famine alarm.

Rainfall in the Sahel occurs almost entirely between July and September, and is associated with the passage of the Inter-tropical Convergence Zone. This is an almost continuous belt of unsettled rainy weather that girds the tropics. It is sandwiched between fine weather to the north and south. Along the belt strong upward movements of air occur, which cause clouds and rainfall wherever there is moisture in the air. The zone moves north and south with the seasons. Where the belt passes, millions of people depend on its rain for their survival. Beyond its reach lies the Sahara.

During the 1970s and early 1980s, it was widely believed that the Sahel drought was manmade: a result of the loss of moisture in the atmosphere because of the destruction of vegetation through overgrazing and the destruction of coastal rainforests.

Since the mid-1980s, opinion has swung towards the view that changes in ocean temperatures, perhaps caused by global warming, may be the key. It seems that short-term changes in the seas govern whether one year will be drier than the next. This discovery is the basis for experimental forecasts of summer rainfall in the Sahel made each April since 1986 by Chris Folland and others at the Hadley Centre for Climate Prediction and Research, part of Britain’s Meteorological Office.

The research has taken two paths. Folland and his team have plugged the summer temperatures of the surface of the sea into the Met Office’s General Circulation Model (GCM) of the atmosphere, one of the world’s leading climate models. So primed, the model generates figures for annual rainfall in the Sahel that come close to those observed on the ground.

A second, even more successful, approach has been to look for correlations between Sahel rainfall and anomalies in sea-surface temperatures. Intuitively, one might expect that sea temperatures off the coast of West Africa might influence rainfall in the adjacent western Sahel. But Folland reported in 1986 that ‘persistent wet and dry epochs in the Sahel may be related to sea-surface temperature anomalies on a near-global scale’. Indeed, global sea temperatures were a better yardstick of Sahel rainfall than local ones.

Folland found a startling correlation between Sahel rainfall and the difference between average ocean temperatures in the northern and southern hemispheres. Tracing records back to the beginning of the century, he noticed that whenever the southern hemisphere’s oceans are warmer relative to those in the north, the Sahel has a drought.

The statistical finding was later reproduced in the GCM. Folland found that inside the model, warm southern seas reduced the strength of convergence of air over the Intertropical Convergence Zone in Africa. Less converging air over the Sahel in summer meant less upward air motion, fewer clouds and less rain.

Folland and his colleages, in a paper published in the Journal of Forecasting last month, comment that ‘the present balance of opinion is shifting steadily from a purely local or regional control on Sahel rainfall to that of very large-scale processes, involving very large-scale sea-surface temperature and atmospheric circulation changes’.

It is unclear if warm southern seas are directly linked to global warming, but as the world warmed during the early and mid-1980s, it was the southern oceans that warmed first and fastest. So the search for a way to forecast Sahel rainfall may have stumbled upon one of the most important levers for climate change in a greenhouse world.

Up to now, the Met Office has issued experimental forecasts only for the Sahel as a whole. This misses the regional variations. Last year’s forecast of a slightly drier summer gave Ethopia little warning that it faced an exceptionally dry summer.

In the new research, Folland’s team divides the Sahel into 27 subregions. The first, unpublished, subregional forecasts in 1990 successfully predicted that the west Sahel would be wetter than the east.

The Met Office sends the team’s annual prediction of summer rainfall to African countries in April, so it has to be based on spring sea temperatures. Unfortunately, it is temperatures in June and July that determine rainfall. ‘In order to get it right, we need to be able to forecast sea-surface temperatures two months ahead,’ says Neil Ward, one of Folland’s researchers.

The year they got it badly wrong was 1988. That year, there was a sudden change in the Pacific: between April and June, the eastern tropical waters went from being 2 °C warmer than normal to between 2 and 3 °C below normal.

That year saw the highest rainfall in the Sahel for two decades. There were floods in Khartoum and dams burst in Nigeria. Yet the Met Office team had forecast an exceptionally dry year, with rainfall at 60 per cent of normal. ‘If it weren’t for 1988 a lot more people would be using our forecast, and we might have been able to take the word ‘experimental’ off the title,’ says Ward.

‘We are still as accurate with our three-month Sahel forecast as we are with our local five-day forecasts for Britain,’ he points out. But he knows that in the Sahel rain forecasts are used for life-and-death decisions on famine relief rather than for deciding whether to take the brolly to work.

Abrupt changes in sea-surface temperatures, such as those that messed up the 1988 forecast, are usually connected to El Nino events in the Pacific Ocean. El Nino is an occasional warm tropical ocean current that starts around the East Indies and surges in a few dramatic weeks all the way to the South American coast, before dissipating. Along the way, it turns off the westerly trade winds, bringing torrential rain to the deserts of Chile and Ecuador and drought to Australia. El Nino also has an opposite, known as La Nina, a current of cold water which heads back westerly, along the equator.

Each event happens roughly every three to four years, but they vary greatly in intensity and sometimes fail to happen at all. Thus, a major El Nino brought climatic havoc across the Pacific in 1983, and another strong El Nino ran in 1987. An El Nino that some scientists were predicting for the last months of 1990 failed to show up.

The importance of La Nina had almost been forgotten since the last notable occurence in 1975, until a strong cold current surged across the Pacific in the summer of 1988, comprehensively upsetting the Met Office’s drought calculations.

El Nino years, says Ward, tend to be years with high global temperatures, and are often associated with other extreme climatic events. A key task for climatologists in the 1990s is to decide whether these tentative connections are correct, and if so, find out how they work.

One important question is how events in Africa, India and the Pacific are connected. The El Nino of 1987, for instance, saw extreme drought in the Sahel and a very weak monsoon in India. In a mirror image the following year, a strong El Nina ran across the Pacific and there were floods in Khartoum and in Bangladesh.

But 1990 was different. There was drought in much of the Sahel, especially the parts close to the Indian Ocean, but there was no El Nino. In parts of India, record rains brought flooding to the Thar desert on the border with Pakistan.

Ward believes the precise timing of an El Nino or La Nina may be crucial to its influence on global events. An El Nino mostly peaks in mid-winter and may have little influence on summer events. The exceptions were the abnormally late El Nino of 1983 and the La Nina of 1988 which brought, respectively, exceptionally dry and wet conditions to Africa.

‘What we have to look for,’ says Ward, ‘is a relationship between the prevailing global pattern of sea-surface temperatures and what happens when an El Nino is superimposed.’

That done, Folland’s team might be able to predict sea temperatures a couple of months ahead and so remove the major uncertainty for its drought forecasts. The next step would be to tie models of changes in sea-surface temperatures to simulations of the responses of the global climate system to the greenhouse effect. Then we may see whether changes in global sea-surface temperatures, the apparent cause of more than two decades of Sahelian droughts, are indeed the consequence of global warming.

The pieces in the global puzzle of climate are slowly being assembled. But for Africa, the immediate problem is not scientists’ inability to forecast events reliably. It is the world’s failure to act. The evidence that drought would create famine in the eastern Sahel has been plain for more than three months. Until the world can learn to act on such certain knowledge, there is little prospect of it responding to a weather forecast, however reliable.