The four-year droughts that scorched harvests in Afghanistan, seared the Mediterranean scrub and baked cornfields in the American south-east may have had a common cause, according to US researchers.
They may have been triggered by unusual behaviour far away in the tropical Pacific. A touch of cooler water east of the international dateline, and a slight warming to the west and in the Indian ocean, could have launched a cascade of climate disturbance that ended in blue skies and arid soils in the northern hemisphere from 1998 to 2002.
With the spectre of global warming and a steady increase in the number and economic cost of droughts, floods, ice storms, hurricanes and tornadoes in the past decade, researchers have been racing to make sense of world weather patterns.
Martin Hoerling and Arun Kumar of the US national oceanic and atmospheric administration report today in the journal Science that they think they have found the perfect circumstances to precipitate a drought that could girdle the globe.
Scientists believe that there is a real chance that the greenhouse effect — a warmer world because of increasing carbon dioxide levels — could make such droughts commonplace.
Most researchers piece together a picture of climate patterns by matching data from the past against a computer model of how they think sun, sea and wind drive the general circulation of the planet’s weather.
But Dr Hoerling and his colleague studied climate behaviour as it happened, and then tested it against three different models in three laboratories, for a total of 51 runs.
Each time, they fed in the actual sea surface temperatures between 1998 and 2002. For a century, meteorologists had argued about the conditions for the ”perfect” storm.
These two were looking for the ”perfect” drought.
Their 51 simulations gave strikingly similar answers: they predicted less rain in the US, southern Europe and south-west Asia.
In fact, during those few years, precipitation declined by an average of 50%, mostly due to a failure of the spring and winter rains.
”The modelling results offer compelling evidence that the widespread mid-latitude drought was strongly determined by the tropical oceans,” they report.
”It is thus more than figurative, although not definitive, to claim this ocean was ‘perfect’ for drought.”
There is now no doubt that tropical ocean temperature changes can have dramatic effects. Britain and Ireland have mild climates because a huge Atlantic current from near the Gulf of Mexico delivers the equivalent of 27 000 times the warmth from all UK power stations.
A periodical warming of the eastern Pacific, and a cooling to the west, creates the conditions for the notorious El Nino.
This phenomenon, usually first spotted by Peruvian fishermen at Christmas — which is why it is nicknamed ”the child” — is a signal for a failure of the tropical monsoons in India, forest fires in Indonesia and catastrophic flooding in normally dry regions such as the American west.
The last twist in climate research raises worrying questions about drought in a warmer world.
More than one-billion people currently have no access to clean water; two-billion are without sanitation. Within 25 years, about three billion people could be struggling with permanent water shortages.
Rising global average temperatures of up to 5,8 C in the most alarming scenarios are expected to accentuate the problems in the more arid zones and to dump heavier rains in other places.
It would become ever more important to predict drought. The new research suggests a way to see cloudless skies a season ahead.
But questions remain. ”It sounds plausible,” said Brian Hoskins, of the University of Reading, yesterday. ”But it remains to be seen. One general circulation model doesn’t make a summer. Certainly there is enough to be interesting here.”
The challenge is to work out quite how a slight change in sea temperate in two regions could have such a precise effect to the north along a latitude that circles the globe.
One group has calculated that a warmer western Pacific pumped up towering rain clouds which released extra heat into the atmosphere. This heat altered the winds of the jet stream that then steered away the winds that should have carried winter rains to Iraq, Iran, Pakistan and the central Asian states to the north. That might explain dry summers, or even dry winters, but not a weather pattern for a whole year.
”They seem to be suggesting a year round, long-term influence on a number of places in the northern hemisphere,” said Prof Hoskins. ”Certainly we do have to know what the tropical ocean atmosphere is liable to do, and get much more competence at that. But at the moment, God knows. We do need to know because something like these sorts of things we can expect.”
The big picture
– Ocean temperatures affect dinner servings. Researchers in the Pacific this month identified a 50-year cycle in which nets filled first with anchovies, then sardines, and now anchovies again
– El Nino, a warming of the eastern Pacific, has been linked to outbreaks of dengue, malaria, cholera and a 200% increase in gastrointestinal diseases in Peru
– The late arrival of the Indian monsoon in 2002 has been linked to the unusual wet weather in the south of France that same summer
– If the Gulf stream were turned off, Britain would have to endure winter temperatures to match those of Nova Scotia. Scientists warned in 2001 that it may happen
– In the past 30 years, Arctic ocean ice has shrunk by an area the size of the Netherlands every year. It has also thinned by 30%. – Guardian Unlimited Â