http://www.guardian.co.uk/climatechange/story/0,12374,886120,00.html
Scientists
discover the harbinger of drought
Subtle temperature changes in tropical seas may
trigger northern hemisphere's long, dry spells
Tim Radford, science
editor
Friday January 31, 2003
The Guardian
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%.
