Floating lab tracks Sahara sandstorms' effect on ecosystem

*Floating lab tracks Sahara sandstorms' effect on ecosystem*

UK oceanographers are trying to find out how dust and oceans interact

*James Randerson in Cape Verde Islands
Monday February 13, 2006
*http://www.guardian.co.uk/science/story/0,,1708408,00.html


Eric Achterberg calmly retrieves the two large plastic bottles that have
just fallen off the table and careered towards him across the lab. He
staggers back to his seat, avoiding a chair that is now sliding in the
other direction.

"Where were we?" he says. The risk of injury from objects flung around
by the ocean swell makes it difficult to concentrate on his answers.

Dr Achterberg and his team from the National Oceanography Centre (NOC)
in Southampton have given the Guardian exclusive access to a leading
research project. They are trying to plug a hole in the understanding of
climate change - how dust in the atmosphere affects the climate and oceans.

"There's a complete lack of data," says Dr Achterberg. What they find
out will help scientists predict global warming patterns more
accurately. The research vessel Poseidon, which is on loan from Kiel
University in Germany, and the nine scientists on board, are
dust-hunting in the eastern Atlantic Ocean between the coast of Senegal
and the Cape Verde Islands. This is where gigantic dust clouds from the
Sahara embark on a 5,000-mile journey across the Atlantic to South
America. On the way they act as atmospheric sunscreen and fertilise the
ocean.

Both effects should mitigate climate change, by bouncing heat back into
space and by stimulating the growth of algae which, when they die, carry
carbon to the sea floor. Charles Darwin commented on the quantities of
dust he encountered. "The falling of impalpably fine dust," he wrote in
The Voyage of the Beagle, "was found to have slightly injured the
astronomical instruments".

The dust is also responsible for the spectacular Cape Verde sunsets.
What is special about the British expedition is that it will look at the
sunscreen and fertiliser effects simultaneously, something that has
never been done before. The £600,000 project, which is funded by the
Natural Environment Research Council, a government research body,
involves coordinating samples taken at sea with measurements done from a
plane.

Science at sea is notoriously difficult. Dr Achterberg admits that
little work was done in the first two days of the cruise. "I was only
out of bed for two hours on the first day," said Polly Hill, a research
student at NOC. The German captain uses oompah music to keep him awake
on his late shift on the bridge and the chef serves sausage at
breakfast, lunch and dinner.

*Problems*

There are many practical problems. To work out the effect of the dust on
the plankton, the scientists need precise measurements of elements such
as iron and aluminium in the air and water. A change of wind that brings
the smoke from the ship's funnel into the air filter can wipe out an
afternoon's results.

Water samples, too, must be timed to avoid the daily discharge of the
ship's sewerage system. The dust clouds we are following are essential
to the food chain in oceans. Even at the surface, where light is
plentiful, around a third of the ocean is a virtual desert. Plankton
cannot operate because they lack essential nutrients. The rust-coloured
Saharan dust provides these and so kick-starts the ecosystem. "For many
parts of the ocean, dust is the main nutrient source," said Dr Achterberg.

So will global warming increase or decrease the quantity of dust in the
atmosphere? At the moment the Sahara desert is growing, and so more of
the ochre red fertiliser is being pumped into the African sky, but this
may not continue.

"The world heating up means more moisture in the atmosphere, so it
doesn't necessarily mean the Sahara getting bigger and drier," says Phil
Williamson, a biological oceanographer at the University of East Anglia
who is also involved in the project.

Which way it will go is still being argued over by climate modellers,
but the science of what dust does to the ocean is still in its infancy.
"The big question is to try to quantify the effects of dust on the whole
system," says Micha Rijkenberg, who studies ocean chemistry at NOC and
is also on the cruise.

Hurricane researchers will also eye the data from Poseidon with
interest. It is this region where hurricanes like the one that hit New
Orleans form. By affecting the sea surface temperature, dust may have a
hand in this too.

Dust storms are common over the driest regions of the Earth. The bigger
particles of soil or sand whipped up into the atmosphere by wind
eventually return to Earth but the tiniest particles stay airborne for
much longer, and can be swept thousands of kilometres downwind. Dust
storms in the Sahara desert regularly end up on the other side of the
Atlantic Ocean.

These clouds of dust have all sorts of influences on weather and
climate. They block sunlight, thereby cooling the Earth's surface, but
they also absorb the sun's heat, causing the atmosphere to warm up.
According to researchers at Nasa, temperatures under a dust cloud are
typically 1C cooler than normal, similar to the effect of a rain cloud.

In summer, the deserts surrounding the Arabian Sea are the main source
of dust clouds in the northern hemisphere. The Indian monsoon winds
carry these clouds towards Asia and North Africa. The clouds heading
west are augmented by dust from the Sahara. In the southern hemisphere,
most of the dust starts from the Australian outback. Scientists estimate
that half of the dust in today's atmosphere might be the result of human
activity.

Modelling the movement of these dust storms can help to predict extreme
weather. Hurricanes from the Atlantic slam into Florida every year.
These storms form off the west coast of Africa but scientists have found
in recent years that the hurricane risk is reduced if nascent storms run
into dusty air from the Sahara. If meteorologists could predict better
how and when a storm will hit a dust cloud, it would give more warning
of potential disasters.


*Interactive guides*
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