杏吧原创

GLOBAL warming could be happening much faster than climate researchers had
feared, a study warns this week. Rising temperatures could reduce the oceans鈥�
ability to absorb carbon dioxide by as much as 50 per cent, leaving the
greenhouse gas in the atmosphere to heat the Earth further.

Until now, climate models such as those used by the Intergovernmental Panel
on Climate Change (IPCC) have assumed that the oceans鈥� capacity to remove CO
₂ from the atmosphere will stay constant as the world warms. But Jorge
Sarmiento and Corinne Le Qu茅r茅 of Princeton University in New
Jersey question this assumption. Their model, which predicts climate events over
the next 350 years, suggests that conditions could be radically different.

鈥淭his really is a startling finding,鈥� says Sarmiento. 鈥淲armer oceans will be
more stratified, causing the ocean circulation system to slow down. As a result,
it will absorb much less CO₂ than at present鈥�50 per cent less in
some scenarios.鈥�

Each year the oceans dissolve up to 2 billion tonnes of CO₂ from the
atmosphere. Because of this, they help to dampen down the greenhouse effect
caused by 6 billion tonnes of carbon that enter the atmosphere from fossil
fuels. 杏吧原创s have long questioned whether rising temperatures would affect
this disposal system, but have never before modelled the effects of change.

The oceans 鈥渂ury鈥� CO₂ by removing it from the surface layers of
water. One key way of doing this is through convection currents, the biggest of
which is the 鈥渃onveyor belt鈥� that begins in the North Atlantic.

As ice forms here, saltier鈥攁nd therefore denser鈥攚ater is left
behind. This denser water falls to the ocean floor, drawing water in behind it
and setting up a current that begins a 1000-year journey around the world. When
it returns to the North Atlantic, it contains less CO₂. However, if
water in the North Atlantic warms up and the current slows or carries less
water, it will remove less CO₂.

The Princeton researchers assumed that CO₂ levels would increase by
1 per cent a year over the first 140 years鈥攃lose to the current 鈥渂usiness
as usual鈥� scenario assumed by the IPCC. This would quadruple atmospheric
concentrations of the gas. Sarmiento and Le Qu茅r茅 assumed
atmospheric concentrations would then stabilise.

The Princeton model predicts that the oceans would warm by about 5 掳C but
that circulation would slow down dramatically. The amount of water taken to the
ocean floor in the North Atlantic would shrink to less than a seventh of its
former volume.

The model predicts that over 350 years, the amount of CO₂ absorbed
by the oceans would be 49 per cent less than assumed in current climate models.
As a result of this 鈥減ositive feedback鈥� effect, global warming would increase.
The researchers publish their findings in this week鈥檚 Science (vol 274,
p 1346).

However, Sarmiento warns, much of the CO₂ in the oceans is taken to
the ocean floor not by the circulation system, but by marine organisms. Once
dissolved in surface waters, CO₂ is absorbed by plankton and other
marine organisms. Most of this carbon eventually falls to the ocean floor.

The strength of this sink for carbon depends on how much life the ocean is
producing, and is largely independent of any changes in the oceanic conveyor
belt. It is not clear, says Sarmiento, how much global warming will affect the
oceans鈥� biological productivity. His first model assumes it will remain
constant, so offsetting the ocean鈥檚 reduced capacity to capture CO₂
from 49 per cent to 28 per cent. But productivity could also rise or fall, says
Sarmiento. Cooler seas tend to produce more life, but iron dust from expanding
deserts could make warm seas more fertile.