At issue may be microscopic vegetation, such as calcareous algae (coccolithophorids and foraminifers) or miniscule mollusks, like the pteropods. These organisms construct their exoskeletons from aragonite. That element is very sensitive to ocean waters' acidification.

The increase in CO₂ emissions has a perfectly quantifiable impact on the oceans, "more finely understood than its effects on climate," specifies James Orr, a researcher at the Sciences Laboratory of Climate and the Environment. "Out of 70 CO₂ molecules that we emit, 20 are absorbed by the terrestrial biosphere, 30 remain in the atmosphere, and 20 dissolve in the oceans," details Paul Tréguer, scientific director for EUR-Océans, a European network for the study of oceanic ecosystems. That dissolution modifies the chemical balance by increasing the concentration of hydrogen ions (H+). Since the beginning of the industrial era, that concentration has increased by 25%, a modification of the same order as that of the atmosphere, ever more overburdened with CO₂.

That chemical destabilization causes the oceanic concentration of carbonates to drop. But, Mr. Orr explains, "Carbonate, along with calcium, is one of the two bricks necessary to calcareous formation." Result: The pteropods, coccolithophorids and foraminifers, shelled marine microorganisms that specifically need carbonates to form their aragonite exoskeleton will have disappeared from certain regions of the Pacific and the entire southern hemisphere's oceans as of 2030, according to studies recently published by the journal Nature. A particularly important region since, as Mr. Tréguer explains, "the oceans of the Southern hemisphere are connected to all the others."

What will the consequences be? "It's extremely difficult to predict," answers Mr. Orr. "The food chains in the oceans are complex and to succeed in modeling them is a great challenge. What is certain, however, is that the pteropods are close to the bottom of the food chain and that they constitute the nutritional resources for certain important fish, like the hake, the salmon, the cod, even, at certain periods of the year, the whale ... "

Not knowing whether other micro-organisms will benefit from this forecast disappearance, it is difficult - if not impossible - to predict future scenarios. All the more so as at the moment, the researchers are not taking the increase in CO₂ emissions into account. Now, "we know, for example, that the oceans' temperature will evolve, as well as nutritional salt content," explains Jean-Pierre Gattuso, a researcher at the Laboratoire océanographique de Villefranche-sur-Mer (Maritime Alps). "In our experiments, we do not master the interactions between these different parameters that will also evolve under the impact of global changes."

Nonetheless, one thing is certain. Small marine organisms can be extremely sensitive to modifications in their environment, as was demonstrated by the work of Grégory Beaugrand, from the Seashore and Coastal Ecosystems laboratory. The present migration of some plankton north is directly linked to prevailing variations of less than a half degree Celsius in the temperature of surface waters. And the movement of this manna that fish go in for partially explains the drop in stocks of certain fish that people eat, especially the cod. The waters' acidification could accentuate these movements already at work and have a significant economic impact on fishing.

Apart from its impact on the food chain, this process will have other dramatic consequences over a longer term. "Two thirds of deep water corals, present in cold waters, notably along European coasts, are threatened with disappearance before 2100," says Mr. Orr. "These corals play an important role, by, for example, supplying habitat to certain fish. But they've only been studied for a decade or so and remain poorly understood." If industrial activity continues to produce greenhouse gases during the coming decades, all calcareous organisms - planktons, sea shells, coral - at all latitudes are potentially threatened.

Surprisingly, the first studies on these subjects are extremely recent. "We've only known since 1998 that the biology of certain organisms responds negatively to this acidification of the oceans," Mr. Gattuso indicates. "We long thought that the ocean's "buffer power" was such that its pH would not change significantly."

Research on oceanic acidification produces more questions than answers today. In the United States, it is still virtually non-existent. For once, Europe has reacted more quickly. It will inscribe oceanic acidification as a separate research subject in its 7th Program Framework for Research and Development.

"Hydrogen potentia" indicates the concentration of hydrogen ions (H+) in a solution. An environment is said to be neutral if its pH equals 7; acidic when it is less; alkaline when it is more.

The average of ocean surface waters' (the top 100 meters) pH has fallen from 8.2 to 8.1. At current rates, it should fall to 7.9 in 2100.

In 2006

Over 25 million tons of carbonic gases combine daily with ocean water.

Agenda

EUR-océans scientists have launched an information campaign concerning ocean acidification. A documentary will be distributed to several aquarium-museums.

On the Internet

Site of the global forum on oceans, coasts, and islands, which, in partnership with UNESCO, is drawing up a panorama of the state of marine ecosystems.