Carbon dioxide is a greenhouse gas, which means that it traps heat in Earth's atmosphere and causes harmful global warming. A large amount of atmospheric carbon dioxide is absorbed by the oceans. This was once seen as entirely beneficial, and there have even been ideas of compressing carbon dioxide from power stations and pumping it down to the deep ocean floors. There, at the near-freezing temperatures and high pressures of that realm, the compressed carbon dioxide would stay as a dense liquid for a long time, hugging the ocean floor and only slowly being dissolved into the waters above. Only a few obscure deep-sea organisms would suffer. it was said, while the rest of us would benefit.

Beginning in the late 1970s, oceanographers began raising concerns over possible unintended consequences of such an experiment. The chemistry was fairly straightforward. The carbon dioxide dissolved into the oceans will combine with water to form carbonic acid, which releases hydrogen ions (atoms or molecules) into the water. This causes the acidity to rise, changing the balance o the carbonate and bicarbonate ions already dissolved in the water, shifting the balance away from carbonate towards bicarbonate. It is the carbonate in the ocean waters that is the feedstock for those organisms-mollusks, corals, foraminifera, sea butterflies-that build skeletons of calcium carbonate. The less dissolved carbonate there is, the more difficult it is for these organisms to build their skeletons. But how big is this problem?

Let us take a couple of examples. Coral reefs are being observed particularly closely because of their enormous ecological importance. They are, like the tropical rain forests on land, storehouses of much of the oceans' biodiversity, some 25 percent of all marine species being crammed into about a tenth of one percent of ocean area. They are remarkable, counterintuitive biological systems. Their diversity is predicated upon the elaborate architectural framework built by the calcium carbonate skeletons of the coral colonies, and it needs low-nutrient and not high-nutrient settings (because the coral systems are so efficient at recycling the few nutrients there are). Coral reefs are therefore sensitive to nutrient levels, and have suffered as human-made phosphates and nitrates have washed into them from the land; seaweeds then grow at the expense of the corals. Corals are also sensitive to temperature: if it gets too hot, the coral animals expel from their tissues the din flagellates (a type of single-celled organism) that they need to help them grow. The corals then lose their vivid colors: they "bleach" and usually die. In recent years there have been major bleaching events, killing off large areas of coral during tropical heat waves. Corals are also sensitive to disturbance, both physical (dynamiting, building, dredging) and biological (human hunting of ecologically important reef fish). In short, coral reef systems worldwide today are struggling (some are already dead), and the growing acidification threatens to be fatal.

The latest studies suggest that at atmospheric carbon dioxide levels of somewhere near 550 ppm (parts-per-millin) the coral animals will not be able to maintain a positive balance of calcium carbonate formation. From around that point (and it will vary between different species and in different settings), the coral reefs will stop growing and start to shrink back. At current rates of carbon emissions, this particular tipping point for reefs will occur around the middle part of this century. Earth, then, would go through another ofthe events that geologists term a "reef gap", when these magnificent, diverse structures disappear from the world. The last reef gap was 55 million years ago-and was also associated with an ancient global warming and marine acidification event, termed the Paleocene-Eocene Thermal Maximum. It took millions of years for the reef systems to recover. Different organisms have different tolerances for acidity. Some, like pteropods, already show signs of decline, just as a result of the small acidity increase so far. Pteropods are tiny animals that secrete delicate shells of calcium carbonate, which fall onto the sea floor in such amounts that they build up in layers. These layers look to be in danger of vanishing this century.