Trends can be difficult to detect in real-world data, and the noisier the data, the tougher the task becomes. A longer time series can help limit the impact of noise, but these can be difficult to come by. Verifying the human alteration of ocean chemistry requires tackling challenges like these.

Ocean acidification entails a decrease in the pH of ocean water as the carbonate that buffers it is consumed. That carbonate does more than just maintain pH, though. Lots of marine organisms, from plankton to mollusks to coral, use it to build shells and skeletons. As the buffer is depleted, the saturation state of carbonate minerals like calcite (and its polymorph aragonite) decreases, making it more difficult for organisms to incorporate them. In most areas of the surface ocean, calcite and aragonite are supersaturated, making it easy for organisms to build shells and skeletons. In undersaturated water, the equilibrium tilts the other way, and dissolution of these structures becomes possible.

Calcite and aragonite saturation states vary regionally and seasonally, so how can we make sure the acidification trend we’re measuring is real and human-caused? One way to look into this question is to take the measurements we have and model the whole ocean to see what natural variation would have looked like before humans started emitting CO 2 . A recent study in Nature Climate Change does just that.

The researchers ran a climate model from 800 A.D. to 2100 A.D. using the best data available for the various forcings: solar activity, volcanic activity, changes in land use, and anthropogenic emissions of greenhouse gases and reflective aerosols. They project the rest of this century using the IPCC A1B emissions scenario, a "middle-of-the-road" emissions estimate. To track acidification, they use the saturation state of aragonite in surface waters.

The model shows large variability between regions. For example, fluctuations in upwelling that occur near the Galápagos Islands cause large swings in the aragonite saturation state. In the Caribbean, on the other hand, it holds quite steady.

In all areas where coral reefs are found (these are often described as the "rainforests of the sea" for their astonishing diversity and abundance of life), the researchers find that the current saturation state of aragonite is well below the pre-industrial average. To put it into concrete terms, they estimate that calcification rates of reef organisms have already dropped by about 15 percent. Under the A1B emissions scenario, calcification rates would decrease by a total of 40 percent (relative to pre-industrial) by 2100.

Comparing this to the magnitude of natural variability in preindustrial oceans, the model indicates that we are already considerably outside that envelope (as the authors describe it, there’s a high signal-to-noise ratio). On average, aragonite saturation states at reefs in the Caribbean and western Pacific have dropped by 5 times the range of natural variability. In areas where that range is small, such as Melanesia, the drop is as high as 30 times the natural envelope. With a few small exceptions, the signal-to-noise ratio is already at least 2:1 across all of Earth’s oceans, even near the Galápagos Islands where natural variability is high.

The model also indicates that the Southern Ocean will be undersaturated with aragonite by 2030. The nutrients that come up from the deep ocean make this region incredibly fertile, supporting massive fisheries and attendant populations of birds and marine mammals. The plankton at the base of that food web require calcium carbonate to build their shells. While the Southern Ocean is the most sensitive region, it's not the only one with problems. The authors estimate that 30-50 percent of ocean water above 40° latitude becomes undersaturated in the model by 2100.

For another comparison, the group simulated the end of the last glaciation, which was the last time Earth saw a sizeable increase in CO 2 . Over 6,000 years, atmospheric CO 2 rose from about 190 ppm to around 280 ppm. The authors write that the model shows “[t]he observed present-day anthropogenic rate of change in [surface aragonite saturation state] is one or two orders of magnitude larger than estimated for the last glacial termination.”

The researchers emphasize that other factors—such as changes in light penetration, temperature, and nutrients—will be affecting marine ecosystems at the same time. (And acidification can affect more than just the calcareous critters.) The authors write, “These stress factors probably do not simply add up, but combine in a species-dependent manner. Tropical surface temperatures are projected to increase at a rate that would lead to massive coral bleaching and mortality in the next three to five decades. Combined with a detectable change due to reduced ocean aragonite saturation and the corresponding estimated drop in carbonate accretion of ~15 percent since the industrial revolution, severe reductions are likely to occur in coral reef diversity, structural complexity, and resilience by the middle of this century.”

Nature Climate Change, 2012. DOI: 10.1038/NCLIMATE1372 (About DOIs).