The acidity of the ocean has increased far beyond the range of natural variations due to carbon dioxide emissions from human activity, a team led by Hawaii researchers has found.

That’s bad news for corals and other shell-forming organisms, according to their report, published Sunday in the online issue of Nature Climate Change.

The Hawaiian Islands, on the northern edge of the tropics, will be one of the first places to feel the effects this century, the researchers predict.

Using computer models and new data, the scientists were able to compare ocean acidity today to levels when glaciers were at their maximum extent 21,000 years ago during the last ice age.

"In some regions the man-made rate of change in ocean acidity since the Industrial Revolution is a hundred times greater than the natural rate of change between the last glacial maximum and pre-industrial times," said lead researcher Tobias Friedrich, a postdoctoral fellow at the International Pacific Research Center at the University of Hawaii. "When Earth started to warm 17,000 years ago, terminating the last glacial period, atmospheric CO2 levels rose from 190 parts per million to 280 ppm over 6,000 years. Marine ecosystems had ample time to adjust. Now, for a similar rise in CO2 concentration to the present level of 392 ppm, the adjustment time is reduced to only 100 to 200 years."

About a third of carbon dioxide emissions from cars, factories and other human activities enters the ocean.

When it reacts with sea water, carbon dioxide increases the water’s acidity, which can reduce the calcification rate of marine organisms such as corals and mollusks, Friedrich and the other researchers report.

In their models, they studied a type of calcium carbonate called aragonite, which forms naturally in mollusk shells and the endoskeletons of corals.

As ocean acidity rises, the saturation level of aragonite drops.

But the saturation level also varies from year to year and season to season.

The computer models captured the current observed seasonal and annual variations in this quantity in several key coral reef regions, including Hawaii, the Caribbean, Bermuda and the Canary Islands.

Today’s levels of aragonite saturation in these locations have already dropped five times below the pre-industrial range of natural variability, they calculated. As more greenhouse gases enter the ocean, the problem will only get worse, they warn.

"Our results suggest that severe reductions are likely to occur in coral reef diversity, structural complexity and resilience by the middle of this century," UH-Manoa scientist Axel Timmermann, another research team member, said in a statement Monday.

» Ocean acidification video, 1800 through 2100: iprc.soest.hawaii.edu/users/tobiasf/Outreach/OA/Ocean_Acidification.html