Carbon dioxide produced by human activity has caused an unprecedented increase in the acidity of ocean water, according to new calculations by International Pacific Research Center scientists. And that spells trouble for the marine organisms that create coral reefs…and the communities who depend on them.

Combining computer modeling with observations, the international team of climate modelers, marine conservationists, ocean chemists, biologists and ecologists led by Tobias Friedrich and Axel Timmermann at the University of Hawaiʻi at Mānoa concluded that ocean acidity has increased far beyond the range of natural variations due to CO2 emissions over the last 100–200 years.

Their study appeared online in the January 22 issue of the journal Nature Climate Change.

Chemistry set: acidification limits calcification

Scientists typically measure the concentration of aragonite in ocean water to gauge the water’s acidity.

Tobias Friedrich

Aragonite is a form of calcium carbonate. Marine organisms, such as corals, use it to build skeletal structures through a process called calcification.

CO2 is absorbed by the ocean, where it reacts with the seawater, increasing the water’s acidity. As acidity of seawater rises, the saturation level of aragonite drops.

Lower aragonite levels may significantly reduce the calcification rate of marine organisms, resulting in the potential loss of ecosystem.

“Any significant drop below the minimum level of aragonite to which the organisms have been exposed for thousands of years and have successfully adapted will very likely stress them and their associated ecosystems,” said lead author Friedrich, an IPRC postdoctoral fellow.

A ticking clock: organisms will have little time to adapt

Friedrich’s team used Earth system models that simulate climate and ocean conditions 21,000 years back in time, to the Last Glacial Maximum when much of Earth was covered in ice, and forward to the end of the 21st century. The models incorporated current observed seasonal and annual variations in several key coral reef regions.

Aragonite saturation in these locations have already dropped five times below the pre-industrial range of natural variability. Another recent study indicates that decrease could translate into a decrease in overall calcification rates of corals and other aragonite shell–forming organisms by 15 percent.

Given continued use of fossil fuels, the model predicts that saturation levels will drop further, potentially reducing calcification rates of some marine organisms by more than 40 percent of their pre-industrial values within the next 90 years.

Current fluctuations not only far exceed natural variability (as much as 30 times on regional scales), but they are happening far more quickly, the researchers stress.

“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,” emphasized Friedrich. The Earth warmed slowly after the last ice age, allowing marine ecosystems ample time to adjust as the level of CO2 rose over 6,000 years. Now, they face a similar increase in CO2 concentration in just 100–200 years.

The map: impact will vary, depending on location

Coral reefs are generally found in places where open-ocean aragonite saturation reaches levels of 3.5 or higher. Such conditions exist today in about half of the ocean, mostly in the tropics. By end of the 21st century this fraction is projected to drop below 5 percent.

Axel Timmermann

The Hawaiian Islands, which sit just on the northern edge of the tropics, will be one of the first to feel the impact.

The study suggests that some regions, such as the eastern tropical Pacific, will be less stressed than others because greater underlying natural variability of seawater acidity helps to buffer man-made changes. However, aragonite saturation in the Caribbean and the western Equatorial Pacific, both biodiversity hotspots, have very little natural variability, making these regions particularly vulnerable to human-induced ocean acidification.

“Our results suggest that severe reductions are likely to occur in coral reef diversity, structural complexity and resilience by the middle of this century,” said co-author Timmermann, a professor of oceanography.

More about the study

Read the abstract.

The study was funded by The Nature Conservancy, IPRC sponsor Japan Agency for Marine-Earth Science and Technology and the National Science Foundation.

IPRC is a climate research center founded within the UH Mānoa School of Ocean and Earth Science and Technology as a collaborative effort between agencies in Japan and the United States.