HONOLULU — A new study from the University of Hawai‘i found that 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, the
HONOLULU — A new study from the University of Hawai‘i found that 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, the university reported Tuesday.
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 findings were reported by an international team of climate modelers, marine conservationists, ocean chemists, biologists and ecologists — led by Tobias Friedrich and Axel Timmermann at the University of Hawai‘i at Manoa — the university stated in a news release.
Their study appeared online in the Jan. 22 issue of the journal Nature Climate Change.
Scientists typically measure the concentration of aragonite in ocean water to gauge the water’s acidity. 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 seawater, increasing the water’s acidity. As the acidity of seawater rises, the saturation level of aragonite drops.
Lower aragonite levels might significantly reduce the calcification rate of marine organisms, resulting in the potential loss of ecosystems.
“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,” Friedrich said in the release.
A ticking clock
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 said.
“In some regions, the man-made rate of change in ocean acidity since the Industrial Revolution is 100 times greater than the natural rate of change between the Last Glacial Maximum and pre-industrial times,” Friedrich said.
“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,” Friedrich said.
Impact will vary
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 the end of the 21st century, this fraction is projected to drop below 5 percent.
The Hawaiian Islands, which sit just on the northern edge of the tropics, will be among the first to feel the impact, the scientists said.
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 hot spots, 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,” Timmermann said.
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 partner of the UH Manoa School of Ocean and Earth Science and Technology as a collaboration between agencies in Japan and the U.S.
