The study, undertaken by an international team of scientists, was led by Tobias Friedrich and Axel Timmermann at the International Pacific Research Center, University of Hawaii at Manoa. Using computer modeling and observations, researchers concluded that carbon dioxide emissions have already raised ocean acidity far beyond the range of natural variations. Meaning, that oceans have acidified more in the last 200 years, than they did in the previous 21,000 years.
Scientists used their computer models to assess changes in the saturation level of aragonite. Aragonite is a naturally occurring crystal form of calcium carbonate, used to measure ocean acidification. Typically, when acidity levels in the ocean rise, argonite saturation levels drop, meaning bad news for corals and other sea life. Many marine organisms, including corals, rely on and build skeletons from calcium carbonate – in the form of calcite or aragonite.
"Coral reefs are currently found only in places where open-ocean aragonite saturation reaches levels of 3.5 or higher, today in about 50% of the ocean – mostly in the tropics," said the study. By end of the 21st century, this fraction is projected to be less than 5% with the Hawaiian Islands, which sit just on the northern edge of the tropics, being one of the first to feel the impact.
Using Earth system models that simulated climate and ocean conditions 21,000 years ago in the Last Glacial Maximum (a period in earth's history when glaciers were at their thickest and sea levels at their lowest), scientists took them forward to the end of the 21st century.
"When Earth started to warm 17,000 years ago, terminating the last glacial period, atmospheric CO2 levels rose from 190 parts per million (ppm) to 280 ppm over 6,000 years," said Friedrich
, meaning that marine ecosystems had ample time to adjust." Now, he adds, "calculating for CO2 concentration to the present level of 392 ppm, the adjustment time is reduced to only 100 – 200 years."
Scientists blame human-caused carbon dioxide emissions from fossil fuel consumption. Their continued use they say, will cause saturation levels to drop further, "potentially reducing calcification rates of some marine organisms by more than 40% of their pre-industrial values within the next 90 years." Scientists also added, that in several key coral reef regions, aragonite saturation is already five times below its lowest pre-industrial range.
Scientists from the Australian Institute of Marine Science say that not all not all corals are victims of CO2. In Nov. 2011, Dr Ken Anthony, Research Team Leader for the Climate Change and Ocean Acidification team at AIMS, said "overall, CO2 enrichment and ocean acidification is bad news for coral reefs" but, "some reef areas take up more CO2 than they produce [...] which can lower the vulnerability of neighbouring reef areas to ocean acidification."
Anthony acknowledges however in "Increased acidity not an even test for coral reefs
", that reef areas with greater coral cover produce more CO2 than they consume [...] and that adds locally to the ocean acidification threat." Scientists hope to be able to differentiate between specific areas of coral on reefs such as the Great Barrier Reef to gain a a better grasp of risks from ocean acidification.
The current study suggests that the Caribbean and the western Equatorial Pacific are particularly vulnerable to human-induced ocean acidification. Professor Axel Timmermann, co-author of the study, said, "our results suggest that severe reductions are likely to occur in coral reef diversity, structural complexity and resilience by the middle of this century."
The study was funded by The Nature Conservancy, the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) through its sponsorship of the International Pacific Research Center, and National Science Foundation and is published online, in the January 22 issue of Nature Climate Change