Nearly one-third of CO2 emissions due to human activities enters the world's oceans. By reacting with seawater, CO2 increases the water's acidity, which may significantly reduce the calcification rate of such marine organisms as corals and mollusks. The extent to which human activities have raised the surface level of acidity, however, has been difficult to detect on regional scales because it varies naturally from one season and one year to the next, and between regions, and direct observations go back only 30 years.
Credit: Dwayne Meadows, NOAA
Combining computer modeling with observations, an international team of scientists concluded that anthropogenic CO2 emissions over the last 100 to 200 years have already raised ocean acidity far beyond the range of natural variations. The study is published in the January 22 online issue of Nature Climate Change.
The team of climate modelers, marine conservationists, ocean chemists, biologists and ecologists, led by Tobias Friedrich and Axel Timmermann at the International Pacific Research Center, University of Hawaii at Manoa, came to their conclusions by using Earth system models that simulate climate and ocean conditions 21,000 years back in time, to the Last Glacial Maximum, and forward in time to the end of the 21st century. They studied in their models changes in the saturation level of aragonite (a form of calcium carbonate) typically used to measure of ocean acidification. As acidity of seawater rises, the saturation level of aragonite drops. Their models captured well the current observed seasonal and annual variations in this quantity in several key coral reef regions.
Today's levels of aragonite saturation in these locations have already dropped five times below the pre-industrial range of natural variability. For example, if the yearly cycle in aragonite saturation varied between 4.7 and 4.8, it varies now between 4.2 and 4.3, which – based on another recent study – may translate into a decrease in overall calcification rates of corals and other aragonite shell-forming organisms by 15%. Given the continued human use of fossil fuels, the saturation levels will drop further, potentially reducing calcification rates of some marine organisms by more than 40% of their pre-industrial values within the next 90 years.
"Any significant drop below the minimum level of aragonite to which the organisms have been exposed to for thousands of years and have successfully adapted will very likely stress them and their associated ecosystems," says lead author Postdoctoral Fellow Tobias Friedrich.
The upper panels shows simulated surface aragonite saturation for the years 1800, 2012 and 2100, respectively. White dots indicate present-day main coral reef locations. The lower panels shows atmospheric CO2 concentration in parts per million simulated for the years 1750 to 2100.
This animation was generated as part of a project funded by The Nature Conservancy, the National Science Foundation and JAMSTEC.
Contacts and sources:
Gisela Speidel
University of Hawaii ‑ SOEST
These are yellow tangs frolicking among corals.
Combining computer modeling with observations, an international team of scientists concluded that anthropogenic CO2 emissions over the last 100 to 200 years have already raised ocean acidity far beyond the range of natural variations. The study is published in the January 22 online issue of Nature Climate Change.
The team of climate modelers, marine conservationists, ocean chemists, biologists and ecologists, led by Tobias Friedrich and Axel Timmermann at the International Pacific Research Center, University of Hawaii at Manoa, came to their conclusions by using Earth system models that simulate climate and ocean conditions 21,000 years back in time, to the Last Glacial Maximum, and forward in time to the end of the 21st century. They studied in their models changes in the saturation level of aragonite (a form of calcium carbonate) typically used to measure of ocean acidification. As acidity of seawater rises, the saturation level of aragonite drops. Their models captured well the current observed seasonal and annual variations in this quantity in several key coral reef regions.
Today's levels of aragonite saturation in these locations have already dropped five times below the pre-industrial range of natural variability. For example, if the yearly cycle in aragonite saturation varied between 4.7 and 4.8, it varies now between 4.2 and 4.3, which – based on another recent study – may translate into a decrease in overall calcification rates of corals and other aragonite shell-forming organisms by 15%. Given the continued human use of fossil fuels, the saturation levels will drop further, potentially reducing calcification rates of some marine organisms by more than 40% of their pre-industrial values within the next 90 years.
"Any significant drop below the minimum level of aragonite to which the organisms have been exposed to for thousands of years and have successfully adapted will very likely stress them and their associated ecosystems," says lead author Postdoctoral Fellow Tobias Friedrich.
The upper panels shows simulated surface aragonite saturation for the years 1800, 2012 and 2100, respectively. White dots indicate present-day main coral reef locations. The lower panels shows atmospheric CO2 concentration in parts per million simulated for the years 1750 to 2100.
Credit: Tobias Friedrich 
"In some regions, the man-made rate of change in ocean acidity since the Industrial Revolution is hundred times greater than the natural rate of change between the Last Glacial Maximum and pre-industrial times," emphasizes Friedrich. "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. 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 – 200 years."
On a global scale, coral reefs are currently found in places where open-ocean aragonite saturation reaches levels of 3.5 or higher. Such conditions exist today in about 50% of the ocean – mostly in the tropics. By end of the 21st century this fraction is projected to be less than 5%. 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 anthropogenic changes. The aragonite saturation in the Caribbean and the western Equatorial Pacific, both biodiversity hotspots, shows 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," says co-author Professor Axel Timmermann."
An animation showing the changes in aragonite surface saturation level from 1800 to 2100 is available athttp://iprc.soest.hawaii.edu/users/tobiasf/Outreach/OA/Ocean_Acidification.html. The animation is also playing at the Science on a Sphere in the Jhamandas Watumull Planetarium at the Bishop Museum in Honolulu.
Humans have released ~500 billion tons of carbon to the atmosphere. About 30% have been taken up by the oceans. The uptake leads to changes in ocean chemistry resulting in a decrease of seawater pH and carbonate ion concentration, commonly referred to as ocean acidification The availability of carbonate ions is crucial for marine calcifying organisms to form their skeletons or shells that are made of different crystalline forms of calcium carbonate, such as calcite and aragonite. Aragonite is more soluble than calcite and organisms forming aragonite, such as corals and mollusks. Thus, the saturation state of aragonite can be taken as an indicator for ocean acidification.
"In some regions, the man-made rate of change in ocean acidity since the Industrial Revolution is hundred times greater than the natural rate of change between the Last Glacial Maximum and pre-industrial times," emphasizes Friedrich. "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. 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 – 200 years."
On a global scale, coral reefs are currently found in places where open-ocean aragonite saturation reaches levels of 3.5 or higher. Such conditions exist today in about 50% of the ocean – mostly in the tropics. By end of the 21st century this fraction is projected to be less than 5%. 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 anthropogenic changes. The aragonite saturation in the Caribbean and the western Equatorial Pacific, both biodiversity hotspots, shows 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," says co-author Professor Axel Timmermann."
An animation showing the changes in aragonite surface saturation level from 1800 to 2100 is available athttp://iprc.soest.hawaii.edu/users/tobiasf/Outreach/OA/Ocean_Acidification.html. The animation is also playing at the Science on a Sphere in the Jhamandas Watumull Planetarium at the Bishop Museum in Honolulu.
Humans have released ~500 billion tons of carbon to the atmosphere. About 30% have been taken up by the oceans. The uptake leads to changes in ocean chemistry resulting in a decrease of seawater pH and carbonate ion concentration, commonly referred to as ocean acidification The availability of carbonate ions is crucial for marine calcifying organisms to form their skeletons or shells that are made of different crystalline forms of calcium carbonate, such as calcite and aragonite. Aragonite is more soluble than calcite and organisms forming aragonite, such as corals and mollusks. Thus, the saturation state of aragonite can be taken as an indicator for ocean acidification.
The animation shows how aragonite saturation at the ocean's surface is projected to decrease towards the end of the 21st century as man-made carbon dioxide accumulation in the atmosphere continues to rise. Nowadays, coral reefs are only found in regions where open-ocean aragonite saturation is 3.5 or higher. Currently, this value is reached in about 50% of the world's ocean. Projections indicate that the size of areas providing aragonite saturation levels of 3.5 or higher will shrink to less than 5% until the end of the 21st century. By that time, large parts of the Southern Ocean will experience conditions that may lead to a dissolution of skeletons and shells due to extremely low aragonite saturation (less than 1). Ocean acidification constitutes a serious hazard to global marine ecosystem and marine resources such as fisheries, food supply and tourism.
This animation was generated as part of a project funded by The Nature Conservancy, the National Science Foundation and JAMSTEC.
Gisela Speidel
University of Hawaii ‑ SOEST
Friedrich, Tobias: International Pacific Research Center, University of Hawaii at Manoa, Honolulu, Hawaii;
Timmermann, Axel: International Pacific Research Center, University of Hawaii at Manoa, Honolulu
Timmermann, Axel: International Pacific Research Center, University of Hawaii at Manoa, Honolulu
This study was funded by The Nature Conservancy (www.nature.org), the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) through its sponsorship of the International Pacific Research Center, and National Science Foundation grant #0902551.
Citation: T. Friedrich, A. Timmermann, A. Abe-Ouchi, N. R. Bates, M. O. Chikamoto, M. J. Church, J. E. Dore, D. K. Gledhill, M. González-Dávila, M. Heinemann, T. Ilyina, J. H. Jungclaus, E. McLeod, A. Mouchet, and J. M. Santana-Casiano: Detecting regional anthropogenic trends in ocean acidification against natural variability. Nature Climate Change - DOI: 10.1038/NCLIMATE1372.
Citation: T. Friedrich, A. Timmermann, A. Abe-Ouchi, N. R. Bates, M. O. Chikamoto, M. J. Church, J. E. Dore, D. K. Gledhill, M. González-Dávila, M. Heinemann, T. Ilyina, J. H. Jungclaus, E. McLeod, A. Mouchet, and J. M. Santana-Casiano: Detecting regional anthropogenic trends in ocean acidification against natural variability. Nature Climate Change - DOI: 10.1038/NCLIMATE1372.
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