Unprecedented, man-made trends in oceans acidity

The Hawaii Reporter: 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, resulting in the potential loss of ecosystems. 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.

By combining computer modeling with observations, an international team of scientists concluded that anthropogenic CO2 emissions, resulting from the influence of human beings, 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, 2012 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 at the University of Hawai‘i at Mānoa, 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. In their models, they studied changes in the saturation level of aragonite (a form of calcium carbonate) typically used to measure ocean acidification.

As acidity of seawater rises, the saturation level of aragonite drops. Their models captured 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,” said lead author Friedrich...

An illustration of some coral from the Austrian National Library, Kommentar von Otto Mazal (S. 47 und 52) in: Pedanius Dioscorides – Der Wiener Dioskurides, Codex medicus Graecus 1 der Österreichischen Nationalbibliothek, Graz: Akademische Druck- und Verlagsanstalt 1998 (Band 2). ISBN 3-201-01725-6