Nearly one-third of CO2 emissions due to human activities enters the world’s oceans. This is part of the natural recycling of carbon. By reacting with seawater, CO2 increases the water’s acidity (lower pH), 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 as well as due to man made sources from one season and one year to the next, and between regions, and direct observations go back only 30 years. Ocean acidification is the name given to the ongoing decrease in the pH and increase in acidity of the Earth\'s oceans, caused by the uptake of carbon dioxide (CO2) from the atmosphere. About a quarter of the carbon dioxide in the atmosphere goes into the oceans, where it forms carbonic acid. As the amount of carbon has risen in the atmosphere there has been a corresponding rise of carbon going into the ocean. Between 1751 and 1994 surface ocean pH is estimated to have decreased from approximately 8.25 to 8.14, representing an increase of about 30% in acidity in the world\'s oceans. 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 Hawaii at M?noa, 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. \"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,\" emphasized 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 of 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. Changes in ocean chemistry can have extensive direct and indirect effects on organisms and their habitats. One of the most important repercussions of increasing ocean acidity relates to the production of shells and plates out of calcium carbonate (CaCO3). This process is called calcification and is important to the biology and survival of a wide range of marine organisms. Calcification involves the precipitation of dissolved ions into solid CaCO3 structures, such as coccoliths. After they are formed, such structures are vulnerable to dissolution unless the surrounding seawater contains saturating concentrations of carbonate ions.