Manmade CO2 emissions over the last 100 to 200 years have already raised ocean acidity way beyond its normal natural range, says an international team of scientists.
The team used a combination of observation and computer modeling, with Earth system models simulating climate and ocean conditions from 21,000 years ago to the end of the 21st century.
They studied changes in the saturation level of aragonite - a form of calcium carbonate - which drops as the acidity of seawater rises. They say their models were a good reflection of currently-observed seasonal and annual variations in several key coral reef regions.
Indeed, today's levels of aragonite saturation in these areas have already dropped five times below the pre-industrial range of natural variability. This could translate into a 15 percent fall in overall calcification rates of corals and other aragonite shell-forming organisms.
And, says the team, given the continued human use of fossil fuels, these 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.
"In some regions, the man-made rate of change in ocean acidity since the Industrial Revolution is a hundred times greater than the natural rate of change between the Last Glacial Maximum and pre-industrial times," says Tobias Friedrich of the International Pacific Research Center at the University of Hawaii.
"When Earth started to warm 17,000 years ago, terminating the last glacial period, atmospheric CO2 levels rose from 190 parts per million 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."
Coral reefs are currently found in places where open-ocean aragonite saturation reaches levels of 3.5 or higher. Today, that's about 50 percent of the ocean – but b they end of the 21st century, it's expected to be less than five percent. The Hawaiian Islands, which sit just on the northern edge of the tropics, will be one of the first to feel the impact.
The eastern tropical Pacific should do better, because greater underlying natural variability of seawater acidity helps to mask man-madechanges. The Caribbean and the western Equatorial Pacific, both biodiversity hotspots, are particularly vulnerable.
"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 University of Hawaii professor Axel Timmermann.