Emissions of carbon over the last two centuries have raised the acidity of the oceans to the highest levels in 21,000 years and likely beyond, according to a new study inNature Climate Change. The change threatens a number of marine species, including coral reefs and molluscs.

The oceans play a massive role in the carbon cycle, sequestering a significant amount of the carbon released naturally. But for the past couple centuries human activities, such as burning fossil fuels and deforestation, have substantially increased the amount of carbon emitted into the atmosphere, forcing the oceans to sequester more. Currently, the oceans sequester about one third of all carbon emissions, however this influx of carbon is not benign: the carbon lowers the sea waters pH levels (a rise in acidity).

“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,” explains lead author Tobias Friedrich at the International Pacific Research Center, University of Hawaii at Manoa, Honolulu, Hawaii.

The researchers used computer models to estimate the the abundance of a calcium carbonate known as aragonite in sea water—a commonly used proxy for acidification—from the Last Glacier Maximum (21,000 years ago) to today. According to the authors, it’s not just the level of acidity that is worrisome, but the speed at which it is happening.

“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,” Friedrich says.

Species that produce shells and plates out of calcium carbonate are at risk from acidification, since acidity lowers the amount of available calcium carbonate. Carbonate ion concentrations are currently at their lowest rate in nearly a million years endangering shell fish, types of algae, foraminifera, pteropods, and corals. According to a recent report by 2100, 70 percent of cold water corals will be exposed to high acidification levels. As the most biodiverse ecosystem in the oceans, many species stand to lose out if coral reefs vanish.