We should be thankful for our oceans. In addition to providing us with food and recreation and a host of other services, they absorb up to one quarter of our carbon dioxide emissions. As a result, climate change is not nearly as bad as it might be. But they, and the marine creatures that live in them, also pay a tremendous price for this inadvertent favor: acidification. Motherboard Vice reports that our oceans are 30 percent more acidic today than they were 200 years ago, and now for the first time, we know which oceans are acidifying at a faster rate than others. Tara Takahashi from Columbia University and his team used four decades of data to map how acidity levels vary across the world’s oceans; their research appeared in the August issue of the journal Marine Chemistry.

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Using 2005 as a benchmark, Takahashi and his colleagues used four decades of data to map out acidity levels in all of the world’s oceans. They found that tropical and temperate oceans see the least variation in pH levels, and an oversaturation of the mineral aragonite, which is necessary for pteropods to build their protective shells.

The greatest fluctuations occur in colder waters with levels highest in winter when CO2 rises from the deep to the surface. As a result, aragonite levels fall, hindering pteropod shell growth and therefore decreasing the availability of food for fish. The northern Indian Ocean is at least 10 percent more acidic than the Atlantic and Pacific oceans, while waters near the north and south poles are acidifying at a rate of five percent per decade.

Related: 10 Million Scallops Killed as a Result of Rising CO2 Levels

Takahashi explained to Motherboard Vice that seasonal acidity fluctuations are normal and the marine ecosystem is accustomed to adapting to them. But by 2050, he said, “it’s going outside of the normal seasonal changes.” And when that happens, nobody really knows what to expect.

“This is exactly what we’d expect based on how much CO2 we’ve been putting in the air,” Rik Wanninkhof, a Miami-based oceanographer with the National Oceanic and Atmospheric Administration (NOAA), said of the study.

“This is an important point for scientists to underscore—these calculations are not magic.”

Via Motherboard Vice