Deep sea affected by ocean acidification

Ocean acidification is penetrating to depths of 1,500 meters, posing new threats to creatures such as sea butterflies, sea snails and cold-water corals.

The ocean is the largest natural sink of carbon dioxide, absorbing about a quarter of our annual emissions. The uptake of CO2 makes the ocean surface more acidic, which affects sensitive ecosystems such as coral reefs. But until now, researchers didn’t know how far acidification was reaching the deep ocean.

jens daniel müller Researchers at the Federal Institute of Technology Zurich in Switzerland have developed a 3D reconstruction of how CO2 moves through the ocean, based on global measurements of ocean currents and other circulation patterns. They used this model to estimate how the carbon dioxide absorbed by the ocean has affected deep-sea acidity since 1800, around the beginning of the industrial revolution.

They found clear acidification signals down to 1,000 meters deep in most of the ocean. In some regions, such as the North Atlantic, where the powerful Atlantic Meridional Overturning Current (AMOC) transports carbon from the surface to the deep ocean, acidification was observed down to 1500 meters. Some pockets of deep water, which are more acidic in nature, showed even more acidification than the surface. Their high natural acidity reduces their ability to absorb added CO2, Mueller says.

This is more or less what researchers expected would happen as the oceans absorbed more carbon dioxide. Wang Hongjie at the University of Rhode Island. “But it’s another thing to actually see the data coming in that affirms this.”

Remarkably, about half of all acidification since 1800 has occurred since 1994, as CO2 emissions have increased exponentially. “We’re seeing this progress be quite rapid,” Muller says.

The scale of acidification is sufficient to threaten the survival of a wide range of life in the oceans. Chiropods such as sea snails and sea butterflies are particularly at risk because their shells are made of calcium, which dissolves when water becomes too acidic. Increasing acidification has doubled the number of areas where it is difficult for cold-water corals to survive.

And ocean acidification will continue as seawater absorbs more carbon dioxide. “Even if we were able to stop carbon dioxide emissions immediately, we would still see ocean acidification processes inland for hundreds of years,” Muller said.