Almost a third of the CO2 we emit is absorbed by the oceans. That helps to reduce global warming, but it also makes the oceans more acidic - posing a serious threat to marine life.
As scientists assess why global temperatures haven't climbed as rapidly as forecast by climate models, the theory that excess heat is being absorbed by deep ocean water has gained backing from a study by the International Programme on the State of the Oceans (IPSO).
"We've got evidence that the oceans are warming, with temperature rises of up to 1.3 degrees Celsius in places like the Baltic. We've also seen increasing evidence that the deep waters, deeper than 700 meters, are also taking up heat," IPSO's scientific director Alex Rogers, a professor of conservation biology at the University of Oxford, told DW.
"The oceans are taking up about a third of the carbon dioxide we're producing at the moment. While this is slowing the rate of earth temperature rise, it is also changing the chemistry of the ocean in a very profound way."
Carbon dioxide reacts with sea water to form carbonic acid. Gradually, this makes oceans more acidic.
Threat to marine life
Sea water is already 26 percent more acidic than it was before the onset of the Industrial Revolution. According to the IPSO report, it could be 170 percent more acidic by 2100.
Over the last 20 years, scientists around the world have been conducting laboratory experiments to find out what that would mean for the flora and fauna of the oceans. Ulf Riebesell of the Helmholtz Institute for Ocean Research in Kiel, a lead author of the report, conducted the world's first experiments in nature, off the coast of the Arctic island of Svalbard in 2010.
Giant test-tubes were lowered into the ocean to capture a water column with living organisms inside it. Different amounts of CO2 were added to simulate the effects of different emissions scenarios in the coming decades.
The experiments showed that increasing acidification decreases the amount of calcium carbonate in the sea water, making life very difficult for sea creatures that use it to form their skeletons or shells. This will affect coral, mussels, snails, sea urchins, starfish as well as fish and other organisms, Riebesell told DW. "Some of these species will simply not be able to compete with others in the ocean of the future," he added.
Hard times for coastal residents
The scientists warn of severe economic and social consequences. Ultimately, acidification will affect the food chain. Tropical and sub-tropical areas with warm-water corals would be badly hit, said Riebesell. He pointed out that coral reefs are home to numerous species, serve as nurseries for fish and are a valuable tourist magnet. "And without coral reefs, coastlines are increasingly exposed to waves and storms," he added.
At the same time, the polar regions are suffering more than others, as cold water absorbs CO2 faster. Experiments in the Arctic indicate that the sea water there could become corrosive within a few decades, Riebesell said. "That means the shells and skeletons of some sea creatures would simply dissolve."
The Antarctic is already affected, said IPSO's Alex Rogers: "We're seeing instances where we're finding tiny shelled molluscs, tiny snails that swim in the surface of the oceans, with corroded shells."
These creatures play a key role in the marine food chain, supporting everything from tiny fish to whales. "One of our primary sources of marine-derived protein is in rapid decline," said Monty Halls, manager of the UK-based Shark and Coral Conservation Trust. He describes ocean acidification as the "most serious threat to our children's welfare."
Vicious circle of climate change
Scientists are also concerned about a feedback effect that will further exacerbate global warming. In the long run, the ocean will become the biggest sink for human-produced CO2, but it will absorb it at a slower rate.
"Its buffer capacity will decrease, the more acidic the ocean becomes," said Riebesell.
Alex Rogers sees a further problem: "Carbonate structures actually weigh down particular organic carbon. In other words, they help carbon to sink out of the surface layers of the ocean into the deep sea. Anything that interferes in that process can potentially accelerate the rate of CO2 increase in the atmosphere."
And that would be very dangerous. "The rates of CO2 increase we are seeing at the moment are probably as high as they've been for the last 300 million years," Rogers noted.
The IPSO report draws an unsettling comparison between conditions today and climate change events in the past that have resulted in mass extinctions: "On a lot of these major extinction events we see the fingerprints of high temperatures and acidification, similar effects to the ones that we are experiencing today."
The experts stress that it is not too late to halt the acidification process, although the CO2 will remain in the oceans for thousands of years. The most important step would be to reduce greenhouse gas emissions, said Riebesell. At the same time he called for a reduction in pollution from agricultural runoff or plastic waste and more marine reserves.
Everyone can make a contribution, Rogers stressed: "Switching off lights, going to work by bike, not using plastic bags, avoiding other polluting chemicals - all of this adds up to a benefit for the oceans and the earth system."