Not all carbon emissions find their way into Earth's atmosphere—about half of it is absorbed by vegetation and the world's oceans. On the one hand, that helps limit carbon's climate-changing effects. But on the other, it can deliver what a National Oceanic and Atmospheric Administration scientist calls a "double whammy" to the oceans.
That's because carbon dioxide is a weak acid, and when it's absorbed by water, it contributes to ocean acidification, which can kill coral reefs and shellfish, wreaking havoc on undersea plant and animal life.
As humans have increased their carbon emissions over the past 100 years, vegetation and the world's surface oceans have been working overtime to absorb about half of it, about the same proportion as 50 years ago, according to the study, published Wednesday in Nature.
"Humanity is getting an assist on climate change from natural systems, otherwise the carbon dioxide in the atmosphere would be twice as high," says Pieter Tans, one of the study's authors. "But CO2 is an acid and the amounts [being absorbed by the ocean] are so massive that I don't see how we can remedy coming acidification."
Reforestation in parts of North America and China and deforestation slowdowns in other parts of the world have allowed plants to bear some of the burden, but he says the ocean is working overtime to pull in more carbon than ever before.
But even though Earth is absorbing a similar proportion of carbon as it was 50 years ago, overall human emissions have greatly increased, meaning sea temperatures are rising even as they acidify. According to a Scripps Institution of Oceanography study released earlier this year, ocean temperatures have increased by about half a degree over the past 100 years; many scientists say that increase has been responsible for an increase in the severity and frequency of hurricanes.
"Sea temperature change comes from climate change, but they're also acidifying," Tans says. "The oceans get a double whammy."
While increasing carbon emissions may take longer to wreak havoc on the world's climate, it could deal a death blow to vulnerable coral reefs, which shelter millions of plant and animal species, Tans says.
"Acidification is a concern for sea life—for the atmosphere, it's a good thing our oceans are absorbing so much carbon, but as the oceans acidify, it'll affect [coral reefs and shellfish], and work its way up the food chain," he says. "At some point, [reefs] are endangered. We're not too far away from that."
In the short term, it's good news that so much carbon dioxide is staying here on earth. But researchers aren't sure how long earth's vegetation can continue to keep up with increasing emissions. Because plants use carbon dioxide in photosynthesis, it's possible that plants simply use more carbon when it's available in a process known as the "CO2 fertilization effect," which allows plants to grow faster.
Despite nature's assist, the amount of carbon dioxide in the atmosphere is still increasing due to unprecedented fossil fuel emissions, Tans says, and "we're still in bad shape."
Jason Koebler is a science and technology reporter for U.S. News & World Report. You can follow him on Twitter or reach him at firstname.lastname@example.org