Just How Bitter Is the Tea of the Sea?

A new study examines the ocean’s ability to store carbon as it gets more acidic.


Illustration by Perrin Ireland

If you add up all the carbon stored in living things on earth, you’ll wind up with the same amount of carbon contained in the dissolved organic matter in the ocean. When a leaf, for example, falls into a river, all the stuff that leaches out of the leaf as it decomposes eventually ends up in the sea. Think of this as the “tea of the ocean”—and the sea has a lot of tea.

So this carbon “tea” from leaves and fish and other once-living matter is naturally occurring in the oceans and typically kept in balance. But then there’s the additional carbon load the oceans get from humanity’s CO2 pollution. In fact, since the Industrial Revolution, the sea has absorbed about half of the carbon emissions from our burning of fossil fuels. That excess atmospheric carbon is changing the ocean’s chemistry, making it more acidic—and doing it faster than we expected. In a study published today, German geochemist Maren Zark and her team at the University of Oldenburg explore what might happen to the sea’s carbon-storing abilities when you add ocean acidification into the mix.

There’s been some concern among scientists that as the ocean acidifies, its carbon-storage capacity won’t be able to keep pace with our CO2 emissions. The fear is that in 100 years, the seas could start exacerbating climate change by spitting out some of the carbon they’ve been storing in their dissolved organic matter for, in some cases, 7,000 years.

The researchers set up giant test tubes, called mesoscosms, within the fjords of coastal Sweden. These mesoscosms are basically big bags of ocean water that get the same amount of light, wind, waves, and critters as natural seawater does. They then treated the water in some of these mesocosms with the same carbon levels expected to be in the oceans by 2100 if we continue polluting at today’s rates. Using one of the highest resolution mass spectrometers in the world, Zark was able to get a really close look at what’s in ocean tea. Over the next 109 days, she and her team monitored 7,300 components of seawater and found there was no difference between the amounts of dissolved organic matter in the regular water and the carbon-loaded samples.

First, the good news: The findings suggest that ocean acidification won’t necessarily hamper the water’s ability to store carbon over the long term. The bad news is that you cannot live on tea alone—and ocean acidification is still going to hit marine food webs hard. More acidic waters will eat away at coral reefs, snails, oysters, clams, crabs, and many other invertebrates, leaving everything that eats them very hungry—including those of us living way upstream.

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