In the battle to reduce the amount of carbon dioxide in the atmosphere and slow global warming, humans have a few natural allies. The best-known of these allies are trees, those charismatic carbon sinks that create shade and oxygen for us and our fellow landbound creatures. But land covers less than a third of the earth, and trees live on a shrinking sliver of that. The ocean covers most of the rest of the planet and absorbs up to 50% of all fossil fuel-related carbon dioxide emissions—20 times more than trees, other land plants, and soil combined.
“All the plans moving forward for stabilizing earth’s climate depend on the ocean continuing to remove carbon dioxide from the atmosphere,” says Scott Doney, a University of Virginia professor who researches how ocean ecology and the carbon cycle responds to climate change. “It’s a really important stabilizer of the planetary climate.”
The ocean’s ability to absorb carbon and stabilize the climate is attracting attention from scientists and companies alike looking for ways to counteract the rise of anthropogenic greenhouse gas emissions. Within this, the world of blue carbon offsets is gaining popularity. Companies can buy and sell these credits, which represent a certain amount of emissions being removed from the atmosphere and absorbed by the ocean. It’s easier said than done, though. Some blue carbon projects, especially those that involve geoengineering, have been criticized for a lack of data on efficacy and consequences. But there’s no doubt that the ocean has an outsized influence on the climate, and no one working to fight global warming should turn their back on the sea.
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The ocean’s impact on the climate goes beyond its role as a carbon sink. It also serves as a heat sink due to the same conveyor-belt currents that send dissolved inorganic carbon to the depths. These currents carry warm, salty water from the equator up to the North Atlantic, where it cools and sinks due to its salt-saturated density, bringing the heat with it. About 90% of the planet’s heat is stored in the ocean.
Here’s the downer: Fossil fuel emissions and other anthropogenic effects are having a negative effect on all of the ocean’s mechanisms for absorbing carbon and heat. Carbon dioxide is more soluble in cold water, so global warming is slowing down the physical pump. The amount of carbon in the atmosphere makes it harder for water to cool down because of the greenhouse effect (carbon dioxide blocks the waves of thermal radiation that the ocean sends back out to space after being heated by the sun). These temperature changes are also shifting the long-established currents of the ocean, which is changing both global weather patterns (ocean circulation is interlinked with atmospheric circulation) and the path carbon traditionally takes toward the ocean floor.
And because carbon dioxide reacts with water to form carbonic acid, the ocean grows more acidic as it absorbs more carbon—a process known as ocean acidification. This has its own negative effects on marine ecosystems and may affect the growth of the photosynthetic plankton that form the basis of the biological pump. A 2015 study found that ocean acidification will cause several species of phytoplankton to die out.
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Jennifer Fergesen, Time, 14 October 2022. Full article.
