Ocean waves break onto a beach not far from Breidamerkurjokull glacier on Aug. 18, 2021 near Hof, Iceland. Iceland is undergoing a strong impact from global warming, as are the waters that surround it.
Sean Gallup—Getty Images
October 14, 2022 7:00 AM EDT

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.

A Blue Carbon Sink

The ocean helps stabilize the climate in a number of ways. First, it contains countless phytoplankton, microscopic organisms which—like trees—absorb carbon dioxide through photosynthesis, the process by which they convert sunlight into energy they can use. Phytoplankton are the foundation of the ocean food web and the main diet of zooplankton (non-photosynthetic microorganisms), which then become food for larger marine animals.

Read more: The Ocean Is Climate Change’s First Victim and Last Resort

On land, the carbon in plants’ and animals’ bodies can be re-released into the atmosphere when they die and decay, but in the ocean dead things drift into the deep ocean below, where they are broken down by deep-sea dwellers. Their carbon is stored long-term in the depths, buried under sediment. This process is known as the ocean’s biological carbon pump.

But the ocean’s carbon-sucking capacity doesn’t stop there. There’s also the physical pump, also known as the solubility pump. Like all gasses, carbon dioxide can dissolve into water. Unlike most, it can also react with water, forming a number of hydrogen- and oxygen-containing compounds that, together with dissolved free carbon dioxide, are known as dissolved inorganic carbon. Dissolved inorganic carbon that forms at the surface of the ocean is gradually drawn into the depths as it follows the global conveyor belt of the ocean’s currents.

Trapping Heat

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.

Read more: The World’s Oceans Are in Trouble. And So Are Humans, Warns U.N. Report

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.

Using Technology to Trap Emissions

Some scientists and activists are promoting an anthropogenic method to boost the biological pump: ocean fertilization. This form of geoengineering involves seeding the upper depths of the ocean with nutrients that phytoplankton need to grow, such as iron, nitrogen, or phosphorus. The method is controversial, not only for its possible negative consequences (phytoplankton blooms can deplete the oxygen levels in water, killing other marine life), but also because some experiments suggest that much of the carbon absorbed by surface-dwelling plankton will reenter the atmosphere before it can sink into the deep ocean.

Though most carbon dioxide removal projects remain land-based, such as forest protection and direct air capture, iron fertilization is part of a growing number of solutions looking to the other 70% of the planet. The emerging blue carbon credit market is attempting to monetize this field. One blue carbon startup is Maine-based Running Tide, which is growing seaweed—absorbing carbon in the process—and sending it to the depths of the ocean, selling carbon credits to pay for the operation. Other blue carbon credit projects involve efforts like planting mangroves or protecting existing marine and coastal plants.

Read more: The Ocean Farmers Trying to Save the World With Seaweed

However, like other carbon offset markets, the blue carbon market has received criticism for a lack of regulatory oversight and data. Few detailed studies have been conducted to measure the amount of carbon dioxide these projects can sequester and how long it can be stored, or the effect they have on existing ecosystems.

“There’s an incredible need for the studies to happen as soon as possible,” says Ken Buesseler, a senior scientist at the Woods Hole Oceanographic Institution in Massachusetts who serves on the organizing committee for Exploring Ocean Iron Solutions, a team of scientists promoting further research on iron fertilization. “We should move ahead,” he urges, calling for more research, “so we can protect the ocean from bad behavior.”

—With reporting by Jennifer Junghans

This article is part of a series on key topics in the climate crisis for time.com and CO2.com, a division of TIME that helps companies reduce their impact on the planet. For more information, go to co2.com

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