Here’s a pop quiz. What is the earth’s biggest carbon sink?

Nope, it’s not forests, or peatland. It’s that body of water that covers 71% of our planet’s surface: the ocean.

We have a lot to thank the ocean for. 3 billion people depend on its ecosystems for food and economic security. It has also helped to mitigate climate change, absorbing 93% of the heat trapped by greenhouse gases and about 30% of the carbon dioxide emitted by burning fossil fuels so far. If we didn’t have the ocean, we’d be in a much stickier situation.

In return, humankind has polluted the oceans with oil, sewage and plastic. We’ve plundered our waters for all they have to offer — harvesting fish stocks to depletion, tearing up the seabed with trawl nets and mining deep-sea mineral deposits. In the meantime, noisy and polluting shipping vessels plough across waters and sometimes into marine creatures.

It’s become abundantly clear that the ocean is suffering from our actions. Some 10% of marine species are at risk of extinction — and that’s just the ones we know about. It’s often said that we know more about the moon than we do about the deep sea, meaning the full extent of the damage could be much worse. A third of fish stocks are overfished, meaning that they’re being caught faster than populations can recover. As seawater absorbs CO2 and heat, it is becoming more acidic — 30% more acidic in the last 200 years alone — and, naturally, warmer. That’s causing all sorts of issues for ocean ecosystems.

Currently, less than 7% of the ocean is protected. Of the waters 200 nautical miles out from shore, beyond national jurisdictions, just 1% is highly protected. These waters are known as the high seas, a largely lawless place that makes up two-thirds of the world’s ocean and 95% of the Earth’s habitable space by volume. They teem with life, supporting whales, sea turtles, huge shoals of fish, deep-water corals. The health of the high seas is integral to the well-being of the planet.

That’s why it’s a huge breakthrough that, after nearly two decades of negotiation, the high seas are en route to finally getting the protection they deserve. On Saturday, United Nations member states struck a deal for a new agreement that provides a framework for more robust governance of international waters. The high seas accord — formally known as the less-catchy Biodiversity Beyond National Jurisdiction treaty — classifies 30% of the world’s oceans as protected areas, requiring environmental impact assessments for emerging activities and ensuring that benefits from the use of marine genetic material are shared.

Alan Evans, technical advisor to the U.K. delegation in the negotiations, told me the agreement “gives a fundamental commitment to make sure that the global good can come together to manage the ocean area responsibly.”

The endeavor is primarily intended to help protect and restore the ocean’s biodiversity. By enabling nations to set up marine protected areas (MPAs) in the high seas, protecting 30% of land and sea by 2030 becomes significantly easier. These MPAs can be astonishingly effective: A 2017 study showed that marine reserves in national waters have on average 670% more fish, as measured by biomass, than adjacent unprotected areas. Not only is that a promising sign of a thriving ecosystem, but these flourishing populations also spill over into fishing areas — providing fishermen with increased catches. But there could be another benefit: climate mitigation.

We’ve already established that the ocean is a huge carbon sink, but that status is increasingly threatened.

Liz Karan, oceans project director at the Pew Charitable Trusts, explained to me that the healthy ocean ecosystem is really what allows it to play an important role in the carbon cycle. If biodiversity is lost, then the ocean’s ecosystem services — carbon sequestration and oxygen production — would also be potentially lost or greatly reduced, she says.

A recent review of 22,403 publications spanning 241 MPAs found that marine conservation efforts significantly enhance natural carbon removal and storage. The effects that MPAs would have in the high seas has been less well-studied, but it’s easy to imagine the climate benefits.

Take, for example, a whale. The 13 species of great whales store an average of 33 tons of carbon in their bodies in a lifetime. Unlike terrestrial animals, if a whale dies in the ocean, it pulls that carbon down to the depths, where it’s stored for centuries. And that’s not all. In a process known as “the whale pump,” whales dive down to feed and then return to the surface to breathe. At the surface, whales release — ahem — nutrient-rich fecal plumes. This buoyant waste is great for phytoplankton — the microscopic creatures that capture about 37 billion metric tons of CO2 a year and produce at least 50% of the oxygen in our atmosphere. Wherever whales go, phytoplankton blooms follow.

Unfortunately, great whale populations have been diminished after decades of industrial whaling, and so has phytoplankton activity. The blue whale population is now less than a tenth of what it was during the 19th century. More whales equal more phytoplankton, and if phytoplankton activity was increased by just 1%, that would be equivalent in carbon capture to the sudden appearance of 2 billion mature trees.

That’s just one way in which a healthy ocean ecosystem can draw down significant amounts of carbon. MPAs won’t save the planet — there’s a lot to do up on land, too — but the potential climate benefits of this U.N. biodiversity measure are something to get excited about.

However, the finalized treaty text is just the beginning. Protecting ocean biodiversity will require the dedication and focus of governments for years to come. Though the high seas are outside regional borders, let’s hope nation states make them a priority in the decades to come.

Lara Williams is a Bloomberg Opinion columnist covering climate change.

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