We replaced the stove with plywood, turning the kitchen of the dive boat into an impromptu research lab. Plugging in wires and connecting tubing, we assembled a scientific instrument within the cramped cabin.

Then we cast off into Halifax Harbour, Canada, surveying the turquoise waters for signs of an unusual test: could we use the ocean itself to remove carbon dioxide from the air?

Carbon dioxide (CO₂) is the most important driver of climate change, but it cannot be seen. Its build-up in the atmosphere is gradual. Its worst consequences take time to emerge. Even if emissions fell sharply tomorrow, the CO₂ already released would continue to warm the planet.

That is why scientists and policymakers are increasingly turning to carbon dioxide removal (CDR): taking CO₂ that has already been released back out of the air. So far, most large-scale CDR has focused on land, such as reforestation. But land is finite, competes with food production and biodiversity, and stored carbon can be lost through fire or deforestation. As emissions continue to outpace what these approaches alone can manage, attention has turned toward the ocean.

The overlooked role of the ocean in carbon storage

The ocean covers about 70% of the Earth’s surface and holds roughly 50 times the amount of carbon found in the atmosphere. Before the industrial revolution, carbon moved between air and sea in near balance. As industrial activity increased atmospheric CO₂, more of it dissolved into seawater and the ocean became more acidic.

All that dissolved carbon has resulted in the ocean storing about a third of human CO₂ emissions since the industrial revolution —........

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