| The seascape Carbon removal has become an essential piece of our response to climate change, as the UN's climate change committee pointed out in a report last year. Estimates of exactly how much carbon we'll need to remove vary, but the consensus is that it will need to top a billion tons annually within the next few decades if we're going to keep warming below 2 °C over preindustrial levels and avoid the worst effects of climate change. To be clear: efforts to pull carbon out of the atmosphere won't replace the need to cut emissions moving forward, but we've already polluted too much to neglect the cleanup. Researchers are looking into a wide range of approaches to removing carbon from the atmosphere. Some are straightforwardly technical, like direct air capture in giant facilities can use massive fans and specialized membranes to trap carbon dioxide. Other approaches lean on nature, like growing trees and trapping carbon in their biomass underground. Oceans cover the majority of our planet, and in fact they already suck up roughly 30% of human-caused greenhouse-gas emissions through a whole host of routes. Researchers think there's a huge opportunity to capture and store even more carbon in the ocean's depths by enhancing these natural mechanisms or inventing their own. Some groups are looking to tweak the ocean's chemistry through a process called alkalinity enhancement: with more alkaline chemicals, seawater can absorb more carbon dioxide. Other approaches are more direct, like one that zaps seawater to remove carbon dioxide. And a growing number of groups are aiming to leverage the natural potential of seaweed for carbon removal. Like other photosynthesizers, seaweed absorbs carbon dioxide as it grows, transforming it into biomass. And while most carbon captured by plants and algae eventually returns to the atmosphere as those organisms die and decay, a small part ends up stuck, or sequestered, when remains sink into places where they can't decompose. In the case of seaweed, as much as 175 million tons of carbon could be sequestered each year as fronds sink to deep parts of the ocean where the water doesn't mix much. And a growing number of groups are looking to mimic nature, growing seaweed and sinking it on purpose to capture more carbon.
The snag While turning up the dial on seaweed sinking sounds like a straightforward approach to achieve carbon removal, a lot of questions are left on the table, as my colleague James Temple outlined in a 2021 story on the topic. First, there would be a frankly wild amount of seaweed involved in full-scale efforts to remove carbon in this way: probably on the scale of millions of tons. And we just don't know what the ecological effects of sinking millions of tons of seaweed into the ocean might be. There could be practical constraints, too. Growing enough seaweed to capture a billion tons of carbon would likely require a million square kilometers of ocean space, according to a new study. That's over twice the size of California, just for growing seaweed that we would sink into the ocean. My colleague Rhiannon Williams wrote more about this study in a story last week. Also, it's not a given that we totally understand how all these dynamics work in the deep ocean. There's a massive debate going on about how carbon removal projects in the ocean can track and verify that they're actually capturing as much carbon as they say they are. The ocean could still be a crucial partner in cleaning up the mess we've made with climate change. But no one solution will magically solve the problem for us, even deep below the waves. | 
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