When scientists model our climate future and consider the actions we might take, like how quickly we stop burning fossil fuels, the math is unsparing. It's hard to plot a path that avoids the worst impacts of global heating without relying to some extent on carbon dioxide removal, or CDR.

Simply put, in addition to curbing fossil fuel emissions, we need to quickly take some of the heat-trapping CO2 out of the air and durably store it away.

Of course, trees help, and there are already trillions of them. But the University of Oxford, the University of Wisconsin-Madison, and others find in a 2023 report that this category of “conventional” CDR, which includes “afforestation, reforestation and management of existing forests” won’t be enough on its own to get where we’d like to be in 2030 or 2050.

“Virtually all scenarios that limit warming to 1.5°C or 2°C require ‘novel’ CDR,” the report continues. Novel CDR includes direct air capture technologies from ambitious companies like Climeworks and Heirloom—both build facilities to suck CO2 out of the atmosphere—and other approaches like enhanced rock weathering.

(Sticklers will note that CDR, as defined both in the report and by the IPCC, does not include approaches that capture carbon emissions at their source, like from an industrial smokestack, or that turn that carbon into transient plastic, fuel, or fizz in your soda. This means carbon capture and utilization or storage, or CCUS, generally doesn’t qualify as CDR.)

So how fast do we need novel CDR to scale up? The report looks at a few different scenarios. Averaging across them, it wants to see novel CDR increase thirty-fold by 2030, and by a factor of 1,300 by mid-century; some scenarios put those numbers higher still. How we make that happen is an open question: Climeworks and Heirloom aim to profit off people and companies paying to counteract their emissions. More public funding wouldn’t hurt.

Then there’s an important knock against CDR: It takes energy. If your direct air capture machine runs on fossil fuels, research says it’s “net CO2 additive,” or worse than pointless. That’s why Climeworks and Heirloom have taken pains to run their facilities on renewable energy, but that’s renewable energy that couldn’t be used elsewhere, ideally to supplant aging fossil fuel infrastructure.

And that leads us to a final sticking point. While we need CDR to succeed at scaling drastically, it's no substitute for curbing fossil fuels. We have to take coal and gas power plants offline and cars that run on internal combustion off the road—so it's awkward to cheer for CDR when some in the oil industry see it as a way they can stay in business.

This all points to a paradox at the heart of carbon dioxide removal: Though we’re not sure how, we’re depending on it to flourish over the next few decades. We’d also like to work toward a future where it’s superfluous.