Stopping climate change will take a social, economic, and technological revolution the likes of which the world has never seen. But imagine, for a moment, that humanity eventually succeeds in not just flattening the curve of rising atmospheric carbon dioxide, but reversing it. How much of the climate heating greenhouse gas should we scrub out of the atmosphere if we’re one day able to do so?

Of the many fascinating questions Kim Stanley Robinson raises in The Ministry for the Future, a novel that explores a future in which humanity effectively solves the climate crisis, this is one that stuck with me the most. On its face, the question sounds so simple as to almost be trite. Surely there’s an ideal setting for Earth’s thermostat—a carbon concentration all the smart climate folks out there agree on?

At least, that’s what I had assumed. But when I posed the question to experts, I learned that there is little agreement on this matter from a physics perspective and even less consensus around what will be socially and economically feasible. “The answer,” Texas Tech University climate scientist Katharine Hayhoe told me, “is that we really don’t know.”

The plot of The Ministry for the Future revolves around a titular UN-backed agency whose mission is to safeguard the future of all life on Earth. In practice, this means stopping climate change, which means convincing the world’s power brokers to dismantle capitalism from the inside. As Ministry for the Future head Mary Murphy and her fellow bureaucrats attempt to do that, a heat wave in India kills 20 million people, prompting the nation to spray sun-dimming particles into the atmosphere to reduce global temperatures; scientists pump water out from beneath Antarctica’s glaciers to slow their collapse; and a terrorist organization called the Children of Kali starts shooting passenger planes out of the sky to protest carbon emissions.

The book raises hard questions about what sorts of tactics are ethically permissible in pursuit of a habitable future—is dimming the sun? is terrorizing the rich into giving up their destructive ways?—as well as at what point humanity can declare victory. While the protagonists spend most of the book’s pages racing to halt carbon emissions, eventually, they succeed. At that point, the conversation shifts from how to eliminate fossil fuels to at what carbon level we should stabilize the atmosphere.

Robinson doesn’t answer that question, and most scientists I reached out to weren’t sure, either. But Earth’s geologic past offers clues about the sort of atmosphere we’ll want to aim for.

Atmospheric carbon concentrations currently sit at around 415 parts per million (ppm). Prior to the industrial revolution, the level was 280. In the several million years leading up to the discovery of petroleum, air carbon levels swung up and down due to climactic and ecosystem shifts related to changes in Earth’s orbit around the Sun. Atmospheric carbon levels averaged 350 ppm during the Pliocene (1.8 to 5 million years ago), and they bottomed out around 180 ppm during recent ice ages.

If we want a future climate that roughly resembles Earth’s recent past, it’s worth keeping these numerical guideposts in mind. Indeed, many scientists, policymakers, and activists have embraced 350 ppm as the highest “safe” carbon dioxide level for maintaining a human-friendly climate based on paleoclimate records.

But some experts, like Jessica Tierney, a paleoclimatologist at the University of Arizona, feel that 350 ppm is far too high. Tierney notes that during the Pliocene, the Greenland ice sheet was “basically gone,” West Antarctica’s ice was severely reduced, and global sea levels were about 66 feet higher than they are today.

“Most of Manhattan would be underwater, Philadelphia would be underwater, large parts of Washington DC would be underwater, all of south Florida is underwater” at 350 ppm, Tierney wrote in an email. “Of course it takes a long time for the ice sheets to melt (hundreds to a thousand years) but since we are talking about sci-fi...in the long term, living with 350 ppm means building sea walls like in The Expanse.”

To keep our coastal cities above the water line, Tierney suggests dialing back air carbon levels closer to 280 ppm. If we want to start planning on geologic timescales, she says we could try to forestall the next Northern Hemisphere glaciation event by setting the thermostat a bit higher—maybe somewhere around 300 ppm.

Hayhoe also suggested bringing atmospheric carbon levels down to the low or mid-300s “since that is the level that human society is currently most well-adapted to.” Once we revert to a 20th century atmosphere, she says, we’ll want to monitor the climate very carefully and fine-tune the thermostat as needed.

Of course, lowering air carbon levels won’t be easy. Even after we eliminate greenhouse gas emissions from every economic sector—a feat the Intergovernmental Panel on Climate Change (IPCC) says will take “rapid, far-reaching and unprecedented changes in all aspects of society”—it will require massive deployment of so-called negative emissions technologies. These could include everything from regenerative agriculture techniques that stuff carbon in the soil for long term storage to “direct air capture” machines that scrub carbon out of the sky (technology that is currently in a very nascent phase). Scaling these solutions up globally won’t be cheap, and several experts I reached out to said that the price tag is ultimately going to determine how much atmospheric repair work we’re able to get done.

“[E]ven if we can pull CO2 out of the atmosphere affordably, I doubt that it is going to be free,” said Klaus Lackner, the director of the Center for Negative Carbon Emissions at Arizona State University. “So people will have to make a conscious choice whether or not they are going to contribute to such an effort.”

While Lackner expects there will be “lots of volunteers” willing to support carbon drawdown when atmospheric concentrations are dangerously high, once the atmosphere starts to approach 350 ppm “my guess is that you are running out of people willing to pay for further reductions.”

“The way people work, we are more likely to run out of steam before we hit a number that is uncomfortably low,” he went on.

University of Maryland, Baltimore environmental scientist Erle Ellis agreed. “The preindustrial 280 would seem the safest bet, if there were no costs or consequences for any particular ppm,” he said. “But of course, those costs and consequences, and their social allocation, are the real issue here.”

In Ellis’ view, the key challenge will be to make clean energy as cheap as possible while ensuring that the costs of climate change mitigation and adaptation are allocated equitably “whether CO2 is reduced to 350, 280 or kept at 400 or even 450 ppm.”

Speed will matter as much as where we decide to set the planetary thermostat, said Georgia Institute of Technology climate scientist Kim Cobb. “Every decade spent with excess heat in the system means more long-term damage,” Cobb said, noting that even if we brought air carbon concentrations down to 280 ppm, it would take “decades” for the surface of Earth’s oceans to cool off given how much heat from global warming they have already absorbed. The longer we draw out the drawdown process, the more likely we are to lose heat sensitive coral reefs and irreversibly damage Earth’s ice caps.

At the same time, Cobb says, it’s important to keep in mind that once we begin reducing atmospheric carbon levels the planet will start to cool quickly. “It blows my mind,” Cobb said, “that in one single generation our society could go from sounding the alarm bells on global warming, to decades of denial, bickering, and procrastinating, to beginning global cooling” in a few decades.

Indeed, while the notion of humanity reversing climate change still has the ring of science fiction, the IPCC’s blockbuster 2018 report on 1.5 degrees Celsius of warming outlines scenarios that could put us on track to do this by the middle of the century. Whether or not we’ll have to engineer glaciers, blockchain everybody’s money to eliminate tax havens, or implement the myriad other solutions Robinson imagines is still TBD, but one way or another we could start turning the Keeling Curve around within our lifetimes.

And it’s never too early to start thinking about what comes next if we do.

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