With global greenhouse gas emissions still on the rise, despite decades of talk about curbing them, maybe the time has come to think differently about the climate crisis. Yes, we need to burn less coal, oil and natural gas, but clearly fossil fuels are going to be around for awhile. So why not try to clean up the mess they make?

That’s what a handful of prominent scientists are trying to do by developing technologies to remove carbon dioxide from the air. These scientists have launched start-up companies and attracted well-to-do investors — most notably Bill Gates — along with venture capital and, most recently, the attention of Wall Street. They say their technology does not need government support, though it would help. What it needs, above all, is a mindset that regards CO2 not simply as a pollutant but as a valuable commodity.

Nathaniel “Ned” David, the chief executive of a startup called Kilimanjaro Energy, puts it this way: “The single largest waste product made by humanity is CO2. Thirty gigatons a year. It’s immensely valuable, and today we just blow it out the tail pipe. What if there were some way to actually capture it, use it, and make money?”

Carbon dioxide removal, or CDR, is sometimes seen as a subset of geoengineering — deliberate, planetary-scale actions to cool the Earth — but it’s actually quite different. Geoengineering strategies are risky, imperfect, controversial and difficult to govern. The most-discussed geoengineering technology, solar radiation management, alleviates a symptom of the climate problem (warmer temperatures) but does nothing to address the cause (rising atmospheric concentrations of CO2). What’s more, geoengineering as a climate response is stuck because governments have declined to provide more than token funds for research, and there’s no business model to support it.

Carbon dioxide removal, by contrast, targets the root cause of global warming. It doesn’t create global risks. It’s being financed by the private market, and it’s more akin to recycling waste than to playing God with the weather.

Despite widespread skepticism in the scientific community, three startup companies are betting that they can make money by recycling CO2, and thereby cool an overheating planet. Kilimanjaro Energy is the pioneer. The company was launched in 2004 by Klaus Lackner, a Columbia University physicist who first wrote about air capture of CO2 in a 1999 paper. It was initially financed with $8 million from Gary Comer, the founder of Land’s End, who grew concerned about climate change after he sailed a yacht through the normally ice-bound Northwest Passage with relative ease. (Comer died in 2006.) Last year, Kilimanjaro raised another $3.5 million from a venture firm called Arch Venture Partners.

Global Thermostat, a second startup, also took root at Columbia. Its founders are Peter Eisenberger, a former head of research for Exxon who started Columbia’s Earth Institute, and Graciela Chichilnisky, who holds dual Ph.Ds in economics and math. Edgar Bronfman Jr., the former Warner Music CEO and heir to the Seagram’s fortune, has put $15 million into venture, and big private equity firm is in talks with the founders about taking a major stake in Global Thermostat. (Eisenberger and Chichilnisky wouldn’t identify the investor.)

Global Thermostat has built a small demonstration plant at SRI International in Menlo Park, Calif., that today is sucking carbon dioxide from the air. About the size of a two-story elevator shaft, the pilot module sucks air past porous ceramic blocks known as monoliths, where amines bind with the carbon dioxide; the blocks are then lowered into a chamber where they are flooded with steam that releases the CO2, and the process then repeats itself.

Finally, there’s Carbon Engineering, a startup run by David Keith out of Calgary, Alberta, the nerve center of Canada’s oil and gas industry. Bill Gates is an investor, as is his friend Jabe Blumenthal, a former Microsoft executive who is passionate about climate issues. So is N. Murray Edwards, an oil and gas billionaire. Keith, a physicist and climate scientist, has a joint appointment at the University of Calgary and at Harvard’s Kennedy School.

There’s no doubt that CO2 can be removed from the air using chemical processes. That’s how people can breathe on submarines or in spaceships. But the conventional wisdom among scientists is that it’s expensive and therefore impractical to do air capture on a global scale. Last year, a committee of the the American Physical Society produced a 100-page technology assessment, called Direct Air Capture of CO2 with Chemicals, which estimated that the cost of an air capture system would be “of the order of $600 or more per metric ton of CO2.” The report concluded: “Direct air capture is not currently an economically viable approach to mitigating climate change.”

Howard Herzog, an MIT professor, argues that it makes more sense to capture CO2 from the flue gas of power plants, where concentrations are higher — about 12 percent for coal plants or 4 percent for natural gas plants. (In the air, CO2 levels remain under 400 parts per million, which means that less than 0.04 percent of the air is CO2.) Herzog says anyone who claims that they can capture CO2 from the air at a low cost is “either not being totally honest or they’re deluding themselves.” He co-authored a peer-reviewed study in the Proceedings of the National Academy of Sciences that estimated the cost of air capture at “on the order of $1,000 per ton of CO2.”

“I am absolutely sure that’s wrong,” replies Carbon Engineering’s David Keith. In an FAQ on its website, Carbon Engineering offers a “conservative estimate” of the cost of air capture at “less than $250 per ton” of CO2 and says that it will drive costs lower. In his 1999 paper, Lackner estimated the cost of air capture as “on the order of $10 to $15 per ton,” a target that now appears wildly optimistic. This argument about about costs is crucial to the future of air capture, but it is unlikely to be settled until one of the startups begins to build industrial-scale plants.

Costs matter — a lot — because there’s substantial demand for CO2, at prices that can top $100 a ton. Most of it comes from oil companies that want to inject liquified CO2 into reservoirs to squeeze out stranded oil, a proven technology called enhanced oil recovery (EOR). The U.S. government estimates that state-of-the-art EOR using CO2 could add 89 billion barrels of oil to the recoverable resources of the U.S. That’s more than four times current proven reserves.

“The single largest deterrent to expanding production from EOR today is the lack of large volumes of reliable and affordable CO2,” says Tracy Evans, the former president of Denbury Resources, which specializes in enhanced oil recovery.

Each air-capture startup is pursuing its own technology and plant design. Global Thermostat plans to use residual waste heat from power plants to run its machines, while Carbon Engineering is betting on a technology known as “wet scrubbing” in which a water-based solution absorbs CO2 from air that is passed through devices known as air contactors. Each machine will require massive amounts of hardware, and thousands of machines would need to be built to have a meaningful climate impact.

All three startups intend to get their businesses rolling by selling CO2 to the oil industry. Farthest along is Global Thermostat, which has had serious conversations with a Seattle-based energy firm called Summit Power about building a demonstration plant to capture CO2 and extract stranded oil, as part of Summit’s massive, government-backed Texas Clean Energy Project. Liquid CO2 used for EOR would be sequestered underground, offsetting emissions generated when the oil is later burned. By some estimates, oil recovered that way would have roughly half the carbon footprint of conventional petroleum. This oil, the theory goes, could be made into lower-carbon transportation fuels with special appeal to customers — airlines, most obviously — that face regulatory pressure to reduce emissions.

Over time, if costs come down, air capture technology could serve CO2 markets beyond the oil industry. At least two startups have been talking to algae companies that would like to enrich air with CO2 to feed algae to
produce biofuels. “Algae is the most efficient creature for making fuels, and it can’t on its own harvest enough CO2 from the atmosphere,” says Ned David of Kilimanjaro, who previously worked at Sapphire Energy, an algae firm. Capturing carbon from the air to feed algae makes possible a carbon-neutral, closed-cycle fuel — that is, one in which the CO2 released when the fuel is burned is offset by the CO2 absorbed when it is produced.

At Global Thermostat, Eisenberger and Chichilnisky talk about making transportation fuels by combining CO2 with hydrogen extracted from water. (They have formed a joint venture with an unnamed startup that they say can produce hydrogen from water at a lower cost than previously possible.) If the process could be powered by solar energy, it could produce renewable, carbon-neutral hydrocarbons for cars, trucks, ships and planes. “This has always been for me the holy grail, even back when I was at Exxon in the last energy crisis,” Eisenberger told me. “It solves the energy security issue since everyone has water and CO2 from air.” Any nation could become an oil producer.

Because greenhouse gases are dispersed around the globe, air capture can be done anywhere. This fact is key to the business plans of all three startups. Carbon Engineering’s business model, for example, revolves around what Keith calls “physical carbon arbitrage.” The company plans to build its first carbon-capture plants in places with cheap labor, cheap land, cheap construction costs, cheap natural gas to operate them and, ideally, strong demand for CO2. “If we can find all those at once,” he says, “we’re printing money.”

What this means for the environment is that carbon pollution need not be cleaned up at its source. CO2 spewing from a tailpipe in Sao Paulo or a coal plant in China can be captured by machines in Iceland or the Middle East because the atmosphere functions as a conveyor belt, moving CO2 to any sink. Air capture may prove to be the only way to absorb dispersed emissions from cars, trucks, trains, ships or planes.

It’s an exciting prospect, at least in theory. But remember — the scientific establishment says this is all pie in the sky. What’s more, for air capture to do what we’ve failed to do so far — reduce emissions on a scale that matters to climate — these tiny startups would have to spawn a giant, global industry, employing thousands of engineers and requiring many billions of dollars of investment. “If air capture is going to succeed, it’s going to take industrial might,” says Keith. To reduce atmospheric concentrations of CO2 by one part per million — they’re now at about 390 ppm, which some scientists think is too high — would require the removal of about 8 gigatons, or 8 billion tons, of CO2.

Given the obstacles ahead, most everyone who has looked at carbon dioxide removal warns that the technology cannot be seen as a license to keep burning fossil fuels. As Steve Hamburg, chief scientist at the Environmental Defense Fund, puts it, “We’ve got to mitigate emissions — that’s first, second, third and tenth.” But until we do, coming up with a backup plan can’t hurt.