Earth keeps getting hotter. Humanity isn't doing enough to stop it. So, scientists are increasingly musing about conducting dramatic interventions in the atmosphere to cool the planet. And new research suggests that a project of atmospheric cooling would not only be doable, but also cheap enough that a single, determined country could pull it off. That cooling wouldn't reverse climate change. The greenhouse gases would still be there. The planet would keep warming overall, but that warming would significantly, measurably slow down.

Those are the conclusions of a paper published Nov. 23 in the journal Environmental Research Letters by a pair of researchers from Harvard and Yale universities. It's the deepest and most current study yet of "stratospheric aerosol injection" (also known as "solar dimming" or "solar engineering"). That's the spraying of chemicals into the atmosphere to reflect the sun's heat back into space, mimicking the global cooling effects of large volcanic eruptions.

The researchers found that humanity could, using this method, cut our species' annual contributions to the greenhouse effect in half at a price that states and large cities spend all the time on highways, subways and other infrastructure projects: a total of about $3.5 billion over the course of the next 15 years to develop the technology. (Most of those funds would go into building planes able to carry big tanks of aerosol spray into the stratosphere, about double the cruising altitude of a Boeing 747.) Once the tech is ready, the researchers found, the project would then cost another $2.25 billion or so each following year (assuming the effort would run for the next 15 years).

For comparison, the Massachusetts Department of Transportation budget in 2017 was $1.8 billion. Texas will have spent nearly a billion dollars replacing a single bridge in Corpus Christi. New York City subway-repair budgets routinely run into the tens of billions of dollars. Belgium spends about $4 billion every year on its military. In other words, geoengineering the atmosphere to slow climate change is cheap enough that a small, determined state or country could probably afford to do it, not to mention a superpower like the U.S. or China. [8 Ways Global Warming Is Already Changing the World]

That might seem nuts, but outside researchers who read the paper said its methods were sound and its conclusions not all that surprising.

"[The paper] seemed reasonable and methodical to me," said Kate Ricke, a professor at the University of California, San Diego, who studies climate change and policies for addressing it. "I think it's definitely a helpful contribution, in that it confirms this idea that stratospheric engineering would be much cheaper than emissions reductions for the same global temperature effect."

Ken Caldeira, a senior scientist at the Carnegie Institution for Science, agreed.

"One could expect any governmental operations to have cost overruns, but overall, I've got no reason to question these findings. They seem reasonable to me," he told Live Science.

Does that mean this is a good idea? Should we start building the spray planes?

The science here is in a certain respect straightforward: Dump sulfur dioxide (SO2) into the atmosphere, and it will reflect light back into space. SO2 is cheap, and there's lots of it available. Most of the costs of the project would come from lofting the SO2 high enough that it would stick around, Wake Smith, a co-author of the paper and lecturer at Yale, said. [Cool the Planet? Geoengineering Is Easier Said Than Done]

"If you deploy material at 35,000 feet [10,700 meters], say, where your 737 flies, it rains back down in a few days, because it's just being acted on by gravity," he told Live Science. "If you get it up into the stratosphere, on the other hand, then it stays aloft for a year or 18 months."

(This, incidentally, is one of the reasons that chemtrail conspiracy theories — which mistakenly link chemtrails to a secret government plan to modify the weather — are so implausible, he added. Anything sprayed at the heights at which jetliners fly would disappear within half a week.)

Still, getting the SO2 high enough isn't an insurmountable challenge, this paper shows, and the approach really could cool down the planet.

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But cooling down the planet isn't the same thing as reversing climate change, the researchers explained.

Carbon emissions do a lot more than just form a chemical greenhouse around the planet. They also make the oceans more acidic and alter the global movement of air and water. Already, these emissions have baked heat into the system that wouldn't just go away if humanity slapped a layer of SO2 into the stratosphere. [The Craziest Climate Change Fixes]

"It may be that we can reduce global surface temperatures​ overall, relative to where they would be in an un-engineered world," Smith said, "but that doesn't mean that the climate in every place will go back to the way it was. Some places will be warmer. Some will be cooler. Some will be drier. And some will be wetter, and even a perfectly engineered climate future, which is impossible, will change things all over the world, and that won't be good for people either."

Plus, he said, there are tipping points in climate change that an SO2 bandage wouldn't fix.

"If all the ice in Greenland melted and slid into the sea," Smith said, referring to a scenario that would drastically raise sea levels, flooding coastlines all over the world, "and then we refreeze the planet, or cool the planet by engineering, the ice won't climb back up from the sea onto the land. The ice on Greenland is the result of millions of years of snowfall."

So, even though he thinks this sort of geoengineering is worth studying, he said it's important that people understand that it isn't a solution.

"I do worry that some fossil fuel company will say exactly that, and the geoengineering community is going to have to figure out how to guard against that infiltration or any association in the public's mind," he said.

Is this our future anyway, like it or not?

The idea of pumping aerosols into the upper atmosphere to mitigate climate change taken seriously enough that the concept turned up in the recent 2018 IPCC report on climate change as a possible mitigation approach — though the IPCC stopped short of endorsing this sort of spraying. Right now, it looks cheaper than alternative geoengineering technologies, Ricke said, like proposals to suck carbon dioxide out of the atmosphere. (The IPCC, or Intergovernmental Panel on Climate Change, is an international organization established by the United Nations to assess the science, risks and impacts of climate change.)

But that doesn't mean that such approaches will, or should, happen, the researchers all agreed.

"I don't think it's a good idea at this point," Ricke said. "I don't think we know enough about how to do it. And we don't have anything close to a system for reaching agreement about the amount we should do or how we should make that decision about the specifics of where we would put more aerosols, et cetera. I don't think we're anywhere close."

But all of that could change, she said.

"There's a lot of scary climate-change impacts, like melting glaciers in Greenland and Antarctica, that are staring us in the face," she said. "Because [cutting emissions] and CO2 removal will take some time, even if we get serious about implementing them — which I'm not convinced about — I think that solar geoengineering has the potential to be one of the only options left."

That's worrying for a number of reasons, Smith said, one of which is that there would almost certainly be side effects that the sprayers couldn';t anticipate. Though one benefit of the spraying, he added, is that as soon as it's stopped its effects would go away within 18 months.

Caldeira agreed that the use of such engineering looks more and more likely, but said he doubted it would happen, due to the political dynamics involved. No politicians, he said, would want to take the blame for a bad weather event that occurred the year after they voted to spray SO2.

"Imagine if Hurricane Sandy happened on the year after we started putting this material up there," he said, suggesting people could place blame on the atmospheric engineering.

Still, he said, a small country badly hit by climate change might decide to do this without global approval. However, the paper noted that such an effort would be impossible to keep secret, and other, larger nations might decide to stop the project. Doing this work properly would require flying all over the world's middle latitudes, and this would have to go on indefinitely. (Masking the warming effect of greenhouse gases doesn't make them go away, and they can last for a thousand years in the atmosphere, unlike sulfates. So, the solar engineering would have to continue, to counteract those effects.)

"I'm not going to say whether [I think we'll get to the point of atmospheric spraying]," Smith said, "not because it's too much of a hot potato, but because I really don't know."

Other techniques for geoengineering might become cheaper, or nations might just never get around to this sort of climate mitigation, he said.

For now, Ricke said, the big open questions involve stratospheric chemistry — how sulfur would interact with other chemicals in the atmosphere — and the local effects of this sort of program. How would a big new batch of SO2 in the atmosphere affect the ozone layer, for example? How would individual regions, agriculture or local water systems react to the sudden change in sunlight? How would the public react?

For now, she said, she wants to see a lot more research.

Originally published on Live Science.

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