The first of these, known as “carbon removal” or “negative emissions technologies,” is something that scholars now largely agree we’ll need to do in order to avoid dangerous levels of warming (see “One man’s two-decade quest to suck greenhouse gas out of the sky”). Most no longer call it “geoengineering”—to avoid associating it with the second, more contentious branch, known as solar geoengineering.

This is a blanket term that includes ideas like setting up sun shields in space or dispersing microscopic particles in the air in various ways to make coastal clouds more reflective, dissipate heat-trapping cirrus clouds, or scatter sunlight in the stratosphere.

The word geoengineering suggests a planetary-scale technology. But some researchers have looked at the possibility of conducting it in localized ways as well, exploring various methods that might protect coral reefs, coastal redwoods, and ice sheets.

Where did the idea come from?

It’s not a particularly new idea. In 1965, President Lyndon Johnson’s Science Advisory Committee warned it might be necessary to increase the reflectivity of the Earth to offset rising greenhouse-gas emissions. The committee went so far as to suggest sprinkling reflective particles across the oceans. (It’s revealing that in this, the first ever presidential report on the threat of climate change, the idea of cutting emissions didn’t seem worth mentioning, as author Jeff Goodell notes in How to Cool the Planet.)

But the best-known form of solar geoengineering involves spraying particles into the stratosphere, sometimes known as “stratospheric injection” or “stratospheric aerosol scattering.” (Sorry, we don’t come up with the names.) That’s in part because nature has already demonstrated it’s possible.

Evan Cohen

Most famously, the massive eruption of Mt. Pinatubo in the summer of 1991 spewed some 20 million tons of sulfur dioxide into the sky. By reflecting sunlight back into space, the particles in the stratosphere helped push global temperatures down about 0.5 °C over the next two years.

And while we don’t have precise data, huge volcanic eruptions in the distant past had similar effects. The explosion of Mount Tambora in Indonesia in 1815 was famously followed by the “Year Without a Summer” in 1816, a gloomy period that may have helped inspire the creation of two of literature’s most enduring horror creatures, vampires and Frankenstein’s monster.

Soviet climatologist Mikhail Budyko is generally credited as the first to suggest we could counteract climate change by mimicking this volcanic phenomenon. He raised the possibility of burning sulfur in the stratosphere in a 1974 book.

In the following decades, the concept occasionally popped up in research papers and at scientific conferences, but it didn’t gain much attention until the late summer of 2006, when Paul Crutzen, a Nobel Prize–winning atmospheric chemist, called for geoengineering research in an article in Climatic Change. That was particularly significant because Crutzen had won his Nobel for research on the dangers of the growing ozone hole, and one of the known effects of sulfur dioxide is ozone depletion.

In other words, he thought climate change was such a threat that it was worth exploring a remedy he knew could pose other serious dangers.

So could geoengineering be the solution to climate change, relieving us of the hassle of cutting back on fossil fuels?

No—although the idea that it does is surely why some energy executives and Republican legislators have taken an interest. But even if it works (on which more below), it’s at best a temporary stay of execution.

It does little to address other climate dangers, notably including ocean acidification, or the considerable environmental damage from extracting and burning finite fossil fuels. And greater levels of geoengineering may increase other disruptions in the climate system, so we can’t just keep doing more and more of it to offset ever rising emissions.

How is geoengineering being researched?

In the years since Crutzen’s paper, more researchers have studied geoengineering, mainly using computer simulations or small lab experiments to explore whether it would really work, how it might be done, what sorts of particles could be used, and what environmental side effects it might produce.

The computer modeling consistently shows it would reduce global temperatures, sea-level rise, and certain other climate impacts. But some studies have found that high doses of certain particles might also damage the protective ozone layer, alter global precipitation patterns, and reduce crop growth in certain areas.

Others researchers have found that these risks can be reduced, if not eliminated, by using particles other than sulfur dioxide and by limiting the extent of geoengineering.

But no one would suggest we’ve arrived at the final answer on most of these questions. Researchers in the field believe we need to do a lot more modeling work to explore these issues in greater detail. And it’s also clear that simulations can only tell us so much, which is why some are proposing small outdoor experiments.

Has anybody conducted real-world geoengineering experiments?

In 2009, Russian scientists conducted what is believed the be the first outdoor geoengineering experiment. They mounted aerosol generators on a helicopter and car and sprayed particles as high as 200 meters (660 feet). The scientists claimed, in a paper published in Russian Meteorology and Hydrology, that the experiment had reduced the amount of sunlight that reached the surface. (It’s worth noting that Yuri Izrael, a climate skeptic and scientific advisor to Vladimir Putin, was the lead author of the study as well as the editor of the journal.)

One of the first attempts to conduct an experiment that was openly advertised in advance as geoengineering-related, known as the SPICE project, was ultimately scrapped. The idea was to pump particles up a pipe to a high-altitude balloon that would scatter them in the stratosphere. But the proposal prompted a public backlash, particularly after it emerged that some of the researchers had already applied for patents on the technology.