President Trump’s efforts to undermine U.S. climate action come at a critical time. While nations are making progress in the fight against global warming, carbon emissions are not falling quickly enough to maintain a stable climate.

Faced with rising temperatures and a dearth of American leadership, scientists are investigating geoengineering — deliberate, large-scale interventions to cool the Earth’s climate. Geoengineering can take many forms. Solar geoengineering is the most risky and controversial.

To understand how it works, look to nature. In 1991, the Mount Pinatubo volcano in the Philippines erupted in a big way, spewing millions of tons of sulfur dioxide gas into the atmosphere. Once there, the gas turned into small droplets that circled the globe for weeks, reflecting sunlight back into space. The result: Earth became cooler.

“Nature performed this experiment for us,” said Trude Storelvmo, associate professor of geology and geophysics at Yale. Storelvmo studies the role of aerosol particles and their effect on climate. “We had a cooling of a half degree of Celsius or more for a year or two after the eruption, until the particles settled down again. Every time we have a volcanic eruption, it puts particles up there. That’s how we know for a fact that this works.”

The 1991 eruption of Mount Pinatubo. Source: United States Geological Survey

By “this,” she means solar geoengineering, a process that involves the deliberate injection of reflective aerosols such as sulfuric acid or diamond dust into the stratosphere as a way of temporarily offsetting the warming effects of greenhouse gases. Scientists have been studying the concept in the lab and in computer models in recent years, and increasingly, they have come to believe it eventually could be a valuable adjunct — but not an alternative — to cutting emissions.

“Unlike reducing our emissions of greenhouse gases, climate change doesn’t stop when you use geoengineering, which is why you couldn’t use it forever,” said Kate Ricke, assistant professor at UC San Diego’s Scripps Institution of Oceanography. “It’s one of the big reasons it’s not a substitute for mitigation efforts, and it’s not a long-term solution.”

A single act of solar geoengineering could cool down the entire planet, albeit for a short while, if the materials were dispatched from the right location. “If it takes place at the Equator, whatever enters the stratosphere will spread out to the entire globe,” Storelvmo said. “The atmosphere will do that for you if the injection happens at the right latitude, which would be in the tropics.”

A computer rendering showing how ash from Mount Pinatubo had circled the globe two weeks after the volcano erupted. Source: United States Geological Survey

Also, the effects are temporary, so the particles must be reloaded periodically. “The lifetime of sulfate particles is about a year, so you must continuously replenish it to maintain a constant amount,” said Alan Robock, professor of environmental sciences at Rutgers University. “Mount Pinatubo put 17 to 20 million tons into the stratosphere. You might have to do a Pinatubo every year.”

Geoengineering raises a number of ethical, political, economic and social issues, including the need for safeguards and for some kind of international monitoring or regulation.

“No one owns the stratosphere,” said Edward Parson, professor of environmental law at UCLA. “There is no international law that says you can’t do this. But it must be regulated at the international level. Whether it can be is an open question. It will be a big problem without an international structure to legitimize, control and tame it, and ensure that it doesn’t weaken mitigation. It’s a huge challenge.”

There are numerous other problems. Sulfate aerosols would likely deplete the ozone layer. Because solar geoengineering disrupts hydrological cycles, it could trigger drought in some regions and flooding in others. It would also do nothing to curb ocean acidification, which is caused by the buildup of carbon pollution in the world’s oceans.

The particles also potentially could contribute to respiratory problems, although their impact would be far less than that already produced by burning fossils fuels, Robock said. “The sulfur will eventually come into the troposphere and fall out as acid rain,” he said. “It really depends on how much is put into the stratosphere. If it is 5 teragrams (or 5 million tons) SO2 per year, we can compare that to about 100 teragrams (or 100 million tons) SO2 per year put into the troposphere as a by-product of burning fossil fuels. So…while it would increase the sulfur in the troposphere a little, and it would certainly be bad for breathing and for the surface, it would not be enough to be a deal-breaker.”

On the other hand, “if it can be done — which we don’t know — and it reduces global warming, it might be such a big benefit it would outweigh all the risks,” said Robock, who included a list of potential risks and benefits in a paper published last year.

With solar geoengineering, scientists would spray sulfate aerosol into the upper atmosphere to block sunlight. Source: Hugh Hunt

Scientists studying geoengineering have called for the government to invest in research to better understand its effects. In 2015, the National Academies of Sciences (NAS) issued a report calling for more research into solar geoengineering. Authors were emphatic that nations should continue to cut carbon emissions even if they implement geoengineering.

“As the report emphasizes, there is no substitute for dramatic reductions in heat-trapping emissions,” says Peter Frumhoff, director of science and policy for the Union of Concerned Scientists. “Preventive medicine is far more attractive than getting treated in the emergency room.”

The potential benefits of solar geoengineering “warrant a large-scale international research effort,” said David Keith, professor of applied physics at Harvard and a proponent of geoengineering research. Keith cites economic damages from climate change at more than a trillion dollars per year later this century. “A geoengineering project large enough to cut the economic damage in half could be implemented at a cost of a few billion dollars per year, several hundred times less than the economic damage it would prevent.”

“I think there’s a strong ethical case for research because results today strongly suggest that the benefits of solar geoengineering would be largest for the world’s poorest and most vulnerable,” Keith added.

In what would be the first-ever solar geoengineering field experiment, he and his colleagues hope to initiate several small-scale localized outdoor flights out of Tucson next year, launching a propeller-driven balloon that would inject sulfate aerosol and water into the air. The intent is not to cool the climate, but to study what happens to the particles. Initial flights will release water vapor, and later flights will use solid aerosols such as calcium carbonate. “We may do sulfates, but they would be later in the experiment series and are not a prime focus,” Keith says. “The reason for the calcium carbonate work is because of ideas about how that could be used to do solar geoengineering while restoring the ozone layer.”

A rendering of the stratosphere balloon Keith and his colleagues would use to study solar geoengineering. Source: Keutsch/Keith group

“We see the experiment as very much a part of the normal activities of atmospheric and environmental science,” Keith says. “You need to couple laboratory work, experiments and modeling in order to minimize the chance of being fooled.”

For his part, Keith says he would welcome the oversight. “I don’t regard regulations as a hurdle,” he says. “I’m generally someone who would like to see stronger environmental regulation. My view is simply that we would be better off knowing more both about the science and about the governance, as the worst decisions are often the ones made in haste and ignorance.”

Marlene Cimons writes for Nexus Media, a syndicated newswire covering climate, energy, policy, art and culture.