In June 1991, Mount Pinatubo in the Philippines erupted. It was the second biggest volcanic event of the century. Hot ash shot 22 miles into the sky.
The ash spewed 20 million tons of sulfur dioxide into the stratosphere. That gas formed a kind of shield around the earth, which reflected sunlight back into space. The reduction in sunlight beaming down to earth led the planet to cool for two years. The temperature dipped .5 degrees Celsius — a big drop in climate terms.
Scientists have long wondered if people could do something like that on purpose — cool the earth by injecting a substance into the stratosphere, the second layer of the atmosphere. Called "solar geoengineering," the controversial technology could be used by humans to deliberately alter the climate.
"They would reflect away a little bit of sunlight, just the way a very thin cloud reflects away a little bit of sunlight ... to offset some of the warming that comes from the slow buildup of carbon dioxide from our industrial activity," says Harvard University applied physics professor David Keith.
For a long time, research into solar geoengineering was taboo. It's a powerful technology with the potential to disrupt the atmosphere, climate and people's health.
Keith and Harvard atmospheric chemistry professor Frank Keutsch are planning a first-of-its kind field experiment to help scientists understand solar geoengineering's efficacy and risks. They plan to release an aerosol — less than 2 kilograms of calcium carbonate — into a tiny portion of the stratosphere, to see how it behaves.
For now, though, Keith and Keutsch are working in a lab at Harvard. There, much of their attention is focused on a metal tube about a yard long and a couple of inches wide, which is acting as a mini stratosphere. They're conducting experiments inside the tube similar to the one they want to carry out high above the earth.
"So we go to stratospheric temperatures. We go to stratosphere pressures in there," Keutsch explains. "We put in the aerosol that we want to understand the reactivity of. And then we look at these reactions [in the tube]."
That aerosol — the calcium carbonate — is the same compound that's in limestone and shells. It's in common consumer products including toothpaste and antacids. Calcium carbonate isn't in the stratosphere — but one day, humans could spray enough of it there to reflect away the sun.
In the Harvard experiment, though, the scientists want to measure how the compound interacts with gases in the stratosphere. That could help determine if it would further damage the ozone layer.
David Keith says their plan is to launch a scientific balloon, likely in the American southwest.
"The balloon is about a few stories in diameter, and these are the same as other stratospheric balloons that have been used for science for a long time," he says.
It would rise 12 miles high — twice as high as a passenger plane flies. Keutsch explains that the balloon would have a metal gondola hanging from it, with propellers.
"That gondola has a mechanism that allows you to inject particles into the stratosphere," Keutsch says. "So as you're moving along, you make a plume that is about a kilometer, few kilometers in length. Then you would try to turn the balloon around and sample back going through this plume, comparing how the air outside of the plume is different from the air and the aerosol in the plume."
Their project is called SCoPEx — short for Stratospheric Controlled Perturbation Experiment.
The Harvard scientists insist the experiment itself is not solar geoengineering; they say it's too modest to affect the climate, and it's safe. Others disagree.
"As soon as you start putting particles into the stratosphere, aren't you intervening with climate?" asks Daniel Cziczo, the head of the department of earth, atmospheric and planetary sciences at Purdue University. For most of the last decade, he taught at MIT.
Cziczo says the SCoPEx project fits into a category he refers to as "climate intervention" instead of solar geoengineering, and he says the science is dangerous.
"Even though you're saying, 'I'm doing it as an experiment,' the result is exactly the same," he says. "It might be a smaller scale ... But I would argue that if you're putting enough material there that you're going to be looking at the properties of that material and trying to tell what effect they have, that what you're doing is an experiment on climate intervention."
Cziczo says full-scale climate intervention would disrupt the formation of clouds, further hurt the ozone layer and do nothing to reduce carbon in the atmosphere.
"I'm not willing to sign up for, you know, something that's a solution to the [earth's] temperature problem that's going to cause more skin cancer, that's going to allow for continued acidification of the oceans, or that's going to cause regional droughts or precipitation," Cziczo says. "I'm just not personally comfortable with that, especially when we have another path in climate — which is the reduction of these greenhouse gases that we know are the underlying cause."
The three scientists agree on that point: that the real solution to global warming is to cut back on fossil fuels. They're concerned people will view solar geoenginering as an easy out; they refer to that as the "moral hazard" of the science.
"This experiment can be misinterpreted as that there is a technical remedy that gets us off the hook, where it should be clear that ... even in the best case scenario with the solar geoengineering in the stratosphere, it doesn't address the causes of climate change," Keutsch explains. "It's addressing a symptom."
David Keith is concerned about the energy industry shirking its responsibility in helping reduce global warming.
"I'm talking about big oil, big fossil fuel companies — that they will claim that because of this techno fix, there's no need to cut emissions. That's not true," Keith says. "Nothing about solar geoengineering changes the fact that if you want a stable climate, you eventually have to cut emissions to zero."
Keith says he sees promise in solar geoengineering.
"Nothing about solar geoengineering changes the fact that if you want a stable climate, you eventually have to cut emissions to zero."
"The combination of emissions cuts and solar geoengineering could actually be significantly safer, have lower human and environmental risks, less sea level rise, less people dying from heatstroke, less change in the high arctic glaciers than would a world with just emissions cuts," he says. "So there's really a prize here, a huge human and environmental benefit."
But the scientific community needs to research the consequences — good and bad, Keith says -- before world leaders make rash decisions.
"Some government faced by maybe a huge killing heat wave may make decisions to actually move toward deploying these technologies within the next decades, whatever research we do," Keith says. "We are more likely to make a reasoned decision as a species, as humanity, about this if we get it out in the open, warts and all."
There is no national or international governing structure for solar geoengineering. The proposed SCoPEx experiment may eventually be reviewed by the federal government under national laws regulating environmental policy and weather modification.
Harvard has formed an independent committee to review and make recommendations on all aspects of the experiment, including its safety and environmental impact.
Meanwhile, the coronavirus pandemic kept David Keith and Frank Keutsch out of their lab for a few months. They're back now, working toward the day they get the go-ahead to launch their big balloon.
This segment aired on July 22, 2020.