Support Our Work

Scientists Consider Whether to Cause Global Cooling

By Alyson Kenward

The eruption of Mt. Pinatubo in 1991.
Credit: USGS.

Almost anywhere you turn these days, you are bound to hear a dire prognosis for climate change and the planet's rising temperatures. Well, if those aren't alarming enough, then prepare yourself for one of the potential solutions to our climate change woes: recreating the effects of hundreds of erupting volcanoes to cool our earth back down.

The idea that humans can purposefully manipulate our environment to reduce global temperatures, a term called geoengineering, has recently been gaining ground amongst climate researchers. Scientists are exploring the possibility of blocking out the sun’s warming rays or scooping excess carbon dioxide out of the atmosphere as a potential “plan B,” in case humans are unable to, or simply refuse to, reduce greenhouse gas emissions in the coming decades. The most well-studied geoengineering scenario involves injecting reflective aerosols into the atmosphere, but there are a handful of other methods scientists are exploring. The ideas are appealing, some say, because if they work, people won't have to abandon fossil fuels right away — which is likely to be extremely expensive.

Yet, even if there are ways to cool the planet artificially, a tough question remains: should we rely on geoengineering? The climate researchers studying geoengineering technologies say there is no easy answer to that question. Furthermore, each geoengineering strategy comes with it's own set of risks but there is not yet any consensus on how much risk is acceptable, especially considering that some regions of the planet will be more vulnerable to negative side effects from geoengineering. But this week at the University of Montana, Missoula, ethicists specializing in questions of climate science are helping geoengineering researchers and environmental scientists to explore the moral challenges facing this idea of a climate change "quick-fix.”

The Aerosols Approach

“The reason people thought of it in the first place is because of natural volcanic eruptions,” says climatologist Alan Robock, from Rutgers University in New Brunswick, NJ. Massive volcanic eruptions, like that of Mt. Pinatubo in the Philippines, which sent an enormous plume of ash into the sky in 1991, have actually cooled the planet in the past, but only for a short period of time, he says. Researchers credit Pinatubo, for example, with cooling the global average surface temperature by about one degree Celsius during the year following the eruption.

Research into geoengineering methods, such as releasing volcanic-like aerosol clouds, has gathered support from both U.S. and international funding agencies, but scientists aren’t convinced that a quick fix is the ideal solution to our climate woes, if instead we could limit our carbon habits. And while most scientists say research should be increased in case geoengineering turns out to be the best bet to fend off climate change, one of the most important unanswered questions is whether researchers can even test the plans to see if they are going to work.

“There is no actual strategy for geoengineering right now,” says Robock. He says that conducting small-scale experiments to test geoengineering methods is still tough because it is hard to detect how the climate changes with just small perturbations. “We don’t even have the technology available yet to measure it,” he says.

What scientists do understand is that the smoky plumes rising from active volcanoes contain aerosols of sulfur dioxide, which reflect some of the sun’s incoming radiation away from the earth’s surface. When a volcano is particuarly violent and a lot of sulfur dioxide is churned into the upper atmosphere, the effect is significant enough to cool the entire planet by a degree or more for up to a couple of years.

To emulate this volcanic effect, scientists, such as the University of Calgary’s David Keith, have proposed that we could use airplanes to spray clouds of sulfur dioxide aerosols into the upper atmosphere. Many researchers have used elaborate computer models to predict  how the global climate would respond to this, and many have demonstrated that these sulfur clouds would keep the planet from further warming for at least a few years. Keith says there are also small but real experiments to try that would not have an impact on the climate but could provide valuable information to improve the current computer models.

“Can you learn by emitting just kilogram quantities of sulfur aerosols? The answer is absolutely yes,” says Keith, who has been studying many aspects of geoengineering for more than a decade.

A TED talk given in 2007 by geoengineering expert David Keith of the University of Calgary.

In fact, an increasing number of climate change reports, including the Fourth Assessment Report from the U.N. Intergovernmental Panel on Climate Change (IPCC) from 2007 and the America’s Climate Choices reports released by the National Research Council in May 2010, have identified the need to research how effective this type of rapid climate intervention could be if it turns out to be needed in the coming decades. 

Moral Questions

According to Gordon McBean, a climate scientist at the Institute for Catastrophic Loss Reduction at the University of Western Ontario in London, Canada, it’s an area researchers need to focus on intensely in the coming years. “If it isn’t done by good scientists, we might not have the correct guidance [for policy makers],” he said.

So far, research suggests geoengineering by reflecting sunlight will keep global temperatures down, but there may be an environmental cost to pay. The sulfur-containing aerols are likely to affect regional climate and could lead to further droughts in many parts of the world, says Robock. Furthermore, this type of geoengineering strategy won't slow ocean acidification caused by increasing levels of carbon dioxide in the atmosphere. And perhaps most importantly, the cooling effect is not permanent: climate models indicate that the supply of aerosols would need to be perpetually refreshed or else the global warming, once the aerosol supply stopped, would be more rapid than if no intervention was taken at all.

With millions of dollars already invested in geoengineering research, there are also some problems that better models and more experiments can’t help solve. For example, who gets to decide that point at which global warming has become so severe that it is time to turn to large-scale geoengineering? Is global consent needed, considering that once the aerosols are emitted into the atmosphere, they are unlikely to observe the arbitrary geographical borders that divide nations?

And, perhaps the stickiest question of all, should we be searching for a technological mask to climate change when simply reducing greenhouse gas emissions could avoid the need for geoengineering?

“The idea of geoengineering is in contrast to everything that’s been written on environmental ethics in the past 50 years,” says Christopher Preston, a philosopher from the University of Montana in Missoula whose work explores how humans treat the environment. He says that, traditionally, conservation means scaling back and living within the limits of a healthy environment. “Geoengineering doesn’t do that. It changes the limits so we can live the life we want to live.”

Those that study geoengineering admit that one drawback of using sulfur-containing aerosols to cool the atmosphere could be that humans will no longer feel motivated to reduce carbon emissions if they believe a quick fix is available. But despite this moral hazard, Preston says it is possible geoengineering “will still be the lesser of two evils.” Regardless, he says, there is still a lot of work to do in evaluating all the harms and benefits.

With a grant from the National Science Foundation, Preston and a handful of colleagues in Missoula are now hoping to explore the ethical questions that face those studying geoengineering. Over the next year, they will be talking to scientists, policymakers, philosophers, and citizens around the world and, as a result, hope to offer guidance on how to proceed with geoengineering research.

Until these tough questions are answered, Robock thinks that his fellow scientists should only proceed with theoretical geoengineering experiments, though he agrees that more research can help better understand what the impact of sulfur aerosols would be and whether moving to real experiments or full deployment will be feasible.

“There are some experiments you never want to do,” he says. “This may well be one of these things.”


Bala, G., Duffy, P.B. & Taylor, K.E. "Impact of geoengineering schemes on the global hydrological cycle." Proc. Natl. Acad. Sci. USA 105, 7664 (2008).

Blackstock, J. J. & Long, J. C. S. "The Politics of Geoengineering." Science 327, 527 (2010).

Robock, A. "20 reasons why geoengineering may be a bad idea." Bulletin of Atomic Scientists, May/June, 14 (2008).

Shepherd, J.G. , Working Group on Geoengineering the Climate,. "Geoengineering the climate: Science, governance and uncertainty." London, GB, Royal Society (2009).