If the world had a giant thermostat, who would control the weather?

As the prospect of a warmer planet becomes reality, scientists are seeking ways to control the climate and keep the planet cooler. It’s a risky and highly controversial idea and, if successful, could imperil the ozone layer and lead to changes in rainfall patterns worldwide. It could also pit nations against one another as they try to control the weather or even use it as a weapon.

“Whose hand would be on the thermostat?” a leading climate scientist at Rutgers University, Alan Robock, asked the House Committee on Science, Space and Technology in 2009. “What if Canada or Russia wanted the climate to be a little warmer, while tropical countries and small island states wanted it cooler?”

To understand how people might take control of the planet’s climate, scientists have been studying volcanic eruptions that have spewed ash and sulfur aerosols into the atmosphere. The resulting blanket of particles has caused the earth to cool in the past, but not without consequences.

Three eruptions in the Northern Hemisphere in the 1970s and ’80s were followed by three of the driest years on record in the Sahel, an area of scrubland that stretches across the width of Africa, just south of the Sahara. Those rainless years contributed to one of the harshest droughts in northern Africa in recent memory. More than 300,000 people died as a result, and millions more, weakened by famine, migrated from villages to cities in search of food, where they were marooned in poverty and died from disease.

“(The drought) had a huge impact on people there,” says Robock.

That sort of fallout from unstable climatic conditions has increased scientists’ concern about a new technology called solar-radiation management (SRM). This technology used to be taboo, but with the current concern over rising global temperatures, many worry that SRM could be gaining favor.

In its latest report (PDF), the Intergovernmental Panel on Climate Change (IPCC) discussed the potential use of SRM for the first time. Much like a volcanic eruption, it entails putting sulfur aerosols or other particles into the stratosphere to shield the earth from incoming solar rays. As observed in the ’80s, this would likely cool the planet and temporarily stave off global warming’s harshest effects, such as the melting of polar ice caps. But as seen in the Sahel, SRM could also reduce rainfall in some areas, while other regions would potentially benefit from more precipitation. And that’s just one of its possible drawbacks.

Solar-radiation management is only one type of geoengineering. Other techniques for blocking solar rays or removing carbon dioxide from the atmosphere were until recently relegated to the realm of science fiction, given their immense downsides. Nevertheless, federal and private money is driving some scientists toward the first field test of SRM. 

Established by the United Nations in 1988, the IPCC is an international body of thousands of scientists who review the state of the world’s knowledge about climate change. Roughly every five years they issue a report summarizing their collective assessment. The latest one is being released in three parts, the first of which states for the first time in the panel’s 25 years of existence that solar-radiation management has the “potential to substantially offset a global temperature rise.” That is a signal for some that current methods for containing climate change may be inadequate and that drastic measures may now be required to prevent a global catastrophe.

The report’s recognition of SRM’s potential is steeped in ominous caveats. SRM would also modify the global water cycle, the report states, and ocean acidification would continue unabated. If SRM were discontinued for any reason, “there is high confidence that global surface temperatures would rise very rapidly” to the level they would have reached without the technology. The report concludes that SRM methods “carry side effects and long-term consequences on a global scale.”

Robock, who is studying geoengineering on a National Science Foundation grant, is one of a long line of climate scientists who have expressed concern about changing global precipitation patterns, damage to the ozone layer, reductions in the efficiency of solar-power equipment and the continued whitening of our skies. Many also worry that SRM could be misconstrued as a substitute for reducing greenhouse-gas emissions.

Despite that, the increased emphasis on the study of SRM began when Nobel Prize–winning chemist Paul Crutzen suggested it in a 2006 paper. Attempts to lower greenhouse-gas emissions have been “grossly unsuccessful,” he argued. A worst-case scenario of warming — a 5 degree Celsius increase in this century — could melt the ice sheet of Greenland or the poles and cause the sea level to rise dramatically. SRM could possibly prevent that, he wrote.

Since Crutzen’s paper was published, more climate scientists and physicists have begun studying the technology and its effects. But most are quick to state that they hope it is never needed.

“The solution to global warming is to stop putting carbon dioxide in the atmosphere,” says Robock. “Every person working on geoengineering will tell you that. Nobody really wants to be working on geoengineering. They want mitigation to work.”

In the 1980s Robock contributed to the world’s understanding of how multiple, large-scale detonations of nuclear weapons would send a sufficient number of particles from smoke and soot into the atmosphere that a nuclear winter would result. Sunlight would be unable to reach the Earth, plunging temperatures down to winter conditions for years. That sobering reality was a sufficient motivation for the Soviet Union to move to end the arms race.

Similarly, Robock says that geoengineering could prove too problematic to use if “the research shows that you would have unacceptable change to agriculture or to the amount of ultraviolet (light) reaching the ground.”

While it seems apparent that an increase in sulfur aerosols in the atmosphere leads to a decrease in global temperatures, questions remain about specific types of aerosols and the effects in different regions, such as rainfall in the Sahel.

With the National Science Foundation funding, Robock has been able to spearhead a significant effort by a consortium of climate scientists to examine the long-term effects of solar-radiation management. As in the study of nuclear winters, Robock and others use computer models to predict the planet’s response to changes in temperature.

The Geoengineering Model Intercomparison Project (GeoMIP) began two years ago. Scientists ran four SRM scenarios through 12 computer climate models.

Simone Tilmes, a climate scientist with the National Center for Atmospheric Research, used GeoMIP models to determine that if SRM were employed throughout the stratosphere to rapidly reduce warming to preindustrial levels, it would decrease precipitation worldwide by 4.5 percent. It could also cut monsoon rains in East Asia, South Africa, North America and South America by up to 7 percent and evaporation by 10 percent. A paper that Tilmes and her team prepared compared those results with what would happen by 2100 if the world does nothing to abate greenhouse-gas emissions. In that scenario, precipitation would increase by almost 7 percent globally, with a significant uptick in heavy rains.

The study’s parameters are extreme and represent highly improbable scenarios, Tilmes points out. How accurately one can extrapolate from extreme scenarios is a subject of debate among researchers. But in Tilmes’ view, “it is very likely that precipitation will get reduced.” The effects of such a reduction on plants is unclear, given that there would also be a drop in evaporation. Those two effects could counterbalance each other.

But David Keith disputes how researchers are reporting the modeling-experiment results. A Harvard physicist and public-policy professor, Keith is an outspoken advocate of geoengineering, particularly of solar-radiation management. His new book, titled “A Case for Climate Engineering,” was published last fall. By not making it clear enough to the public that the scenarios are unrealistic, the GeoMIP researchers are “spinning” their results, Keith argues.

In particular, he questions why Tilmes’ group chose to test extreme scenarios. “Maybe because they weren’t too thoughtful, maybe because they want to make (the use of geoengineering) look worse than it is,” he says. “There’s nothing scientifically wrong with it. The issue is just how you report the results. And I think that some people in that group need to be a little more careful.”

Robock defended his team’s practices as ordinary. When trying to predict climate, modelers account for as many variables as possible in computer simulations, including constantly changing weather patterns. With such variability, it can be difficult to identify trends. The standard way in climate modeling to overcome that challenge is to create extreme scenarios that tease out potential impacts.

“We look at it as a very artificial experiment,” Robock explains. “But it’s a standard practice. And we’re certainly not advocating (these scenarios). We hope to learn from it.”

The findings from GeoMIP and earlier studies have caught the attention of the International Red Cross and Red Crescent movement and its Climate Centre.

“My prediction is that the Red Cross/Red Crescent will have to deal with the consequences (of geoengineering),” says Pablo Suarez, who works with the Climate Centre and has a background in climate science. “And we will not have additional money, equipment, people or brainpower to get it done.”

Suarez and his colleagues are urging other humanitarian groups to begin preparing for the fallout from geoengineering, including situations in which underdeveloped nations are unable to compensate for resulting weather changes and have to rely on humanitarian organizations for assistance.

Given that SRM would have global implications regardless of who deploys it, the U.S. and other nations, as well as policy organizations, are paying close attention to its evolution. Congress held hearings about the topic in 2009. The Government Accountability Office issued reports on geoengineering’s governance in 2010 and technical feasibility in 2011. The National Academy of Sciences is midway through an assessment of geoengineering, funded in part by the Central Intelligence Agency. “There are going to be winners and losers if SRM technologies were ever to be used,” says Alex Hanafi, an attorney with the Environmental Defense Fund (EDF). “So that creates a real risk of international conflict, where you have some countries saying … ‘I suffered more, so I need some kind of compensation’ or ‘You need to stop doing SRM.’”

In 2010, the EDF helped launch the Solar Radiation Management Governance Initiative, which has held workshops in China, India, Pakistan, Senegal, Ethiopia and South Africa to discuss SRM. It is too soon to consider government oversight of this technology, Hanafi says. But “early cooperation and some transnational dialogue will help the global community make better decisions and avoid the kind of international tension that can come from secrecy.”

In 2010, almost all U.N. member states signed the Convention on Biological Diversity, a nonbinding agreement to effectively ban geoengineering until a global regulatory approach is in place. (The U.S. did not sign.) The convention stipulates that there must be an “adequate scientific basis to justify geoengineering,” one that takes the inherent risks into account. It also allows small-scale studies designed to answer specific research questions about SRM. That opens the door for researchers like Keith.

Keith has proposed a balloon test to measure possible effects of the technology on the ozone layer. He and his colleagues suggest releasing roughly a kilogram of sulfuric acid and 100 kilograms of water into the atmosphere. The experiment would last just a day, with negligible impact on the environment, he says. They could be testing flight equipment in as little as two years. If the field test is approved by the federal government — Keith declines to state what agency he approached — it would be the first to examine questions specific to SRM.

“I’m not interested in saying a lot more that isn’t now public,” he says. “The reality is that this a hostile environment. People are looking for us to make mistakes, and in a pretty active way.”

He has received more than $4 million in private funding from, among others, billionaire Bill Gates and Canadian tar-sands magnate Murray Edwards. He maintains that he will not use the private money to support his field test, and that he would pursue it only with government funding and oversight.

Keith explained the basic objectives of the experiment. The sulfate aerosols associated with SRM act as a surface on which chlorine gases can be activated. The gases, in turn, cause chemical reactions that destroy ozone. He maintains that we need to better understand that process. He wants to know, in particular, whether there’s a right combination — of altitude for injection and of particle type — that replaces rather than destroys ozone.

“There are two reaction types that I don’t think people understand well enough in the lab,” Keith says. “We need to understand those better if we’re going to have accurate predictive models.”

He went on to say, “If we find some huge risk situation, like the ozone loss really is very large and can’t be avoided, then in a way it’s easy. It just won’t be used.”

Many climate scientists are leery of real-world experiments and believe there’s still a great deal to be learned from models. Tilmes examined the effects of sulfate aerosols in the atmosphere on the ozone. Between a quarter and three-fourths of the ozone layer above the Arctic would be destroyed, she found, and the recovery of the ozone hole over the Antarctic would be delayed by about 30 to 70 years.

While Tilmes sees a possible role for small, targeted field studies such as the one Keith proposes that would ultimately improve climate models, she warns, “Real experiments in nature might help you for some of those (process) questions. But they won’t really answer questions like ‘What really happens to the climate or precipitation?’”

To answer those questions, she says, geoengineering would have to be deployed, and the world would have to wait and watch.