Budgetmakers in the U.S. Senate want the Department of Energy (DOE) to study the possibility of making Earth reflect more sunlight into space to fight global warming. Earth's reflectivity is known as its albedo, and the request to study "albedo modification" comes in the details of a proposed spending bill passed by the Senate appropriations committee to fund DOE, the Army Corps of Engineers, and related agencies for fiscal year 2017, which begins 1 October. The bill does not specify how much money should be spent on the research.

Critics argue that albedo modification and other "geoengineering" schemes are risky and would discourage nations from trying to reduce their emissions of carbon dioxide, the heat-trapping gas that comes from the burning of fossil fuels and that is causing global warming by absorbing increasing amounts of energy from sunlight. Still, climate researchers say they should find out what its potential of albedo modification might be.

"The recommendation is great," says Joyce Penner, an atmospheric scientist at the University of Michigan, Ann Arbor. Albedo modification "is not a solution to global warming, it is only a way to avoid, perhaps, a tipping point in the climate." David Keith, an atmospheric physicist at Harvard University, says, "Ignorance is not a good basis for making decisions, so learning more about this is extremely valuable even if we find out that it will never work." Keith adds, however, that the few existing studies suggest albedo modification could help ameliorate some effects of global warming.

The call for further research comes in a bill that would provide $5.4 billion for DOE's Office of Science next year. It also builds on the recommendations of a February 2015 report from the National Academy of Sciences (NAS) entitled Climate Intervention: Reflecting Sunlight to Cool Earth. That report warned explicitly that albedo modification shouldn't be deployed now because the risks and benefits were far too uncertain. Still, the committee urged further research to find out what those risks and benefits might be.

The Senate appropriations committee wants DOE to do that research. "As other nations have launched research programs on albedo modification, the Committee recommends the Department review the findings of the NAS report … and leverage existing computational and modeling capabilities to explore potential impacts of albedo modification," says a report that accompanies the Senate budget bill. The direction is included in a section outlining priorities for the Office of Science program in biological and environmental research, which currently has an annual budget of $609 million.

Albedo modification would work by lacing the atmosphere with tiny particles or aerosols that would reflect sunlight and mimic natural processes. For example, in 1991 the volcano Mount Pinatubo in the Philippines spewed 20 million tons of sulfur dioxide into the stratosphere, which spans altitudes from 10 to 50 kilometers. There, the sulfur dioxide produced aerosols that reflected enough sunlight to reduce global temperature by an estimated 0.3°C for 3 years. Albedo modification might also work by using aerosols to seed cloud formation in a lower atmospheric layer called the troposphere.

However, the NAS report also noted that the potential effects of albedo modification remain poorly understood and quantified. Scientists lack even the observational tools to measure the effects of albedo modification, the report states. To make up for those shortcomings, the report called for a research program, including smaller scale field trials, whose goal "should be to improve understanding of the range of climate and other environmental effects of albedo modification, as well as understanding of unintended impacts."

For decades studying albedo modification has been taboo in the United States, Keith says, for fear that it would distract from the ultimate goal of zeroing out carbon emissions. But given the potentially catastrophic effects of global warming, scientists have an obligation to study the idea, he says. "Because the burden will fall so disproportionately on the poor I think there's a moral imperative to do it," he says.

Even if albedo modification won't work, the research would pay big dividends, Penner predicts. Among the most uncertain elements in climate models are the effects of aerosols and their interactions with clouds—just the things involved in albedo modification—she says. Because of those uncertainties, researchers can estimate only that doubling atmospheric carbon dioxide from preindustrial levels would increase global temperature between 1°C and 5°C. "If we knew the aerosol forcing we could reduce that range," Penner says.

If the federal government turns a blind eye to albedo modification, the research will be done anyway by countries such as China and researchers sponsored by nongovernmental organizations such as the Bill & Melinda Gates Foundation, Keith says. So the real choice is between exploring albedo modification openly or in relative secrecy, he says: "I believe in sunshine and democracy, and I really think this should be done by the U.S. government and in a transparent way."

The Senate language is a long way from becoming law. The full Senate has yet to act on the measure, which ultimately would have to be reconciled with DOE spending levels approved by the House of Representatives. Any final bill would have to be signed by the president.