Should we follow Mount Pinatubo’s example and cool Earth using sulphur dioxide? (Image: ARLAN NAEG/AFP/Getty)

Curbing global warming by shading Earth from the sun could be quick and cheap but also “irrational and irresponsible” without also cutting emissions and capturing carbon, says a report on geoengineering by the US National Academies.

In theory, making Earth more reflective to the sun’s rays – called albedo modification – could be done relatively easily with existing technology, says the report. In 1991, emissions of just 20 million tonnes of sulphur dioxide from the eruption of Mount Pinatubo in the Philippines reduced global temperatures by 0.3 °C for three years. A similar human exercise to curb the warming from a doubling of carbon dioxide concentrations in the air would be “at least an order of magnitude less than the cost of decarbonising the world’s economy”, the report says.

But sending mirrors into space or putting sulphur dioxide in the stratosphere could have “unanticipated, unmanageable and regrettable consequences” on the global climate, it says. Other impacts of carbon emissions, such as ocean acidification, would continue and if the techniques were ever stopped, rapidly soaring temperatures could have cataclysmic consequences.


“Ideas to lessen the amount of energy absorbed from the sun should not be considered for deployment,” says Ralph Cicerone, president of the National Academy of Sciences. Nonetheless, the authors conclude that the techniques should be researched – not least so the world could respond if a climate vigilante were to carry out “an unsanctioned act of albedo modification”.

Cut CO 2

The report marks a departure from previous reviews of geoengineering, such as the UK Royal Society’s 2009 study, says Simon Nicholson at the American University in Washington DC. “The idea advanced by the Royal Society that albedo modification is some kind of Plan B has largely fallen out of favour,” he says.

John Shepherd, chair of the Royal Society’s 2009 study, says: “I agree that the language is a bit more emphatic than ours, but the message is just the same, i.e. strongly in favour of research, too early to consider deployment.”

Shepherd welcomes the report’s recommendation for discussions of what international research governance structures may be needed, and involving civil society in decision-making through a transparent and open process.

The report underlines that the best way to combat climate change is the simplest: curbing CO 2 emissions. A partner report by the Academies also examines a second approach to geoengineering our way out of climate change by enhancing carbon dioxide removal (CDR) from the atmosphere. Techniques range from tree planting and low-till agriculture to fertilising the oceans and direct chemical capture of CO 2 from the atmosphere.

The report says CDR is slow and costly, but likely to be more benign in its side-effects, since it mimics natural processes. Compared to albedo modification, “the environmental risks are relatively low and generally understood,” the report finds, and concludes that “it is increasingly likely that we will need to deploy some form of CDR to avoid the worst impacts of climate change”.