Proposals for the first trials to cool the planet include cloud brightening and spraying aerosols into the ozone layer. They might start in just two years

Laboratory in the sky? (Image:Justin Guariglia/Redux/Eyevine)

IF WE can’t reduce emissions enough, what else can cool the planet? We need to find out if geoengineering works, and soon, say a group of atmospheric scientists.

Engineering the planet’s weather and climate is a highly controversial idea. That’s why we need experiments, the group say, and they want the first to start in two years’ time. The frontrunners are schemes to alter the atmosphere to reflect more of the sun’s rays back into space, or to change clouds so that they let more of Earth’s heat out instead of trapping it (see diagrams).

Last week, the group published a “road map” of proposals for how real-world experiments might be carried out (Philosophical Transactions of the Royal Society A, doi.org/xb9).


One would explore the effects of injecting aerosols of sea salt into marine clouds. The aim is to increase the water droplet content of the clouds, making them reflect more sunlight – so called marine cloud brightening.

The second, and most detailed, devised by John Dykema of Harvard University, would explore the effects of injecting sulphur-containing substances at an altitude of 20 kilometres – the lower reaches of the boundary with outer space (Philosophical Transactions of the Royal Society A, doi.org/xb8).

The aim of the so-called stratospheric controlled perturbation experiment, or SCoPEx, is to see if sulphate ions would undermine measures to rebuild the ozone layer. The fear is that such substances might set off chemical reactions that deplete the ozone.

The third experiment would explore the potential for making cirrus clouds in the upper atmosphere more porous to radiation bouncing back into space from Earth. Water vapour in the clouds behaves like a greenhouse gas, trapping heat almost as efficiently as carbon dioxide. By seeding them with substances like bismuth tri-iodide, which cause water to form into ice particles, the hope is to reduce the water vapour and allow more radiation to escape.

Geoengineering to cool the planet by deliberately altering Earth’s atmosphere is highly controversial, with sceptics fearing it will fail and mess up the climate even more. Altering cloud cover, for example, could change rainfall patterns and increase droughts and floods unpredictably. Opponents also fear that if we rely on geoengineering solutions, people will no longer strive towards the main goal of dramatically reducing our reliance on the fossil fuels that are inexorably heating up the planet.

Nevertheless, some ideas deserve further exploration, say proponents. The road map, conceived at a Harvard workshop in March, is “a big move forward”, says lead author David Keith, who is also at Harvard.

So far, all geoengineering work has been in the lab or based on computer models. “Modelling and lab experiments are critical,” says Dykema. “But to understand the intricate chemistry people are concerned about, the only way to find out is in the atmosphere, where you have the right flux of solar radiation, the right mix of chemical species and the real dynamics of aerosol particle interactions in gas, liquid and solid phases.”

Modelling is critical, but the only way to understand the intricate chemistry is in the atmosphere

“The proposed experiments are quite small scale, and the environmental consequences are likely to be negligible compared with a lot of human activity we already take for granted,” says co-author Doug MacMartin of the California Institute of Technology in Pasadena.

Dykema’s experiment, for example, involves releasing just a kilogram of sulphur, the same amount as emitted in just 1 minute by a standard commercial jet.

However, a major UK project to investigate geoengineering, called the Integrated Assessment of Geoengineering Proposals, is more cautious and pessimistic about solar radiation management.

Scaling up

For example, researchers using computer simulations have found that attempts to reduce solar radiation in the Arctic to stop sea ice from retreating are infeasible. “We found that the scale of deployment in both time and space would have to be huge before current observing systems could detect any effect,” says principal investigator Piers Forster of the University of Leeds.

And he is sceptical of small-scale tests. “We’ve emitted 500 billion tonnes of carbon dioxide and we only recently have any certainty this is affecting our climate, so limited field tests would tell you next to nothing about the climate effects of solar geoengineering.”

The would-be experimenters argue that, if anything, reliance on modelling increases their case to pursue more real-world data, to make the models more accurate. Models are potentially compromised by having too little real-world data to work with, they say.

“There are many aspects of climate interactions, especially those that affect clouds, that are poorly understood because the range of scales, from nanometres to kilometres, can’t be accurately included in global models,” says Lynn Russell of the Scripps Institution of Oceanography in San Diego, California, a member of the experimentation group.

Trials would provide invaluable insights for mainstream climate research, she says, even if geoengineering ultimately proves to be impractical. Observations from such experiments would also allow us to understand cloud processes better and lead to improvement in climate models more generally, she says.

Experiments would give invaluable insights, even if geoengineering ultimately proved to be impractical

“My preference is for experiments that have broad utility, both for understanding proposed solar geoengineering schemes and improving our understanding of the natural climate system,” says Ken Caldeira of the Carnegie Institution for Science in Stanford, California.

Keith says the earliest the SCoPEx experiment could get going would be in two years’ time. In the meantime, the researchers are hoping to secure $10 million in funding from the US government.

They also call for the formation of national regulatory bodies to independently assess the merits of all proposed experiments and give official permission to conduct them. Such approval is imperative to retain public trust, they say. “We believe that external governance is critical,” says Dykema. “But at present, there’s no one to apply to, to do the experiments.”

This article appeared in print under the headline “Plan B for climate change”