Geoengineering: How scientists could zap clouds with aerosols to cool the Earth down

Humans are warming up the planet, but we also have the potential to cool it down through geoengineering – including massive projects to alter the atmosphere to make it retain less heat. But this should be a measure of last resort, scientists have said, as there is a chance that it could go disastrously wrong and end up accelerating climate change.

Some of the most drastic plans to slow down climate change involve geoengineering. These plans have got a lot of attention in the past, as a hopeful technological fix to a human-made problem that, for now, is only going to get bigger.

One form of geoengineering involves getting rid of cirrus clouds – those long, thin, streaky ones that appear very high up in the sky. They form naturally, but the lines of cloud forming behind aeroplanes – called contrails – also have similar physical properties.

"We want to remove them from the skies – cirrus clouds have a net warming effect and that effect is similar to that of greenhouse gases," experimental atmospheric physicist Ulrike Lohmann of ETH Zurich told IBTimes UK.

These clouds and contrails add to the greenhouse effect by trapping radiation from the Earth in the atmosphere, rather than letting it radiate out into space. If there were fewer cirrus clouds, more of that radiation could escape, which would help reduce temperatures down here at the surface.

Reducing the number of cirrus clouds in our skies is theoretically possible, but it would be a very far from straightforward task, Lohmann argues in a perspective article in the journal Science.

The way to do it would be to soak up the water vapour in the atmosphere with aerosols – tiny particles of matter – in the places where cirrus clouds are likely to form. This would use up the vapour to temporarily form clouds or ice crystals that would quickly fall out of the sky. So the idea is that this wouldn't leave enough water locally in the atmosphere for cirrus clouds to form.

That's the theory – but get it wrong, and you could end up creating many more clouds rather than zapping them out of the sky.

"At the moment it's all theory. We don't have too many good observations of those cirrus clouds forming. That's part of the uncertainty," said Lohmann.

"We urgently need more measurement programmes to look at that. Most of what we know about this kind of climate engineering is based on models and simulations. But what happens, say, if all these clouds form on dust instead of allowing to form as they wish?"

What would happen is a lot more cirrus clouds – adding a warming blanket to the skies instead of zapping it away. This would make the problem of climate change worse. There could also be other, unpredictable, problems.

"Impacts would include a slower hydrological cycle, effects on the ozone layer, and changing monsoon patterns," according to an editorial in the same issue of the journal.

This lack of crucial knowledge makes many scientists nervous about geoengineering projects like this.

"There's a general fear to intervene with the climate to start with – and that's well-grounded," Lohmann said. "But what we have with climate change is effectively a very slow experiment with our atmosphere – emitting carbon dioxide and seeing what happens."

The only sure – and safe – way to reverse that is to stronger mitigation of climate change, she said. Geoengineering can't tackle the problem at the root, it can only act as a sticking plaster. It also leaves various other problems associated with climate change – ocean acidification, for example – by the wayside. But in the absence of strong international efforts, we may be forced to turn reluctantly to the riskier options.

"If there were serious efforts at mitigation, geoengineering would never be needed. But not seeing those serious efforts, we will start to see climate engineering being discussed more and more," Lohmann said.

But this, too, would require intense international cooperation and regulation – exactly what is proving to be such a big task for reducing carbon emissions.

The Science editorial concludes: "The world is heading to an increasingly risky future and is unprepared to address the institutional and governance challenges posed by these technologies. Geoengineering has planet-wide consequences and must therefore be discussed by national governments within intergovernmental institutions, including the United Nations."