Here’s my latest piece for BBC Future.
It’s quite a promise. Using existing technology, we could engineer the clouds “to cancel the entire warming caused by human activity from pre-industrial times to present day”. But this, the latest of many ‘geo-engineering’ proposals to mitigate climate change, has a drawback. Get it only a bit wrong, and you make the problem worse.
That, however, has been the worry with such ‘techno-fixes’ all along. We could fire a fleet of little mirrors into an orbit around the Sun that locks them in place to deflect sunlight from the Earth. But if it goes wrong, we could be plunged into an ice age. Manipulating the clouds has been a popular idea with would-be geo-engineers, but these proposals face the fact that the climate effects of clouds are among the hardest parts of the climate system to understand and predict, so we can’t be too sure what the results will be.
The new suggestion examined by climate scientist Trude Storelvmo of Yale University and her coworkers targets one particular kind of cloud: the cirrus ice clouds that extend their wispy tendrils in the upper troposphere, at altitudes of about 5-15 kilometres. The researchers say that these thin clouds are known with confidence to have a net warming effect on our planet, since their ice crystals trap infrared radiation from the sun-warmed surface and re-emit it back down towards ground. So if we can make cirrus still thinner, we’ll let out more heat and cool the globe.
This idea was first suggested in 2009 by David Mitchell and William Finnegan of the Desert Research Institute in Nevada (D. Mitchell & W. Finnegan, Environmental Research Letters 4, 045102 (2009)). It relies on a rather counterintuitive effect: to reduce the warming influence of cirrus cloud, one should add to the upper troposphere tiny ‘seed’ particles that actually encourage the formation of the ice crystals from which the clouds are made.
So how does that work? The ice crystals of cirrus clouds generally form spontaneously in moist, cold air. But seed particles, if present in the right concentration in the air, could grab all the water vapour to produce a small number of large ice crystals, preventing the formation of lots of unseeded little ones. This would have two consequences. First, the resulting clouds would be more transparent, just as big blobs of water in oil create a more transparent mixture as salad dressing separates out, compared with the milky, opaque mixture you get when you shake it into lots of tiny droplets. The thinner clouds absorb less radiation from the warm ground, allowing more to escape into space.
Second, clouds made from larger ice particles have shorter lives, because the big crystals sink down through the atmosphere under gravity.
This wasn’t by any means the first proposal for geo-engineering climate by modifying the reflection or absorption of light in the atmosphere. For example, British meteorologist John Latham and coworkers have suggested that a fleet of solar-powered ships might spray sea salt into the air to seed the formation of stratocumulus clouds, which cool the planet by reflecting sunlight (J. Latham, Nature 347, 339 (1990)). And climate scientist Paul Crutzen has proposed injecting a sulphur-containing gas into the stratosphere, which would form a haze of sulphate particles to reflect sunlight – a process that happens naturally in volcanic eruptions and which is known to cool the earth (P. J. Crutzen, Climatic Change 77, 211 (2006)).
One of the advantages of climate engineering via clouds is that the effects are transient: if it doesn’t go to plan, the process can be stopped and all will return to normal in a matter of weeks. Mitchell and Finnegan suggested that seeding of cirrus cloud might be done by releasing the particles from aircraft. They suggested that the somewhat exotic (but not excessively costly) compound bismuth tri-iodide would be a good material for the seeds, as it is known to promote ice formation on its surface.
But will it work as planned? That’s what Storelvmo and colleagues have now studied by using a climate model that incorporates a description of cirrus cloud formation. They find that to get climate cooling, one has to use just the right concentration of seed particles. Too few, and cirrus clouds form just as they do normally. But if there are too many seeds, they generate more ice crystals than would have formed in their absence, and the clouds are actually thicker, trapping even more heat.
If we get the seed concentration right, the effect is dramatic: the cooling is enough to offset all global warming. But this ‘Goldilocks’ window is quite narrow. What’s more, the researchers say, finding the precise boundaries of the window requires more information than we have at present, for example about the ability of bismuth tri-iodide to seed ice formation and the rates at which the ice crystals will settle through the atmosphere. So attempting this sort of engineering prematurely could backfire – even if the effect would be quite short-lived, we should hold fire until we know more.
T. Storelvmo et al., Geophys. Res. Lett. 40, 178-182 (2013).