“A number of factors inform this range of estimates,” Vaughan said. “A key one is particle size: the smaller the particle size the less mass of sulfur is needed, but if the particles are too small they can ‘glob’ together and fall out of the stratosphere quicker.” Injections would have to be replenished every two to three years.
Earlier this year, Vaughan published a paper with UEA colleague Tim Lenton in the journal Atmospheric Chemistry and Physics Discussions, comparing different schemes to cool the Earth. They concluded that stratospheric aerosols had by far the greatest potential to combat warming in the timescale to 2050, given their relatively low cost and high efficiency.
But they also raised several concerns. Aside from unpredictable changes in the amount and pattern of global rainfall, stratospheric aerosols would slow the recovery of the ozone layer.
Perhaps the biggest issue, however, is that this kind of “solar radiation management” does not do anything to tackle the culprit behind global warming: carbon dioxide. Such techniques — other proposals include throwing salt water into the air to enhance cloud cover and painting roofs white to reflect away sunlight — mask the core problem rather than permanently dealing with it. And they would need to be in place for ever.
“As soon as you stop any type of solar radiation management, the rate of warming is extremely fast — the system readjusts,” says Vaughan. “Rather than having steady warming as our carbon dioxide levels go up, if you bring the intervention in so the world cools, and then stop it 20 years down the line, you get a rapid warming back up to the level that it would have been if you’d never had that intervention.”
So, as well as solar radiation management, geo-engineers would need to come up with ways of removing carbon dioxide from the atmosphere. There are good ideas for how to do this. Some suggest stimulating algae in the oceans could sequester large amounts of carbon dioxide, while Klaus Lackner at Columbia University wants to build huge “artificial trees” to directly suck carbon dioxide from the air. But all are complex to engineer.
Stratospheric aerosols are easier — but making a 29km hose defy gravity still won’t be simple.
“I think they’re going to do it by having 100 balloons lifting it up or something,” says John Loughhead, executive director of the UK Energy Research Centre. “We shouldn’t underestimate how difficult it will be to get that up there.”
“What they put forward as a possible way needs an enormous amount of work to see if it can be done practically,” he said.
Loughhead doesn’t discount the effort but he warns against the temptation to focus attention on just one technology.
“The problem that we face with climate and carbon is of such a scale and in a system of such complexity that to believe anything will be a silver bullet is naive. We need to explore all the possible routes because they will all advantages and disadvantages and we will probably have to apply them all in some way ultimately to get to where we want to be,” he said.
RSS