April 6, 2011

Cloud Whitening Could Do More Harm Than Good

According to new research, whitening clouds by spraying them with seawater could do more harm than good for climate change.

Whiter clouds reflect more solar energy back into space, which inevitably cools the Earth.

However a study presented at the European Geosciences Union meeting found that using water droplets of the wrong size would lead to warming, not cooling.

One scientist said it should be possible to make sure droplets were the correct size.

John Latham of the University Corporation for Atmospheric Research in Boulder, Colorado originally proposed cloud whitening in 1990.

A number of other researchers, including University of Edinburgh wave energy pioneer Stephen Salter, have developed other theories as well. 

One version envisages specially designed ships, powered by wind, operating in areas of the ocean where reflective stratocumulus clouds are scarce.

The ships would continually spray fine jets of seawater droplets into the sky, where tiny salt crystals would act as nuclei around which water vapor would condense, producing clouds or thickening them where they already exist.
Kari Alterskjaer of the University of Oslo in Norway performed a study that confirmed earlier findings that if cloud whitening were to be done, the best areas would be just to the west of North and South America.

However, it concluded that about 70 times more salt would have to be carried aloft than proponents have calculated.

She found that using droplets of the wrong size could reduce cloud cover rather than enhance it.

"If the particles are too small, they will not brighten the clouds - instead they will influence particles that are already there, and there will be competition between them," she told BBC News.

"Obviously the particle size is of crucial importance, not only for whether you get a positive or negative effect, but also whether particles can actually reach the clouds - if they're too large, they just fall to the sea."

Cloud-whitening developers have forecast the possibility of this technique having a warming impact.

Dr. Latham wrote in a 2002 scientific paper that "the overall result could be a reduction in cloud droplet concentration, with concomitant reductions in albedo and cloud longevity, i.e. a warming effect."

He argued that this possibility could be eliminated by careful design of the spray system.

"I agree that the drop size has to be correct and that the correct value may vary according to local conditions," Salter told BBC.

"However, I am confident that we can control drop size by adjusting the frequency of an ultrasonic pressure wave which ejects drop from micro-nozzles etched in silicon.

"We can test this at very small scale in the lab."

Salter is working with engineers in Edinburgh to produce extremely fine yet robust nozzles from semiconductor sheets.

Many climate scientists are frustrated by the slow progress in reducing greenhouse gas emissions, and cloud whitening has sometimes been held up as an example of a technology that could make a real difference.

One scientist at Alterskjaer's presentation to the European Geosciences Union (EGU) meeting commented that it was the most depressing thing he had heard in a long time.

Piers Forster from the U.K.'s University of Leeds, who is leading a major U.K. project on geoengineering techniques, said more research would be needed before cloud whitening could be considered for "prime time" use.

"The trouble is that clouds are very complicated; as soon as you start manipulating them in one way, there are a lot of different interactions," he told BBC.

"We need real-world data and we need modeling that tries to simulate clouds on more appropriate scales, and that means less than 100m or so, because if you look at a deck of stratocumulus it's not one big thing, it has pockets and cells and other features."

"Far more uncertain is the idea that you'd inject a particular drop size, because it won't stay that size for long - it will spread out, and that would be uncertain."

Salter believes more research needs to be done, including building a prototype injector ship and studying how it works in practice

He told BBC last year that such research was urgently needed because there was little sign of real cuts being made in the world's greenhouse gas emissions.


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