Apr 07 2009

Can Cloud Seeding Prevent Global Warming?

Posted by: Jonathan Marshall

What would it take to manipulate the earth's environment to offset the impact of a doubling of carbon dioxide in the atmosphere? Finding some way to reflect just 1.7 percent of incoming sunlight would tame the greenhouse effect and keep the average global temperature roughly in check, scientists estimate.

As noted in yesterday's post on geoengineering, proposals to just that include launching trillions of sun shades into orbit or lacing the upper atmosphere with reflective sulfur aerosols. Critics claim that such schemes would either cost an exorbitant amount or cause major environmental hazards of their own.

But what if we could just make ordinary clouds a little brighter so they would reflect more sunlight? What if the main requirements were wind and seawater? What if the cost were just a few billion dollars? And what if the process could be turned on or off at will to minimize any  unintended consequences?

That's the promise of a cloud seeding proposal by John Latham, senior research associate at the National Center for Atmospheric Research and former head of the Atmospheric Physics Research Group at the University of Manchester. His idea, first broached in 1990, has been gaining traction in peer-reviewed scientific articles, TV documentaries for BBC and the Discovery Channel, and two papers last year in the prestigious journal, The Philosophical Transactions of the Royal Society.

In a nutshell, Latham and a couple of colleagues propose seeding ocean clouds with tiny droplets of saltwater. In theory, the clouds should reflect more sunlight, possibly offsetting greenhouse warming for up to 100 years. In effect, the scientists propose speeding up the natural process of cloud formation over water much as cloud seeders do over land.

(Latham told me he got the germ of the idea while standing on top of a mountain in Wales, explaining to his eight-year-old son why the clouds they saw were so white. His son called them "soggy mirrors" and the phrase stuck with Latham ever since.)

The team proposes ingeniously to do the job with a fleet of 1500 300-ton ships powered by vertical spinning wind turbines called Flettner rotors. The rotors would both propel the ships and spray seawater into the atmosphere to promote cloud formation around salt nuclei. (A ship powered by Anton Flettner's rotors actually crossed the Atlantic in 1926. A German wind turbine manufacturer, Enercon, has plans to finish building a state-of-the-art ship based on similar technology this year.)

They estimate that the ships would cost $2 million to $4 million each, and the research and design program could be financed for about $100 million--cheap insurance by almost any measure.

The best sites to operate the ships, according to their calculations, would be off the coasts of California, Peru and Namibia.

Since the salt nuclei last in the atmosphere at most about five days, the scheme could be tested with no lasting impact to the environment. If put into operation, the ships could be controlled remotely to fine-tune their effects.

Latham told me that computer climate models show very encouraging results for the scheme, but only field testing can prove the concept. He estimates that preliminary scientific work to lay the foundation would cost only a quarter million dollars.

I asked Ken Caldeira, a leading expert on climate change and geoengineering at Stanford University, for his opinion. While he prefers the idea of injecting dust into the stratosphere to cool the earth quickly in case of a climate emergency, he likes the fact that Latham's cloud-seeding scheme can be tested at small scale, uses benign mechanisms, appear to be affordable and could be stopped quickly if needed..

So what's the downside? Perhaps the biggest is shared by all schemes to offset global warming by reflecting sunlight: even if the average temperature of the globe did not change, regional changes caused by the greenhouse effect could still be significant and disruptive. In addition, Latham's scheme would do nothing to prevent continued emissions of carbon dioxide from making the oceans more acidic, threatening many species of sea life.

That's why Latham, and other students of geoengineering like Caldeira, strongly advocate curbs on greenhouse gas emissions as the best remedy for global warming. But while they wait for the world's governments to summon the political will to enforce emissions controls, they're busy working on Plan B.

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