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May 2004

Cooling us off:

John Latham ponders a plan to counter global warming

John Latham

John Latham

When MMM’s John Latham first wrote about the idea of tinkering with marine clouds to offset global warming in 1990, he faced some scorn. Two irate readers berated John’s note on the topic in Nature magazine, contending that society should cut back on carbon dioxide emissions instead of further interfering with the environment.

But with carbon dioxide levels continuing to rise in the atmosphere, John’s research is getting a bit more attention, as are other still-unproven plans that seek to counter global warming.

“I was quite prepared, and still am, for this idea to be viewed as crazy,” John concedes.

John’s idea, which has not been fully tested, is to increase the number of water droplets in about 10% of the world’s marine stratocumulus clouds. This could be accomplished by bolstering the number of tiny saltwater droplets that act as cloud condensation nuclei, meaning they would serve as centers for the production of additional droplets.

Such a process would make the clouds whiter, increasing their albedo, or ability to reflect solar radiation back into space. If the clouds’ reflectivity could be boosted by a few percent (which could be amply achieved by doubling the droplet numbers), this would compensate for a doubling of carbon dioxide levels in the atmosphere and, at least in theory, produce a cooling that would compensate for global warming.

The plan could also have the benefit of blocking additional sunlight by increasing the lifetimes of the clouds. The reason is the added droplets would be small and descend slowly. As a result, it would take longer for them to combine into drizzle or raindrops and fall back to Earth, dissipating the clouds.


One theorectical approach to whitening marine stratocumulous clouds would be
for giant turbines to send microscopic droplets into the air, as demonstrated by
this prototype instrument. (Photo courtesy Stephen Salter).

John hopes to get funding to test the idea within the next couple of years. Such tests could involve a plane seeding marine stratocumulus clouds with particles and then collecting data on the resulting whitening of the clouds.

If the plan proved valid and policy makers ever wanted to implement it, John says they would have to overcome formidable technological challenges. When bubbles on the ocean surface burst naturally, they release hundreds or thousands of microscopic droplets, many of which rise to form droplets in low-lying marine clouds. How could such droplets be produced artificially?

John has a few ideas, but one of his collaborators, a Scottish inventor at the University of Edinburgh named Stephen Salter, is working on a particularly innovative solution. It involves designing turbines with a height of about 70 meters (220 feet) that would generate a saltwater spray. John says hundreds of such turbines, powered by wind or wave energy, could whiten the required coverage of clouds.

The amount of water involved would be surprisingly minor: only about 10 cubic meters of ocean water throughout the world would have to be sprayed every second. Because the required saltwater particles are so small—about one micron in diameter—some 1021 particles can fit it into a single cubic meter of water. (If you’re wondering how big a number 1021 is, it’s more than the estimated number of grains of sand on all the world’s beaches.)

Even if the technological challenges could be overcome, scientists would need to make sure such a system wouldn’t create unintended consequences. For example, John wonders how creating cool areas over an otherwise warm world would affect global wind and precipitation patterns.

Despite qualms about further interfering with nature, some experts appear open to at least exploring plans to cool Earth. A recent symposium in England on engineering strategies to counter global warming included presentations on floating tiny aluminum balloons in the stratosphere to reflect sunlight, burying carbon dioxide in the ground or beneath the ocean, encouraging the growth of plankton to absorb carbon dioxide, and several other schemes.

As far-fetched as these may sound, scientists are beginning to wonder if society will need to take some sort of action to offset higher greenhouse gas levels in the atmosphere—especially since public leaders are failing to curb emissions.

“People came to the conference with a very skeptical view,” says John, one of the presenters at the conference, “and they came away thinking these ideas need to be explored.” •David Hosansky

Also in this issue...

Shielding the Pentagon

Streamlining the NCAR Science Store

Wilmot “Bill” Hess

Short Takes

Spring Fling

Mentoring Latina students

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