Planktos Inc. of Foster City, Calif., recently launched its ship, the Weatherbird II, on a trip to the Pacific Ocean to dump 50 tons of iron dust. The iron should grow plankton, part of an algae bloom that will drink up carbon dioxide from the atmosphere.

The idea of seeding the ocean with iron to beef up a natural plankton and algae system has been tried on a small scale several times since 1990.

Planktos chief executive officer Russ George said his ship will try it on a larger scale, dumping a slurry of water and red iron dust from a hose into the sea.

Tim Barnett, a marine physicist at the Scripps Institution of Oceanography, said large-scale ocean seeding could change the crucial temperature difference between the sea surface and deeper waters and have a dramatic effect on marine life.

Cicerone, a climate scientist who is president of the National Academy of Sciences and advocate for more geoengineering research, said iron seeding has led to some promising science. However, he believes it is unlikely to be an effective way of removing significant amounts of carbon dioxide from the air.

When Mount Pinatubo erupted 16 years ago in the Philippines it cooled the Earth for about a year because the sulfate particles in the upper atmosphere reflected some sunlight.

Several leading scientists, from Nobel Laureate Paul Crutzen to the late nuclear cold warrior Edward Teller, have proposed doing the same artificially to offset global warming.

Using jet engines, cannons or balloons to get sulfates in the air, humans could reduce the solar heat, and only increase current sulfur pollution by a small percentage, said Tom Wigley of the National Center for Atmospheric Research.

Scientists at the Center for Atmospheric Research put the idea into a computer climate model. The results aren’t particularly cheap or promising, said NCAR scientist Caspar Ammann. It would take tens of thousands of tons of sulfate to be injected into the air each month, he said.

This technique, while reducing heating, wouldn’t reduce carbon dioxide. So it also wouldn’t counter a dramatic increase in the acidity of the world’s oceans, which happens with global warming, scientists said. It harms sea life, especially coral reefs.

For far-out concepts, it’s hard to beat Roger Angel’s. Last fall, the University of Arizona astronomer proposed what he called a “sun shade.” It would be a cloud of small Frisbee-like spaceships that go between Earth and the sun and act as an umbrella, reducing heat from the sun.

These nearly flat discs would each weigh less than an ounce and measure about a yard wide with three tab-like “ears” that are controllers sticking out just a few inches.

About 800,000 of these would be stacked into each rocket launch. It would take 16 trillion of them, so there would be 20 million launches. And then there’s the cost: at least $4 trillion over 30 years, probably more.

Scientifically, it’s known as “air capture.” But the instruments being used have been dubbed “artificial trees” – even though these devices are about as treelike as a radiator on a stick. They are designed to mimic the role of trees in using carbon dioxide, but early renderings show them looking more like the creation of a tinkering engineer with lots of steel.

Nearly a decade ago, Columbia University professor Klaus Lackner, hit on an idea for his then-middle school daughter’s science fair project: Create air filters that grab carbon dioxide from the air using chemical absorbers and then compress the carbon dioxide into a liquid or compressed gas that can be shipped elsewhere. When his daughter was able to do it on a tiny scale, Lackner decided to look at doing it globally.

Newly inspired by the $25 million prize offered by Richard Branson, Lackner has fine-tuned the idea. He wants to develop a large filter that would absorb carbon dioxide from the air. Another chemical reaction would take the carbon from the absorbent material, and then a third process would change that greenhouse gas into a form that could be disposed of.

Even if each filter was only the size of a television, it could remove about 25 tons of carbon dioxide a year, which is about how much one American produces annually, Lackner said. The captured carbon dioxide would be changed into a liquid or gas that can be piped away from the air capture devices.

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