Health & Science: A way to clean up after ourselves

Since global warming was first recognized as a potent threat to our planet, scientists have been designing solutions ranging from the feasible to the far-fetched. As we enter an age of environmental consciousness, we have already begun to reduce our use of fossil fuels, recycle petroleum-based products, and pollute less. Even so, experts speculate that our efforts are not enough: Not only do we need to stop polluting, but we need to clean up after our mess, too.

Humanity has deposited billions of tons of carbon dioxide into the atmosphere since the start of the Industrial Revolution. Carbon dioxide, methane, ozone, and even water vapor are greenhouse gases. At natural levels, greenhouse gases trap heat from the sun and help warm our planet, enabling life to thrive; at heightened levels, greenhouse gases cause global warming. Carbon dioxide is of particular importance to the global warming problem, because its quantity in the atmosphere has increased the most since pre-industrial times.

Carbon dioxide is regulated naturally through what is known as the carbon cycle. The gas, containing one carbon atom and two oxygen atoms per molecule, is released in a variety of ways: animal respiration, decay of organic matter, volcanic eruptions, and combustion of fossil fuels. Carbon dioxide is then reclaimed through plant and plankton respiration, dissolution with seawater, or other biological use.

As plants and plankton die, their organic matter returns to the earth: The carbon-rich shells of certain plankton fall to the sea floor as sediment, and buried plants and animals are absorbed into the soil and sometimes converted to fossil fuels after millions of years.

The root of our global warming problem is an unbalanced carbon cycle. We are depositing far more carbon into the atmosphere than nature can reclaim and store. Left to its own devices, the Earth cannot cope with the current amount of carbon dioxide in a reasonable amount of time. Fortunately, the solution might rest with artificially restoring the balance to the carbon cycle.

There are several ways to accomplish this feat: While not very practical, we could cease to use all petroleum worldwide; we could also begin removing carbon from the atmosphere at a rate similar to that at which we put it there, a process known as carbon sequestration.

One researcher from Pennsylvania State University, Dirk Van Essendelft, believes he may have found a practical method for the latter strategy. Capturing carbon dioxide directly from a car's exhaust or a factory's smokestack is costly. If that factory is a power plant, its efficiency would greatly diminish. Instead, Van Essendelft proposes pulling carbon dioxide directly from the air. He has found that "serpentine" rock, which contains magnesium, can be used as a power catalyst to dissolve atmospheric carbon dioxide.

By crushing the rock, then preparing it with a cocktail of chemicals, carbon dioxide is readily absorbed after reacting with the magnesium. The result of this process is magnesium carbonate. Van Essendelft says his process can be scaled easily and magnesium carbonate can be used for everything from making cement to antacid tablets, or can simply be thrown away.

Another scientist, David Keith, at the University of Calgary, is investigating the use of "wet scrubbers" to pull carbon dioxide directly from the air. Often used on flues and smokestacks, scrubbers are pollution- control devices that remove particulates or gases from exhaust streams. Wet scrubbers accomplish this task by saturating the exhaust stream with a liquid reagent.

Keith's wet scrubber relies on a slurry of caustic soda (sodium hydroxide for those more chemically inclined) mixed with water. When exposed to air, the caustic soda reacts with any carbon dioxide and forms the solid sodium carbonate. His design uses an ultra-efficient fan to force air through the system and output a gaseous mixture with far less carbon dioxide than before. Like magnesium carbonate, sodium carbonate also has industrial uses, or it can also be safely thrown away.

The primary benefits of Keith's and Van Essendelft's "air capture" devices is that they quickly and efficiently remove and neutralize the harmful carbon dioxide gas by transforming it into a far less harmful solid. They are also present a viable means to offset the pollution created by less eco-friendly power plants without expensively retrofitting their outdated infrastructure.

The UK Treasury's Stern Review on Climate Change estimates that we need to remove 11 billion tons of carbon dioxide from the atmosphere to reach levels twice that of pre-Industrial Revolution times. Modest-sized air capture devices can remove approximately 1,000 tons of carbon dioxide per year. Using that figure, the Earth needs about one million devices to reach the 11 billion ton benchmark. With over 450 million expensive fossil-fuel-driven cars and trucks on the road today, we can build one million cheap, efficient air capture devices to clean up after ourselves.