What would it take to reverse global warming?

Reducing the amount of carbon in the atmosphere is an important part of reversing the global warming trend. Nature has an elegant solution. Plants take in water and carbon dioxide from the atmosphere and through photosynthesis convert it into oxygen and organic compounds. Could we mimic nature and pull carbon from the air?

In 2008, the National Science Foundation sponsored a workshop at which scientists discussed the possibility of capturing and recycling carbon. It's a lofty goal. If we could capture the carbon in the atmosphere and convert it into a fuel source, we could create a looped system. Instead of mining the Earth for fuel, we could pull it from the air. Burning the fuel would release the carbon back into the atmosphere as carbon dioxide. But then we'd just capture and convert it again.

If you apply critical thinking to this approach, you'll see that there are several challenges we face to make it a reality. We need a way to capture carbon from the atmosphere. We need a way to convert it into fuel. And we need a way to power the capture and conversion process that doesn't require us to mine more carbon-based fuels or we'll simply add to the existing greenhouse gas emissions.

To capture carbon from the atmosphere, we need to design absorbants. These materials do just what you'd think -- they absorb a particular material like a sponge. The workshop scientists concluded that the best place to start capturing carbon is from point sources -- concentrated streams of carbon dioxide from things like geothermal wells or power plants. We need to develop the technology to separate carbon dioxide from other gases for collection.

But that doesn't address more than half of the carbon dioxide we humans produce. We produce more carbon dioxide from our homes, vehicles and businesses than we do from factories or wells [source: Prakash, et al.]. But because we don't produce carbon dioxide in a steady stream, like some point sources do, it's harder to capture. The carbon dioxide concentration in the atmosphere is about 385 parts per million -- it's so diffuse that it's difficult to capture.

Assuming that scientists are able to develop the technology to capture carbon dioxide, the next step is to convert it into something else like methanol or dimethyl ether. This is also challenging because carbon dioxide is a very stable molecule -- it resists change. To convert it into something else requires energy. Nature uses the power of the sun. We would need to use a renewable energy source to avoid adding more carbon to the atmosphere.

Just as we need better technology to capture carbon dioxide, we need to develop efficient ways to convert it into something useful. If the capture or conversion process is too expensive or inefficient, it won't make sense to support it. The workshop scientists pointed out that our current fuel infrastructure would support fuel developed from captured carbon. That gives their proposal an advantage over alternative fuels like hydrogen, which would require a new infrastructure to be effective.

Now let's take a look at some solutions that sound more like science fiction than science fact.

The Earth's heat ultimately comes from the sun, so the proposed plans to fight global warming focus on reducing the amount of heat the Earth receives from the sun. That means finding a way to redirect or block some light from the sun.

This need has prompted some interesting proposals from scientists and engineers. Some of them sound like science fiction. One method would require us to put reflective surfaces in orbit around Earth to reduce the amount of energy hitting the planet from the sun.

In 2005, astrophysicist Gregory Benford suggested that we build a concave lens and position it in orbit around the Earth so that it reduces the light hitting Earth from the sun. The lens would be 1,000 kilometers (621 miles) across and would require engines to keep it spinning in the proper alignment with the Earth.

That same year, another proposal suggested we create a ring of either reflective particles or spacecraft with reflective surfaces to block some light from the sun. The proposal had a hefty price tag: $500 billion to $200 trillion, depending on the method [source: Britt].

Another proposal in 2006 came from a scientist at the University of Arizona's Steward Observatory named Roger Angel. Angel's idea was to launch trillions of round lenses to circle the Earth. He also suggested using an electromagnetic gun to fire the lenses up into position [source: Christensen]. The gun would need a renewable energy source to power it. Since then, even more scientists have suggested similar approaches involving putting reflective objects in orbit.

But not every suggestion involves putting junk into space. Another option is to change the nature of low-flying clouds over the ocean. By seeding the clouds with the right mixture, scientists can make the clouds more reflective. The best part of this plan is that it involves spraying sea water into the air -- there's no need to use harmful chemicals. John Latham of the National Center for Atmospheric Research suggested that we design a fleet of autonomous ships that can spray sea water across the ocean to redirect sunlight and reduce global warming [source: PhysicsWorld].

In an interview with the Science Channel, scientist and advisor to the Canadian government David Keith cautioned against relying too heavily on these climate engineering techniques. It's not that the techniques might not work -- if engineered correctly they should work. The bigger problem in Keith's mind is that if we design a system that reduces global warming, we may not feel an incentive to change our carbon-emitting lifestyles. But eventually, the problem will just build up again until our quick fix isn't enough to save us and we'll be back at square one [source: The Science Channel].

Reversing climate change is one of the biggest challenges humans have ever faced. There are technological, economic and political considerations that we must make if we are to reverse a trend that might otherwise threaten our very existence.

Learn more about climate change and what we can do about it by following the links on the next page.