What would it take to reverse global warming?

The news isn't all bad, as there are some measures humans can take to slow, halt, or perhaps even reverse the warming trend. These measures range from changing our behaviors and making some sacrifices, to plans that seem to belong in the realm of science fiction.

One way to affect global warming is to reduce greenhouse gas emissions. Without them, the Earth would lose heat too quickly, and life as we know it would be impossible to sustain. Greenhouse gases, which include water vapor and carbon dioxide, absorb heat in the lower atmosphere and reflect it back to the Earth. But according to the Intergovernmental Panel on Climate Change (IPCC), a consortium of more than 2,000 scientists, humans are increasing the greenhouse gas effect through carbon dioxide emissions from burning fossil fuels.

Assuming that the IPCC's conclusions are accurate, reducing global emissions and planting trees could help slow and eventually reverse climate change. It takes time for the environment to absorb carbon dioxide and, right now, humans are producing carbon dioxide faster than the environment can absorb it. It doesn't help that humans are also clearing large regions of the world's forests, reducing the planet's carbon-absorbing abilities.

But even if we were to convince everyone to stop cutting down tropical forests, switch to wind energy and solar power, and generally try to reduce carbon dioxide emissions, the Earth's temperature could continue to climb. It could take as long as 1,000 years after a complete halt of greenhouse gases for environmental measures like sea level and ocean surface temperature to return to pre-industrial levels [source: NOAA].

There's no question that if warming continues, it'll cause big problems for us. Sea levels will rise as ice melts at the Arctic circle. Farmers will see reduced yields in crops as the temperature climbs. Certain regions will experience extreme weather events like heat waves more frequently. And it's possible that we'll see bigger hurricanes more often. But if the globe keeps warming even if we cut greenhouse emissions, what can we do?

The first step might be finding a way to capture carbon from the atmosphere and recycle it ourselves, giving nature a boost.

Reducing the amount of carbon in the atmosphere is an important step if we are aiming to reverse global warming. 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 fossil fuels, 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.

What Makes Carbon Dioxide Removal Complicated

If you apply critical thinking to this carbon removal 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. Otherwise, we'll simply add to the existing climate crisis.

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, say, 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.

Exploring Natural Energy Sources

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. 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 reflective particles to block some light from the sun. The proposal had a hefty price tag of between $500 billion and $200 trillion [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 to reverse global warming, we may not feel an incentive to change our carbon-emitting lifestyles. Eventually, the problem could 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.