This is an excellent question, and actually many scientists from both public (government) and private (for-profit) organizations are working on this. There are several popular approaches to removing atmospheric CO₂. The most actively studied one is to fertilize the oceans with iron, which causes microscopic plants (phyto-plankton) to grow faster. As they grow, photosynthesis causes them to remove CO₂ from the air and convert it to sugars so that they can grow. When they die, we hope that they fall to the bottom of the ocean and thus a lot of CO₂ is stored in their dead bodies. The reason that iron works as a fertilizer is because the oceans are very far from land where most of the iron is, and the process of photosynthesis requires iron to make the chlorophyll molecule work properly. One problem with this approach is that if something eats the phytoplankton or bacteria decompose them before they sink then all that CO₂ will be released (breathed out by respiration) and will wind up back in the atmosphere. There are two large private companies trying to develop this, and it has been tried several times in the Antarctic and Pacific oceans (where they dump iron in the ocean over 100's of square miles by boat), but there are a lot of concerns over how it may impact the ecosystem in unforeseen ways. This is also a problem because no country "owns" the oceans and thus nobody is responsible if something goes wrong.

I searched your question on Google: atmospheric CO₂ extraction And I found there is a $25 million prize for doing this. Some scientists published an article about doing it, but the prize is for removing at least a billion tons of CO_{2 per year.}

IF CO₂ is in fact responsible for global warming (which it probably is, but there is still some debate), then the answer is yes. Corporations and governments alike are now working on developing CO₂-sequestration technologies that will do this, using biological means. I don't know how they plan to pull this off - this is an engineering question and I'm not an engineer. I also don't know how successful they are going to be: as with any invention, until you do it you never know if it is in fact possible in a practical sense or not.

It's possible to "scrub" CO₂ from air using several chemicals, usually amine-based solvents. But these are all expensive. Also, many of these chemicals require mining or additional fossil fuels to make them, which defeat the purpose. An even bigger problem is that we don't have any good way to *store* the CO₂ after we remove it. Since the world burns about 1 cubic mile of coal every year, that means we would need to store 1 cubic mile of solid carbon, or many times that much gas. Unfortunately, the most stable, long-lasting, compact means of storing carbon is... coal. So the best thing we could ever do for global warming is to leave the coal (and oil) in the ground.

The best shot we have at sequestering large amounts of CO₂ is to promote nature to do it for us. Plankton in the ocean converts CO₂ to oxygen, but there is not enough plankton to compensate all the CO₂ we are producing from fossil fuels. One proposal is to "fertilize" the ocean with iron particles which help plankton grow. But it's not clear whether we can do this on a huge scale, or whether it would cause other problems.

Yes, many scientists think that it might be possible to extract atmospheric CO₂ and sequester it in some solid form so it no longer contributes to the greenhouse effect. Probably the best way to do this is to have plants convert atmospheric CO₂ into plant matter by photosynthesis (all of the carbon in cellulose, wood and other plant matter was once atmospheric CO₂). Then you have to make sure the plant matter doesn't decompose and release the CO₂ back into the atmosphere. For example, if you grew a tree, cut it down, and then sank the tree to the bottom of the ocean, all of the CO₂ in the tree would be sequestered. This would be hard to do with trees, but some oceanographers think it is possible to do the same thing with phytoplankton. Phytoplankton are the microscopic algae that float near the ocean surface and get their energy from photosynthesis. In most oceans, the growth of phytoplankton is limited by the availability of iron, so if we fertilize the ocean with iron, phytoplankton growth will 'bloom.' In theory most of the phytoplankton in the bloom would die and sink to the bottom of the ocean, taking all of their carbon with them. However, there are still some bugs to be worked out in this plan - for one thing, it would take an unrealistic amount of iron to really make a dent in atmospheric CO₂ concentrations.

An interesting way of thinking about this problem is to remember where all of the 'extra' CO₂ in the atmosphere came from in the first place: fossil fuels. How did all of the carbon get into fossil fuels? Millions of years ago, many plants converted CO₂ into plant matter, and that plant matter became sequestered in the ground and fossilized instead of decomposing and returning to the atmosphere. Now we've released much of that CO₂ by burning coal and oil and so forth, so these new ideas about CO₂ sequestration are really just a way of duplicating the process that generated fossil fuels.

Anyway, like I said, there are still a lot of problems with the iron-fertilization plan. So if you can come up with a better plan that would be pretty helpful!

It is certainly possible to extract CO₂ from the atmosphere; however, to do so in quantities sufficient to make any measurable change in the content is staggeringly difficult, simply because of the quantity.Recently (2002), the amount of CO₂ was measured to be about 0.037% of the atmosphere. Given that normal air pressure is 14.7pounds/sq. in. or10,500 Kg/sq m, the mass of the atmosphere is the earths surface area times the mass of air over each sq. meter (i.e. the pressure as sea level). This works out to be: 5.3x10^18 Kg. So the current amount of CO₂is: 2x10^17Kg or 200 trillion metric tons. The problem of CO₂ injection into the atmosphere is joint among all fossil fuel burning activities on the planet such as driving, heating, cooking and power generation. This is why most efforts have been to limit expansion of such activities (i.e. the Kyoto accords).

On the other hand, there is the theory that the current mass fraction of CO₂ is limited by plant growth on land and (mostly) in the oceans since the current atmospheric makeup seems strongly affected by the Earth's biomass. However, the mechanisms behind the current atmosphere are not completely understood and there is very little information about the way the biosphere will change if the mass fraction of CO₂ is changed. It might be that the global warming question revolves around the behavior of microscopic plants and bacteria in the oceans. There is potential for specialized engineered plankton or bacteria to make differences on this scale -- but the associated risks enormous as unforeseen side effects could endanger the planet's food supply.

In general, you must be careful accepting statements about vast complex systems where there is a substantial unknown factor. Scientists cannot isolate the planet to see what really makes it work -- and while physical analysis (like the masses above) are relatively easy to determine, figuring out how the biological and geological system works and what will happen due to a hundred years of industrial dumping of CO₂is very difficult. Recently, there has been substantial evidence for global warming of the planet -- at least over the last century. However, beyond that period, the Earth's mean temperature is not easy to determine directly, and a century is a very short time period compared to those processes which we strongly suspect change the atmospheric composition and temperature. It is somewhat likely that the real consequences of CO₂ dumping will await further study and interpretation.