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How we get there

The process of undoing the damage we've done by emitting lots of carbon into the atmosphere will likely be a tough, but necessary process. First, we need to limit the amount of CO2 we are emitting from fossil fuels. Next, we need to be able to capture CO2 from the air and store it in places that it can't get back into the atmosphere.
There are no really easy answers for this

Capture Methods

There are no really easy answers for this. Imagine you make a really good soup and then spill half a shaker's worth of salt into it. Odds are, it's not going to taste very good anymore. Well, the first thing you would do is make sure that no more salt fell in. But then imagine you wanted to try to filter out that salt that you spilled and get it back into the shaker. That's sort of what we're trying to do with CO2 in the atmosphere.

The current solutions involve trying to either capture CO2 directly from the air or using plants, such as plankton, to capture the CO2 and then making sure it is not released by them. Projects for taking CO2 directly from the air include attempts to build artificial trees that have surfaces that bind CO2 and turn it into a liquid and another that aims to build towers that suck in air and produce charcoal.

The other primary option being proposed is referred to as ocean fertilization. It involves dropping iron into the ocean to stimulate the growth of plankton, which consumed the CO2 during their growth, and then become part of the ocean food chain that eventually moves the carbon to the ocean floor, where it is stored for a long period of time. This takes care of both the capture and storage problems, but is subject to a lot of concerns about altering natural food webs and skepticism about the actual amount of carbon it sequesters. A recent study has found that carbon sequestered in plankton blooms tends to not fall very far or very fast.
Abandoned mines can be used for storing CO2 captured from the atmosphere.Credit: Wikimedia Commons

Storage

There are a couple of different ways to store carbon once it is captured from the atmosphere. The first is to store it deep in the ground. This is referred to as geo-sequestration. It involves injecting CO2 underground into geological formations including spent oil fields, gas fields, saline formations, unminable coal seams, old mines, etc. These formations have to have highly impermeable caprock and geochemical trapping mechanisms that would prevent CO2 from escaping to the surface. The IPCC estimates that CO2 stored in properly selected and managed geological sites could be trapped for millions of years, with the ability to retain over 99% of the injected CO2 over 1000 years.

The second method that is proposed is mentioned above, which involves storing CO2 at the bottom of the ocean by getting it into the oceanic food chain. The other ocean storage mechanism is a bit more intense, but also more likely to be able to store large amounts of CO2, and that is to directly deposit CO2 onto the sea floor at depths greater than 3000m, where it is more dense than water and is expected to form a lake. Both of these systems could have any number of unintended biological effects.

A final possible CO2 storage mechanism would actually be to reuse the carbon to form hydrocarbons that could be turned into plastics. This is not ideal as it is not a long term solution, but it is better to use the carbon we already have pumped out of the ground to make these things than to pump out new carbon.

It is important to note that while we don't have much in the way of options at the moment, that we are good at making a lot of progress on technical solutions when we need to. It is just important to realize that the need is there and to start giving the issue some attention.
BBC News - Artificial Trees
BBC News article on the invention of artificial trees that take up CO2 from the atmosphere.
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Gather - Carbon Sequestration
Article on the Discovery Channel special talking about a professor who has built a tower that pulls CO2 out of the air.
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Wired - Ocean Fertilization
Wired article reviewing the coverage of a report showing mixed results from an iron fertilization study.
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Green Car Congress - Ocean Fertilization
Review of an upcoming study showing that carbon from plankton blooms tends to not fall very far or very fast in the ocean, limiting the potential impact of geo-engineering with iron dumps in the ocean.
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Wikipedia - Carbon Capture and Storage
Wikipedia article on carbon capture and storage
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Wikipedia - Carbon Capture and Storage
Wikipedia article on carbon capture and storage
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