Experiment Earth
Climate change represents humanity's first planetwide experiment. But, if all else fails, it may not be the last. So-called geoengineering, radical interventions to either block sunlight or reduce greenhouse gases, is a potential last resort for addressing the challenge of climate change.
Some of the ideas are –

releasing sulfate particles in the air to mimic the cooling effects of a massive volcanic eruption;
placing millions of small mirrors or lenses in space to deflect sunlight;
covering portions of the planet with reflective films to bounce sunlight back into space;
fertilizing the oceans with iron or other nutrients to enable plankton to absorb more carbon;
increasing cloud cover or the reflectivity of clouds that already form.

All may have unintended consequences, making the solution worse than the original problem. But it is clear that at least some form of geoengineering will likely be required: capturing carbon dioxide before it is released and storing it in some fashion, either deep beneath the earth, at the bottom of the ocean or in carbonate minerals. Such carbon capture and storage is critical to any serious effort to combat climate change.

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