Today I went to see Thomas (Tad) Homer-Dixon speak at a work related conference. His presentation was about climate science, and underlined the need to work on mitigation and adaptation activities immediately (and according to THD, they are not in contrast to each other, as stated in the links I gave). He also discussed geo-engineering as a possibility that climate scientists are now realizing might be necessary, although undesirable (due to the unknown externalities) by putting sulfur in the atmosphere. They discovered this because this is what volcanoes spew.
But, I hear you ask, why is it necessary, and what does it do?
The polar ice caps reflect the sun’s energy back into the atmosphere so the heat is not absorbed by the earth. As the polar ice-caps melt due to global warming, there is a positive feedback loop whereby more ice melts, so there is less reflectivity, and more absorption of heat, leading to….you guessed it, more ice melting. So, spraying sulfur into the atmosphere will reflect the sun’s rays, hence heat, back into the atmosphere, and we will geo-engineer our way out of climate change.
But I have an even better, and potentially more cost effective, idea. I think that we can make a giant white sheet, and hold it over the polar ice caps region, and use that to reflect light. And I don’t think we should stop there: we should get citizens of the earth involved. 
Recalling how much fun it was to play parachute games in the school yard during elementary school, I think that we could have an event where millions of people from around the world make a statement about climate change by holding onto the edges of the giant white sheet to reflect the sun’s light into the atmosphere to combat global warming.
I know that anyone with a practical side will say that this is too expensive. But think about it, what can you put on a blank white sheet which is visible to satellite cameras in space? A logo. We can auction off the advertising space to the company seeking to be known as the most advanced corporate social responsibily (CSR) company in the world.
We can also sell off the spots for the trip (by boat, of course, not by air) by telling people that they can “join people all around the world in showing that you care about our planet and want to play a part in helping to fight climate change.”
Of course, all of the proceeds will go towards funding climate change mitigation and adaptation activities.
