With replicating assemblers, we will even be able to remove the billions of tons of carbon dioxide that our fuel-burning civilization has dumped into the atmosphere. Climatologists project that climbing carbon dioxide levels, by trapping solar energy, will partially melt the polar caps, raising sea levels and flooding coasts sometime in the middle of the next century. Replicating assemblers, though, will make solar power cheap enough to eliminate the need for fossil fuels. Like trees, solar-powered nanomachines will be able to extract carbon dioxide from the air and split off the oxygen. Unlike trees, they will be able to grow deep storage roots and place carbon back in the coal seams and oil fields from which it came.
(That's actually Jay quoting Eric Drexler's Engines of Creation rather than Jay directly).
This has obvious relevance to what I was saying about environmental issues and sustainability the other day.