Executive summary: ice has a high albedo and reflects most sunlight hitting it back into space. Less ice means less light reflected, more energy absorbed. Arctic ice is in a melt/refreeze cycle, but recently warming has meant less ice refreezing (18% below the long-term average). At some point this enters into a domain of positive feedback and:
Current computer models suggest that the Arctic will be entirely ice-free during summer by the year 2070 but some scientists now believe that even this dire prediction may be over-optimistic, said Professor Peter Wadhams, an Arctic ice specialist at Cambridge University....Sea ice keeps a cap on frigid water, keeping it cold and protecting it from heating up. Losing the sea ice of the Arctic is likely to have major repercussions for the climate, he said. "There could be dramatic changes to the climate of the northern region due to the creation of a vast expanse of open water where there was once effectively land," Professor Wadhams said. "You're essentially changing land into ocean and the creation of a huge area of open ocean where there was once land will have a very big impact on other climate parameters."
But don't worry, the mega-engineers are on the job.
So, Schelling asked, how much would it cost to increase the Earth's albedo by enough to offset the damage from increased greenhouse-gas emissions? The necessary change involves a fraction of a percent of incident solar energy, not enough to be observable without precise instruments. Some apparently minor changes might do the trick: slightly degrading the performance of jet aircraft engines could put more carbon black into the statosphere. A higher-tech solution would be to put lots of reflective mylar in low-earth orbit; a lower-tech solution would be to scatter lots of ping-pong balls in tropical waters; an extremely cute solution, if practicable, would be to stimulate the formation of cirrus clouds over parts of the Pacific Ocean.
So we replace the Arctic ice with ping-pong balls, problem solved!
We are gonna be living in a science-fiction world, like it or not. But it's more Bruce Sterling and Kim Stanley Robinson than Isaac Asimov or Robert Heinlein.