When will the polar ice caps melt completely? Take it as a given that global warming is real.

"Take it as a given that global warming is real" is a statement I think we should post at the door of Congress. The scientific consensus on global warming is absolutely overwhelming: it is not only happening, but it's also our fault and it will have terrible consequences if we don't do something fast.

To make this specific forecast, we have to be clear about what we mean by "melt completely"; this could mean three things:

1. A complete melting of all polar ice in summer
2. A complete melting of all polar ice for a whole year
3. A complete melting of all polar ice indefinitely

Moreover, all three of these could be applied to either Arctic ice or Antarctic ice, which turns out to be important.

The good news is that Antarctic ice will not disappear completely any time soon, not even in the summer. Right now Antarctica is sloughing off its huge amount of land-based ice into the sea, so sea ice around Antarctica is actually increasing for the time being. There is so much ice in Antarctica that in fact it can probably continue melting for thousands if not millions of years at the current rate without all the ice disappearing. 26.5 million km^3 is 26.5 million gigatons, and the current melt rate is about 70 gigatons per year. Even if it speeds up a thousandfold, we still have centuries.

Now for the bad news. Arctic polar ice in summer is almost gone already. Current models say it won't last more than the next four years. So by the most narrow definition of polar ice melting, it's about to happen.

There will still be Arctic ice during the winter until about 2040 or 2050 by current estimates, and for a decade or two after that there will be cold years that have Arctic ice, but not every year will.

It's actually the Antarctic melting that's a problem, though; Arctic melting doesn't raise sea levels very much, because it's mostly sea ice and sea ice is already at equilibrium with the water. But Antarctic ice is mostly land ice, and as land ice falls into the sea it raises sea levels. If it all melted, sea levels would rise an astonishing 58 meters.