Teribus, the low lying islands are in danger because the polar caps themselves are melting.

One scenario that gets very little press is a large methane release. The oceans store large amounts of frozen methane hydrate at the bottom. A slight increase in temperature can trigger the methane to suddenly melt into gas. Methane is a powerful greenhouse gas. A releatively small release -- only requiring a couple of degrees warmer -- could raise temperatures enough to trigger a much larger release... and so on.

In the period of global warming 55 million years ago, a large amount of methane was released from the oceans during tectonic shifts, and the methane caused the earth's temperature to rise 13 degrees. The effects lasted 100,000 years. In biological terms, 13 degrees is a catastrophe, one which humans are even unlikely to survive except in small numbers, because it would cause massive disruption in the food chain.