Figueres, in Samoa for a UN conference on small island states, said the impact of climate change was greatest on Pacific nations, even though they had contributed little to the problem.
"Climate change is the greatest threat these islands face and they are recognised as the bellwether of global efforts to address this issue," she told AFP.
"Unless the world acts on climate change in a timely way, they are going to be the hardest hit."
Warming also meant more cyclones and storms battered the islands, while planning was underway for a worst-case scenario where populations of climate change refugees would have to be relocated from their homelands.
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"Kiribati (which has purchased land in neighbouring Fiji) is probably the most famous, but countries as large as Papua New Guinea are already starting to identify which are their most threatened populations," she said.
"These are extreme measures that these islands are having to look at. Of course they, and the rest of the world, want migration of populations out of the islands to be kept at a minimum."
The UN wants to limit global warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels, which scientists say is the minimum needed to stabilise the climate.
"The science tells us that we have to stay under two degrees temperature-wise and that the door is closing quickly," she said. "It's still possible for us to stay under two degrees but we have to do it.