The Arctic summer sea ice shrank by more than 20 percent below the previous 2005 record low in mid-September to 4.13 million sq km (1.6 million sq miles), according to a 30-year satellite record. It has now frozen out to 4.2 million sq km.

The idea of climate tipping points -- like a see-saw that suddenly flips over when enough weight gets onto one side -- is controversial because it is little understood and dismissed by some as scaremongering about runaway effects.

The polar thaw may herald a self-sustaining acceleration that could threaten indigenous peoples and creatures such as polar bears -- as Arctic sea ice shrinks, the darker ocean soaks up ever more heat than reflective snow and ice.

In Germany, the Potsdam Institute for Climate Impact Research says Arctic sea ice has "already tipped".

Among potential "tipping elements" that are still stable, it lists on its Web site a melt of Siberian permafrost, a slowdown of the Gulf Stream and disruptions to the Indian monsoon.

"I'd say we are reaching a tipping point or are past it for the ice. This is a strong indication that there is an amplifying mechanism here," said Paal Prestrud of the Center for International Climate and Environmental Research in Oslo.

"But that's more or less speculation. There isn't scientific documentation other than the observations," he said.

SHIPPING, POLAR BEARS

Many experts now reckon Arctic ice may disappear in summer before mid-century, decades before earlier forecasts. The thaw would open the region to oil and gas exploration or shipping.

Reuters will host a summit of leading newsmakers on Oct 1-3 to review the state of the environment. Speakers will include Rajendra Pachauri, the head of the U.N. Climate Panel and Michael Morris, chief executive of American Electric Power.

"All models seem to underestimate the speed at which the ice is melting," said Anders Levermann, a Potsdam professor.

"I do not believe that this is alarmist... not all tipping points are irreversible," he said. And societies can weigh up remote risks, such as planes crashing or nuclear meltdowns.

Hansen said he is seeking more study of causes of the melt, widely blamed on greenhouse gases from burning fossil fuels but perhaps slightly stoked by soot from forest fires or industries in Russia and China. Ice darkened by soot melts faster.

"It is a very good lesson, because the ice sheets (on Greenland and Antarctica) have their own tipping points, somewhat harder to get started but far more dangerous for humanity around the globe," he said.

A melt of floating Arctic sea ice does not affect sea levels but Greenland has enough ice to raise oceans by 7 metres and Antarctica by about 57 metres, according to U.N. estimates.

Pachauri's authoritative climate panel, in a summary report due for release in November, does not use the phrase "tipping point" but does say: "Climate change could lead to abrupt or irreversible climate changes and impacts."

It says, for instance, that it is "very unlikely" that the Gulf Stream bringing warm water north to Europe will switch off this century. That could bring a big regional cooling.