09:03, September 29, 2007
A record melt of Arctic summer sea ice this month may be a sign that global warming is reaching a critical trigger point that could accelerate the northern thaw, some scientists say.
"The reason so much (of the Arctic ice) went suddenly is that it is hitting a tipping point that we have been warning about for the past few years," said James Hansen, director of NASA's Goddard Institute for Space Studies.
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, 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 website 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.
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.
"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. 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 meters and Antarctica by about 57 meters, according to UN estimates.