AS ARCTIC sea ice hits a record low, focus is turning to climate ''tipping points'' - a threshold that, once crossed, cannot be reversed and will create fundamental changes to other areas.
''It's a trigger that leads to more warming at a regional level, but also leads to flow-on effects through other systems,'' said Will Steffen, the chief adviser on global warming science to Australia's Climate Commission.
There are about 14 known ''tipping elements'', according to a paper published by the US Proceedings of the National Academy of Sciences.
In the case of the Arctic ice cap, less ice means less white surface to reflect heat and more dark water to soak it up. This, in turn, leads to higher temperatures, which scientists say will unlock more ancient greenhouse gases frozen into ocean depths and permafrost, speeding climate change, interfering with ocean currents, rainfall patterns and weather.
"I'm pretty certain that we have now passed the tipping point for Arctic sea ice.''
-Will Steffen, chief adviser, Australia Climate CommissionNext to the Arctic ice cap, Greenland experienced melting across 97 per cent of its surface in June and July. It is unclear what the tipping point is for the melting of the Greenland ice sheet, and melts similar to this year's seem to occur every century or so. What is known is that if temperatures keep rising as they are, the ice sheet will start to disintegrate on a massive scale some time in the second half of this century. Tentative estimates from Australian and international studies say that another 1.5 degrees of warming would push Greenland across the threshold into irreversible melt, a process that would continue for centuries. There is enough ice in Greenland alone to raise sea levels off NSW and Victoria by four-to-nine meters.
Frozen methane trapped in pockets around the Arctic circle is also seen as a critical tipping element. Methane is a potent greenhouse gas, and as frozen earth thaws, more is leaking out. There are no exact measurements on the rate of leakage. Rough estimates suggest 30 to 60 billion tons of methane may leak by 2070.
Other potential tipping elements include monsoon patterns and the El Nino-Southern Oscillation cycle, which scientists expect will start to shift quite suddenly in response to global warming.
Changes in tree cover, especially in giant forests like the Amazon, are also expected in response to changing rainfall and more heat - and this would have the effect of amplifying global warming because fewer trees would mean less carbon dioxide was being soaked up out of the air.
As the polar ice cap shriveled at unforeseen speed, Professor Steffen said he had changed his mind about the Arctic tipping point in past weeks. Existing predictions of an ice-free North Pole by 2050 were looking hopelessly wrong. ''I would say that, certainly, it is looking like 2050 would be an outlier now - I'm pretty certain that we have now passed the tipping point for Arctic sea ice.''
Sea ice reached a minimum size of 3.41 million square kilometers, down from an average of 7.4 million in the 1980s, 6.8 million in the 1990s and 5.7 million last decade. Professor Steffen believes that ''the most radical projection is about 2016, and probably the most conservative projection is about 2030, for when it will be ice free.''
The speed of events is why scientists are getting so worried. The only known way to stop these thresholds being crossed is to cut the human greenhouse emissions triggering these changes, and there are few signs of that taking place.
''Australians should see the vanishing Arctic sea ice as a warning sign that stronger action to cut greenhouse gas emissions is needed,'' the commission's chief, Tim Flannery, said this week. ''This is absolutely the critical decade for action. We're only seeing the beginning of rising sea levels; the real question is the rate - how fast will sea level rise? This poses risks for coastal communities, infrastructure and ecosystems right around the world including in Australia.''