It seems that a combination of increased solar heating, clear skies, warm water influx from the south and strong winds blowing ice out into the Atlantic Ocean caused the exceptionally low ice levels in 2007. And, of course, once ice has been replaced by water, the decrease in albedo allows the surface to absorb more sunlight, creating a positive feedback effect.

The observed rate of loss of sea ice has been faster than expected and there's growing evidence that models aren't representative, said Mark Serreze of the US National Snow and Ice Data Center, who gave the AGU Meeting's Nye Lecture this year. There are concerns that the increasingly young and thin sea ice may reach a critical level, becoming so thin that natural climate variability causes rapid ice loss. Serreze explained that new data from IceSat indicates that the ice may be close to that critical thickness.

"Was 2007 the tipping point?" he asked. "We won't know for ten years or so."

James Hansen of NASA believes that most climate parameters such as Arctic sea ice, ice sheets, shifting climatic zones, alpine water supplies and ocean acidification have already reached a tipping point. He defines this as a point where no additional forcing is needed for relatively large climate change with large climate impacts. But Hansen believes that these parameters are not yet past the point of no return, that is, they haven't reached the level of an unstoppable climate impact that's irreversible on a practical timescale.

"Fossil fuel interests assume it's a God-given fact that we're going to burn all the fossil fuels," he said. "Other people, especially young people, may have different opinions. We need to roll back or else we are going to get big impacts."

But what about the solutions? A session of geoengineering brought a number of researchers together to look at the technical possibilities.

"There's a growing noise level about geoengineering...much of this is being vented in the media," said Richard Turco of UCLA. "We thought it would be good to discuss it in a scientific forum - there is a woeful lack of real scientific work and we hope this session will catalyse more interest from scientists."

Turco says that geoengineering proposals often lack basic attributes such as a scientific underpinning, a sense of scale, reasonable cost estimates, risk assessment, and ethical and legal vetting. "This is an issue that deserves to be done in a more serious manner," he said.

Ken Caldeira of Stanford University agreed on the need for further research. "If anyone tells you they know how the climate system will respond to a geoengineering scheme they're trying to sell you snake oil," he said. "It's possible the schemes will just make things worse. There's a tendency for politicians to want to act and we need to have scientific information as to whether [the schemes] would work."

While much about the climate may be uncertain, one thing's for sure - thousands of researchers will be back reporting on their work in San Francisco in December 2008.