Australian scientists are warning that the planet is nearing a tipping point beyond which polar melting will be irreversible in some areas, triggering a rise in sea levels of metres.

John Church, a marine scientist at the Australian CSIRO Marine and Atmospheric Research, told news agency Reuters: "Observations are at the very upper edge of the projections [in the IPCC report published in February]. I feel we are uncomfortably close to the threshold."

He argues that ice-sheet melting in Greenland and parts of Antarctica is close to being irreversible, and that warming in the Antarctic peninsula is happening faster than anywhere else on Earth.

In 2002, a 500 billion tonne iceberg - about the size of Luxembourg - broke off from the Larsen ice shelf. While this scale of break up has not been repeated yet, the reduction in the ice around the continent is making it possible for glaciers to flow into the sea much faster.

Elsewhere, the glaciers are actually retreating. One thousand kilometres to the north of the Antarctic continent, glaciers at Heard Island are in massive retreat.

And while western Antarctica has always been regarded as particularly vulnerable, new data from the much colder and higher eastern parts of the continent shows that the Tottenham Glacier's height has fallen by 10 metres over the last 16 years.

Church notes that the last time the planet was as warm as it is expected to be by the end of this century, sea levels were between four and six metres higher than they are today. The ice sheets of western Antarctica hold enough water to account for roughly that kind of sea level rise.

Meanwhile, about 100 million people live within a metre of today's sea level. Steve Rintoul, Church's colleague at CSIRO, told Reuters "Those people will need to go somewhere." ®

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