Areas of permafrost could start to thaw within decades, freeing long-stored greenhouse gases to the atmosphere, according to a study released on Wednesday that measured ancient stalagmites in a Siberian cave.

Continuous permafrost -- land that is frozen all year round -- starts to thaw when temperatures rise around 1.5 degrees Celsius (2.7 degrees Fahrenheit) above pre-industrial levels, its authors said.

Earth has already warmed by around 0.8 C (1.4 F) since the start of the Industrial Revolution in the mid-18th century, and on current trends, the threshold could be reached "within 10-30 years," they said.

"An urgent global effort (in) reducing emissions of greenhouse gases is required," they warned.

A team led by Gideon Henderson at Oxford University's Department of Earth Sciences looked at speleothems -- stalagmites and stalactites -- at Ledyanaya Lenskya cave near Lensk, eastern Siberia.

Speleothems grow when water from the surface seeps through the roof of the cave.

Caves themselves are usually at about the same temperature as the mean average air temperature at the surface.

Thus when the surface temperature drops below zero C, the ground freezes and there is no water seepage to promote the growth of speleothems.

As a result, speleothems in permafrost regions are faithful recorders of when their region was frozen and when it was above freezing, with traces of uranium and lead isotopes providing the pointers in time as to when these periods occurred.

The evidence from Ledyanaya Lenskaya suggests that its speleotherms grew substantially around 945,000 years ago, and again around 400,000 years ago.

Those bursts of permafrost thaw coincide with periods when Earth's surface warmed by 1.5 C (2.7 F) in relation to the pre-industrial benchmark, with a margin of error of 0.5 C (0.9 F), according to the research.

The study will be presented at the Geological Society of London on June 27, the society said in a press release.

The state of the permafrost is a big question in climate science.

Nearly a quarter of the northern hemisphere's land surface is permafrost, sequestrating an estimated 1,700 billion tonnes of carbon gas from vegetation that died millions of years ago.

If this land starts to thaw, the locked-up gas is released to the atmosphere, which adds to global warming emitted by fossil fuels, according to a much-feared scenario.

This in turn causes more permafrost to melt, emitting more gas, creating a vicious circle, which in scientific terms is called a positive feedback.

UN members have pledged to limit warming to 2 C (3.8 F) under a pact that would be agreed by 2015 and take effect by 2020.

On Wednesday, a report by the World Bank said there was a growing likelihood of 4 C (7.2 F) or even more by 2100, "in the absence of near-term actions and further commitments" on carbon emissions.