By Environment Correspondent Alister Doyle

OSLO (Reuters) - Ancient records from icy caves in Siberia show that a small amount of global warming can thaw vast areas of frozen ground and release harmful stores of greenhouse gases, a study showed.

Any melt of permafrost, or permanently frozen soil that covers almost a quarter of the northern hemisphere from Alaska to China, can also destabilize everything from oil and gas pipelines to buildings and roads.

"Global climates only slightly warmer than today are sufficient to thaw significant regions of permafrost," experts in Britain, Russia, Switzerland and Mongolia wrote in Friday's edition of the journal Science after studying Siberian caves.

A global rise of 1.5 degrees Celsius (2.7F) above late 19th century temperatures - less than a 2.0 C (3.6F) ceiling for global warming set in 2009 by almost 200 nations - could bring a substantial thaw as far north as 60 degrees latitude, they said.

Sixty degrees girdles the world through Siberia, Nordic nations, the southern tip of Greenland, Canada and south Alaska. U.N. studies show that temperatures have already risen by about 0.7 degree C (1.4F) since the 19th century and are still rising.

The scientists studied caves in Siberia and found a 500,000-year record of stalactites and stalagmites, rocks which only grow when water drips and so show periods when permafrost melted.

DEEP FREEZE

In the cave furthest north at 60 degrees latitude, near the Siberian town of Lensk, they discovered that stalactites last grew 400,000 years ago in a naturally warm period.

Ancient records from elsewhere in the world, such as growth of forests in Greenland or Pacific Ocean temperatures inferred from fossils, indicated that global temperatures at the time were about 1.5 degree Celsius above 19th century levels.

"We need to study more caves, further north in Siberia," Anton Vaks, lead author of the study at Oxford University in Britain, told Reuters. He said there were worrying implications for the stability of infrastructure and global warming. Continued...