Small temperature rise could start permafrost melt

Small temperature rise could start permafrost melt

22 February 2013, by Tamera Jones

Frost crystals at the entrance of Ledyanaya Lenskaya Cave.

If this happens, vast quantities of carbon dioxide and methane could be released from the permafrost to the atmosphere, potentially adding to the greenhouse effect.

Not just that, but buildings, transport infrastructure and major oil and gas facilities are all built on top of Siberian permafrost.

'If this land thawed, infrastructure would be seriously damaged, with obvious economic implications,' says Dr Anton Vaks from the University of Oxford, lead author of the study, published in Science Express.

Permafrost covers nearly a quarter of the northern hemisphere's land surface. Studies suggest it has warmed and thawed in the last 20 years as a direct result of changes in Earth's climate. But, until now, how this permanently frozen land will respond to long-term warming was less clear.

So a team of researchers from the UK, Russia, Switzerland and Mongolia decided to look to the past for clues. They analysed 36 speleothems – stalactites and stalagmites – from six caves along the so-called permafrost frontier, from Lensk to the Gobi Desert, where the ground begins to be permanently frozen in layers tens to hundreds of metres thick.

They focused on these structures, because they only grow when water from rain or melting snow drips into the caves. When cave temperatures drop below 0ºC, the water freezes and so stalactites and stalagmites stop growing.

'Speleothems in places like Siberia are relics from warmer periods before permafrost formed, so they give us a way of looking back in time,' explains Vaks.

This means stalactites and stalagmites can record up to half a million years of changing permafrost conditions, including warm periods with conditions similar to those today.

The thickness of the structures reflects long-term water availability in the region. So, from the northernmost cave at Lensk, the researchers recorded only eight centimetres of growth in half a million years. In contrast, speleothems from caves in southern Siberia had grown around 70cm in the same time. But in the dry Gobi Desert, this dropped back to 30cm.

Vaks and his colleagues used a specific technique called uranium-thorium dating to figure out how old the structures were. The technique relies on the time it takes for uranium-238 and uranium-234 – a tiny amount of which is found in calcium carbonate structures like speleothems or corals – to decay to thorium-230.

They found that most of the stalactites' growth in the northernmost cave near Lensk, which is on the boundary of continuous permafrost, happened around 400,000 years ago. This was a particularly warm period between ice ages, when scientists know temperatures were 1.5ºC higher than they are today.

But when global temperatures were just 0.5 to 1ºC higher than today's average, the researchers found that stalactites from the northernmost cave hadn't grown at all. Indeed, the researchers found that stalactites and stalagmites hadn't grown for 400,000 years, suggesting this region has existed in a permafrost state for all that time.

The findings reveal that more permafrost thawed 400,000 years ago during this warm period than at any point since. Previous studies have discovered that boreal forest – the most northerly place where trees can grow at any given time – existed in southern Greenland during this time.

The study also revealed potential changes to the Gobi Desert if average global temperatures rose to 1.5ºC higher than they are today.

'In a world warmer than today, warm enough to melt the coldest permafrost, adjoining regions would also see significant changes. Mongolia's Gobi Desert would become much wetter than it is today and, potentially, this extremely arid area coming to resemble today's Asian steppes,' says Vaks.

The study was funded by the Natural Environment Research Council, the Royal Society, and the Russian Foundation for Basic Research.