New research projects widespread permafrost thaw with 1.5 degrees of warming

  • 21 Feb 2013, 20:00
  • Roz Pidcock

A global temperature slightly warmer than today's could be enough to cause widespread thawing of Arctic permafrost, potentially releasing billions of tonnes of carbon dioxide into the atmosphere, says a new study. We investigate what this really means for the climate.

The new research suggests that based on what's happened in the Earth's past, global temperatures 1.5 degrees Celsius above pre-industrial levels could cause vast areas of carbon-rich permafrost to thaw. Global temperature has already risen 0.75 degrees since the turn of the 20th century.

What is permafrost?

Permafrost is permanently-frozen ground land in high latitudes. Scientists are concerned that as atmospheric temperatures rise, heat penetrates further into the ground, causing permafrost to thaw.

As the permafrost thaws, microbes degrade the carbon within it - a process that releases carbon dioxide and methane into the atmosphere. Methane is a powerful greenhouse gas - around 25 times  more effective at trapping heat than carbon dioxide over a 100-year cycle.

So scientists are concerned that greenhouse gases released from thawing permafrost could amplify global warming. Further warming means more permafrost thawing and so on, in a self-reinforcing cycle.

Cave deposits

Understanding how permafrost has changed throughout earth's history can offer clues about how sensitive it is to global temperature. In the new study, scientists reconstructed how the amount of permafrost in high northern latitudes has changed over the past 500,000 years.

In that time, the earth has gone through several natural temperature cycles with cold periods, known as glacials, interspersed with warmer periods, known as interglacials.

The team of UK, Russian, Swiss and Mongolian scientists analysed samples of stalagmites and stalagtites from caves in Northern Asia and Siberia, using radioactive dating techniques to precisely determine their ages.

Stalagmites and stalagtites only grow when the temperature is above zero degrees. So being able to date periods of growth provides a record of when permafrost existed and when it didn't.

When the scientists compared their permafrost record against global temperature changes over the last 500,000 years, they found the two varied closely with one another. Perhaps unsurprisingly, the most extensive permafrost thaw occurred in the warmest interglacial, about 400 thousand years ago. At that time, global temperatures were 1.5 degrees above pre-industrial levels.

Today, permafrost exists north of about 60 degrees latitude in the northern hemisphere - extending halfway down Russia and Canada and spanning the whole of Greenland. During the warmest interglacial, the permafrost boundary was pushed much further north, the scientists say - meaning less permafrost overall.

The researchers concluded that a similar level of warming in the future - this time due to human rather than natural causes - could have the same shrinking effect. And if permafrost does thaw to this extent, it could lead to the "substantial release" of carbon into the atmosphere.

Scientific uncertainty

While studies like this help pin down how sensitive permafrost could be to temperature rise, they don't answer the question of how much carbon is held in permafrost. As Professor Julian Murton from the University of Sussex told Carbon Brief:

"[T]he area of permafrost in the northern hemisphere is larger than the whole of Canada's land mass (about 10 million square kilometres) and the number of [carbon measurements] is very limited."

The new study is valuable, Murton added, but more studies are needed to determine how the exchange of carbon between the land and the atmosphere would be affected by a similarly large shrinking event today.

Murton also warned that while the past can offer clues as to how permafrost may develop in the future, there could be important differences in vegetation and snow cover, which could affect how quickly permafrost thaws.

Cause for concern?

Scientists have already found evidence that permafrost is thawing along continental shelves undersea in the Arctic and, more recently, in the  Antarctic. This has led some media outlets to talk about permafrost as "the methane time bomb". We have discussed the appropriateness - or otherwise - of this phrase before .

We asked Professor Murton how worried we should we be about thawing permafrost. He told us:

"With substantial global warming projected during the 21st century, which almost certainly will be amplified in Arctic regions (due to feedback effects in the global climate system), we should be concerned about greenhouse gas release from thawing permafrost."

But, he added that until scientists have more information about where exactly the carbon is stored and how much there is, it's difficult to predict exactly how much carbon dioxide or methane could be released.

A question of timing

There is also an issue of how long it takes for carbon to be released from permafrost. As Professor David Archer, expert on the climate impact of frozen carbon on land and under the ocean at the University of Chicago, told us recently:

"[T]he release rate of carbon from both of these sources [is] fairly slow, both in comparison with present-day methane emissions from tropical wetlands, and relative to fossil fuel carbon release as CO2."

So carbon released from permafrost is likely to have a much smaller impact on climate in the near future than carbon dioxide released from burning fossil fuels. Or as Archer put it:

"Don't worry about the methane, worry about the carbon dioxide. If carbon dioxide emissions aren't stopped, methane will just be a thin bit of frosting on the cake, and if carbon dioxide is fixed, methane won't be a problem."

So carbon dioxide emissions from burning fossil fuels may still the biggest source of global warming in the short term. But with temperatures expected to rise to levels where widespread permafrost thaw has occurred in the past, the release of carbon from permafrost may well be important in climate projections for the coming centuries.

Vaks. A et al., (2013) Speleotherms reveal 500,000-year history of Siberian permafrost. Science DOI:  10.1126/science.1228729

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