Patrick Guilfoile: How permanent is permafrost?
About one-quarter of land in the Northern Hemisphere is permanently frozen ground, or permafrost, located primarily in Canada, Alaska and Siberia. In areas with permafrost, buildings are carefully constructed to prevent the ground from thawing. Otherwise, the soil will turn to mush when the permafrost melts, destroying the building.
Aside from the concerns about damage to human infrastructure, there is also concern about the potential for massive release of carbon into the atmosphere if permafrost melts. Permafrost is a major storage site for carbon. Large scale melting of permafrost would allow microbes to convert much of that carbon to carbon dioxide, leading to further increases in carbon dioxide concentrations in the atmosphere.
So, how do you determine the stability of permafrost? Researchers from several universities in Europe studied caves in Russia. They analyzed stalactites and stalagmites from the caves. The scientists started with the premise that, in areas where permafrost has melted, water will flow through the ground and add to the stalactites and stalagmites. In cases where the permafrost remains intact, no new material is added, since the water is locked up as ice.
But how do you determine the dates when liquid water was flowing into the caves? The scientists used a diamond saw to cut through collected stalactites and stalagmites. The resulting cross sections looked much like tree rings, with wide rings representing times when the formations were growing.
The researchers then took small samples from the wide rings and measured the amount of radioactive elements uranium and thorium present in each. Since uranium is soluble in water and thorium is not, water percolating into caves contains only uranium. Over time, at a rate measured in hundreds of thousands of years, the uranium decays to thorium. Using highly sensitive instruments, scientists measured the amounts of uranium and thorium in the samples, and used those measurements as a clock to determine how long ago the rock was formed. The process has analogy to measuring time with an hourglass, if you imagine an hourglass where the sand stays in place and marks time by gradually changing from sand to salt. The ratio of sand to salt would be a measure of elapsed time.
Researchers used this dating method on samples taken from caves in areas from 60 degrees north latitude to 50 degrees north latitude in Siberia, and were able to determine dates where the areas surrounding the caves did not have permafrost. In cave furthest north, in an area now with continuous permafrost, the climate warmed between 400,000 and 450,000 years ago to the point where there was no permafrost near the cave.
During this time, when there was widespread melting of permafrost, the carbon dioxide levels in the atmosphere were over 280 parts per million, among the highest levels in the past 500,000 years, and the climate was slightly warmer than it is today.
Currently, the carbon dioxide concentration in the atmosphere is about 397 parts per million, up from 290 parts per million since 1900. Most climate scientists feel that existing data show that as carbon dioxide levels in the atmosphere rise, global temperature rises as well, although there may be a slight lag between increases in carbon dioxide and temperature.
Consequently, there is substantial concern that temperatures in the arctic will increase in the near term, and this may lead to rapid melting of permafrost with destructive effects on buildings and other human development. In addition, melting permafrost may release additional carbon dioxide, further adding to concerns about climate change.
More information is available in an article by A. Vaks and others. “Speleothems reveal 500,000-year history of Siberian permafrost.” Science 340:183-186, April 12, 2013.
— Patrick Guilfoile has a doctorate in bacteriology and is the associate vice president for academic affairs at Bemidji State University