The effects of global warming on the microstructures and mechanical properties of permafrost

Global climate change is set to continue apace with 2016 as the warmest year since record keeping began.  The effects are most dramatic in the Polar Regions, where in Antarctica ice shelves are carving off with increasing rapidity and in the Arctic both the extent and thickness of the sea ice are continually decreasing.  The major issues arising from these phenomena have received a great deal of attention, i.e. the decrease in albedo as the sea ice cover recedes each summer, enhancing global warming, and the rise in sea level as land ice melts or slides into the ocean (melting of sea ice makes no difference to the sea levels although thermal expansion of the oceans is a significant contributor).

An issue that has received much less attention is the effects of global warming on the permafrost that covers up to 25% of the land in the Northern Hemisphere (permafrost also exists beneath the sea). Permafrost is a combination of rock, sediment, soil and some organic matter “cemented” together by ice. Permafrost, which can be up to 1500 m deep, contains 1500 GT (1.5 x 1015kg) of carbon1, twice as much as the carbon in the atmosphere and in all vegetation2.  Each year depending on location and local climate, the top 0.3-4 m of the permafrost, the so-called “active layer”, thaws.  As temperatures climb, the depth of this active layer increases and if it becomes too deep then some of the below surface layers do not refreeze in winter – perversely increasing snow cover with warmer temperatures can insulate the ground in winter preventing refreezing. There are four major issues as permafrost warms and thaws:

  1. The carbon locked up in the permafrost will be attacked by microbes leading to the production and release into the atmosphere of carbon dioxide and methane, two potent greenhouses gases - methane is 84 times more potent than carbon dioxide - that will accelerate Global warming;
  2. Permafrost is impermeable to water: permafrost thawing allows water to percolate through it leading to a loss of surface water, including whole lakes;
  3. Warmer permafrost is mechanically weaker and existing structures, such as buildings, roads, bridges and pipelines built on permafrost can collapse when the permafrost no longer supports the load – when the permafrost has thawed it cannot support a load but flows under its own weight;
  4. As coastal permafrost regions thaw, their weakened state along with the lack of sea ice cover allows wave action to erode the coast.

All of these phenomena are currently happening. While the only thing that we can do about the first two problems is to slow global warming, the latter two problems both affect Northern communities now and are also a challenge to construction in these regions as the temperature increases and the resources in these regions are exploited.  Thus the ability to understand the behavior of permafrost as it warms and relate it to its microstructure will enable prediction of the effects of warming and possible mitigation of the effects. Thus, the aim of this pilot project is to generate data that relates the strength of permafrost to its microstructure at different temperatures.

Funded by Dartmouth College.

Faculty contact: Ian Baker