February 2, 2012 | A new computer modeling study from NCAR investigates how an increase in shrubs in the Arctic may affect permafrost.

Over the past few decades, a warming climate has meant that the Arctic’s grassy tundra is being increasingly overtaken by shrubs. A recent field experiment found that the shade provided by these shrubs keeps the permafrost in the ground beneath them comparatively cool. However, shrubs also alter the land surface albedo (fraction of solar radiation reflected back into space), as their dark leaves and stems absorb more radiation than the surrounding snowpack and tundra. This may partly counteract the shading effect by warming the atmosphere and soil.

NCAR scientists David Lawrence and Sean Swenson applied the Community Earth System Model to the shrub question. They found that ground temperatures were indeed cooler beneath shrubs, and that summertime soil thaw was shallower than beneath grassy tundra. But when they used the model to simulate a 20% increase in Arctic shrub coverage, the climate warmed considerably due to surface albedo changes and increased atmospheric moisture content from higher shrub transpiration. This heated the soil and deepened the depth to which soil thawed by 10 centimeters (4 inches).

The study, published in Environmental Research Letters, concludes that shrub expansion may actually increase, rather than decrease, permafrost vulnerability.

The scientists’ next step will be to study how an increase in shrubs may alter the Arctic’s carbon balance, with a focus on what will have a stronger impact—carbon sequestration in growing shrubs or carbon emissions from decomposing, freshly-thawed soil organic matter.

Lawrence, David M., and Swenson, Sean C., "Permafrost response to increasing Arctic shrub abundance depends on the relative influence of shrubs on local soil cooling versus large-scale climate warming," Environmental Research Letters, 2011; DOI: 10.1088/1748-9326/6/4/045504

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