"Changes in abundance and size of shrubs are frequently observed in these regions," says lead author Celine Bonfils of the Lawrence Livermore National Laboratory, US. "Small shrubs are already present in most tundra areas, ready to grow under more favourable conditions but until now, most climate model studies have only focused on the climate effects induced by a complete tundra-to-forest conversion."

The researchers filled this knowledge gap by conducting a series of idealized experiments with the NCAR-DOE Community Climate System Model to investigate how large-scale tundra-to-shrub conversion impacts on both the boreal climate and the vulnerability of permafrost. They found that an increase in the shrub fraction from 32% to 51% on the land north of 60°N triggered substantial regional atmospheric warming in spring and summer by reducing the land-surface albedo and boosting the water vapour content of the atmosphere through increased transpiration (see also ">Insight: rise in Arctic shrubs likely to increase vulnerability of permafrost).

"These runs, performed at the National Energy Research Scientific Computing Center (NERSC), show for the first time that the strength and timing of these two mechanisms greatly depends on the height of the shrubs, and the time at which branches and leaves protrude above the snow," explained Bonfils. Taller, and aerodynamically rougher, shrubs lower the albedo earlier in the spring and transpire more efficiently than shorter shrubs, thereby increasing soil warming and making the permafrost less stable.

If ocean and sea-ice are allowed to interact with the atmosphere, the model produces additional indirect warming, especially in the months during which the shrubs have no direct impact. This additional warming occurs through sea-ice reduction, which further lowers the surface albedo, and an increase in ocean evaporation, which adds more water vapour to the atmosphere. The fact that tall shrubs tend to systematically warm the soil and destabilize the permafrost more substantially than short shrubs is a conclusion relevant for the DOE IMPACTS Project. This project is funding research on possible future abrupt climate change in the boreal region resulting from carbon-climate interactions.