Expansion and Growth of Boreal Shrubs Would Enhance High Latitude Warming
There is evidence that boreal trees and shrubs are invading tundra regions due to global warming at high latitudes. New computer simulations by DOE-funded scientists indicate that an invasion of shrubs, in turn, can further warm the northern high latitudes at a rate that depends on the plants’ height. The scientific team, composed of Lawrence Livermore National Laboratory, Lawrence Berkeley National Laboratory, and National Center for Atmospheric Research (NCAR) scientists, conducted a series of idealized experiments with the NCAR-DOE Community Climate System Model (CCSM) to investigate the potential impact of a large-scale, tundra-to-shrub conversion on permafrost and the boreal climate. They found that an increase in the total shrub fraction from 32% to 51% of the land north of 60°N (Alaska and north) triggered a substantial regional atmospheric warming in the spring and summer by 1) reducing the land surface albedo (reflectance of light), and 2) increasing the water vapor content of the atmosphere through increased transpiration (loss of water vapor from plants). The team also found that the strength and timing of these two mechanisms depends highly on the height of the shrubs (i.e., the time at which branches and leaves protrude above the snow). Taller and aerodynamically rougher shrubs lower the albedo earlier in the spring and transpire more efficiently than shorter shrubs, increasing soil warming and destabilizing the permafrost. The addition of an interactive ocean model produces additional warming through reduction in the amount of sea ice (which lowers further the surface albedo) and an increase in ocean evaporation (which adds more water vapor to the atmosphere). The study highlights the significant warming influence of high-latitude vegetation changes that should be included in climate simulations.
Bonfils, C., T. J. Phillips, D. M. Lawrence, P. Cameron-Smith, W. J. Riley, and Z. M. Subin. 2012. “On the Influence of Shrub Height and Expansion on Northern High Latitude Climate,” Environmental Research Letters 7, 015503. DOI:10.1088/1748-9326/7/1/015503.