Thermodynamic and hydrological impacts of increasing greenness in northern high latitudes

Satellite remote sensing data indicate that greenness has been increasing in the northern high latitudes, apparently in response to the warming of recent decades. To identify feedbacks of this land-cover change to the atmosphere, the authors employed the atmospheric general circulation model ARPEGE-CLIMAT, an adaptation of the Action de Recherche Petite Echelle Grande Echelle model for climate studies, to conduct a set of control and sensitivity modeling experiments. In the sensitivity experiments, they increased the greenness poleward of 60°N by 20% to mimic the manifestation of vegetation changes in the real world, and by 60% and 100% to represent potential aggressive vegetation change scenarios under global warming. In view of the direct exposure of vegetation to sunlight during the warm seasons, the authors focused their study on the results from late spring to early fall. The results revealed significant thermodynamic and hydrological impacts of the increased greenness in northern high latitudes, resulting in a warmer and wetter atmosphere. Surface and lower-tropospheric air temperature showed a marked increase, with a warming of 1°-2°C during much of the year when greenness is increased by 100%. Precipitation and evaporation also showed a notable increase of 10% during the summer. Snow cover decreased throughout the year, with a maximum reduction in the spring and early summer. The above changes are attributable to the following physical mechanisms: 1) increased net surface solar radiation due to a decreased surface albedo and enhanced snow-albedo feedback as a result of increased greenness; 2) intensified vegetative transpiration by the additional plant cover; and 3) reduced atmospheric stability leading to enhanced convective activity. The results imply that increased greenness is a potentially significant contributing factor to the amplified polar effects of global warming. © 2006 American Meteorological Society.