Biophysical and Biogeochemical Tradeoffs of Extratropical Afforestation

Abstract:

Afforestation is a viable and widely practiced method of sequestering carbon dioxide from the atmosphere. However, recent studies have projected inadvertent consequences of planting forests over previously non-forested land. Because of a change in surface albedo from more reflective grasslands, crops, and tundra, the relatively darker forest canopy can cause an increase in net radiation at the surface, resulting in localized warming. This effect is further enhanced in far northern latitudes where the prevalence of snow cover exacerbates the albedo difference during the winter months. Using a dynamic vegetation model with model enhancements to accurately simulate the growth cycle of conifer vegetation, simulations were run over the northern latitudes of the US and Canada, with additional analysis focusing on Minnesota. The goal of this study is to examine the competing effects of enhanced carbon sequestration versus localized surface warming. This analysis can assist in determining where forests may be a benefit to the global climate (i.e., where carbon sequestration exceeds increased net radiation and surface warming). Using the State of Minnesota as an example, this approach is used to assess the efficacy of carbon sequestration to offset the projected increase in statewide greenhouse gas emissions, which are mandated to be reduced by 80% of 2005 levels by 2050. Our analysis suggests that over an 80-year time span, the average annual net ecosystem exchange of carbon would only offset 15% of 2005 emissions as a result of planting conifer forests over the entire state of Minnesota and lead to significant regional warming because of a lower surface albedo. Additionally, results show that the optimal time frame in which carbon sequestration exceeds the detrimental biophysical effects is on the order of 45-55 years, thus suggesting that large-scale land management (i.e., planting and harvesting) would be required every century to maintain a climate benefit. These results can provide needed information to policymakers evaluating carbon sequestration strategies.