Radiative Forcing Due to Reduction in Albedo by New Zealand Pasture/Forest and Equivalent CO2 Absorption

摘要

New Zealand is a comparatively “green” country with respect to land-cover, with approximately 39% land area under pasture and 31% under exotic and native forest. Throughout New Zealand's history conversion between forest and pasture has been a major land-use change. Forests contain a large amount of carbon stored in the plant biomass compared with pasture; however, trees reflect less incoming radiation compared with grass. The reduction in albedo increases the radiative forcing and hence negates some of the benefit of carbon storage. This paper examines the relationship between the radiative forcing due to reduction of albedo and the CO2 absorption when converting pasture land to forest. Previous studies have used a linear approximation of the highly non-linear relationship in the analysis. However, this approximation significantly overestimates the amount of CO2 uptake required to compensate for typical changes in albedo. This paper describes three commonly used non-linear functions that can more accurately calculate CO2 uptake required to balance albedo changes. Results are presented for the New Zealand plantation and indigenous forests. Five different forest types were investigated, and without accounting for the albedo effect the forests captured on average between 0.64 and 1.86 kg CO2/m~2/yr over a 50-year period. Accounting for the increased radiative forcing due to the albedo effect reduced the equivalent CO2 removal from the atmosphere to between 0.49 and 1.70 kg CO2/m~2/yr.