Time Scales of Terrestrial Carbon Response Related to Land-Use Application: Implications for Initializing an Earth System Model

Abstract

he dynamic vegetation and carbon cycling component, LM3V, of the Geophysical Fluid Dynamics Laboratory (GFDL) prototype Earth system model (ESM2.1), has been designed to simulate the effects of land use on terrestrial carbon pools, including secondary vegetation regrowth. Because of the long time scales associated with the carbon adjustment, special consideration is required when initializing the ESM when historical simulations are conducted. Starting from an equilibrated, preindustrial climate and potential vegetation state in an offline land-only model (LM3V), estimates of historical land use are instantaneously applied in five experiments beginning in the following calendar years: 1500, 1600, 1700, 1750, and 1800. This application results in the land carbon pools experiencing an abrupt change?a carbon shock?and the secondary vegetation needs time to regrow into consistency with the harvesting history. The authors find that it takes approximately 100 years for the vegetation to recover from the carbon shock, whereas soils take at least 150 years to recover. The vegetation carbon response is driven primarily by land-use history, whereas the soil carbon response is affected by both land-use history and the geographic pattern of soil respiration rates. Based on these results, the authors recommend the application of historical land-use scenarios in 1700 to provide sufficient time for the land carbon in ESMs with secondary vegetation to equilibrate to adequately simulate carbon stores at the start of the historical integrations (i.e., 1860) in a computationally efficient manner.