Contribution of different external forcings to the terrestrial carbon cycle variability in extratropical Eurasia in the last millennium

Abstract

The simulations for the last millennium with an Earth system model developed at the A. M. Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences (IAP RAS CM) are performed. These simulations are forced by changes of the parameters of the Earth orbit, total solar irradiance, volcanic (stratospheric) aerosols optical depth (only since 500 C. E.), atmospheric concentrations of greenhouse gases (CO2, CH4, and N2O), land use, tropospheric sulphate burden, and population density. It is found that the externally forced part of the terrestrial carbon cycle (TCC) interannual variability (IAV) was mostly driven by volcanic activity in the preindustrial part of the last millennium with an increase of importance of anthropogenic forcing during the 20th century. The latter enhanced IAV in the 20th century. For different time intervals and for different kinds of external forcing, coefficient of variation of IAV in different TCC characteristics is smaller (typically, up to few percent) in forested regions and larger in the regions covered by grasses (e.g., in tundra), where it could be as large as several tens of per cents for fire return interval. We show that the externally forced IAV of gross primary production during the 20th century dramatically increased as compared to that during the preindustrial period. In addition, the land use activity increases the relaxation time scale of the vegetation carbon stock by one order of magnitude.