Constraining the strength of the terrestrial CO2 fertilization effect in an Earth system model

Abstract:

In its most simple form the global carbon cycle can be characterized by the response of the land and the ocean carbon components to changes in atmospheric CO₂ concentration and the associated climate change. These characterizations are represented in terms of carbon-concentration and carbon-climate feedback parameters and have been used to compare Earth system models with interactive land and ocean carbon cycle components. Of these feedbacks, the response of the land carbon cycle component to changes in atmospheric CO₂ concentration, i.e. the carbon-concentration feedback over land, which is primarily determined by the strength of the terrestrial CO₂ fertilization effect, is the most difficult to constrain based on available observations and consequently also the most uncertain across models. Here, we estimate the strength of the terrestrial CO₂ fertilization effect in version 4.2 of the Canadian Earth system model that yields best comparison with four observation-based determinants of the global carbon cycle and the historical global carbon budget over the 1850-2005 period. These determinants include 1) globally-averaged atmospheric CO₂ concentration, 2) cumulative atmosphere-land CO₂ flux, 3) atmosphere-land CO₂ flux for the decades of 1960s, 1970s, 1980s, 1990s and 2000s and 4) the amplitude of the globally-averaged annual CO₂ cycle and its increase over the 1980 to 2005 period. In the simulation that yields best comparison with these determinants, the terrestrial net primary productivity (NPP) increases from ~58 Pg C/yr in 1850 to about ~74 Pg C/yr in 2005; an increase of ~27% over the 1850-2005 period.