Our understanding of Earth’s carbon-climate system depends critically upon interactions among the factors that govern the land carbon (C) cycle, of which ecosystem response to increasing CO2, changing land-use, and nitrogen (N) limitation of vegetation are central. Ocean-atmosphere records suggest that globally, the land has been a C source over most of the industrialized period and only recently became a sink. This net change in the biosphere is caused by two main and opposing phenomena: A C loss through land-use and a C gain in response to increased atmospheric CO2 and anthropogenic N deposition. Typically, analyses of the land carbon sink assume that the effects of land-use and the response of the terrestrial biosphere to CO2 and N enrichment on land are additive and do not interact.
Results/Conclusions
We present results from the Princeton-GFDL LM3V, a dynamic global land model that allows effects of land-use, CO2 increase, anthropogenic N deposition and recent climate change to interact and operate in concert. While our model recreates the independent records of land C dynamics over the past 200 years, our results also show critical interactions between effects of land-use and CO2 increase that reduce the capacity of the terrestrial biosphere to take up anthropogenic carbon. We quantify three sources of interactions: (i) an accelerated response of secondary vegetation to CO2 and N; (ii) a C sink foregone on agricultural lands that would have occurred if they had been left in their original state; and (iii) a compounded clearance loss of C from deforestation. We find that these effects combine to reduce the current net C uptake on land by (0.4 Pg yr-1), and that each effect is becoming increasingly important in determining the trajectory of CO2 uptake of the land biosphere.