Regional Respiration Responses to Historical CO2 Fertilization in a Data Assimilation Framework

Abstract:

The increase in atmospheric concentrations of CO₂ has been shown to increase terrestrial gross primary productivity (GPP) through modeling studies and site level experiments. Additionally, the terrestrial carbon cycle is estimated to have served as a sink to approximately 30% of the CO₂ emitted by humans over the last century. Here, we quantify the respiration response to CO₂ driven by these increases in GPP and how it differs across regions to explain the net carbon sink. To do so, we use Carbon Data Model framework (CARDAMOM), a Bayesian model carbon cycle data assimilation system, constrained by remote sensing observations (including net biosphere exchange from CMS-Flux, solar-induced fluorescence, carbon monoxide and leaf area index). Here we use forcing scenarios with climate change, rising atmospheric concentrations of CO₂, or both in an ensemble of past (1920 – 2015) meteorology from the Community Earth System Model Large Ensemble Numerical Simulations Modeled meteorology is bias-corrected to a combination of Climatic Research Unit TS3.2 and the National Centers for Environmental Prediction reanalysis (CRUNCEP V7) to enable consistency with observationally-derived parameters. We find that the total change in carbon fluxes are dominated by the response of photosynthesis to rising CO₂. However, the increase in the net carbon sink across regions is strongly shaped by the response of respiration to this enhanced vegetation growth. This indirect response of respiration offsets nearly 75% of the increased GPP globally. Regionally, this offset is particularly strong in the Wet Tropics where large increases in GPP are offset by nearly commensurate large losses of carbon to respiration. The respiration rate changes will also be related to changes in different soil and vegetation carbon pools across the globe. Taken together, our results suggest indirect respiration responses to CO₂ are an underappreciated modulator feedback between climate and the carbon cycle.