Historically inconsistent productivity and respiration fluxes in the global terrestrial carbon cycle

Abstract

The terrestrial carbon cycle is a major source of uncertainty in climate projections. Its dominant fluxes, gross primary productivity (GPP), and respiration (in particular soil respiration, R_S), are typically estimated from independent satellite-driven models and upscaled in situ measurements, respectively. We combine carbon-cycle flux estimates and partitioning coefficients to show that historical estimates of global GPP and R_S are irreconcilable. When we estimate GPP based on R_S measurements and some assumptions about R_S:GPP ratios, we found the resulted global GPP values (bootstrap mean ${149}_{-23}^{+29}$ Pg C yr⁻¹) are significantly higher than most GPP estimates reported in the literature (${113}_{-18}^{+18}$ Pg C yr⁻¹). Similarly, historical GPP estimates imply a soil respiration flux (Rs_GPP, bootstrap mean of ${68}_{-8}^{+10}$ Pg C yr⁻¹) statistically inconsistent with most published R_S values (${87}_{-8}^{+9}$ Pg C yr⁻¹), although recent, higher, GPP estimates are narrowing this gap. Furthermore, global R_S:GPP ratios are inconsistent with spatial averages of this ratio calculated from individual sites as well as CMIP6 model results. This discrepancy has implications for our understanding of carbon turnover times and the terrestrial sensitivity to climate change. Future efforts should reconcile the discrepancies associated with calculations for GPP and Rs to improve estimates of the global carbon budget.

Terrestrial plants sequester carbon through photosynthesis, and that carbon is eventually returned to the atmosphere through respiration by plants and soil microbes. Here the authors show a large, unexpected gap in estimations of these two carbon fluxes.