Abstract

Abstract. While heterotrophic respiration (Rh) makes up about a quarter of gross global terrestrial carbon fluxes, it remains among the least-observed carbon fluxes, particularly outside the midlatitudes. In situ measurements collected in the Soil Respiration Database (SRDB) number only a few hundred worldwide. Similarly, only a single data-driven wall-to-wall estimate of annual average heterotrophic respiration exists, based on bottom-up upscaling of SRDB measurements using an assumed functional form to account for climate variability. In this study, we exploit recent advances in remote sensing of terrestrial carbon fluxes to estimate global variations in heterotrophic respiration in a top-down fashion at monthly temporal resolution and 4∘×5∘ spatial resolution. We combine net ecosystem productivity estimates from atmospheric inversions of the NASA Carbon Monitoring System-Flux (CMS-Flux) with an optimally scaled gross primary productivity dataset based on satellite-observed solar-induced fluorescence variations to estimate total ecosystem respiration as a residual of the terrestrial carbon balance. The ecosystem respiration is then separated into autotrophic and heterotrophic components based on a spatially varying carbon use efficiency retrieved in a model–data fusion framework (the CARbon DAta MOdel fraMework, CARDAMOM). The resulting dataset is independent of any assumptions about how heterotrophic respiration responds to climate or substrate variations. It estimates an annual average global average heterotrophic respiration flux of 43.6±19.3 Pg C yr−1. Sensitivity and uncertainty analyses showed that the top-down Rh are more sensitive to the choice of input gross primary productivity (GPP) and net ecosystem productivity (NEP) datasets than to the assumption of a static carbon use efficiency (CUE) value, with the possible exception of the wet tropics. These top-down estimates are compared to bottom-up estimates of annual heterotrophic respiration, using new uncertainty estimates that partially account for sampling and model errors. Top-down heterotrophic respiration estimates are higher than those from bottom-up upscaling everywhere except at high latitudes and are 30 % greater overall (43.6 Pg C yr−1 vs. 33.4 Pg C yr−1). The uncertainty ranges of both methods are comparable, except poleward of 45∘ N, where bottom-up uncertainties are greater. The ratio of top-down heterotrophic to total ecosystem respiration varies seasonally by as much as 0.6 depending on season and climate, illustrating the importance of studying the drivers of autotrophic and heterotrophic respiration separately, and thus the importance of data-driven estimates of Rh such as those estimated here.

Highlights

  • The terrestrial carbon-cycle–climate feedback is a dominant contributor to the uncertainty of temperature projections in 2100 (Booth et al, 2012)

  • The future effect of carbon–climate feedbacks depends on the climate sensitivity of net terrestrial carbon fluxes, which are a close balance of net primary productivity, disturbance-related fluxes, and heterotrophic respiration (Rh)

  • More complex dynamics are revealed by considering the coefficient of variation (CV) of Rh

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Summary

Introduction

The terrestrial carbon-cycle–climate feedback (together with atmospheric processes) is a dominant contributor to the uncertainty of temperature projections in 2100 (Booth et al, 2012). The future effect of carbon–climate feedbacks depends on the climate sensitivity of net terrestrial carbon fluxes, which are a close balance of net primary productivity, disturbance-related fluxes, and heterotrophic respiration (Rh). The overall sensitivity of the terrestrial carbon uptake is dependent on the climatic response of these fluxes. The climatic sensitivity of Rh is the primary driver of the large divergence across modeled global soil carbon pools Konings et al.: Global satellite-driven estimates of heterotrophic respiration et al, 2013), which make up the largest terrestrial carbon pool (Jobbágy and Jackson, 2000)

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