The impact of spatiotemporal variability in atmospheric CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; concentration on global terrestrial carbon fluxes

Eunjee Lee,Benjamin Poulter,Fan-Wei Zeng,Lesley E Ott,Brad Weir,Randal D Koster

doi:10.5194/bg-15-5635-2018

Abstract

Abstract. Land carbon fluxes, e.g., gross primary production (GPP) and net biome production (NBP), are controlled in part by the responses of terrestrial ecosystems to atmospheric conditions near the Earth's surface. The Coupled Model Intercomparison Project Phase 6 (CMIP6) has recently proposed increased spatial and temporal resolutions for the surface CO2 concentrations used to calculate GPP, and yet a comprehensive evaluation of the consequences of this increased resolution for carbon cycle dynamics is missing. Here, using global offline simulations with a terrestrial biosphere model, the sensitivity of terrestrial carbon cycle fluxes to multiple facets of the spatiotemporal variability in atmospheric CO2 is quantified. Globally, the spatial variability in CO2 is found to increase the mean global GPP by a maximum of 0.05 Pg C year−1, as more vegetated land areas benefit from higher CO2 concentrations induced by the inter-hemispheric gradient. The temporal variability in CO2, however, compensates for this increase, acting to reduce overall global GPP; in particular, consideration of the diurnal variability in atmospheric CO2 reduces multi-year mean global annual GPP by 0.5 Pg C year−1 and net land carbon uptake by 0.1 Pg C year−1. The relative contributions of the different facets of CO2 variability to GPP are found to vary regionally and seasonally, with the seasonal variation in atmospheric CO2, for example, having a notable impact on GPP in boreal regions during fall. Overall, in terms of estimating global GPP, the magnitudes of the sensitivities found here are minor, indicating that the common practice of applying spatially uniform and annually increasing CO2 (without higher-frequency temporal variability) in offline studies is a reasonable approach – the small errors induced by ignoring CO2 variability are undoubtedly swamped by other uncertainties in the offline calculations. Still, for certain regional- and seasonal-scale GPP estimations, the proper treatment of spatiotemporal CO2 variability appears important.

Highlights

Quantifying the sources and sinks of carbon at the land surface is key to an accurate carbon balance and to the overall assessment of where anthropogenically released fossilized carbon ends up in the Earth system
Given that no direct measurements of gross primary production (GPP) exist at the global scale (Anav et al, 2015), we evaluate the GPP values produced in our control simulation against GPP estimates from the data-derived FLUXNET Multi-Tree Ensemble (MTE) GPP project
Our results indicate that ignoring temporal variability in atmospheric CO2 in the bottom-up estimation of carbon fluxes with a representative offline model can lead to overestimates of global GPP of up to 0.5 Pg C year−1

Summary

Introduction

Quantifying the sources and sinks of carbon at the land surface is key to an accurate carbon balance and to the overall assessment of where anthropogenically released fossilized carbon ends up in the Earth system. Some studies consider tropical ecosystems to be carbon sinks (Stephens et al, 2007; Lewis et al, 2009; Schimel et al, 2015) and others consider them to be carbon sources (Baccini et al, 2017; Houghton et al, 2018). A substantial interannual variability is found in the tropical carbon balance, primarily in response to climate-driven variations (Baker et al, 2006; Cleveland et al, 2015; Fu et al, 2017); tropical ecosystems represent a large fraction of the uncertainty in estimates of the total land carbon sink and its future trajectory (Pan et al, 2011; Wang et al, 2014). Carbon fluxes in boreal ecosystems remain highly uncertain and are likely to be strongly influ-

Methods

Discussion

Conclusion