The regional European atmospheric transport inversion comparison, EUROCOM: first results on European-wide terrestrial carbon fluxes for the period 2006–2015

Guillaume Monteil ,J Tarniewicz ,Ingrid T Luijkx ,Michael Mischurow ,Anita L Ganesan ,Marko Scholze ,Grégoire Broquet ,Christian Rödenbeck ,Matthew Lang ,Naomi Smith ,Rona L Thompson ,Frank‐Thomas Koch ,A G C A Meesters ,M L Rigby ,Philippe Ciais ,Alistair J Manning ,Wouter Peters ,P Peylin ,Alex Vermeulen ,Ute Karstens ,Christoph Gerbig ,Emily White ,Evie M Walton

doi:10.5194/acp-20-12063-2020

Abstract

Abstract. Atmospheric inversions have been used for the past two decades to derive large-scale constraints on the sources and sinks of CO2 into the atmosphere. The development of dense in situ surface observation networks, such as ICOS in Europe, enables in theory inversions at a resolution close to the country scale in Europe. This has led to the development of many regional inversion systems capable of assimilating these high-resolution data, in Europe and elsewhere. The EUROCOM (European atmospheric transport inversion comparison) project is a collaboration between seven European research institutes, which aims at producing a collective assessment of the net carbon flux between the terrestrial ecosystems and the atmosphere in Europe for the period 2006–2015. It aims in particular at investigating the capacity of the inversions to deliver consistent flux estimates from the country scale up to the continental scale. The project participants were provided with a common database of in situ-observed CO2 concentrations (including the observation sites that are now part of the ICOS network) and were tasked with providing their best estimate of the net terrestrial carbon flux for that period, and for a large domain covering the entire European Union. The inversion systems differ by the transport model, the inversion approach, and the choice of observation and prior constraints, enabling us to widely explore the space of uncertainties. This paper describes the intercomparison protocol and the participating systems, and it presents the first results from a reference set of inversions, at the continental scale and in four large regions. At the continental scale, the regional inversions support the assumption that European ecosystems are a relatively small sink (-0.21±0.2 Pg C yr−1). We find that the convergence of the regional inversions at this scale is not better than that obtained in state-of-the-art global inversions. However, more robust results are obtained for sub-regions within Europe, and in these areas with dense observational coverage, the objective of delivering robust country-scale flux estimates appears achievable in the near future.

Highlights

The carbon budget of Europe has been explored in several large-scale synthesis studies, such as the CarboEuropeIntegrated Project (Schulze et al, 2009) and the REgional Carbon Cycle Assessment and Processes project (RECCAP; Luyssaert et al, 2012), to name a few
Comparisons between the prior and posterior bias and root mean square (RMS) differences between the time series of measured and simulated data are shown for each inversion in Fig. 3, for each site and for the whole ensemble of assimilated observations
The largest bias reductions are obtained for the inversions that had the largest prior biases (CarboScope-Regional, with a mean bias reduced from −0.91 to −0.18 ppm, and NAME-HB, with a mean bias reduced from −0.87 to +0.04 ppm)

Summary

Introduction

The carbon budget of Europe has been explored in several large-scale synthesis studies, such as the CarboEuropeIntegrated Project (Schulze et al, 2009) and the REgional Carbon Cycle Assessment and Processes project (RECCAP; Luyssaert et al, 2012), to name a few These have helped refine the knowledge of the European carbon cycle, large uncertainties remain regarding the quantification of the flux between terrestrial ecosystems and the atmosphere, usually quantified as the net ecosystem exchange (NEE), i.e. the sum of emissions (total ecosystem respiration (TER), i.e. autotrophic and heterotrophic respiration) and uptake (gross primary production (GPP), i.e. photosynthesis) of carbon by ecosystems to and from the atmosphere, or alternatively NBP (net biome production), which includes the impact of ecosystem disturbances (fires, land use change, etc.). The lack of a robust and precise quantification of the natural CO2 fluxes in Europe limits our ability to understand the links between the NEE flux and external forcings such as, for example meteorological variability (including the impact of extreme events like droughts and cold spells) and trends (Ciais et al, 2005; Maignan et al, 2008) or land use change (Naudts et al, 2016), and to forecast the evolution of the land sink in Europe, in the context of global climate change

Methods

Results

Discussion

Conclusion