X-BASE: The first terrestrial carbon and water flux products from an extended data-driven scaling framework, FLUXCOM-X

Authors: NELSON, J.A. - Max Planck Institute For Biogeochemistry; WALTHER, S. - Max Planck Institute For Biogeochemistry; GANS, F. - Max Planck Institute For Biogeochemistry; KRAFT, B. - Max Planck Institute For Biogeochemistry; WEBER, U. - Max Planck Institute For Biogeochemistry; NOVICK, KIMBERLY - Indiana University; BUCHMANN, N. - Eth Zurich; MIGLIAVACCA, M. - European Commission-Joint Research Centre (JRC); SIGUT, L. - Global Change Research Institute; Scott, Russell; Goslee, Sarah; WOHLFAHRT, GEORG - University Of Innsbruck

Submitted to: Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/2/2024
Publication Date: 11/18/2024
Citation: Nelson, J., Walther, S., Gans, F., Kraft, B., Weber, U., Novick, K., Buchmann, N., Migliavacca, M., Sigut, L., Scott, R.L., Goslee, S.C., Wohlfahrt, G., et al. 2024. X-BASE: The first terrestrial carbon and water flux products from an extended data-driven scaling framework, FLUXCOM-X. Biogeosciences. 21(22):5079–5115. https://doi.org/10.5194/bg-21-5079-2024.
DOI: https://doi.org/10.5194/bg-21-5079-2024

Interpretive Summary: Many of the most pressing scientific knowledge gaps surrounding the land carbon balance and the water cycle require information on carbon and water exchanges through time and at large continental to global scales. However, direct observations of these exchanges are confined to individual locations and limited periods of time. Methodologies to transcend the gap between local and global scales are needed to ultimately support societal relevant activities of building greenhouse gas monitoring systems, taking informed climate and land management actions, and verifying the effectiveness of mitigation strategies. Here, we present a new dataset of global high-resolution carbon and water exchanges. This dataset was produced using a framework designed to explore and mitigate current limitations to upscaling from sites to the global scale. Beyond fostering activities to enhance the quality and coverage of available site observations, the framework is designed to be flexible, allowing for the incorporation of new site and remote sensing data streams as they become available.

Technical Abstract: Mapping in-situ eddy covariance measurements of terrestrial land-atmosphere fluxes to the globe is a key method for diagnosing the Earth system from a data-driven perspective. We describe the first global products (called X-BASE) from a newly implemented up-scaling framework, FLUXCOM-X. The XBASE products comprise of estimates of net ecosystem exchange (NEE), gross primary productivity (GPP) as well as evapotranspiration (ET) and, for the first time, a novel fully data-driven global transpiration product (ETt), at high spatial (0.05°) and temporal (hourly) resolution. X-BASE estimates the global at -5.75 +/- 0.33 PgC yr-1 for the period 2001-2020, showing a much higher consistency with independent atmospheric carbon cycle constraints compared to the previous versions of FLUXCOM. The improvement of global NEE was likely only possible thanks to the international effort to improve the precision and consistency of eddy covariance collection and processing pipelines, as well as to the extension of the measurements to more site-years resulting in a wider coverage of bio-climatic conditions. However, X-BASE global net ecosystem exchange shows a very low inter-annual variability, which is common to state-of-the-art data-driven flux products and remains a scientific challenge. With 125 +/- 2.1 PgC yr-1 for the same period, X-BASE GPP is slightly higher than previous FLUXCOM estimates, mostly in temperate and boreal areas. X-BASE evapotranspiration amounts to 74.7x10³ +/- 0.9x10³ km3 globally for the years 2001-2020, but exceeds precipitation in many dry areas likely indicating overestimation in these regions. On average 57% of evapotranspiration is estimated to be transpiration, in good agreement with isotope-based approaches, but higher than estimates from many land surface models. Despite considerable improvements to the previous up-scaling products, many further opportunities for development exist. Pathways of exploration include methodological choices in the selection and processing of eddy covariance and satellite observations, their ingestion into the framework, and the configuration of machine learning methods. For this, the new FLUXCOM-X framework was specifically designed to have the necessary flexibility to experiment, diagnose, and converge to more accurate global flux estimates.