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Abstract Terrestrial ecosystems contribute most of the interannual variability (IAV) in atmospheric carbon dioxide (CO2) concentrations, but processes driving the IAV of net ecosystem CO2exchange (NEE) remain elusive. For a predictive understanding of the global C cycle, it is imperative to identify indicators associated with ecological processes that determine the IAV of NEE. Here, we decompose the annual NEE of global terrestrial ecosystems into their phenological and physiological components, namely maximum carbon uptake (MCU) and release (MCR), the carbon uptake period (CUP), and two parameters, α and β, that describe the ratio between actual versus hypothetical maximum C sink and source, respectively. Using long‐term observed NEE from 66 eddy covariance sites and global products derived from FLUXNET observations, we found that the IAV of NEE is determined predominately by MCU at the global scale, which explains 48% of the IAV of NEE on average while α, CUP, β, and MCR explain 14%, 25%, 2%, and 8%, respectively. These patterns differ in water‐limited ecosystems versus temperature‐ and radiation‐limited ecosystems; 31% of the IAV of NEE is determined by the IAV of CUP in water‐limited ecosystems, and 60% of the IAV of NEE is determined by the IAV of MCU in temperature‐ and radiation‐limited ecosystems. The Lund‐Potsdam‐Jena (LPJ) model and the Multi‐scale Synthesis and Terrestrial Model Inter‐comparison Project (MsTMIP) models underestimate the contribution of MCU to the IAV of NEE by about 18% on average, and overestimate the contribution of CUP by about 25%. This study provides a new perspective on the proximate causes of the IAV of NEE, which suggest that capturing the variability of MCU is critical for modeling the IAV of NEE across most of the global land surface.
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This content will become publicly available on October 16, 2026
Leading Satellite‐Based Evapotranspiration Products Insufficiently Capture Interannual Variability: Evidence From GRACE/FO and In Situ Observations
Abstract Satellite‐based evapotranspiration (ET) products such as OpenET and GLEAM are widely used for drought monitoring and ecosystem‐climate studies. However, their ability to accurately capture interannual variability (IAV), a key requirement for such applications, remains under‐evaluated. Here, we assessed IAV in OpenET and GLEAM using an independent water balance approach that combined precipitation, discharge, and GRACE/FO total water storage anomalies across nine river basins in the western United States. Even after accounting for observational uncertainty through a Monte Carlo approach, both products systematically underestimate IAV relative to water balance‐based ET, by more than 60% on average. This result is further supported by long‐term tower measurements from AmeriFlux. We also demonstrated that ET sensitivity to climate and vegetation drivers in OpenET and GLEAM differ substantially from water balance‐based estimates. These findings reveal important limitations in satellite‐based ET products and highlight the need for improved IAV representation to support ecosystem and climate applications.
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- Award ID(s):
- 2429739
- PAR ID:
- 10650729
- Publisher / Repository:
- Wiley
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 52
- Issue:
- 19
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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