[1] A number of studies have suggested that the growing season duration has significantly lengthened during the past decades, but the connections between phenology variability and the terrestrial carbon (C) cycle are far from clear. In this study, we used the ''ORganizing Carbon and Hydrology In Dynamic Ecosystems'' (ORCHIDEE) process based ecosystem model together with observed climate data to investigate spatiotemporal changes in phenology and their impacts on carbon fluxes in the Northern Hemisphere (>25°N) during 1980-2002. We found that the growing season length (GSL) has increased by 0.30 days yr À1 (R 2 = 0.27, P = 0.010), owing to the combination of an earlier onset in spring (0.16 days yr À1 ) and a later termination in autumn (0.14 days yr À1 ). Trends in the GSL are however highly variable across the regions. In Eurasia, there is a significant trend toward earlier vegetation green-up with an overall advancement rate of 0.28 days yr À1 (R 2 = 0.32, P = 0.005), while in North America there is a significantly delayed vegetation senescence by 0.28 days yr À1 (R 2 = 0.26, P = 0.013) during the study period. Our results also suggested that the GSL strongly correlates with annual gross primary productivity (GPP) and net primary productivity (NPP), indicating that longer growing seasons may eventually enhance vegetation growth. A 1-day extension in GSL leads to an increase in annual GPP of 5.8 gC m À2 yr À1 (or 0.6% per day), and an increase in NPP of 2.8 gC m À2 yr À1 per day. However, owing to enhanced soil carbon decomposition accompanying the GPP increase, a change in GSL correlates only poorly with a change in annual net ecosystem productivity (NEP).Citation: Piao, S., P. Friedlingstein, P. Ciais, N. Viovy, and J. Demarty (2007), Growing season extension and its impact on terrestrial carbon cycle in the Northern Hemisphere over the past 2 decades, Global Biogeochem. Cycles, 21, GB3018,

Key Points:• ECOSTRESS is a state-of-the-art combination of thermal bands, spatial and temporal resolutions, and measurement accuracy and precision • Data from 82 eddy covariance sites were coalesced concurrently with the first year of ECOSTRESS for Stage 1 validation • Clear-sky ET from ECOSTRESS compared well against a wide range of eddy Abstract The ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station (ECOSTRESS) was launched to the International Space Station on 29 June 2018 by the National Aeronautics and Space Administration (NASA). The primary science focus of ECOSTRESS is centered on evapotranspiration (ET), which is produced as Level-3 (L3) latent heat flux (LE) data products. These data are generated from the Level-2 land surface temperature and emissivity product (L2_LSTE), in conjunction with ancillary surface and atmospheric data. Here, we provide the first validation (Stage 1, preliminary) of the global ECOSTRESS clear-sky ET product (L3_ET_PT-JPL, Version 6.0) against LE measurements at 82 eddy covariance sites around the world. Overall, the ECOSTRESS ET product performs well against the site measurements (clear-sky instantaneous/time of overpass: r 2 = 0.88; overall bias = 8%; normalized root-mean-square error, RMSE = 6%). ET uncertainty was generally consistent across climate zones, biome types, and times of day (ECOSTRESS samples the diurnal cycle), though temperate sites are overrepresented. The 70-m-high spatial resolution of ECOSTRESS improved correlations by 85%, and RMSE by 62%, relative to 1-km pixels. This paper serves as a reference for the ECOSTRESS L3 ET accuracy and Stage 1 validation status for subsequent science that follows using these data.