Using automated machine learning for the upscaling of gross primary productivity
Abstract. Estimating gross primary productivity (GPP) over space and time is fundamental for understanding the response of the terrestrial biosphere to climate change. Eddy-covariance flux towers provide in situ estimates of GPP at the ecosystem scale, but their sparse geographical distribution limits larger scales inference. Machine learning (ML) techniques have been used to address this problem by extrapolating local GPP measurements over space using satellite remote sensing data. However, the accuracy of the regression model can be affected by uncertainties introduced by model selection, parametrization, and choice of predictor features. Recent advances in automated ML (AutoML) provide a novel automated way to select and synthesize different ML models. In this work, we explore the potential of AutoML by training three major AutoML frameworks on eddy-covariance measurements of GPP at 243 globally distributed sites. We compared their ability to predict GPP and its spatial and temporal variability based on different sets of remote sensing predictor variables. Predictor variables from only MODIS surface reflectance data and photosynthetically active radiation explained over 70 % of the monthly variability in GPP, while satellite-derived proxies for land surface temperature, evapotranspiration, soil moisture and plant functional types, and climate variables from reanalysis (ERA5-Land) further improved the frameworks' predictive ability. We found that the AutoML framework AutoSklearn consistently outperformed other AutoML frameworks as well as a classical Random Forest regressor in predicting GPP, reaching an overall r2 of 0.75. In addition, we deployed AutoSklearn to generate global wall-to-wall maps highlighting GPP patterns in good agreement with satellite-derived reference data. This research benchmarks the application of AutoML in GPP estimation and assesses its potential and limitations in quantifying global photosynthetic activity.
