Reconciling Global Transpiration Estimates of Process and Data Driven Models

Plants are key regulators of global terrestrial water cycles, acting as the conduit for transporting soil moisture to the atmosphere via transpiration. Plants are also already being impacted by the changing environment, as demonstrated by the large forest mortality of central and southern Europe in the last decade. In addition to these negative effects, increasing CO₂ concentrations are expected to make plants more efficient at taking up carbon per unit of water loss, a phenomenon currently accounted for in most earth system models. The complex potential positive and negative effects of a changing climate on plants, as well as the potential reverberations across the broader water cycle, is a key unknown when making climate projections into the future.

Despite how central plants are to the global terrestrial water cycle, current model estimates of global transpiration to evapotranspriation (T/ET) in the CMIP 6 (Coupled Model Intercomparison Project) models disagree, ranging from 20-60% for the historical period. The broad uncertainty in estimated global transpiration represents a major source of uncertainty, both in our current understanding of the control of plants on water cycles, as well as in how global water cycle feedbacks might play out in the next 100 years.

New data driven estimates of transpiration from FLUXCOM-X, which models ecosystem fluxes using remote sensing, in situ eddy covariance measurements and machine learning, representing a new opportunity for an independent diagnostic to evaluate global transpiration estimates, such as those from the CMIP 6 models. A key advantage to the new FLUXCOM-X transpiration estimates is full spatiotemporal coverage as 0.05° spatial and hourly temporal resolution over more than 20 years, allowing diagnostics to account for spatial regions and temporal periods with highest disagreement, such as green up, peak growing season, and during precipitation events. Going forward, utilizing the data driven products of transpiration and evapotranspiration as a new diagnostic of model functioning will help guide model development and lower climate projections into the future.