Correcting CMIP5 evapotranspiration trends and sensitivity to changing climate
Sullivan, Ryan — Argonne National Laboratory
Area of research:
Evapotranspiration (ET) from the fifth phase of the Coupled Model Intercomparison Project (CMIP5) simulations exhibits substantial biases, fostering little confidence in future ET projections. We develop a methodology to calculate ET offline using the models’ archived meteorological outputs: temperature (T), water vapor pressure (e), atmospheric pressure (P), and surface net radiation (R). This methodology is used here to reconstruct ET projections from 2006 through 2100 over North America using output from select CMIP5 models, and to attribute projected ET trends to specific atmospheric controls.
Evapotranspiration (ET) is the fundamental mediator of water vapor transport across the land-biosphere-atmosphere interface, returning the majority of terrestrial precipitation to the atmosphere. Future projections of ET are critical for agricultural and freshwater management, especially in light of likely increased demand for irrigation, but ET is poorly constrained and highly uncertain in current climate models.
CMIP5 ET exhibits substantial bias in annual ET relative to in situ flux measurements from ARM and FLUXNET across North America (38-73%; 2006-2015), but ET reconstructed from the CMIP5 meteorology with a Penman-Monteith (PM)-based algorithm greatly reduces this bias (-8-+14%). Present-day North American ET is more sensitive to changes in atmospheric demand for ET (temperature and water vapor pressure) than energy limitation (net radiation), and to a lesser extent vegetation properties (leaf area index). Accordingly, ET is projected to increase 0.26-0.87 mm yr-1 yr-1 over North America through 2100 driven primarily by trends in temperature.