Correcting CMIP5 Evapotranspiration Trends and Sensitivity to Changing Climate

Sullivan, R., Argonne National Laboratory

Surface Properties

Warm Boundary Layer Processes

Sullivan R, V Kotamarthi, and Y Feng. 2019. "Recovering evapotranspiration trends from biased CMIP5 simulations and sensitivity to changing climate over North America." Journal of Hydrometeorology, 20(8), 10.1175/JHM-D-18-0259.1.


a) Mean annual ET (mm yr-1) at U.S. Climate Reference Network (USCRN) sites calculated from the Penman-Monteith (PM) equation, and percent change in mean annual ET at USCRN sites calculated from the PM equation with ±20% change in temperature (second row; b-d), water vapor pressure (third row; e-g), net radiation (fourth row; h-j), and leaf area index (fifth row; k-m). First column and red in third column are -20% for each input variable; second column and blue in third column are +20% for each input variable.



Distribution of trends in mean annual ET (mm yr-1 yr-1; 2006-2100) across all grid cells from CMIP land surface models (CMIP-ET), CMIP meteorology and the Penman-Monteith (PM) equation (CMIP-PM-ET), and CMIP-PM-ET with no change in temperature (no ΔT), water vapor pressure (no Δe), and net radiation (no ΔR) (i.e. constant mean annual cycle from 2006-2015), and with a linear increase in LAI of 30% by 2100 (greening). Note outliers have been truncated for legibility.



a) Mean annual ET (mm yr-1) at U.S. Climate Reference Network (USCRN) sites calculated from the Penman-Monteith (PM) equation, and percent change in mean annual ET at USCRN sites calculated from the PM equation with ±20% change in temperature (second row; b-d), water vapor pressure (third row; e-g), net radiation (fourth row; h-j), and leaf area index (fifth row; k-m). First column and red in third column are -20% for each input variable; second column and blue in third column are +20% for each input variable.


Distribution of trends in mean annual ET (mm yr-1 yr-1; 2006-2100) across all grid cells from CMIP land surface models (CMIP-ET), CMIP meteorology and the Penman-Monteith (PM) equation (CMIP-PM-ET), and CMIP-PM-ET with no change in temperature (no ΔT), water vapor pressure (no Δe), and net radiation (no ΔR) (i.e. constant mean annual cycle from 2006-2015), and with a linear increase in LAI of 30% by 2100 (greening). Note outliers have been truncated for legibility.

Science

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.

Impact

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.

Summary

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.