Quantifying the Drivers of Evapotranspiration Over Land Under Future Climate Conditions

University Of Washington, Seattle WA

Investigators

Abstract

The land surface loses water to the atmosphere in two ways: through evaporation from soil, surface water, and plant canopies, and through transpiration, in which plants absorb soil moisture through their roots and release it through stomates, tiny openings on their leaves. Globally transpiration accounts for perhaps 60% of continental evapotranspiration (ET, the sum of evaporation and transpiration), thus vegetation plays a central role in the release of water from the land. The prominence of the "T" in ET gives plants an active role in climate change, as changes in vegetation could have as much influence on ET by regulating the land's supply of moisture to the atmosphere as changes in temperature have by regulating the atmosphere's demand for moisture from the land. Research conducted here examines the regulation of moisture supply by vegetation change under increasing CO2. The focus is specifically on two mechanisms through which CO2 directly influences plants, as opposed to the plant response to higher temperatures caused by the warming effect of CO2 in the atmosphere. The first mechanism is CO2 fertilization, which happens because plants grow by photosynthesis, combining CO2 from the atmosphere with water from the soil, so that higher CO2 levels promote plant growth. The result is an increase in transpiration due to an increase in leaf area, which provides a larger surface through which transpiration can occur. The second mechanism is a reduction in transpiration because plants absorb CO2 through their stomates and (roughly speaking) they close their stomates when they have enough CO2 for photosynthesis. Thus more CO2 in the air means that fewer stomates are open at any given time, reducing transpiration. The competing effects of fertilization and stomatal closure create a challenge for understanding how CO2-induced vegetation change affects the moisture supply for ET, since the net effect could depend sensitively on the details of plant physiology and the answer could differ substantially from one biome to another. The primary activity of the project is an examination of fertilization and stomatal closure in simulations performed with the Community Earth System Model (CESM), which uses the Community Land Model (CLM) as its land surface component model. Models used in climate change assessments show a wide range in both ET change and the contribution of transpiration to ET change. The CESM experiments are designed to determine what aspects of vegetation physiology account for the large model-to-model differences. The question is addressed through a perturbed physics ensemble (PPE) in which parameters controlling plant physiology are varied to determine the dependence of the transpiration response to CO2 on various physiological controls. Additional simulations are performed using a version of CESM in which CLM is replaced by SLIM, the Simple Land Interface Model, in which relevant processes are represented in less detail for the sake of generating results which are easier to interpret and understand. The work is of societal as well as scientific interest given concerns about future aridification due to CO2 increase. Recent studies suggest that changes in the moisture supply for ET due to vegetation effects can be comparable to the increase in atmospheric moisture demand that comes with warming temperatures. These changes matter for the dryness of the surface air, the amount of moisture retained in the soil, and the amount of runoff produced by rainstorms. They also matter for the intensity of heat waves and droughts and the likelihood of wildfires. Beyond its societal value the work has broader impacts through outreach conducted with the Burke Museum's Girls in Science program, in which middle school girls interact directly with female scientists to learn about careers in science and how science research is conducted. The project also provides support and training to a graduate student. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.