Drivers of Changes in Atmospheric Evaporative Demand over Land

Abstract:

A key component of the terrestrial hydrological cycle is evapotranspiration (ET), which links the water, energy and carbon cycles, and returns about two-thirds of precipitation to the atmosphere. Understanding the variability of ET is a key factor in climate change, playing a central role in the potential acceleration of the water cycle under global warming, providing feedbacks between the land and atmosphere, and mitigating and exacerbating extreme events such as droughts and heatwaves. Despite its importance our knowledge of ET variations and its controls is limited. These controls can be separated into atmospheric demand (radiative and aerodynamic controls) and surface limitations (environmental and ecophysical controls). Much of the uncertainty in ET derives from uncertainties in changes in atmospheric evaporative demand, often characterized by potential evapotranspiration (PET), which combines the effects of available radiation for evaporation and the ability of the atmosphere to accept evaporated water. This is particularly important in energy-limited regions, such as high latitudes and the humid tropics, where water availability is not limiting and changes in available radiation and temperature are key drivers.

To better understand changes and controls on atmospheric demand, we have developed an updated global dataset of surface meteorology and radiation for the past 70 years that merges reanalysis, satellite retrievals and station data. The reanalysis/satellite data are corrected locally (for biases and spurious trends) by assimilating data records for thousands of surface meteorology stations, and hundreds of surface radiation stations. The dataset is used to calculate long-term estimates of PET that show a general increase globally over the past 30-40 years. We decompose the variations in PET into components driven by radiative and aerodynamic controls. Although the spatial distribution of controls on PET changes is complex, temperature plays a major role overall in driving increases in PET globally. Humidity and windspeed are associated with changes in PET in some regions, although these are more uncertain because of data uncertainties. We discuss the implications of these variations and their controls on water availability and drought over the past decades and under future climates.