A changing climate effects the availability of water, agriculture, and virtually everyone on the planet. To predict changes in vegetation cover and adapt water usage appropriately it’s necessary to constrain changes in evaporative flux. Measuring evaporation is more challenging than other components of the hydrologic cycle such as precipitation which can be observed directly from space. Local estimates for evaporation rest on assumptions about vegetation and soil and their abilities to exchange moisture and heat with the atmosphere. Although recent progress has been made in reducing them, these estimates are plagued by uncertainties.
Cumulative estimates for evaporation over fluvial basins (e.g., the Amazon drainage basin) can be estimated from a mass balance if river runoff, precipitation and infiltration are known; yet these large-scale estimates are not ideal when tackling local changes in water cycle.
Globally, about 2/3 of evaporation on Earth occurs in the tropical and subtropical regions where climate is sensitive to strong interannual variability of the El Nino Southern Oscillation (ENSO). During an El Nino, precipitation over tropical continents decreases causing less evaporation. During La Nina years the opposite occurs. Evaporation may also change as greenhouse gas concentrations increase. A simple energetic argument predicts enhanced evaporation as climate warms from the greenhouse effect. Whether the greenhouse effect has significantly impacted continental evaporation over the 20th century and whether changes in the frequency of ENSO over the same period have amplified or dampened the greenhouse effect are equally unclear.
Miralles et al. (2013) mapped global evaporation over the past 30 years. They found a general increase in evaporation over most tropical and subtropical regions since 1980, in agreement with a direct greenhouse effect. Consistent with expectations, evaporation changes in the tropics were counteracted by ENSO: Miralles et al. found that stronger El Nino in the late 1990’s led to lower continental precipitation and evaporation in the tropics, leading to increased water stress in those regions. Conversely, in the late 2000’s, a persistent La Nina phase caused evaporation to increase over land with evaporation inversely related to soil moisture.
Miralles et al. calls for a better understanding of ENSO and its relationship to global warming. Changes in the frequency, magnitude or phasing of ENSO during the 21st century could significantly impact continental evaporation, amplifying or inhibiting it, potentially exacerbating or ameliorating the expected increase in evaporation predicted from a direct greenhouse effect.
Miralles, 2013: El Nino-La Nina cycle and recent trends in continental evaporation, Nature Climate Change, DOI: 10.1038/NCLIMATE2068