When air warms it can hold more water vapour. Globally, temperatures have risen by around 0.8°C since the 1880s, boosting the moisture-retaining capacity of air by more than 4%. Some of that extra moisture comes from plants and soil, as warmer temperatures encourage leaf pores to open, whilst the extra heat provides more energy to evaporate water. Today we are starting to see the results, with increased rainfall across many parts of the world.

But are climatic changes the only driver of the faster evapotranspiration cycle? To investigate, Jiafu Mao from Oak Ridge National Laboratory, US, and colleagues used remote sensing data and land-surface models to simulate the changes in evapotranspiration between 1982 and 2010.

Both observations and models show clear increases in evapotranspiration over this period, with up to 1.18 mm more water evapotranspired across the global terrestrial ecosystem each year. Mao and the team investigated the impact of a range of drivers on evapotranspiration, including changing climate, carbon dioxide, nitrogen deposition and land-use change. Climate, in particular precipitation, was the strongest driver of changing evapotranspiration rates, they found. Rising atmospheric carbon dioxide ranked second.

The rise in carbon dioxide acts to suppress evapotranspiration. Although increased carbon dioxide boosts plant growth and photosynthesis, it also encourages plants to close their stomata to reduce water loss. It's this physiological effect that exerts the greatest control. Mao and colleagues found that in areas with dense vegetation, such as tropical forests and high-latitude shrubland, the carbon dioxide effect was the anthropogenic factor that dominated, resulting in locally reduced rates of evapotranspiration. But globally the trend was increasing evapotranspiration, driven mainly by climate influences.

Increased nitrogen deposition amplified global evapotranspiration slightly, by causing more plant growth. Meanwhile, land-use change had regional effects. "Land-use-induced evapotranspiration changes were minor globally, but pronounced locally, particularly over regions with intensive land-cover changes such as deforestation," said Mao. So although the global trend is for increasing evapotranspiration and a faster hydrological cycle, there is significant regional variation.

As global temperatures continue to rise, these hydrological changes are likely to become more pronounced, with implications for life on Earth. "The significant increasing trend of evapotranspiration in each hemisphere may imply decreased availability of freshwater resources for humans and terrestrial ecosystems at the continental scale, while at the same time implying higher demands for agriculture and silviculture," said Mao. Understanding and predicting how those changes will play out is important if we want to prepare ourselves for the challenges they may bring.

The team published their results in Environmental Research Letters (ERL).

Related links

Related stories