The new results account for the freshwater effects of different types of human activity over the twentieth century until present time, including effects of changes in land use (e.g., intensified or extended agriculture, deforestation) and water use (e.g., related to hydropower development). These activities have led to an increased loss of freshwater to the atmosphere due to human-driven increase of evapotranspiration, which includes evaporation from surface and soil water and transpiration by plants.
On average over Earth's land surface, the combined effect of the different human activities is a net consumption of freshwater, which is greater than the freshwater planetary boundary.
Climate change also drives freshwater changes. Parallel new results, which were also published by Fernando Jaramillo and Georgia Destouni, in Geophysical Research Letters (December 2014), show that climate-driven and direct human-driven changes in evapotranspiration counteract each other globally and in most continents. This counteraction dampens the net total freshwater change, compared to only climate-driven change or only human-driven consumption.
"These results put into question an isolated use of only human water consumption as a freshwater planetary boundary," says Georgia Destouni, professor at the Department of Physical Geography, Stockholm University.
In summary, the new results are more alarming than previous assessments of the freshwater planetary boundary but less alarming for net global freshwater change. The latter is due to the found dampening effect.
"The new results show that estimates of global freshwater consumption are still highly uncertain," says Fernando Jaramillo, researcher at the Department of Physical Geography, Stockholm University.
In combination, the results should raise awareness on the high current level of human freshwater consumption, and guide new efforts for reducing the uncertainty about both human freshwater consumption and climate-driven freshwater change.
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