London: Dams and irrigation considerably raise the global human consumption of freshwater by increasing evapotranspiration – an effect that increases the loss of freshwater to the atmosphere, thereby reducing the water available for humans, societies and ecosystems on land.

"Small things that we do on the surface of the Earth can have large global effects. Previously, the global effects of local human activities such as dams had been underestimated.

This study shows that, so far, the effects are even greater than those from atmospheric climate change," said one of the researchers Fernando Jaramillo from Stockholm University in Sweden.

For the study, the researchers compiled and analysed data from 1901 to 2008 for climate, hydrology and water use in one hundred large hydrological basins spread over the world.

Their results raise the previous estimate of the global human freshwater footprint by almost 20 percent.

The increase in total freshwater loss from the landscape to the atmosphere from human activities is calculated to be around 4,370 cubic kilometres per year. This corresponds to two-thirds of the annual flow of the Amazon River, the world's largest river by discharge, the study said.

"Our study shows that we have already passed a proposed planetary boundary for freshwater consumption. This is serious, regardless of whether we have crossed a real boundary or if the boundary has been underestimated," Gia Destouni, the professor at Stockholm University, pointed out.

The findings appeared in the journal Science.

(IANS)