SYDNEY, March 10 (Reuters) - Australian scientists have found that
deforestation along the Amazon River in South America was reducing rainfall and
causing climate change in the region.
A study in the Amazon found that a loss of forests meant less water evaporated
back into the atmosphere, resulting in less rainfall, said Ann
Henderson-Sellers, director of environment at the Australian Nuclear Science and
Technology Organisation.
Key to the study was plotting the cycle of a heavy molecular version of water
common in the Amazon that evaporates more readily through plants than from lakes
and rivers.
Water from household taps consists of two "regular" hydrogen atoms and one
"regular" oxygen atom, explained Henderson-Sellers, but some water molecules see
the second hydrogen atom replaced by a heavier version called deuterium.
"Plants transpire the water molecules and pumps them back into the air, without
discriminating between heavy or regular molecules," Henderson-Sellers told
Reuters.
As the study tracked the water cycle as it flowed from the Amazon River into the
Atlantic Ocean, evaporated, fell as rain and returned back to the sea,
scientists discovered there had been a reduction in heavy-molecule water since
the 1970s.
Henderson-Sellers said the only possible explanation for the decline was that
heavy-molecule water was no longer being returned to the atmosphere to fall as
rain due to less vegetation, signalling a relationship between deforestation and
rainfall.
"The bottom line is for the first time we can tell the difference between
moisture that has been transpired through the plants, and water that has come
through the rest of the water cycle," she said.
"Trees play a critical role in moving heavy-water molecules through the cycle.
This is the first demonstration that deforestation has an observable affect on
rainfall."
The Amazon is the world's second longest river at 6,400 km (4,000 miles), but
boasts the greatest total flow of any river, releasing 6.5 million cubic feet
per second in the rainy season. It is responsible for a fifth of the total
volume of fresh water entering the world's oceans.
The Amazon's rainforest drainage area covers six million square km (2.3 million
square miles) and has been called the "lungs of the earth" by environmental
groups.