10 March 2005
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.
