Fewer trees, less rain: study uncovers deforestation equation

(2005) Australian scientists say they have found proof that cutting down forests reduces rainfall. The finding, independent of previous anecdotal evidence and computer modelling, uses physics and chemistry to show how the climate changes when forests are lost, by analyzing variations in the molecular structure of rain along the Amazon River.

Not all water, Professor Henderson-Sellers said, was made from the recipe of two atoms of “common” hydrogen and one of “regular” oxygen. About one in every 500 water molecules had its second hydrogen atom replaced by a heavier version called deuterium. And one in every 6500 molecules included a heavy version of the oxygen atom.

Knowing the ratio allowed scientists to trace the Amazon’s water as it flowed into the Atlantic, evaporated, blew back inland with the trade winds to fall again as rain, and finally returned to the river. The study showed that since the 1970s the ratio of the heavy molecules found in rain over the Amazon and the Andes had declined significantly. The only possible explanation was that they were no longer being returned to the atmosphere to fall again as rain because the vegetation was disappearing. “With many trees now gone and the forest degraded, the moisture that reaches the Andes has clearly lost the heavy isotopes that used to be recycled so effectively,” Professor Henderson-Sellers said.

“This is the first demonstration that deforestation has an observable impact on rainfall.”

Original article (sydney morning herald)

(saved version: Fewer trees, less rain)