Two Australian scientists have discovered a new tool for testing global warming and deforestation using rainfall information collected over 40 years from sites in the Amazon jungle and the Andes mountain range.

Professor Ann Henderson-Sellers, Director of Environment at ANSTO, said the treasure trove of data comes from a network originally established to monitor radioactive fallout from nuclear bomb testing in the 1950s and 1960s.

Among the records mouldering away in "dusty volumes in Vienna", were measurements of rare, naturally occurring stable water isotopes that gave a baseline for testing current climate change. Ann said the information provides the world's first independent test of the accuracy of Global Climate Models, used to predict climate change.

By testing the ratio of rare water isotopes in rain and snow from the Andes mountains, scientists can read signs of deforestation in the Amazon jungle.

She and Dr Kendal McGuffie, a physicist at the University of Technology Sydney, compared concentrations of two heavy water isotopes deposited on the Andes in the 1970s to 1980s with levels from the 1990s to 2000s.

In a study published in the November 2004 issue of the Journal of Geophysical Research, they reported a marked change in the ratio of these isotopes found in rain and snow on the Andes during the past 25 years. They believe the reason fewer heavy isotopes were found in recent rainy seasons is due to deforestation in the Amazon.

How can they tell? Ann explains that stable water isotopes are found in all natural water. They differ from normal water molecules only in that they are rare and contain extra neutrons, which make them heavier.

Because of their extra weight, these isotopes are less likely to be evaporated into the air stream. And, when water condenses in the form of rain, the hefty molecules drop out first.

When water is evaporated from a lake or river as well as from raindrops caught on the surface of leaves, some of the heavy isotopes stay behind. In its journey from the Amazon mouth to the Andes, water re-evaporates up to five times, so the rare isotopes are gradually depleted from the atmosphere.

But when trees transpire, pumping water vapour out through the openings in their leaves, the heavy isotopes are discharged in the same ratio as they occur in normal water.

So the 'signature' of isotopes in water that has been evaporated is different to the signature of water recycled through trees.

By looking at the different ratios of these isotopes in rainwater the two scientists can detect when changes are due to global warming or deforestation. This information is now being applied to the Murray-Darling basin in Australia where they are looking at human-induced and natural climate change. The United States Geological Survey is doing similar studies of climate and land-use change in America.

Ann, who works at the interface of climatology and nuclear science, explains that measurement of the water isotopes began in the 1960s. Then scientists wanted to monitor increased fallout of the radioactive isotope tritium, caused by above-ground atomic weapons tests. So two United Nations agencies, the International Atomic Energy Agency and the World Meteorological Organisation, set up the Global Network of Isotopes in Precipitation.

Regular rain samples were taken from stations in the Amazon, Andes and elsewhere across the globe. Scientists tested these for tritium. But they also recorded stable water isotopes, one of which occurs once in 6 500 water molecules and the other once in 500 water molecules. Ann said this was "serendipitous". In terms of understanding big issues like global warming and land use change, stable water isotopes are "tons more interesting than tritium.

"The bottom line is that for the first time we can tell the difference between moisture that that has been transpired through the plants, and water that has come through the rest of the water cycle... "

This is "amazingly important" because it can help indicate the extent of deforestation.

"Equally important, the rate of transpiration relates to the carbon budget and greenhouse warming," notes Ann. As trees and other plants absorb fossil fuel based carbon dioxide from the air they are forced to open the pores in their leaves and release water vapour. "How much the carbon is absorbed is exactly related to how much the plants transpire." So by measuring transpiration, scientists can "get a handle on carbon absorption and hence global warming."

Bits & pieces

What do triplets and heavy water have in common? Professor Ann Henderson-Sellers says it has nothing to do with the baby and the bathwater. Instead, she draws a comparison between different molecules of water and multiple births. She says most people know that a water molecule consists of two hydrogen and one oxygen atom. But very few are aware that hydrogen comes in two stable forms 1H or normal hydrogen and 2H or deuterium (D) while oxygen comes in "three flavours" namely normal 16O, less common 18O and extremely rare 17O. She says these atoms can combine in different ways. If you sample water from the ocean, a spring or an ice cube in your fridge most of the water molecules in it will be regular H2O (1H1H16O) but HDO (1H2H16O) occurs once in 500 molecules and 1H1H 18O once in about 6500 molecules. "I try to explain this concept by saying HDO is about five times less common than a twin birth. 1H2 18O is about as frequent as pre-IVF triplets." She says if, like her, you were raised in the pre-test tube baby era you would hardly know anyone who was a triplet. She says heavy water isotopes are not the result of nuclear fallout or testing but occur quite naturally just like twins or triplets.