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Amazonian tree rings reveal past rainfall

3 October 2012, by Tamera Jones

Scientists have used tree rings from just eight cedar trees in Bolivia to build up a detailed picture of rainfall patterns across the Amazon basin over the last century.

Bolivian rainforest

Bolivian rainforest.

The rings in the lowland tropical cedar trees provide a natural archive of data which is closely related to historic rainfall.

'Climate models vary widely in their predictions for the Amazon, and we still do not know whether the Amazon will become wetter or dryer in a warmer world,' says Professor Manuel Gloor from the University of Leeds, who co-authored the report, which is published in Proceedings of the National Academy of Sciences.

'But we've discovered a very powerful tool to look back into the past, which allows us to better understand the magnitude of natural variability of the system,' he adds.

Gloor and colleagues from the UK, the Netherlands, Germany and Brazil measured the ratios of two different forms of oxygen – oxygen-16 and the heavier oxygen-18 – trapped in the wood's annual rings. These different forms are known as isotopes. This approach let them see how much rain fell in the Amazon basin over the last 100 years: rain contains more of the heavier oxygen isotope.

They found that the variation in the ratios of the two types of oxygen accurately reflects changes in rainfall.

'The extreme El Niño year of 1925-26 which caused very low river levels, clearly stands out in the record.'

Roel Brienen, University of Leeds

Tree rings in tropical trees are much less pronounced than those in trees from temperate regions like Europe, because the seasons aren't nearly as different as they are in places further from the equator.

'We already knew that some tropical tree species form annual rings and we also anticipated that the isotopic signature in these rings might record changes in the climate,' says Dr Roel Brienen from the University of Leeds, lead author of the study.

'But what surprised us, however, is that just eight trees from one single site actually tell us how much it rained not just at that small site but over the entire Amazon catchment. That is an area about 25 times the size of the UK.'

Indeed the isotopic ratios contained in the tree rings record rainfall levels so accurately that even El Niño events are easy to pick out. El Niño events are characterized by unusually warm temperatures in the equatorial Pacific Ocean, which have a knock-on effect on wind and rain.

Roel Brienen taking a core sample a Bolivian cedar tree

Roel Brienen taking a core sample a Bolivian cedar tree.

'The extreme El Niño year of 1925-26 which caused very low river levels, clearly stands out in the record,' says Brienen.

Although the century-long history provided by the trees is fairly short, some trends are clearly evident.

'The oxygen isotope series show an increase over time, which may be due to an intensification of the hydrological cycle,' says Gloor. 'That could explain the observed long-term trend in river discharge. We need however to replicate this research at different places in the Amazon to really be able to say more.'

Because of its vast size and location along the equator, how the region's water cycle responds to climate change could significantly affect the magnitude and speed of climate change for the entire globe, say the researchers.

They explain that in the same way that layers in ice cores have been used to study past temperatures, they can now use tree rings as a natural archive of rainfall over the Amazon basin.

'If we find older trees with similar signal strength then this will greatly help us to advance our knowledge of the system,' explains Gloor.