Recent decades likely wettest in four millennia in Tibet

The precipitation records have been reconstructed using sub-fossil, archaeological and living juniper tree samples from the north-eastern Tibetan Plateau. They reveal a trend towards wider growth rings, implying moister growing conditions -- with the last 50 years seeing increasing amounts of rainfall.

Notable historical dry periods occurred in the 4th Century BC and in the second half of the 15th Century AD.

Dr Tim Osborn from UEA's Climatic Research Unit said: "Our collaboration with scientists from China has been very fruitful, leading to what is currently the longest tree-ring-width record in the cold and arid north-eastern Tibetan Plateau. Not only is the record very long, it is based on samples from more than 1000 trees, some of which have an individual lifespan of more than 2000 years. These are among the longest-lived trees in the world."

Not only are these trees long-lived, but they are useful for understanding how climate has changed. The widths of the tree rings show a close correspondence with observations from rain gauges over the last 55 years, such that tree rings in wetter years tend to be wider than tree rings in drier years.

Dr Osborn said: "The most recent few decades have, on average, the widest rings in the 3,500-year record which suggests that this may have been the wettest period, perhaps associated with global warming during the last century. Indeed, over the last two thousand years when the Northern Hemisphere is warm it appears to be wetter in the Mountains of North East Tibet. This suggests that any further large-scale warming might be associated with even greater rainfall in this region -- though we note that other factors could also have contributed to the increased ring widths."