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[ISLAMABAD] Oxygen locked away inside ancient trees shows that human activities have resulted in a dramatic increase in snowfall in northern Pakistan, say researchers.

A study published today (27 April) in Nature says the region received more snow in the past century than at any other point during the past thousand years. It points to the onset of industrialisation and rising greenhouse gas emissions as the cause of the trend.

A senior Pakistani geologist says, however, that the trend could be due to large-scale irrigation schemes that began in the area at around the same time as the West's Industrial Revolution.

The authors of the Nature paper, led by Kerstin Treydte of the Swiss Federal Research Institute, studied the relative amounts of two forms of oxygen trapped in the wood of trees.

The researchers took cores from tree trunks, and sampled the individual rings that form with each year's growth to assess the ratio of normal to 'heavy' oxygen.

They then used local snowfall records to find the relationship between precipitation and the proportion of heavy oxygen in tree rings for the years covered by the weather records.

This allowed them to estimate yearly snowfall stretching back more than 1,000 years — the age of the oldest trees in the study area.

In an accompanying article in Nature, Michael Evans of the University of Arizona at Tuscon, United States points out that the biggest increases in snowfall "occurred in the last 150 years, approximately coincident with the Industrial Revolution and greenhouse gas increases".

But according to Nayyar Alam Zaigham, former director of the Institute of Environmental Studies at the University of Karachi, Pakistan, there may be another explanation.

He told SciDev.Net that the increase in snowfall could be due to large-scale irrigation systems for farming that began in the area in the 1830s.

Zaigham points out that this activity also coincides with the Industrial Revolution, and has intensified in the past 50 years.

"For the past 50 or so years, we have built — and are still building — a number of dams in northern parts of the country to retain water for agriculture," he says. "As a result, we have more agriculture, more vegetation and more precipitation in these areas."

Link to full paper in Nature

Reference: Nature 440, 1179 (2006)

Link to presentation made by Zaigham at the 2005 'RiverSymposium'

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