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A weather story, now. There was unusually heavy snowfall throughout the Rockies this past winter, and that has caused a lot of flooding and headaches downstream. The problem this year is an exception to a trend over the past century. For many years now, there's been less snowpack and the concern has been drought. NPR's Richard Harris reports.

RICHARD HARRIS: Snowpack out in the Rockies isn't simply of interest to skiers and snowmobilers. Greg Pederson from the U.S. Geological Survey in Bozeman, Montana says, as this annual snowpack melts, it provides water for crops and water for drinking.

Mr. GREG PEDERSON (Research Ecologist, USGS): Over 70 million people are dependent on this water. And this water feeds the Columbia River, the Missouri, and the upper Colorado, as well as the Rio Grande.

HARRIS: And water managers have noticed in recent decades that the snow pack is, on average, getting thinner in the Rockies. Pederson wondered whether this is simply natural variation, or if a changing climate means this is a trend that will continue.

Mr. PEDERSON: Fifty to a hundred years of record is, in many cases, compared to the rest of the Earth's history, pretty short time interval. So we were curious how, over the past 500 to a thousand years, snowpack may have changed.

HARRIS: Obviously people weren't out there taking regular measurements of snowpack in the Rockies hundreds of years ago. So Pederson and his colleagues turned to a natural record of snow depth.

Mr. PEDERSON: Basically we can use a whole network of tree rings in the Western U.S.

HARRIS: They used tree rings in two ways. Looking first at trees throughout the west that have fat rings during wet years, signaling high snowpack. And skinny rings during dry years with thin snow. They also looked at trees that are high up in the mountains and are actually stunted by extra snowpack, because don't start growing each spring until the snow that covers them melts away.

Looking back through many centuries of tree rings, Pederson and his colleagues are now able to put the recent decades of reduced snowpack into its historical context.

Mr. PEDERSON: The 20th century across the northern Rockies looks quite low on average, compared to the amount of snow that was there over the past millennium.

HARRIS: And part of the reason for that, he says, is the recent global warming trend. He can see the fingerprints of that in the data. Higher temperatures mean that precipitation over the mountains is increasingly falling as rain instead of snow. And even when it falls as snow, Pederson says it doesn't stick around as long.

Mr. PEDERSON: After we get the delivery of snow, we're oftentimes seeing warmer air masses coming in afterwards. So even if it's dropped as snow, everything is warmer, so it tends to melt faster once the snow is delivered.

HARRIS: And timing of that melt is critical out west. Wildlife, farmers and anyone else who's thirsty, counts on the snowpack to melt gradually through the spring and into the summer, in order to provide a steady supply of water.

Mr. PEDERSON: We're going to have to start looking at how we get through potentially warmer and dryer summers of the future, without that free bank account of snow pack.

HARRIS: In fact, the situation could lead to drought - not only in the Southwestern United States, where water managers have already started to brace for the worst - but farther north.

Philip Mote, a climate scientist at Oregon State University, says we tend to think of drought simply as a lack of rainfall.

Mr. PHILIP MOTE (Director, Climate Change Research Institute, Oregon State University): Here in the Western U.S., where we rely very heavily on snowmelt for summer water supply, anything that impacts the snowpack can also cause a drought. And what this paper shows is that the warming of the 20th century and beyond is already effecting and will profoundly affect the frequency of droughts in the west, simply by whittling away at the snowpack.

HARRIS: He finds the new report persuasive in its link to global warming.

Mr. MOTE: It's sort of ironic to be talking about this, this year when, you know, the Columbia River is at flood stage at Portland.

HARRIS: But underscores the point that you can't judge the climate by a single year, or even by a few decades. And that's why the latest research looked back hundreds of years to show that what's happening today really is a likely departure from natural variation.

Richard Harris, NPR News.

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