By Timothy Gardner
WASHINGTON (Reuters) - Snowpack declines in the Rocky Mountains over the last 30 years are more significant than during any other period in past centuries and foreshadow a strain on summer water supplies for more than 70 million people across the Western United States, a U.S. government study said.
Despite this year's record snowpacks in the Rockies and resulting floods, declines over the three decades have shown a an unusual pattern compared to reconstructions of snowpacks going back 1,000 years, according to the Geological Survey study released on Thursday, in the journal Science.
Runoff from winter snowpack, layers of snow that build up in mountains, accounts for 60 to 80 percent of the West's annual water supply, the study said.
The declines can be attributed to unusual springtime warming caused by man-made climate change and come during the summer, which is the period of highest demand for water in the West, it said.
"The more we increase temperature, the earlier we melt out our snow pack and the less of it there is to get though our warm dry summers," said USGS scientist Gregory Pederson, the lead author of the study.
Pederson declined to predict how the declines would affect humans in the West, saying that people have long been creative in finding new ways to divert and store water when it is needed. But he said agriculture and industries that depend on annual allocations of water could be suffer. In addition, coldwater fisheries could be damaged as warmer water flows downstream.
Pederson and other scientists at the USGS and at the Universities of Arizona, Wyoming and Western Ontario reconstructed past snowpacks using tree ring data going back 500 to more than 1,000 years.
Lower elevation trees such as ponderosa pines grow larger tree rings during big snowpack years as annual melts provide water downstream, while high elevation trees, including mountain hemlocks, grow small rings during big snowpacks because they impedes growth.
The data showed that with few exceptions northern Rocky Mountain snowpacks were large when southern Rocky snowpacks were small and vice versa.
But since the 1980s there were simultaneous declines along the entire length of the Rocky Mountains, the study showed.
"This scientific work is critical to understanding how climate change is affecting western water supplies," Secretary of the Interior Ken Salazar said in a statement.
He said it helps land managers adapt to changing conditions, assists water managers with planning, and gives the public an understanding of the impacts that human-produced carbon emissions are having on U.S. resources.
The study can be seen at: http://link.reuters.com/hyr99r
(Reporting by Timothy Gardner; Editing by Eric Walsh)