18 December 2005Ian Hoffman
For the clearest signs of global warming in the United States, scientists are watching Alaska, where permafrost is melting and shrubs are blossoming in the path left by retreating glaciers.
But in the Lower 48, the place to witness warming of the planet is in the West, most strongly in the Sierra, the Cascades and north-central mountains in Idaho. The West's mountains are sensitive to warming due to their relatively lower elevation, wetter snows and temperate climes, suggesting that the changes could have potentially large ramifications for water supplies, flooding and the survival of ecosystems.
Poring over a century's worth of thermometer and river flow records, scientists have found warmer temperatures and early snowmelt in the West that they say is clearly independent of natural climatic changes such as the El Nino-La Nina cycles and longer, decadelong swings in eastern Pacific sea temperatures.
"There's a general pattern of warming that amounts in winters and springs to something between 1 and 2 degrees Celsius and 2 to 4 degrees Fahrenheit over the last 50 years," said Mike Dettinger, a climate and hydrology researcher for the U.S. Geological Survey and Scripps Institute of Oceanography Institute in La Jolla.
"The West is especially sensitive to this. We're going to start feeling the effects of this warming long before the rest of the country," he said.
Independently, scientists in Nevada and Colorado are finding the same warming. Temperatures on mountains hundreds of miles apart have been inching upward at least since the early 1970s.
"I have a hard time seeing it except as these large-scale things" such as climate change, said Kelly Redmond, chief climatologist for the Western Regional Climate Center in Reno.
In Tahoe City, for example, scientists have high-quality thermometer readings going back to 1931, one of the longest-running temperature records in the nation. Mean temperatures there have been climbing steadily since the 1950s, capped recently by a string of "highly unusual," hot summers and warm winters, Redmond said.
" It has not gone below zero for 12 years," he reported last week in San Francisco, at the fall meeting of the American Geophysical Union, the world's largest society for Earth and planetary sciences.
Snow-fed Western rivers provide more evidence. California state hydrologists noticed in the mid-1990s that spring snowmelt was coming earlier and traced the trend back to the 1950s, matching the temperature rise. As a result, the traditional April-to-July runoff that nourishes California crops and cities is becoming more meager as mountain snow melts earlier and snow becomes rain.
"It seems like it's quite a bit less than it used to be in the mid-20th century," said Maurice Roos, chief hydrologist for the California Department of Water Resources. " That is consistent with global warming."
Now winter is turning to spring earlier for mountains across the West.
Dettinger and colleagues examined mountain weather stations and flow records for more than 300 Western rivers. On April 1, the rough transition point from winter storm to snowmelt runoff, they found less snow on average. More than three-fourths of the rivers are flush with runoff earlier than in previous decades.
"We can see it in the streams," Dettinger said. "It's an average of nine to 11 days earlier."
The 1-2 degree Celsius rise in temperatures agrees with what scientists project when they add human emissions of greenhouse gases such as carbon dioxide and methane to the chemistry of the Earth's pre-industrial atmosphere in computer simulations and run them forward to the present day.
"Now the (global warming) signal is detected," said Ed Maurer, a hydrologist and civil engineering professor at Santa Clara University. "And it's attributed to the warming, it's not related to El Nino."
Computer simulations of natural changes in climate don't produce the temperatures measured in the West today.
"You can't even come close to reproducing the warming trends for the last 35 years now, unless you include those human influences," Dettinger said.
Looking into the future, the 11 most used simulations vary on what temperatures will be like in 2100, with projected increases ranging from 1.4 to 5.8 degrees Celsius or 2.5 to 10.4 degrees Fahrenheit. Much depends on whether the rate of greenhouse-gas emissions continues to accelerate or is reduced by a global shift to carbon-free energy sources, and on whether oceans and plants can continue to absorb most of the carbon dioxide.
Warming of 1 degree Celsius translates into a 500-foot retreat of Sierra snows up the mountainsides, said Roos of the California Department of Water Resources. At three degrees of warming - roughly in the middle of the climate-change projections for 2100 - Northern California would lose half of the snowpack on which it relies for most of its irrigation and drinking water.
Until California finds more places to store water, water managers could be forced by the earlier runoff to make tough decisions.
