WSP12633 - Laserfiche WebLink

Home Browse Search

Virtuillly gone. Computer models suggest that even moderate warming win dras- tically reduce the spring (peak) snowpack in the Oregon and Washington Cascades. NEWS. Focus tions, which can only be explained by warming," Mote says. As for whether this warming is best explained by the decade- long climate swings, Mote defers to the lat- est work by the Intergovernmental Panel on Climate Change (IPCC), the global body of hundreds of scientists that has assembled the "standard model" of climate change. Al- though IPCC's latest report does show that both natural and human-induced factors ex- plain portions of the last century's global temperature record, climate models that take both into account do the best job at reproducing the complete temperature record. Dry times ahead? No matter what the historical picture, Mote, Cayan, and others argue that the picture for west- ern snowpacks looks far more bleak when the anticipated fu- ture warming is taken into ac- count. Here, too, several teams have been working to under- stand how events are likely to unfold. All agree there is consid- erable uncertainty. Precipitation. trends, for example, "are all over the map" in different climate 'models, because precipitation can vary drastically over a short distance, Mote says. However, Mote, Cayan, and others agree that climate models generally do a far better job of estimating tempera- ture, because temperature differences drive winds that tend to reduce those differences. Regional climate models suggest that over the next 100 years, western temperatures are likely to rise between 20 and 70C, de- pending on-among other factors--the rate of increase of greenhouse gases in the at- mosphere. And unlike the precipitation forecasts, the models all show an increase in temperature. Modelers then feed these temperature data and other variables into another set of computer programs called hydrology models that compute the effects of changing climate on snowpack and stream runoff. And these hydrology models consistently show that even low-end temperature changes produce big effects. As part of a study described in last month's issue of Climatic Change, for example, UW Seattle hydrologist Dennis Lettemnaier and colleagues used a global cli- mate model to compute how the western snowpack would respond to modest tempera- ture increases. They found that a temperature rise of I.5OC by 2050 resulted in a loss of nearly 60% of the 1 April snowpack in the Oregon and Washington Cascades, and a 30 rise by 2090 reduced those snowpacks by 72% (see figure). 'That's the best-case sce- nario," Mote says. "By the 2090swith a 1126 warm scenario, you would have essentially no snow left in Oregon by April 1 st" When the Pacific Northwest is taken as a whole, the picture is only a bit better, showing a 35% loss in 1 April snowpack by the 2050s and 47% loss by the 209Os. In a Geophysical Research Letters paper last year, Cayan and former postdoc Noah Knowles-now with USGS in Menlo Park, California--computed a similar analysis for the watersheds that make up the western drainage of California's Sierra Nevada 1950-99 2050$ 000791 ''This represents over 3 lan3 [3 billion cubic meters] of runoff shifting from post-April I to pre-April 1 flows;' the authors write. That . figure nearly doubles by 2090. Other studies show that parts of the Columbia River Basin are likely to fare worse, whereas the Colo- rado River watershed, with smaller anticipat- ed declines in snowpack and generally colder temperatures, is likely to emerge compara- tively unscathed. Overall, however, a steady temperature climb will likely affect tens of millions of people. "There are enormous im- pacts from this potential change," Cayan says. "Water management in the West has been to use the snowpack as a natural reservoir. This reservoir is really important. It's water that will come later when a lot of the water demand is heavi- est." Without that water "peo- ple will need to make some dif- ficult choices," adds Todd Reeve, who directs watershed restoration programs for the Bonneville Environmental Foundation in Portland. That's particularly true in the Pacific Northwest and Cal- ifornia. Reservoirs in the Co- lumbia River Basin capture on- ly about 30% of the region's annual runoff, whereas California's reservoirs hold slightly more. The typical pattern is to fill these reservoirs with late spring runoff and use that water throughout the summer and fall for irrigation and then in the early winter for power generation. An earlier snowmelt means that the water must be spread over a longer dry season when irrigation, recre- ation, and municipal demand peaks. "You're losing natural storage and taxing built stor- age. Something has to give," Lettemnaier says. (Here too, Lettenmaier says, the Colo- rado River Basin is unique, because reser- voirs there can store four times the region's annual precipitation.) With less summertime water, one of the hardest hit areas is likely to be agriculture. Today, farmers in California use about 75% of the state's water. Earlier this month, agri- cultural economists Wolfram Schlenker of the University of California, San Diego, and W. Michael Hanemann and Anthony Fisher of UC Berkeley presented a preliminary study at the American Economic Associa- tion meeting in San Diego of the likely im- pacts of climate change on California agri- culture. Using a range of hypothetical cli- mate and stream-flow scenarios in line with published modeling results, the researchers forecast that snowpack losses could lower farmland values by more than 15%. If that pattern holds for the state's 3.84 million hectares of irrigated farmland, the loss to the 2000s r 180 20 FEBRUARY 2004 VOL 303 SCIENCE www.sciencemag.org . 4060 80 100 120 140 Snow Water EqlivaJent (em) MOlmtains. They found that a predicted tem- perature rise of about 2.1 oC over the next century wOllld reduce the Sierra snowpack by one-third by 2060, primarily at mid to low el- evations, and would halve it by 2090. A sepa- rate analysis by L. Ruby Leung and col- leagues at the Pacific Northwest National Laboratory in Richland, Washington, together with researchers from the National Center for Atmospheric Research in Boulder, Col- orado, and Scripps reached similar conclu- sions when they looked at the effect of cli- mate throughout the West. The one notable difference: In the Rockies, the colder winter- time temperatures are expected to limit the losses to 30%. Without putting too much faith in the exact amount of losses, Mote says, "it's nearly inescapable that we're go- ing to continue losing snowpack." "Enormous impacts" "It doesn't mean we've lost water," Cayan hastens to point out. "It means the water is coming off earlier." Rather than sticking around as snow into the late spring and sum- mer, western snowpacks will wash down mountainsides in the winter and spring. Simply stated, the upshot is wetter winters and drier summers. In the Sierras, for example, Knowles and Cayan's models predict that the portion of water that flows through the watershed's rivers from April through July each year will decline. from 36% today to 26% by 2030.