Where Have Our Winters Gone?
Changes in winter hydrology, ecology and biogeochemistry are focus of sessions at American Geophysical Union (AGU) conference
If you're planning to skate on a frozen lake or river this winter, ski on a snowy slope, or, when spring arrives, depend on snowmelt to refill your water supply, you may need to think twice.
Winter as a "species" may have evolved to be less like the winters we remember. The change has consequences for summer, too, including plants' flowering times.
Scientists will present results on how winter is changing and why it matters at the American Geophysical Union (AGU) conference, taking place in San Francisco from Dec. 3 to 7, 2012.
When Winter Changes: Hydrological, Ecological, and Biogeochemical Responses (Session B21I) takes place on Tuesday, Dec. 4, 2012.
Session conveners include Heidi Steltzer of Fort Lewis College in Durango, Colo.; Michael Weintraub of the University of Toledo; Molly Brown of NASA; and Mark Williams of the University of Colorado at Boulder.
The National Science Foundation (NSF) funded much of the research results being presented at the session.
Subjects to be addressed include: hydrological and ecological implications of radiative forcing by dust in snow; phenological and ecological consequences of changes in winter snowpack in the Colorado Rocky Mountains; and when snow melts early: unusual alpine plant life histories during the summer of 2012.
Other presentations will look at insects, fires and climate change: implications for snow cover, water resources and ecosystem recovery in western North America; climate effects on groundwater storage, hydrochemistry and residence time in geologically variable, snowmelt-dominated mountain catchments in Colorado's Front Range; and the response of aboveground plant productivity to earlier snowmelt and summer warming in an arctic ecosystem.
"Wherever winter occurs, it is likely changing now or projected to change in the future," says Steltzer. "That will affect us all."
Among the session's highlights are talks on snowmelt-dependent water supplies, and mountain ecosystems out of sync.
Windborne dust on high peaks dampens Colorado River runoff
[Presentation B21I-01: Thomas Painter, JPL/UCLA]
"More than 80 percent of sunlight falling on fresh snow is reflected back to space," says atmospheric scientist Tom Painter of the Jet Propulsion Laboratory in Pasadena, Calif., and the University of California at Los Angeles. "But sprinkle some dark particles on the snow and that number drops dramatically."
The darker dust absorbs sunlight, reducing the amount of reflected light and in turn warming the now "dirty" snow surface.
The result? Dust-on-snow events. That's exactly what's happening in the Colorado River Basin.
When the winds are right and the desert is dry, dust blows eastward from the semi-arid regions of the U.S. Southwest. In a dust-up, Western style, small dark particles fall on the mountains' white snowfields, ultimately affecting the entire Colorado River watershed.
While dust has always blown into these mountains, the expansion of grazing and other disturbances in the western U.S. in the mid- to late-1800s led to a five- to seven-fold increase in dust loading. The snow cover became darker and lasted for shorter and shorter periods.
Lee's Ferry, Ariz., is the dividing line between the upper and lower Colorado River Basins.
There, according to Painter, peak spring runoff from the Colorado River occurs an average of three weeks earlier due to the more recent five-fold increase in such dust.
With this effect, total annual runoff at Lee's Ferry--and the Colorado River Basin as a whole--has been reduced by about five percent per year.
"Earlier melt-out allows for an extra three weeks of snow-free conditions," says Painter. "Increased transfer of water from snow to the atmosphere from the warmer snowpack, and transpired water from the uncovered vegetation during those three weeks of no-snow in the basin's mountains, causes the five-percent loss of water from the system."
"This research lays the foundation for future sound water-resource management of a river that serves 27 million people," says Anjuli Bamzai, program director in NSF's Division of Atmospheric and Geospace Sciences, which funded the research.
Runoff from the Colorado River Basin has decreased by more than 35 billion cubic feet due to airborne dust, according to Painter.
"Lake sediments in the mountains indicate that the increased dust load came after the vast increases in grazing and agriculture in the deserts of the southwest U.S. in the late 1800s."
The snow cover, Painter says, is therefore much darker in spring than it was at that time, and melts away several weeks earlier.
"Runoff comes from the mountains in a more compressed period, which makes water management more difficult than if the water came more slowly out of the mountains."
More focus on reducing dust could be effective, says Painter, "and in turn sustain the mountain reservoir system of snow cover, potentially increase runoff, and counter the regional effects of climate change."
If changes are made in the way desert soils are managed and dust emission is reduced, he says, it could ease tensions over water in the entire Colorado River Basin.
Glacier lilies and broad-tailed hummingbirds out of sync
[Presentation B21I-05: David Inouye, University of Maryland]
The glacier lily, a tall, willowy plant that graces mountain meadows throughout western North America, flowers early in spring, when the first bumblebees and hummingbirds appear.
The lily, a plant that grows best on subalpine slopes, is fast becoming a hothouse flower. In Earth's warming temperatures, its first blooms appear some 17 days earlier than they did in the 1970s, ecologist David Inouye of the University of Maryland has found.
The problem is that the glacier lily is no longer synchronized with the arrival of broad-tailed hummingbirds, which depend on the lilies' flowers for nectar.
By the time hummingbirds fly in, says Inouye, many of the flowers have withered away, taking their nectar-laden blooms with them.
"Long-term records of snowmelt and plant-flowering provide a stark interpretation of current events," says Saran Twombly, program director in NSF's Division of Environmental Biology, which funded the research.
"Climate change is disrupting the careful match between organisms and their environments," she says. "The ecological communities of today are dramatically different than those of the past."
Broad-tailed hummingbirds migrate north from Central America every spring to high-mountain breeding sites in the western United States. The birds have only a short mountain summer to raise their young. Male hummingbirds scout for territories before the first flowers bloom.
But the time between the first hummingbird and the first bloom has collapsed by 13 days over the past four decades. In some years, the lilies have already bloomed by the time the first hummingbird lands.
Biologists calculate that if current trends continue, in two decades the hummingbirds will miss the first flowers entirely.
Broad-tailed hummingbirds that breed farther south have fewer challenges.
"In Arizona, for example," says Inouye, "there's no obvious narrowing of the timing between the first arriving males and the first blooms of, in that case, the nectar-containing Indian paintbrush."
Higher latitudes may be more likely to become out of sync ecologically because global warming is happening fastest there.
As snows continue to melt earlier in spring, bringing earlier flowering, the mountains may come alive with glacier lilies long before hummingbirds can complete their journey north.
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