Dust particles, not rising air temperatures, are largely responsible for increased snowmelt in the Colorado Rockies, according to a new study from the Jet Propulsion Laboratory in Pasadena.
"It doesn't matter at all how warm or cold it is, it only matters how dirty or clean the snowpack is," said Thomas Painter, a research scientist at JPL, in reference to rising river levels during the winter and spring in the Colorado Basin.
Under normal conditions between 90 and 95 percent of the energy that's absorbed by freshly fallen snow is from the sun, while only 5 to 10 percent is absorbed from the ambient air. Most of the solar energy is reflected back into the atmosphere by the highly reflective snow.
But when dust blows in from the deserts of the Southwest and blankets the powder with brown flecks, the snow loses its reflectivity and begins to absorb more solar energy. As a result, it melts faster than normal.
"Dust acceleration of a snowmelt, that's what drives the train. And temperature's kind of like a feather pushing lightly on the train," said Painter, who co-authored the paper. "It contributes a little bit, but nothing like what the acceleration by dust creates."
There was dust blowing through the Rockies long before humans came along, but not nearly as much.
Accumulations began cropping up in the mid-1800s as settlement, farming and grazing of the interior west began in earnest. Along with people, cattle and sheep were brought out to graze in large numbers. As they disturbed the pristine soil's top crust, dust was kicked up and carried north by storms. By 1890, in conjunction with a peak in livestock numbers, the levels of dust peaked as well, according to core samples from alpine lakes in the area. Following a massive die off of livestock around that time, the amount of dust stabilized.
From the time humans permanently settled the area, there has been five to seven times more dust blown through the Rockies than before, according to Painter.
Along with increased snowmelt and rising river levels earlier in the season, Painter and his colleagues found that there's also a 3 to 8 percent drop in water available downstream — a concern given that Southern California receives about one third of its water from the Colorado River.
As a greater amount of water flows earlier, more evapotranspiration occurs, as plants that would've been dormant under fresh snow, begin to absorb water earlier in the year. The water leaves the plants and wafts into the atmosphere, instead of flowing downstream and into reservoirs.
"With climate warming and with land use change in the Western U.S., we certainly don’t expect dust to decrease in the future," said McKenzie Skiles, assistant professor at the University of Utah and co-author of the study.
The study was published this week in the journal Geophysical Research Letters.
Clarification: Following the piece's publication, Painter reached out to clarify a quote characterizing the influence of dust on snowmelt, saying that he meant to speak to rising river levels in the Colorado Basin instead. Per his request we’ve amended a description of his quote to clarify what he meant to say.