Dirty or sooty snow decreases the snow’s brightness, warms and shrinks the snow causing it to melt weeks earlier February in spring and dropping the runoff in late spring and summer.
When soot from pollution settles on pristine snow, it can increase snowmelt in the winter month of February (See the chart included to the right). How pollution affects a mountain range’s natural water reservoirs is important for water resource managers in the western United States and Canada who plan for hydroelectricity generation, fisheries and farming.Scientists at the Department of Energy’s Pacific Northwest National Laboratory conducted the first study of soot on snow in the western states. Now they can predict impacts along mountain ranges. Soot warms up the snow and the air above it by up to 1.2 degrees Fahrenheit, causing snow to melt.
“It is important to know how much impact pollution, the dirty snow, is having on our water supply. So we can judge how much water is coming down the rivers so that a good usuage plan can be implemented. Atmospheric scientist Yun Qian said. Snow melt can be up to “75 percent of the water supply, in some regions. Changes in amount and timing of the melts can affect the water supply, as well as aggravate winter flooding and summer droughts.”
Is this sooty snow part of a natural cycle?
The soot-snow cycle begins with the burning fossil fuels and soot is a by-product that is released in the air. It falls to the earth when it rains or snows, sleets or hails and blankets the snow it lands upon with a dark gray black cover. This dark blanket absorbs more of the sun’s energy than clean white snow. The resulting thinner snowpack reflects less sunlight back into the atmosphere and further warms the area, continuing the snowmelt cycle.
Green house gases also seem to be causing differences in regional snowpack caused by higher air temperatures. Qian and his colleagues at PNNL used a climate computer model to zoom in on the Rocky Mountain, Cascade, and other western United States mountain ranges. Qian’s model shows how soot from diesel engines, power plants and other sources affected snowpacks it landed on. (1)
Researchers know that soot settles on snow. And like an asphalt street compared to a concrete sidewalk, dirty snow retains more heat from the sun than bright white snow. Qian and colleagues wanted to determine to what degree dark snow contributes to the declining snowpack.
To get the kind of detail from their computer model that they needed, the PNNL team used a regional model called the Weather Research and Forecasting model – or WRF, developed in part at the National Center for Atmospheric Research in Boulder, Colo.
Compared to planet-scale models that can distinguish land features 200 kilometers apart, this computer model zooms in on the landscape, increasing resolution to 15 kilometers. At 15 kilometers, features such as mountain ranges and soot deposition are better defined.
Recently, PNNL researchers added a software component to WRF that models the chemistry of tiny atmospheric particles called aerosols and their interaction with clouds and sunlight.
Using the WRF-chem model, the team first examined how much soot in the form of so-called black carbon would land on snow in the Sierra Nevada, Cascade and Rocky Mountains.
Then the team simulated how that soot would affect the snow’s brightness throughout the year. Finally, they translated the brightness into snow accumulation and melting over time.
“Earlier studies didn’t talk about snowpack changes due to soot for two reasons,” said atmospheric scientist and co-author William Gustafson.
“Soot hasn’t been widely measured in snowpack, and it’s hard to accurately simulate snowpack in global models. The Cascades have lost 60 percent of their snowpack since the 1950s, most of that due to rising temperatures. We wanted to see if we could quantify the impact of soot.”
Their simulations compared well to data collected on snowpack distribution and water runoff. But their first experiment did not include all sources of soot, so they modeled what would happen if enough soot landed on snow to double the loss of brightness.
In this computer simulation, the regional climate and snowpack changed significantly, and not in a simply predictable way.
Overall, doubling the dimming of the snow did not lead to twice as high temperature changes – it led to an approximate 50 percent increase in the snow surface temperature. The drop in snow accumulation, however, more than doubled in some areas.
Snowpack over the central Rockies and southern Alberta, for example, dropped two to 50 millimeters over the mountains during late spring and early winter. The most drastic changes occurred in March, the model showed.
The team also found that soot decreased snow’s brightness in two ways. About half of soot’s effect came from its dark color. The other half came indirectly from reducing the size of the snowpack, exposing the underlying darker soil of the earth.
Studies like this one start to unmask pollution’s role in the changing climate. While greenhouse gases work unseen, soot bares its dark nature, with a cloak that slowly steals summertime’s snow.
Dirty snow also impacts global warming
“Dirty snow has had a significant impact on climate warming since the Industrial Revolution. In the past 200 years, the Earth has warmed about .8 degree Celsius. Zender, graduate student Mark Flanner, and their colleagues calculated that dirty snow caused the Earth’s temperature to rise .1 to .15 degree, or up to 19 percent of the total warming.
In the past two centuries, the Arctic has warmed about 1.6 degrees. Dirty snow caused .5 to 1.5 degrees of warming, or up to 94 percent of the observed change, the scientists determined.
The amount of warming by dirty snow varied from year to year, with higher temperatures in years with many forest fires. Greenhouse gases, which trap outgoing energy, are primarily responsible for the remaining temperature increase and are considered the Earth’s most important overall climate changing mechanism. Other human influences on Arctic climate change are particles in the atmosphere, including soot; clouds; and land use.
Humans create the majority of airborne soot through industry and fuel combustion, while forest and open-field fires account for the rest.” Time for cities, states and the federal government to step up to the plate along with each individual and clean up our fossil and biofuel act.
Excerpts courtesy of ScienceDaily
1. Dirty Snow Causes Early Runoff In Cascades, – Rockies Staff ScienceDaily, January 13, 2009. DirtySnowCausesEarlyRunoff
2. Dirty sow may warm the atmosphere as much as green house gases -Staff Science Daily June 7, 2007. http://www.sciencedaily.com/releases/2007/
Graphs courtesy of Pacific Northwest National Laboratory as cited in Science Daily
Image snow courtesy of Ski Mountainer ShastinaDillerCanyonLookingUp
Chart courtesy of Science Daily