Coal Dust Darkens the Climate Picture

Dust particles from coal mines are about as endearing to scientists as they are to us.

Just as they muck up the air we breathe, coal dust — and other light-absorbing particles resulting from wildfires and from the burning of diesel fuel by vehicles — can muck up scientific models of Earth’s energy balance, especially in the Arctic region.

These particles can be deposited by wind or precipitation anywhere on Earth, but they have a measurable impact when they pepper pristine Arctic snow and ice. Whereas snow normally reflects some solar radiation back into space, coal dust particles absorb it.

Coal mining is likely to become less regulated under President Donald Trump and the GOP-dominated Congress. And as the warming climate makes Arctic regions more accessible to shipping, mining in those areas may increase. All of this raises the importance of obtaining deeper insights into the impacts of mining, including the effects of the resulting coal dust, in the hope that eventually the information might inform policy decisions.

One vexing problem is that scientists are uncertain about how much coal dust and other particles are in the mix locally and globally. Better estimates of the amount of light-absorbing particles held in Earth’s ice and snow, or cryosphere, would make climate models more accurate, but many polar regions are so remote that it would be almost impossible for scientists to make direct measurements. This is where spacecraft might come in handy. Satellites circling Earth could be outfitted to remotely detect the presence and reflectivity of snow laden with dark particles.

A new study published this month in the Journal of Geophysical Research: Atmospheres offers potential guidance for such a mission if it were to focus on a region with heavy coal pollution.

Alia L. Khan, a post-doctoral researcher at the University of Colorado Boulder, and her colleagues analyzed the reflectivity of snow samples collected in 2013 and in 2015 at several locations upwind, downwind and adjacent to an active mine on an island in Norway’s Svalbard archipelago. The researchers found that local coal dust reduced the snow’s reflectivity by up to 84 percent directly around the mine — contributing to warming. And the team derived a range of wavelengths of light likely to correlate with high concentrations of coal dust in Arctic snow.

At least one spacecraft mission that could help to quantify the extent of light-absorbing particles in snow is in the planning stages, says Khan’s co-author Thomas Painter, a cryosphere scientist and snow hydrologist at NASA’s Jet Propulsion Laboratory in California.

But efforts to use remote sensing to detect a type of dark particles called black carbon in snow in less polluted areas are “destined to fail,” wrote atmospheric scientist Stephen G. Warren of the University of Washington, Seattle, in a 2013 paper. He argued that satellites could not accurately collect such data for several reasons, for instance, patchy and thin snow can appear similar to coal dust or black carbon on snow.

The new coal dust paper reports that snow near and downwind of the Svalbard mine contains a concentration of black carbon which is a factor of 100 times greater than typical values across the Arctic, Warren says. Such high values can be found only very close to pollution sources. “So, Khan et al. do not challenge my conclusions, he says. “In my quick reading of their paper it seems that the authors agree with me.” In other words, accurate space-based measurements may be limited to specific, small areas.

Nonetheless, the new research details another consequence of Arctic mining. “The extreme contrast between snow and dust at this particular site gave us a baseline to develop algorithms that we can now use to take future measurements in areas that aren’t easily accessible,” Khan says. “We hope these ground-based spectral measurements could be used in the management of future energy development in the Arctic, especially for mines that may be unavailable for ground-based observations, but may be large enough to be visible by satellite.”

Such eyes in the sky may thus one day keep researchers from being snowblind.