Arctic snow not darkening due to soot, dust, study finds

October 31, 2015
Dartmouth Adjunct Assistant Professor Chris Polashenski and his colleagues found that degrading satellite sensors, not soot or dust, are responsible for the apparent decline in reflectivity of Arctic snow. Credit: Chris Polashenski

For millennia, Greenland's ice sheet reflected sunlight back into space, but satellite measurements in recent years suggest the bright surface is darkening, causing solar heat to be absorbed and surface melting to accelerate. Some studies suggest this "dirty ice" or "dark snow" is caused by fallout from fossil fuel pollution and forest fires.

But a new Dartmouth-led study shows that degrading satellite sensors, not soot or dust, are responsible for the apparent decline in of inland across northern Greenland. The study's results suggest the ice sheet hasn't lost as much reflectivity as previously thought, and that black carbon and dust concentrations haven't increased significantly and are thus not responsible for darkening on the upper ice sheet.

The findings, which contradict anecdotal observations and earlier scientific studies, appear in the journal Geophysical Research Letters. A PDF of the study is available on request.

Observations suggest the Greenland Ice Sheet's albedo - or its ability to reflect the sun's energy back into the atmosphere—has declined considerably since 2001 due to black carbon and dust from increased industrialization and across the northern hemisphere. The apparent decline is greatest around the ice sheet's edges, but it also is occurring in the high elevation interior known as the dry snow zone, where the reflectivity is effectively reset each winter by new snowfall.

Two properties dominate reflectivity in dry snow - the size of snow grains, which become larger and more absorbent as they melt, and the presence of dark impurities that absorb the sun's energy, predominantly black carbon and mineral dust, which also cause the snow to melt faster. Snow high on the Greenland Ice Sheet typically has black carbon concentrations too low to significantly affect its reflectivity, but in 2012 large wildfires in Canada and Siberia and favorable winds may have combined to trigger record surface melting of the ice sheet that year.

In trying to explain the apparent decline in reflectivity, lead author Chris Polashenski, an adjunct assistant professor at Dartmouth's Thayer School of Engineering and a research geophysicist at the U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory, and his colleagues analyzed dozens of snow-pit samples from the 2012-2014 snowfalls across northern Greenland and compared them with samples from earlier years. The results showed no significant change in the quantity of deposited for the past 60 years or the quantity and mineralogical makeup of dust compared to the last 12,000 years, meaning that deposition of these light absorbing impurities is not a primary cause of reflectivity reduction or surface melting in the dry snow zone. Algae growth, which darkens ice, also was ruled out as a factor.

Instead, the findings suggest the apparent decline in the dry snow zone's reflectivity is being caused by uncorrected degradation of sensors in NASA's aging MODIS satellites and that the declining trend will likely disappear when new measurements are reprocessed. MODIS (or Moderate Resolution Imaging Spectroradiometer) is the key instrument aboard NASA's Terra and Aqua satellites, which provide images of the Earth's surface and cloud cover every two days. MODIS tracks features of the land, oceans and atmosphere that can help develop models that predict global changes. The Terra mission, launched in December 1999, and the Aqua mission, launched in 2002, are designed to collect data for 15 years to differentiate short- and long-term trends and regional and global phenomena.

The study's findings don't apply to the 's lower elevations, where surface melting, soot and dust result in more pronounced declines in reflectivity and where warmer temperatures may promote algae growth that further erodes reflectivity.

Explore further: Fire and ice: Wildfires darkening Greenland snowpack, increasing melting

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14 comments

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SamB
2.2 / 5 (10) Oct 31, 2015
Yes, in the end I am sure many many 'studies; will show improper results due to the incompetence of senior researchers who seem to like to tell us 'the science is settled!'.
In fact, after telling use the Antarctic Ice Sheet was 'dramatically' causing sea levels to rise, it was just discovered that instead the levels of ice and snow have been increasing for decades. It seems our science has degraded into a mess of overpaid haughty morons who really do not understand how to do solid research.
Bongstar420
4 / 5 (4) Oct 31, 2015
Instead, the findings suggest the apparent decline in the dry snow zone's reflectivity is being caused by uncorrected degradation of sensors in NASA's aging MODIS satellites and that the declining trend will likely disappear when new measurements are reprocessed.

They mean to say that the measurements were inaccurate, not that the decline is caused by bad sensors.
ECat
Oct 31, 2015
This comment has been removed by a moderator.
antigoracle
2.3 / 5 (9) Oct 31, 2015
Come on, give NASA a break. They are too busy, cooking the temperature data, to fix any that defy their AGW agenda.
sage101
3 / 5 (4) Oct 31, 2015
antigoracle,
are your comments automatically adjusted to 1/5 stars? Cause that's all I've ever seen.
cjones1
2.3 / 5 (6) Oct 31, 2015
Cool!
Another disappointment for the United Nations and the IPCC. Let us hope the cooler heads prevail over the AGW hotheads.
Jayded
3.4 / 5 (5) Nov 01, 2015
@SamB - so funny, after reading the article I just knew that some dumb ass would try and throw out all the evidence relating to climate change. so predictable. must be scary to know that your level of stupidity is not only expected but becoming so repetitive that it borderlines near certainty. why dont you rather go post comments on disney channel or dora explorer, something more in your grasp range.
Eddy Courant
1 / 5 (7) Nov 01, 2015
But there was a consensus!
thermodynamics
4.2 / 5 (5) Nov 01, 2015
Eddy mumbled:
But there was a consensus!


And there still is a consensus as to the reality of AGW. There has never been a claim that we can accurately predict the motion of aerosols or energy throughout the atmosphere and oceans. That does not mean there is any technical argument against the increase in GHWs, by humans, increasing the enthalpy of the atmosphere and oceans.

What this means is that you do not understand what AGW is and how it interacts with the fluids on the earth.
richard_f_cronin
5 / 5 (4) Nov 02, 2015
re: thermodynamics --- "What this means is that you do not understand what AGW is and how it interacts with the fluids on the earth.

Whoa !! Are we getting into that "earthly fluids" thing?? Hey, this is a family page.
thermodynamics
4 / 5 (4) Nov 02, 2015
re: thermodynamics --- "What this means is that you do not understand what AGW is and how it interacts with the fluids on the earth.

Whoa !! Are we getting into that "earthly fluids" thing?? Hey, this is a family page.


Now that is a funny tilt to the conversation. I wish I could give you a 10 for it.
howhot2
5 / 5 (3) Nov 02, 2015
Cool!
Another disappointment for the United Nations and the IPCC. Let us hope the cooler heads prevail over the AGW hotheads.

Hotheads? Did someone say AGW hotheads? You deniers are the biggest bunch of right wingnut losers mankind has ever stomped into the ground with facts, yet you turds keep popping back. Your like zombie turds that won't float away man! Run you little chicken hawk, run you science denier. The USA doesn't need you kind!

Don't worry zombie turd, the IPPC has negotiated with UN and the USA's FEMA to establish environmental re-education camps to relocate the resisters to. The Obama boots are marching for you man. Run run to Mexico is your only choice to escape FEMA and the IPCC!

katesisco
1 / 5 (1) Nov 02, 2015
Hmmm. Years ago weather men--those people tracking daily weather changes--noticed less sunlight. Solar dimming was discussed among themselves as scientists KNEW that sunlight could not be lessening. So has solar dimming morphed into mistaken recordings?
Shootist
1 / 5 (2) Nov 10, 2015
Fracking, Not Renewables, Is Responsible For Emissions Reduction. Save the birds. Frack!

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