RESEARCH HIGHLIGHT

Wildfire Soot May Contribute to Melting Sea Ice and Glaciers in the Arctic

July 22, 2005

During the summer of 2004, interior Alaska experienced its largest wildfire season on record, when approximately 6.7 million acres (27,114 km²) burned.   The average temperature in Fairbanks was 64.5°F/18.1°C, and according to the Alaska Climate Center, was 2 degrees warmer than the previous record from the summer of 1975.     Records for the past 50 years show an increase in both the number and size of Alaska boreal fires, and during the late 20th century, Arctic sea ice and glaciers in Alaska and western Canada have been melting at an accelerated rate.

Yongwon Kim and Reginald Muskett at the International Arctic Research Center with their colleagues in Japan are studying the effects of wildfire Black Carbon (BC) soot and carbon based aerosols on Arctic sea ice and Alaska glaciers. In the 1990's, the average area that burned in Alaska was approximately 4 x 10² km², or about 100 thousand acres. This was dramatically less than the 2004 season, when the total burned area was 24 x 10³ km², or less than 6 million acres.

Climate models, such as those developed at the NASA Goddard Institute for Space Studies, indicate that soot has twice as much effect as CO₂ on global surface air temperature and may also contribute to the thinning of sea ice and glacial melting.  BC soot particles, or carbon compounds from incomplete combustion, may be a factor in an earlier onset of spring in the Northern Hemisphere. A study on the effects of very small amounts of soot on fresh snow showed a reduced albedo, the reflective quality of snow, of about 30% and a 50% increase in melting compared to clean fresh snow.     According to IARC researcher Reginald Muskett , these particles are about the size of the wavelengths of visible light, and are not detectable by the human eye alone.   He says that the problem is two-fold relating to a reduction of the albedo of snow (that fraction of light with is reflected compared to that which is absorbed) and to the heat capacity of soot particles.   The BC soot particles attach to snowflakes in the atmosphere and can deposit freely on ice or firn, well-bonded snow older than one year, by deposition.   As the soot particles absorb visible light, most of which is not absorbed by snow/firn or ice, they convert the absorbed light into heat (infrared radiation).  

When snow/firn and ice absorb heat, they begin to melt.   The melt water, however, does not flush the soot completely from the snow/firn or ice as melting proceeds because they can easily re-attach to other snow/firn or ice crystals.   BC soot and aerosol particles of 0.4 μm to 10 μm ( 1 μm = 10^-6 mm), which are much smaller than the diameter of a human hair in radius (Fig. 1) can move throughout the atmosphere of the Northern Hemisphere, including all over the state of Alaska, in as little as 8 to 11 days, according the model developed by Kim and colleagues.  

The modeled BC soot deposition on sea ice and land snow/firn around the Pan Arctic compares well with BC soot measurements.   This suggests that a component of the Pan Arctic BC soot is from boreal wildfires in Alaska as well as Canada and east Siberia.   As these particles have the capacity to reduce snow/firn and ice albedo and increase melting, the researchers hypothesize that boreal wildfire soot in the Pan Arctic can be an important natural contributor to the reduction of sea ice and accelerated glacial melting.

They plan to use satellite (MOPITT, or Measurement of Pollution in the Troposphere) and ground-based observations (AERONET, or Aerosol Robotic Network) of BC soot and aerosols with their model and the 3-D global GOES-CHEM model at Harvard University (Goddard Earth Observing System-Atmospheric Chemistry Modeling Group).   They will evaluate the hypothesis and study regional variability, and access trends of boreal wildfire occurrence and severity with changes in the Pan Arctic climate.  

Fig. 1: An electron microscope image showing a particle of Black Carbon soot. (NASA Goddard Institute for Space Studies, D. M. Smith, University of Denver).

Fig. 2:   Wind fields from the European Center for Medium-Range Weather Forecast, a 700 hPa level, 00 UTC on day 1 (July 8) through day 12 (July 19) used in the FROSTFIRE modeling of BC soot and aerosols.   The arrows are the wind field vectors.   The scale arrow at bottom is 20 meters per second.   The small black square is the location of the FROSTFIRE experiment burn site north of Fairbanks, Alaska.   The two long curved arrows on Day 5 and Day 7 illustrate the circulation of the winds due to the passage of cyclonic and anti-cyclonic weather systems.

Fig. 3: Model output of the dispersion of gas and BC-aerosols (soot) on Day 11 (July 19, 2004).   The gas (a) is dispersed greatly over the Northern Hemisphere, but there is little deposition (b).   The BC-soot is also greatly dispersed (c) and more importantly the soot is deposited on surfaces, sea ice in the Arctic and glaciers in Alaska (d) as the model suggests.

Fig. 4: Trajectory distributions, % of particles from release, 9 July through 19 July 1999.   The panels (a, b) and (c, d) are divided geographically for those floating/deposited from 73 degree North to the equator and those floating/deposited from 73 degrees North to the pole, respectively.   The blue vertical bar, the difference in floating particles from air to 1 mm in size is mainly a function of precipitation, whereas the orange vertical bar difference is mainly a function of gravity settling (dry deposition).