Black Snow

When we burn fossil fuels or biomass, we produce CO2, one of the most important greenhouse gasses currently affecting global warming. During the process, however, not all fuel is burned completely, and some of the particles are released into the atmosphere as soot, of which the main component is black carbon. It's the same black stuff that clogs up chimneys or what's left over when you (accidentally) burn your food.

The closer you get to urban regions or factories, the more soot you find that has been emitted by cars or factories; but even in the remote polar regions, significant amounts of soot have been found in what we like to think of as pristine, white snow (Hansen & Nazarenko 2004). It has been transported there through the atmosphere.

The black carbon particles absorb heat when they float through the atmosphere, warming up their surroundings. Moreover, when they reach the surface through precipitation, they do the same thing there. When they fall on snow or ice, the warming process is amplified because the soot particles reduce the albedo. A lower fraction of the incoming solar radiation is reflected, heating up the ice, and inducing melt. A representation of this process can be found in the figure below.

The difference between incoming solar radiation on 'white' snow and snow with black carbon. Source: UK Met Office.

Researches have found that soot plays an important part in Arctic warming via this process. A study done by Mark Jacobson in 2010 found that the reduction in global albedo as a result of soot on snow and ice was between 3.3 and 5.2% (Jacobson 2010), with the strongest effects in the colder regions of the Northern Hemisphere such as Canada and northern Europe. It has been shown that the warming effects of black carbon in the Arctic might have been between 0.5 and 1 ºC (Ramanathan & Carmichael 2008) - which might not seem as a lot, until you realise that it could mean the difference between frosting or thawing.

The effects of soot on global warming are second only to CO2, and larger than many other greenhouse gasses such as methane, CFCs and nitrous oxide. It is estimated that the emission of black carbon is as much as 8 Teragrams annually (Ramanathan & Carmichael 2008), the equivalent of 1785 fully loaded Airbus A380s, the biggest passenger airplanes in the world. Most of the emissions come from North America and Europe, but developing industrial countries in Asia are quickly catching up.

This may sound all doom and gloom, but there is hope. Research has also shown that when soot is stopped being emitted into the atmosphere, its effects disappear within years (Ramanathan & Carmichael 2008; Jacobson 2010). Technologies already exist to accomplish that: for example, filters on diesel engines can prevent soot from being emitted. So compared to CO2, which stays in the atmosphere for hundreds of years, soot removal has a direct positive effect.

Additionally, soot is a health hazard, so taking it out of the atmosphere actively contributes to human well-being. This could be another incentive for policy makers to tackle black carbon emissions, so the Arctic can become pristinely white again.

Further reading
The Climate Change We Can Beat. By David Victor, Charles Kennel and Veerabhadran Ramanathan.

References

• Hansen, J., and L. Nazarenko (2004), Soot climate forcing via snow and ice albedos, PNAS, 101, 423, doi:10.1073/pnas.2237157100.
• Jacobson, M.Z. (2010), Short-term effects of controlling fossil-fuel soot, biofuel soot and gasses, and methane on climate, Arctic ice, and air pollution health, Journal of Geophysical Research, 115, D14209, doi:10.1029/2009JD013795.
• Ramanathan V. and G. Carmichael (2008), Global and regional climate changes due to black carbon, Nature Geoscience, 1, 221, doi:10.1038/ngeo156