Climate Change in the Arctic - The Changing Arctic Environment: The Arctic Messenger - page 292

At the time of writing in 2013, a very large synthesis study on BC was published in
the Journal of Geophysical Research: Atmosphere . Led by Tami Bond, the researchers es-
timated that BC is presently responsible for a total climate forcing of +1.1 W per square
metre for the first year after emission. This is about twice that estimated in the fourth IPCC
assessment and puts BC second only to carbon dioxide. However, BC sources also sim-
ultaneously release other short-lived substances, such as sulphur dioxide, that can cool or
warm the climate. When the team took account of these other agents, they arrived at an
industrial age net climate forcing of +0.22 (−0.50 to +1.08) W per square metre during
the first year after emission. They also estimated that present-day climate forcing from BC
resting on snow and sea ice is about +0.13 W per square metre. The open burning of forests
and savannah vegetation is the largest global source of BC to the atmosphere, but the rank-
ing of other sources varies regionally. In Europe, North America and Latin America, dies-
el engines used for transportation contribute about 70% of emissions. Not all emissions
are equal in terms of their contribution to climate warming, partly because of the co-emit-
ted substances that exert negative forcing from some sources. The study highlighted dies-
el engines used for transportation and possibly residential biofuels as being the two BC
source categories that would produce the greatest degree of climate cooling if their emis-
sions could be eliminated.
The magnitude of the impact of forcing by all SLCFs depends on seasonal patterns
of atmospheric transport and deposition, solar radiation and snow/ice melt. Generally, the
atmospheric transport of pollutants to the Arctic from mid-latitudes is most efficient in
winter and early spring. Without sunlight, there is a buildup of tropospheric ozone (anoth-
er SLCF) and of ozone precursors. Arctic BC concentrations also go up in winter because
their transport is associated with Arctic haze. When solar radiation increases in spring, the
season of forest fires begins in sub-Arctic regions and photochemical reactions involving
methane and other substances (including nitrogen oxides) result in more ozone production.
The warming effects of BC are at a peak in spring and summer, when atmospheric radiat-
ive warming and the snow/ice albedo feedback (both enhanced by BC) are maximized. The