Environmental Engineering Reference
In-Depth Information
in permafrost is of the order of 7.5-400 GtC (Brook et al., 2008), while that
under the sea floor could amount to 500-2,500 GtC (Buffett and Archer,
2004; Milkov, 2004; Brook et al., 2008). Field studies have demonstrated
remarkably large local emissions of methane in association with warming
and melting of permafrost at particular Arctic sites, and there is evidence
for substantial trace gas emissions at some times in the distant past in the
paleoclimatic record (e.g., Walter et al., 2006).
Thus there is a potential for great risk, and this attracts the interest of
scientists, the public, media, and policy makers. At present, our assessment
is that it is not possible to quantify these risks. A challenge for climate sci-
ence is not only to evaluate the physics and chemistry of the underlying
processes, but also to explain why local observations or evidence from past
climates do not necessarily imply that these factors are important for current
and future anthropogenic climate changes.
Methane concentrations are currently about twice their pre-industrial
levels; they were nearly stable for about a decade in 1997-2006, but began
to increase again in 2007 (Rigby et al., 2008; Dlugokencky et al., 2009).
Many studies establish ongoing permafrost retreat (Lemke et al., 2007) as
well as considerable warming in the Arctic in both 2007 and 2008. But
while global methane observations suggest a contribution from an increased
source in the Arctic in 2007, there was no significant Arctic contribution
to the methane increase in 2008 (Dlugokencky et al., 2009). Thus the cur-
rently warm Arctic does not seem to be a consistent source of methane that
is significant on the global scale compared to other sources (which include
wetlands, agriculture, animal husbandry, and waste processing, see Denman
et al., 2007). One factor influencing methane release from permafrost is the
amount of liquid water present, which controls whether decomposition is
aerobic or anaerobic. This implies that not only thermal but also hydro-
logical conditions are involved in whether or not conditions favor methane
releases reaching the atmosphere on a large enough scale to be significant;
similarly methane released from the sea floor can be degraded by bacteria
before reaching the surface (Brook et al., 2008). Therefore, methane ob-
servations from particular sites, while sometimes dramatic and suggestive,
may be insufficient for characterization of the much larger scales needed to
understand global methane increases.
The large increase in methane observed at the time of the Younger-Dryas
transition about 11,600 years ago is an example of a methane-climate feed-
back that has attracted significant interest. One recent study using isotopes
suggests that the primary methane source at that time was from wetlands
rather than permafrost (Petrenko et al., 2009). Several studies suggest a
