Geoscience Reference
In-Depth Information
to melt sea ice (Jackson et al.,
2010
). Other issues include impacts of increased
Atlantic layer inflow on winter ice growth and changes in cloud cover that alter
the surface radiation budget, all of which are very difficult to model. Interestingly,
ocean heat loss in autumn and winter, while representing a negative feedback with
respect to the ice loss, is an important part of the observed process of Arctic ampli-
fication (the ocean loses heat, which is expressed as an atmospheric heat gain). At
issue is the extent to which, via atmospheric circulation, the effects of ice loss on
heating of the lower atmosphere are spread beyond the areas of ice loss to inhibit ice
growth or to warm surrounding land areas.
An uncertainty with global ramifications is the permafrost carbon feedback
discussed in
Chapter 9
. The rate of anthropogenic carbon emissions through the
twenty-first century is of course relevant as this bears on the rate of terrestrial per-
mafrost warming and thaw. As just discussed, the rate of warming over land may
also be influenced by the rate of sea ice loss (Lawrence and Slater,
2005
). As dis-
cussed in
Chapter 10
, a common feature of global climate model simulations of
the twenty-first century is an increase in precipitation over northern high latitudes.
Coupled with warming, it is reasonable to expect more winter snowfall but a shorter
snow-covered seasonal overall. Snow cover is an insulator. This implies that perma-
frost warming will be hastened not only by high surface air temperatures in summer,
putting more heat into the soil column, but by a reduction in winter heat loss from
the soil column attributed to the increased insulating effect of a deeper winter snow-
pack. A complicating issue is how reduced sea ice extent will contribute to changes
in Arctic precipitation, both through influences on atmospheric circulation patterns
and by providing an additional moisture source.
There has been speculation regarding the possibility of a massive and rapid
methane release from Arctic submarine sediments along continental shelves that
were exposed during past periods of lower sea level. Under the proper tempera-
ture and pressure conditions, methane produced by microbial activity can become
trapped in these sediments in a solid (hydrate) form (i.e., as an ice). The concern
is that with ocean warming, these hydrates will warm, melt, and lead to a release
of methane to the atmosphere. As methane is a powerful greenhouse gas, such a
release would lead to further climate warming. G. Westbrook et al. (
2009
) docu-
ment numerous plumes of gas bubbles of predominantly methane emanating from
the seabed of the West Spitzbergen continental margin. Some of the plumes were
observed to extend upwards to within 50 m of the surface before dissolving into the
water column. They argue that this methane release can be linked to warming of
the northward-flowing West Spitzbergen Current over the past several decades that
has reduced the extent of the gas hydrate stability zone, which is a function of both
pressure and temperature. A subsequent modeling study lends support to this con-
clusion (Reagan and Moridis,
2009
). N. Shakhova et al. (
2010
) argue that a release
to the atmosphere of only a small fraction of methane held in hydrate form along the
East Siberian Arctic shelf region (the Laptev, East Siberian, and Russian part of the
Chukchi seas) could trigger an abrupt climate warming. They highlight that more
than 5,000 at-sea observations collected during six ship cruises between the years
