Geoscience Reference
In-Depth Information
mechanisms are examined in more detail by D. Benn and D. Evans (
1998
) and have
been modeled by W. Peltier (
2004
).
There is close correspondence between paleotemperatures over the major gla-
cial-interglacial cycles of the Quaternary and concentrations of atmospheric car-
bon dioxide and methane (Petit et al.,
1999
; Raynaud et al.,
2000
; Sigman et al.,
2010
), with lower concentrations during glacials and higher concentrations during
interglacials. This points to strong feedback processes, whereby changes triggered
by the Milankovith cycles are amplified, and, because these greenhouse gases are
well-mixed in the atmosphere, the ice age cooling and interglacial warming are
globalized. The ranges in CO
2
and CH
4
are, respectively, roughly 300 ppmv and 780
ppbv (interglacial) and 180 ppmv and 320 ppbv (glacial) (Petit et al
.,
1999
). This
compares with CO
2
and CH
4
values in the year 2013 of about 400 ppmv and 1,750
ppbv, respectively. The mechanisms for changes in trace gas concentrations remain
to be completely resolved. The methane variations are viewed as having a link to the
terrestrial biosphere - in colder drier conditions, there were fewer wetlands to act
as methane sources. Thinking on the carbon dioxide problem largely solidified on
ocean biogeochemistry and interactions with ocean circulation, in which with the
high latitude Southern Ocean plays a key role.
As reviewed by Sigman et al. (
2010
), under the present climate, biologically pro-
duced organic matter sinks out of the surface layers into the deep ocean before it
can decompose back into CO
2
. This lowers the partial pressure of the CO
2
in surface
waters, drawing CO
2
out of the atmosphere. The storage of regenerated CO
2
in the
deep ocean also focuses acidity there. This reduces the burial of calcium carbonate
in seafloor sediments, making the global ocean more alkaline, increasing the solu-
bility of CO
2
in the sea water, acting to further lower the partial pressure of CO
2
in
the atmosphere. In the Southern Ocean, the nutrient rich and CO
2
-rich waters then
ascend to the subsurface before major nutrients (nitrogen and phosphorus) are fully
used by photosynthesis for carbon fixation. This allows CO
2
to escape back to the
atmosphere. The available evidence suggests that during ice ages, this release of
CO
2
back to the atmosphere was reduced. This could occur in a number of ways,
including (1) a decrease in the exchange of surface waters in the Southern Ocean
with deeper waters, (2) an enhanced consumption of Southern Ocean surface nutri-
ents by phytoplankton, and (3), an increase in the seasonal coverage of sea ice in the
Southern Ocean acting as a cap.
10.3.3
Ice Sheet Influences on Atmospheric Circulation
Modeling studies indicate that the Quaternary ice sheets strongly influenced the
atmospheric circulation. It is likely that the winter jet stream over North America
was split because of the large extent and height of the Laurentide Ice Sheet. One
branch of the jet went around the northern edge of the ice sheet, while the other
branch curled south of the ice sheet, so that storm tracks were displaced (Kageyama
et al.,
1999
). The LGM ice sheets also seem to have favored an increased tendency
for stationary blocking patterns (Roe and Lindzen,
2001
). The study of P. Ditlevesen,
