12. Ice record of greenhouse gases and last glaciation
Description
This article is from the Climate Change FAQ, by Jan Schloerer jan.schloerer@medizin.uni-ulm.de with numerous contributions by others.
12. Ice record of greenhouse gases and last glaciation
During the past millennium, until about the 19th century, atmospheric
greenhouse gas levels varied little and hence, during that time,
probably contributed little to climatic variations. On a longer time
scale, changes of greenhouse gas levels probably contributed signifi-
cantly to the coolings and warmings of the last two glacial cycles.
Ice cores from Greenland and Antarctica indicate that there was a close
link between greenhouse gases and temperature [Raynaud]. For instance,
the Vostok ice core from Antarctica exhibits a striking correlation
between temperature and the concentrations of carbon dioxide (CO2)
and methane (CH4) over the past 220,000 years [Jouzel]. The level of
nitrous oxide (N2O) probably also varied more or less in phase with
temperature [Raynaud, p 928]. The variations of these trace gases
may account for up to about 50 % of the estimated temperature changes
[Crowley, p 2364] [Raynaud, p 932]. CO2 was most important, while
methane and nitrous oxide contributed less.
During the onset of the last glaciation, the CO2 decrease markedly
lagged the onset of the cooling. During the past two deglaciations,
CO2 may have risen in phase with temperature or with an, in geologic
terms, modest lag of up to about 1000 years [Raynaud, p 931]. Whether
greenhouse gases led or lagged the climatic change, that is, whether
they were a primary cause for the change or whether they acted as a
positive feedback (which amplified a climatic shift already under way),
is important for finding out just exactly what happened, but it is not
by itself relevant for estimating the effect of the trace gases on
surface temperature [Raynaud, p 932].
In spite of this, the effect is hard to quantify. During the last
deglaciation, roughly 18,000 to 10,000 years ago, the rise of trace
gas levels amounted to a radiative forcing of about 2.5 to 3 W/m**2.
The meltdown of the huge glacial ice shields reduced earth's albedo,
accounting for another perhaps 3 to 3.5 W/m**2. These figures are
compatible with the IPCC estimate of about 1.5 to 4.5 o C surface
warming for a CO2 doubling. They do not permit to narrow down the
uncertainty, there remain many unknowns [Crowley, p 2366].
Perhaps most important: How cold was the last ice age ? This is not
yet clear. Tropical oceans, for instance, may have been between 1
and 5 o C cooler than they are now [IPCC 95, p 173-4], and Greenland
may have been several degrees colder than previously thought [Cuffey].
Another point to keep in mind: The sensitivity of earth's present
climate and the sensitivity of the last glacial maximum's climate
to a radiative forcing of so or so many W/m**2 need not be equal.
The starting positions differ.
Glaciations and deglaciations are triggered by variations in earth's
orbit. Tilt of earth's axis, season of the perihelion (closest ap-
proach of earth to sun, now in January), and eccentricity of earth's
elliptical orbit vary. These variations cause, among others, changes
in high northern latitude summer insolation, which are critical for
the waxing and waning of ice sheets. Northern summer insolation was
unusually low at the onset of the last glaciation around 115,000 years
ago, it was high during deglaciation.
The direct effect of the "orbital trigger" was too small to cause
glaciation or deglaciation. Instead, a cascade of feedbacks and
interacting processes with widely varying timescales led to the final
result. Shifts in atmospheric or oceanic circulation may occur within
decades. Southward spread of tundra or poleward expansion of boreal
forests can take centuries to millennia. Over 10,000s of years,
the weight of an ice sheet depresses the underlying bedrock, which
eases melting. Many twists of the story, like the frequent partial
breakdowns of ice sheets, remain enigmatic, even though the trigger
and the gist of the eventual outcome are known [Crowley & North,
chapters 6-7] [Eddy, chapters 17 & 21] [Gallimore] [IPCC 95, p 177-9]
[Keigwin 95]. In today's climate change gamble even the trigger or,
rather, its aerosol component is poorly known.
As we are at it: For the next 25,000 years, high northern latitude
summer insolation will not drop anywhere near its minimum of 115,000
years ago [Eddy, p 40-41].
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