What Causes Ice Age Meltdowns?

Ah, how I love paleoceanography. I've always thought of it as 'ocean
archaeology', since many popularized theories are the result of both
scientific data, and brilliant minds able to to weave a story with
sometimes fairly little to go on. But I digress...

A recent paper published in Science (Scott et al., 2007) proposes
that carbon dioxide wasn't the sole driver of climactic warming as the
Earth headed out of the last ice age some 15-20,000 years ago.  Although this is nothing new per se, the interpretation of events leading to the great global melting are novel.

Paleoclimatologists and paleoceanographers engaged in this whodunit
are determined to place 'blame' on what triggered this global
'meltdown' (relatively, and sensationally speaking, of course).
Esteemed Richard Kerr of Science Magazine has provided a review of this
very subject. However, if you lack a paid subscription, here's my take
on it.

In 1999, the longest time series, at the time, of atmospheric carbon
dioxide was published - over 400,000 years worth of data were gathered
by measuring concentration of CO2 and methane trapped in an Antarctic ice core
miles long. Using a chemical signatures sensitive to environmental
processes (i.e. a chemical 'proxy'), the authors were able to determine
the local overlying air surface temperature.

Source: IPCC

First, this paper was the first to capture more than one
glacial-interglacial period in a single core. Second, it demonstrated a
positive correlation between greenhouse gas concentrations and the
local atmospheric temperature - thus, the following question has come
to many paleoclimatologists: why does this relationship exist, and what
controls it?

A number of hypotheses have been put forth - notably the following two:

1) The Iron Hypothesis: John Martin's 1989 in Paleoceanography paper
suggested that, since vast areas of the ocean were likely Fe-limited
(which there now exists very strong evidence for), and since the dust
content in the ice is (somewhat) anticorrelated to CO2, then
alleviating Fe-limitation of phytoplankton productivity would turn up
the 'biological pump' of carbon from the surface to the seafloor.

2) Changes in deep-water circulation, such that the natural venting
of CO2 from the upwelling of CO2-rich bottom water in the Southern
ocean is hampered, due to the formation of a highly stratified surface
layer (i.e. the larger the difference between two densities, the less
likely they will mix).

Although neither one is likely to be true on their own, some
combination of them is likely causing the shifts in atmospheric CO2.

Some of you are probably asking, "What about the terrestrial biosphere?!"
Well, according to sedimentary carbon isotopic evidence along the coast
of western North America (if my memory serves me correctly...), if anything, terrestrial biomass was much lower than today (as
evidenced by the presence of carbon isotopes unique to land forming
plants). Therefore, the terrestrial biosphere was likely a source of
CO2 - not a sink.

All this now aside, the "Big" question has always been about timing:
what's causing the ice age/meltdown, where did it happen, and when did
it happen? These questions are all intimately linked, as you will see.

So what have Scott and colleagues determined? Well, using another
chemical proxy in foraminfera (calcium carbonate forming animals) -
stable oxygen isotopes - they are able to determine the temperature of
the surrounding seawater in which the organisms grew. Using a core
taken form the tropical Pacific ocean, they isolated the shells of
surface water and sediment dwelling foram species, and dated them using
a Carbon-14 method.

So...what? Why two groups? Well, the sediment dwellers record the
temperature changes of bottom water - seeping down from where deep
water is formed along Antarctica, and creeping northward into the south
Pacific basin. The surface dwellers will, of course, be recording the
temperature change in the tropical regions. Therefore, Scott et al.
have a conceived a way of comparing the temperature change across half
the globe using one sediment core. The elegance of this approach is it
reduces the uncertainty involved with calibrating sediment core data
with other chemical proxy data from, say, tree rings, ice cores, etc. because both are being sampled from the same core.

Comparing the timing of the temperature change between these two
groups, they discovered something no one else had been able to prove
with any appreciable amount of certainty: the sedimentary forams began
warming before the surface forams.

And so their hypothesis goes: Antarctica warmed first, causing the
large ice sheets covering the southern polar front to melt, thus
allowing the CO2 'leak' to increase (see number two above), thus
promoting further warming via the greenhouse (i.e. warming the tropics).

Source: Richard Kerr 

But what caused the initial melting? You might have guessed it - those natural changes in the Earth's orbit.

I think its a great idea, and builds wonderfully on previous work.

The problem now is, global warming denialists will likely use this
work as fodder for spreading their scientific misconceptions. However,
it only suggests CO2 wasn't responsible for the initial warming that
led to the release of CO2 to the atmosphere.