Real Climate
This story is the dream of every science writer. It
features some of the most dramatic and rapid climate shifts in Earth’s history,
as well as tenacious scientists braving the hostile ice and snows of Greenland
and Antarctica for years on end to bring home that most precious material:
kilometre-long cores of ancient ice, dating back over a hundred thousand years.
Back in their labs, these women and men spend many months of seclusion on
high-precision measurements, finding ingenious ways to unravel the secrets of
abrupt climate change. Quite a bit has already been written on the ice core
feat (including Richard Alley’s commendable inside story “The Two Mile Time
Machine”), and no doubt much more will be.
It was the early, pioneering ice coring efforts in
Greenland in the 1980s and 90s that first revealed the abrupt climate shifts
called “Dansgaard-Oeschger events” (or simply DO events), which have fascinated
and vexed climatologists ever since. Temperatures in Greenland jumped up by
more than 10 ºC within a few decades at the beginning of DO events, typically
remaining warm for several centuries after. This happened over twenty times
during the last great Ice Age, between about 100,000 and 10,000 years before
present.
The latest results of the EPICA team (the European
Project for Ice Coring in Antarctica) are published in Nature today (see also the News & Views by RealClimate member Eric Steig). Their
data from the other pole, from the Antarctic ice sheet, bring us an important
step closer to nailing down the mechanism of the mysterious abrupt climate
jumps in Greenland and their reverberations around the world, which can be
identified in places as diverse as Chinese caves, Caribbean seafloor sediments
and many others. So what are the new data telling us?
These data connect the Antarctic ups and downs of climate to the much greater
ones of Greenland. This is hard, as dating an ice core is a difficult art (no
pun intended). If one makes an error of only 5% in determining the age of an
ice layer, for 40,000-year-old ice that’s an error of 2,000 years. But to
understand the mechanisms of climatic changes, one needs to know the sequence
of events – for example, one needs to know whether a particular warming in
Antarctica happens before, after, or at the same time as a warming in
Greenland.
To get around this problem, Thomas Blunier and
colleagues nearly ten years ago pioneered an ingenious method to synchronise
the ice cores of Greenland and Antarctica by analysing changes in the amount of
methane in air bubbles in the ice. Changes in methane are recorded at both
poles, and they should occur almost exactly in step as gases are quickly mixed
through the whole atmosphere. After the ice cores are synchronised by aligning
the methane variations, the relative timing of Greenland and Antarctic
temperature changes can be seen.
While Blunier and colleagues were originally able to
connect only a handful of large climate events, the results published today
take this method to a new level by applying it to the new, high-resolution
Dronning Maud Land ice core. The new data confirm with unprecedented precision
what Blunier found: Antarctica gradually warms while Greenland is cold. But as
soon as Greenland temperatures jump up in a DO event, Antarctic temperatures
start to fall (see graph). This happens for every DO event, and it is a
peculiar and tell-tale pattern that is also found in model simulations of these events (see graph).
Figure: The top two panels show idealised model DO
events on an arbitrary time axis (in years), highlighting the phase
relationship between Greenland and Antarctic temperatures: when a DO event hits
Greenland, Antarctica switches from warming trend to cooling trend. The bottom
panels show the “real thing”, the noisy data from ice cores. Note the expanded
scale for Antarctica in both cases. Time here runs from left to right – normal
for regular folks, but somewhat unusual for the ice core experts (my apologies
to these).
It is (at least in the model) a result of a big change
in northward heat transport in the Atlantic. If the heat transport by the
Atlantic thermohaline circulation suddenly increases for some reason (we’ll
come to that), Greenland suddenly gets warm (an effect amplified by receding
sea ice cover of the seas near Greenland) and Antarctica starts to cool.
Changes in Antarctica are much smaller and more gradual, as it is far from the centre
of action and the vast reservoir of ocean around it acts as a heat store. The
basic physics is illustrated very nicely in a simple “toy model” developed by
Thomas Stocker and Sigfus Johnsen.
There is still debate over what kind of ocean
circulation change causes the change in heat transport. Some argue that the
Atlantic thermohaline circulation switches on and off over the cycle of DO
events, or that it oscillates in strength. Personally, I am rather fond of
another idea: a latitude shift of oceanic convection. This is what happens in
our model events pictured above: during cold phases in Greenland, oceanic
convection only occurs in latitudes well south of Greenland, but during a DO
event convection shifts into the Greenland-Norwegian seas and warm and saline
Atlantic waters push northward. But I am biased, of course: my very first Nature paper(1994) as a young
postdoc demonstrated in an idealised model the latitude-shift mechanism. Other
oceanic mechanisms may also agree with the phasing found in the data. In any
case, these data provide a good and hard constraint to test models of abrupt
climate events.
But irrespective of the details: the new data from
Antarctica clearly point to ocean heat transport changes as the explanation for
the abrupt climate changes found in Greenland. We are thus not talking about
changes primarily in global mean temperature (these are small in the model
results shown above). We are talking about what I call a climate change of the second kind: a change in how heat is moved around the climate system.
As an analogy, think of your bath tub and the types of
change to the water level you can get there. A change of the first kind would
be a change in mean level, e.g. if you add water. A change of the second kind
would be the changes you get by sloshing around the water in the tub.
There are very few possibilities to change the global
mean temperature, a climate change of thefirst kind:
you have to change the global heat budget, i.e. either the incoming solar
radiation, the portion that is reflected (the Earth’s albedo), or the outgoing
long-wave radiation (through the greenhouse effect). Temporarily, you can also
store heat in the ocean or release it, but the scope for changes in global mean
temperature through this mechanism is quite limited.
Changes of the second kind are due to changes in heat
transport in the atmosphere or ocean, and these can occur very fast and cause
large regional change. Think of your tub: if you want 10 cm higher water level
at one end, you can achieve this by turning on the tap – but you can get there
much faster by pushing some water over there with your hand, albeit temporarily
and at the expense of the water level at the other end. That kind of “see-saw”
(but with heat, not water) apparently happens during DO events, as the new data
confirm.
The two kinds of climate change are sometimes
confounded by non-experts – e.g., when it is claimed that DO events represent a
much larger and more rapid climate change than anthropogenic global warming.
This forgets that our best understanding of DO events suggests they are changes
of the second kind. The same error is made by those who claim that the
1470-year cycle associated with the DO events could lead to an “unstoppable
global warming”. A global warming of 3 or 5 ºC within a century, as we are
likely causing in this century unless we change our ways, has so far not been
documented in climate history.
One crucial point has been left unanswered thus far.
If DO events are due to ocean circulation changes, what triggers these ocean
circulation changes? Some have argued the ocean circulation may oscillate
internally, needing no trigger to change. I am not convinced – the regularity
of the underlying 1470-year cycle speaks against this, and especially the fact
that sometimes no events occur for several cycles, but then the sequence is
resumed with the same phase as if nothing happened. I’d put my money on some
regularly varying external factor (perhaps the weak solar cycles, which by themselves
cause only minor climate variations), which causes a critical oceanic threshold
to be crossed and triggers events. Sometimes it doesn’t quite make the
threshold (the system is noisy, after all), and that’s why some events are
“missed” and it takes not 1,500, but 3,000 or 4,500 years for the next one to
strike. But the field is wide open for other ideas – the cause of the 1470-year
regularity is one mystery waiting to be solved.
References
Alley, R.B., 2002: The Two-Mile Time Machine: Ice
Cores, Abrupt Climate Change, and Our Future. Princeton University Press.
Blunier, T. and E. J. Brook, 2001: Timing of
millennial-scale climate change in Antarctica and Greenland during the last
glacial period. Science, 291, 109-112.
Blunier, T., J. Chappellaz, J. Schwander, A.
Dällenbach, B. Stauffer, T. F. Stocker, D. Raynaud, J. Jouzel, H. B. Clausen,
C. U. Hammer, and J. S. Johnsen, 1998: Asynchrony of Antarctic and Greenland
climate climate change during the last glacial period. Nature, 394, 739-743.
Braun, H., M. Christl, S. Rahmstorf, A. Ganopolski, A. Mangini, C.
Kubatzki, K. Roth, and B. Kromer, 2005: Solar forcing of abrupt glacial climate
change in a coupled climate system model. Nature, 438, 208-211.
Stocker, T. F. and S. J. Johnsen, 2003: A minimum
thermodynamic model for the bipolar seesaw. Paleoceanography, 18, art. no. 1087.
