The scientific background

One of the longstanding problems of the Ice Age is why and how do relatively small variations in solar radiation received by the Earth as it orbits the sun become transformed into periodic Ice Ages every 100,000 years or so. The scale of the change, which sees Antarctic-sized ice sheets building up in the Northern Hemisphere, seems out of proportion to the radiation change of a few percent. This is more puzzling when one realises that some of the orbital changes in solar radiation are out of phase between the northern and southern hemispheres. In other words, high solar radiation in the northern hemisphere summer (good for melting glaciers) may coincide with low summer solar radiation in the southern hemisphere (good for growing glaciers). There is now convincing evidence from deep sea and ice-sheet cores that the main Ice Age fluctuations are in phase and that somehow the Earth’s climate system must accentuate the initial radiation change and transform climate world wide.

What are the mechanisms?

This is where our Patagonian project comes in. One way to study the resilience of the global climate system is to focus on the time when it experienced extreme changes and to seek leads and lags in the response of different parts of the system. Do changes in the atmosphere lead changes in the ocean or vice versa? Which climatic zones of the world change first? The transition from the Last Glacial Maximum to our present Holocene climate is one such period during which the world changed from a full glacial mode to an interglacial mode. It also has the best dating control.

Existing hypotheses of hemispheric climate change

The relationships between climate changes in the southern and northern hemisphere and their relationship to mechanisms of change are controversial. There are two main groups of hypotheses.

One group of hypotheses points to the importance of the ocean thermohaline circulation as a driver of global climate. In the North Atlantic the Gulf Stream and its continuation, the North Atlantic Drift, bring warm waters to the shores of Europe. The waters mix with cold Arctic surface water and sink to form a counter deep flow to the world’s oceans. It seems that too much fresh surface water from rivers and ice sheets reduces the density of surface water and can suppress the whole circulation, plunging Europe into an Ice Age mode. The reverse can also happen in that changes in water density can restart the thermohaline circulation, for example, at the end of an Ice Age, and bring sudden warmth to Europe.

Such a mechanism can lead to antiphase behaviour between the hemispheres. This is called the bipolar seesaw and the argument is that the effect of shutting down the circulation is to reduce the heat transport to the North Atlantic thus retaining more heat in the South Atlantic.

Conversely, transporting more heat to the North Atlantic will draw heat and reduce temperatures in the South Atlantic. In a refinement of the hypothesis, such a mechanism could be driven either by fresh water changes in the North or by changes in wind strength and direction, or sea ice extent, in the Southern Ocean. Support for such antiphase hemispheric behaviour comes from Antarctica where ice core records demonstrate an out of phase relationship with the northern hemisphere, as shown in the Figure below.

A second group of hypotheses leans towards synchrony between the northern and southern hemispheres. The hypotheses are based on studies of glacier fluctuations and associated vegetation changes (based on palaeoecological study of plant remains in bogs). Well-dated, millennial records of climatic change in Chile at latitude 420 S correlate closely with changes in the northern hemisphere. The good fit between the records suggests that both hemispheres were responding to climate change synchronously. Such a conclusion is borne out by the apparent synchrony of an ice advance during the Younger Dryas, 11.5 thousand years ago, seen in records in both New Zealand and Europe. This 1500 year long cold period was the final flip of the last Ice Age. If such short-lived events are truly synchronous, then it points to the atmosphere as the main driver of global climatic change. Further the changes affected the whole world with the possible exception of Antarctica.