Glacials and interglacials on the northern and southern hemisphere somehow do not seem to correspond. This has led to a ‘thermal bipolar seesaw theory,’ whereby an off-mode in the thermohaline circulation leads to an ice age in Europe, but excess heat storage down south.
To support this theory ice cores were never much help. That has changed with a new study by an international team of scientists led by the University of Cardiff.
Their research also supports the idea that over the last 800,000 years of the late Pleistocene climate variability has had an abrupt character.
Calibrating the seesaw hypothesis
The Greenland ice sheet is at max 3 kilometres thick – the Antarctic ice sheet approximately 5. But although on Antarctica some ice may be over a million years old, the Greenland ice comes nowhere near that.
Before drills hit rock bottom the oldest fossilised snow flake scientists can bring to the surface is 150,000 years old. Because most Greenland ice cores are however younger, the Greenland temperature record of only the last 100,000 years is known in detail.
That gets us across all of the Holocene and most of the last ice age, but doesn’t tell us anything about climatic conditions in for instance the Eemian or anywhere else in the Pleistocene – the age of ice ages.
These 100,000 years are however very useful to do mathematical testing of the bipolar ice correlations. The research group has made a model that can ‘predict’ 800,000 years of Greenland ice behaviour, based on detailed Antarctic ice cores and ‘what should have happened up north’ if indeed the thermohaline circulation meltwater switches would have created phase differences.
And apparently their trick has worked, as the model’s results show nice comparison to actual Greenland ice core temperature data over the last 100,000. Besides, there is also “strong similarity” with a Chinese temperature record derived from cave sediments, dating back 400,000 years, the researchers write in their Science publication of Thursday.
Extending the Greenland temperature record
The importance of the research may not be so much to prove the bipolar seesaw mechanism may indeed exist, but also to lengthen the Greenland temperature dataset – and understand that in greater detail. This longer Greenland dataset can in turn be used to test future Greenland ice sheet prediction models – to get to better estimates of how the ice would react to further warming.
Abrupt climate variability in late Pleistocene
The temperature dataset shows over the last 800,000 years sudden temperature shifts have been the norm, pointing to a physical mechanism that probably still exists today.
Most likely the thermohaline circulation, ‘the ocean’s conveyor belt’ plays a crucial role in this:
During a warmer episode the influx of fresh, light Greenland melting water into the North Atlantic may stop the Atlantic Meridional Overturning Circulation and the entire circulation. This isolates the Arctic and provokes an ice age over Europe especially, but it may also lead to heat accumulation on the southern hemisphere and a very different climatic episode there.
There are however more theories. Just a week ago there was a publication in Nature which suggested a different cause to the abrupt climate variability of the late Pleistocene: dust storms.
© Rolf Schuttenhelm | www.bitsofscience.org