A tale of two hemispheres
Date: 06 Apr 2012
This week in the journal Nature, Eric Wolff of British Antarctic Survey reviews an article by Jeremy Shakun et al. on the causes for the end of the last ice age.
In their article, Shakun and his colleagues investigate the sequence of events at the end of the last ice age, or glacial period, between 21,000 and 10,000 years ago. There are several theories of what happened, and they provide new evidence of how and why the last glacial period terminated. They produced a synthesis of how mean temperatures in each hemisphere evolved using data from 80 marine, terrestrial and ice-core records and found that global average temperatures rose along with atmospheric carbon dioxide (CO2). Crucially, however, temperatures in the northern and southern hemispheres exhibited very different patterns.
Shakun and colleagues found that the average temperature difference between the two hemispheres closely follows the strength of a crucial global oceanic current: the Atlantic meridional overturning circulation, or AMOC, takes warm surface waters north in the Atlantic while cold water flows south at depth. The AMOC controls the energy exchange between the two hemispheres, and it is thought that fluctuations in its strength played a crucial role in deglaciation.
Shakun and his team combine their temperature records with global-scale models of atmospheric and oceanic circulations to understand the events that led to the onset of deglaciation 19,000 years ago. They suggest that the ice sheets covering northern Europe and North America began to melt following a small warming in the Arctic. They released freshwater into the North Atlantic, weakening the AMOC. This caused warming in the southern hemisphere and rising CO2 concentrations around the world. As a consequence, warming occurred globally, but fluctuations in the AMOC meant that patterns of warming varied between the two hemispheres.
The dataset assembled by Shakun and his team allows them to substantiate the sequence of events at the onset of deglaciation. But one more crucial question remains: why was the AMOC weakened so significantly during this relatively brief period of warming that a complete transformation of the Earth’s climate and ice sheets resulted? There had been longer and stronger periods of Arctic warming in the past which did not initiate a similar sequence of events — so what was special about the end of the last ice age? The Arctic ice sheets were bigger than during earlier warming episodes, which may be a critical factor. If so, the next step would be to understand why very large ice sheets cause longer periods of AMOC weakening — which seems to have been instrumental in bringing about the end of the last glacial period.