Australia: The Land Where Time Began

A biography of the Australian continent 

Stop-and-Go Deglaciation

In the transitions from glacial to interglacial climates of the past were not smooth, which suggests there could potentially be similar abrupt ice loss as a result of future climate change.

The complexity of glacial termination didn't become apparent until climate records at centennial and millennial scales became available, previously low-resolution reconstructions of palaeoclimate suggested meltdown was rapid and relentless. When the records were examined more closely it emerged that there was asynchrony between the Southern and Northern Hemispheres, warming beginning in Antarctica at about 18,000 BP, with an abrupt reversal interrupting the warming at about 14,500 BP. Warming in the Northern Hemisphere countered this cooling occurring in the Southern Hemisphere until it was also interrupted by 1,000 years of cooling. It wasn't until after 11,700 BP that full interglacial conditions were reached in both the Northern and Southern Hemispheres.

The transfer of water from the vast continental ice sheets to the oceans is of particular interest, as it led to a sea level rise of 120 m.

The rise of the sea level following the closing of the glacial period was uneven, with several episodes of meltwater pulses, that are believed to have been from sudden partial ice sheet collapses that pushed the sea level up by 10s of metres within a few centuries, interrupting a background gradual sea level rise. It has been argued (Carlson & Winsor, 2012), based on synthesising work on the past 2 glacial terminations, that rapid disintegration events are characteristic of ice sheets that reach the ocean, though ice sheets terminating on land melted more steadily as they received more solar radiation.

 Studies of later rises on a decimetre scale (Törnqvist & Hijma, 2012) have become possible as the result of measurements that are increasingly precise. The final drainage of Lake Agassiz in North America, a glacial meltwater lake on the margin of one of the largest ice sheets, has been linked to 2 rapid jumps in sea level at about 8,500 BP and 8.200 BP. It was suggested by the study that though such large injections of meltwater into the ocean were a consequence of climate change, they were also a condition that was necessary to cause abrupt climate change that was associated with circulation changes in the ocean.

At a time when temperatures are rising it is tempting to draw comparisons between the warming that occurred at the close of the glacial periods and the melting of the remaining ice sheets on Greenland and Antarctica that has been projected for the future. Subtle changes in insolation as the orbit of the Earth around the Sun evolved, which was the underlying cause of deglaciation, were much slower than the changes resulting from radiative forcing that has been attributed to emissions of anthropogenic greenhouse gases, which means direct comparisons cannot be made. Considering the complications during the last deglaciation, it seems likely the response of the glaciers and ice sheets of the world might be far from gradual as the climate changes in the future.

Sources & Further reading

"Stop-and-Go Deglaciation." Nature Geosci 5, no. 9 (09//print 2012): 585-85.