The Causes of the end of the last Ice Age
The following text is adapted, except where otherwise noted, from an excellent article in the New Scientist magazine, 05 November 2012 by Anil Ananthaswamy Magazine issue 2889
The last great ice age began around 120 000 years ago. One massive ice sheet, more than 3 kilometres thick in places, grew in fits and starts until it covered almost all of Canada and stretched down as far as Manhattan. Another spread across most of Siberia, northern Europe and Britain, stopping just short of what is now London. Elsewhere many smaller ice sheets and glaciers grew, vast areas turned into tundra and deserts expanded as the planet became drier.
With so much ice on land, sea level was 120 metres lower than it is today. Britain and Ireland were part of mainland Europe. Florida was twice the size it is now, with Tampa stranded far from the coast. Australia, Tasmania and New Guinea were all part of a single land mass called Sahul. The planet was barely recognisable.
Then, 20 000 years ago, a great thaw began. Over the following 10 000 years, the average global temperature rose by 3.5° C and most of the ice melted. Rising seas swallowed up low-lying areas such as the English Channel and North Sea, forcing our ancestors to abandon many settlements. So what drove this dramatic transformation of the planet?
*** We have long known the thaw began with an increase in summer sunlight in the northern hemisphere, melting ice and snow. It is what happened next that has remained mysterious. Soon after the thaw began, for instance, the southern hemisphere began to warm while the northern hemisphere cooled - the opposite of what was expected from the changes in sunshine. Now, after nearly two centuries of wrestling with seemingly contradictory findings, we think we finally understand how the ice age ended.
Early in the 20th century, the Serbian astronomer Milutin Milankovitch concluded that summer sunlight in the northern hemisphere must be the crucial factor and spent years painstakingly calculating how this had changed over the past 600 000 years. His ideas weren't accepted at the time, but in the 1970s studies of ocean-sediment cores revealed that the advances and retreats of the ice ages did indeed coincide with 'Milankovitch cycles'.
*** The answer to these puzzles seemed to emerge in the 1980s, when ice cores drilled in Antarctica revealed an astonishingly close correlation between atmospheric carbon dioxide levels and temperature.
'For the last million years, you see these two going up and down, up and down, together through each ice age, and it's almost in perfect lockstep,' says Jeremy Shakun of Harvard University. 'It's about as beautiful a correlation as you ever get from nature.'
If CO2 levels had risen soon after the thaw began in the north, it would explain why the southern hemisphere began to warm too. It would also help to explain the magnitude of the changes. But this promising idea ran into a major problem: by around a decade ago, it had become clear that the Antarctic starting warming a few hundred years before CO2 levels began to rise. So while soaring CO2 levels undoubtedly warmed the planet - they are now thought to be responsible for about half of the warming as the ice age ended - they weren't the initial cause. 'Something else was causing Antarctica to warm,' says Daniel Sigman of Princeton University...
All of it:
http://donsmaps.com/endoficeage.html
