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Monday, November 5, 2012

The End of the Ice Age

New Scientist has an article exploring what caused global temperature to melt and the ice sheets to retreat at the end of the Ice Age. First, some background and the issue:
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?
The problem facing scientists, however, is that various models--increased sunlight, increased CO2--didn't fit with the facts. The increased sunlight was too small, while the increase in CO2 was not until after the ice sheets started to melt. A proposed solution is that the initial changes were driven by changes to ocean currents, which then released CO2 into the atmosphere. The article explains:
Earlier this year, Shakun and colleagues drew together many of these strands of research with an analysis of 80 different records of temperature and atmospheric composition over the past 22,000 years (Nature, vol 484, p 49). Their work pretty much confirms the sequence of events that ended the ice age. It goes like this:

Around 20,000 years ago, the northern ice sheets had spread so far south that just a small increase in sunshine led to extensive melting. As fresh water poured into the North Atlantic, the overturning circulation shut down, cooling the northern hemisphere but warming the southern hemisphere. These changes were mostly due to a redistribution of heat - by 17,500 years ago, the average global temperature had risen just 0.3 °C.

Changing winds or currents, or both, then brought more deep water to the surface in the Southern Ocean, releasing CO2 that had been trapped for thousands of years. As atmospheric levels climbed above 190 parts per million, the whole planet began to warm. The far north was the slowest to respond, but by around 15,000 years ago, as CO2 levels approached 240 ppm and the Atlantic overturning circulation sped up again, temperatures started to shoot up. The recovery of the overturning circulation had the opposite effect in the southern hemisphere: warming stalled and the release of CO2 stopped.

Around 12,900 years ago, the see-saw swung again. Temperatures in northern latitudes suddenly plummeted and remained cold for about 1300 years. This cold snap, called the Younger Dryas, is thought to have been caused by a colossal meltwater lake in North America, which held more water than all the Great Lakes put together, suddenly flooding into the Atlantic and shutting down the overturning circulation once again.

The Southern Ocean, meanwhile, started releasing CO2 again. Levels in the atmosphere shot up to 260 ppm, causing the whole planet to warm rapidly over the next couple of millennia. By around 10,000 years ago, Earth had been transformed. The ice had retreated, the seas had risen and our ancestors were learning how to farm.
 The author goes on to note that, technically, we are still in an ice-age because we still have ice caps, but adds (predictably) that increased atmospheric CO2 may cause the ice caps to fully melt. I have to admit that I'm a little disappointed. I had hoped the cause of the end of the ice age was the rapid industrialization of the Atlantians. (Sarcasm intended).

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