Wednesday, March 31, 2010

New developments in Ice Age climate research

Mega-flood Triggered Cooling 13,000 Years Ago
From Yahoo! News

There's been an interesting development in our understanding of the changes in the climate at the end of the Pleistocene era; more evidence to point to the draining of a massive Canadian lake at the end of the Ice Age, which triggered a short but sharp global cold period called the Younger Dryas.

This cooling event was critically important, as the period that immediately followed - the Holocene - marked the development and spread of modern humans. Some anthropologists have speculated that the environmental stresses of the Younger Dryas, which led to cooler, drier conditions globally, forced previously nomadic hunter-gatherer societies to settle and develop agriculture, one of the key milestones in the development of civilization.

What caused this unusual centuries-long cold snap has been debated in the scientific community. One theory, that this study has found physical evidence to support, is that a massive lake in North America drained into the Atlantic, flooding it with fresh water and shutting down the "thermohaline convection belt" (thermo- meaning heat, -haline meaning salt). This is an ocean current that carries warm water from the Equator along the surface to the North Atlantic, where it freezes into freshwater ice. The remaining cold water, laden with excess salt, sinks to the bottom and returns south to the Equator. This convection circuit is very important for Europe, as it carries heat from the tropics that keeps Europe warmer than other landmasses at the same latitude.

What could stop a whole ocean from circulating? It would take a lot of fresh water to dilute the entire North Atlantic enough to halt thermohaline convection. Lake Agassiz, a huge inland lake formed by the melting of the North American ice sheets towards the end of the Ice Age, had an awful lot of fresh water. Take a look (from UCAR):

It's hard to imagine lakes as big as the ones that can form only after a global ice age. Agassiz was a lake that could submerge the entire United Kingdom. It held more fresh water alone than all the current lakes in the world put together. And, around 11,000 years ago, this new study shows, that fresh water drained from central Canada to the ocean, shutting down thermohaline convection and chilling the world's climate.

What's even more interesting is that this study discovers a new twist: the water didnt drain down the St. Lawrence Seaway, which heads through the Great Lakes and down the St Lawrence River to its mouth north of Maine, as first thought, but rather down the Mackenzie River, north to Canada's Arctic Sea coast near Alaska.

One can hardly imagine the cataclysm that must have occurred in Northwest Canada, millenia ago, when this "mega-flood" first roared down the Mackenzie to the sea.


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