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Roz Pidcock

Roz Pidcock

06.11.2012 | 4:30pm
ScienceMelting Arctic ice triggered the “Big Freeze”
SCIENCE | November 6. 2012. 16:30
Melting Arctic ice triggered the “Big Freeze”

A new study has highlighted the Arctic’s pivotal position in influencing the world’s climate. 
The research suggests that a flood of meltwater from the Arctic thousands of years ago triggered an abrupt return to glacial conditions in Europe. So can the study tell us anything about the role of Arctic meltwater in future climate change?

The study by American researchers, published yesterday in the Proceedings of the National Academy of Sciences (PNAS), describes how meltwater formerly trapped in glacially dammed lakes flooded the North Atlantic as the earth warmed about 13,000 years ago.

The freshwater flood disrupted deep ocean circulation and caused a several degree drop in atmospheric temperature in the northern hemisphere, nicknamed ‘The Big Freeze’.

This finding is important, say the researchers, because it identifies the Arctic as a “key geographical area for triggering global climate episodes”.

Arctic debate

About 13,000 years ago, the world was warming up as it emerged from a cold glacial period into the Holocene, a warmer interglacial period that persists to the present day. But a cold period known as the Younger Dryas interrupted the warming trend, plunging northern Europe back into near-glacial conditions for about 1,200 years.

The new study simulates the release of more than 30,000 cubic kilometers of freshwater – more than the volume of America’s great lakes put together – into the North Atlantic from the Arctic. The researchers say this could have occurred as the world warmed and glacially dammed lakes began to melt.

Scientists first proposed that a freshwater flood could have triggered the Younger Dryas almost 25 years ago. But until now they hadn’t agreed on whether the flood entered the North Atlantic from the Arctic or 4,000 km southeast through the St Lawrence Valley.

Using a model 10-15 times more detailed than previous ones, the new study concludes that meltwater from the MacKenzie Valley in the Arctic was the more likely trigger of the Younger Dryas, seemingly settling decades of scientific debate. 

Arctic Meltwater

A new study shows that the sudden release of meltwater from the Mackenzie Valley in the Arctic rather than from the St Lawrence Valley is the likely cause for a weakening of the MOC 12,000 years ago, triggering the Younger Dryas cold period. 

Source: Condron & Winsor (2012)

Abrupt climate change

The influx of freshwater from the Arctic disrupted a major ocean circulation system known as the Atlantic Meridional Overturning Circulation ( AMOC). The AMOC carries warm surface water northward, giving continental Europe its mild climate, and returns cold deep water southward across the equator.

The new study shows that the freshwater flood suppressed the main driver of the AMOC, which is the sinking of cold salty water at specific sites in the North Atlantic. This weakened the AMOC, reducing the amount of heat transported towards northern Europe by up to 30 per cent.

A second simulation showed that meltwater released from the St Lawrence Valley, was delivered too far south in the North Atlantic to have any significant weakening effect. As the researchers explain in their paper:

“Identifying the Arctic as the location where the meltwater entered the North Atlantic is vital for understanding the sensitivity of the climate system to sudden increases in the delivery of fresh water to the ocean.”

Present context

So what significance does the study have for us today? Scientists refer to the potential shutdown of the global thermohaline circulation – of which the AMOC is a key component – as a tipping point that could have serious consequences for climate, sea level and ecosystems in and around the North Atlantic. But the chances of a similarly abrupt event happening again are slim, Professor Meric Srokosz, head of the RAPID project which monitors the strength of the AMOC, told Carbon Brief.

“There is no ice-dammed lake to burst these days, so the only way to deliver that
amount of freshwater would be if a very large chunk of the Greenland ice sheet slid
into the sea (which is unlikely).”

Research from 2011 shows that the amount of freshwater in the North Atlantic from human-induced Arctic and Greenland ice melt is around 7,500 cubic kilometres, which is far less than the melt that triggered the Younger Dryas. On a longer timescale, the IPCC predict that continued ice melt could lead to a slowdown of the AMOC by up to 50 per cent by the end of the century.

A recent study by McCarthy et al. describes how researchers observed a decline in the AMOC during the winter of 2009/2010 by around 30 per cent. They attribute the decline to an unusual wind event and changes in the deep branch of the AMOC (3000-5000 m). But the decline was followed by a swift recovery. For significant cooling to occur, such a decline would have to persist for a much longer period.

Even with a gradually weakening AMOC, IPCC models show a continued warming trend in the North Atlantic this century because the cooling effects are significantly smaller than the warming influence of greenhouse gases.

An eye on the Arctic

So while we are not in danger of any ‘Day After Tomorrow’-style rapid plunge into glacial conditions, this study serves to highlight the importance of the Arctic in triggering global climate episodes. With this year’s record low sea ice extent in the Arctic and diminishing Greenland ice sheet, studies like this using highly complex and detailed models will undoubtedly help scientists refine projections of how stable our present day climate is to changes in the Arctic.

 


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