Could we be heading for a repeat of the Younger Dryas cold snap, which was last seen 12,000 years ago? And are quantum computers finally becoming a reality?by Philip Ball / January 22, 2006 / Leave a comment
A colder kind of warming
If climate change were easy to understand, the debate would have been over years ago. But because climate effects don’t often follow in direct proportion to their causes, it is inevitable that model predictions are hedged by uncertainties, and that the consequences of the greenhouse effect often seem contradictory and counterintuitive. What now are we to make of the suggestion that, as mean temperatures continue to climb, northern Europe has colder winters in store?
It seems clear such a thing can happen. As the last ice age thawed 12,000 years ago, the north Atlantic was plunged abruptly into, and then out of, a cold snap called the Younger Dryas that lasted over a thousand years. Those switches took just a few decades.
Modern global warming could trigger the same thing, and for the same reason: shutting down of north Atlantic ocean circulation. The tropics are hotter than high latitudes because they receive more sunlight, but this difference would be even greater were it not for the wind-driven currents of warm ocean water that carry heat towards the poles. In the north Atlantic this current is called the gulf stream, crossing the ocean from southwest to northeast.
Water brought from the south has to get back down there if the ocean is not to become lopsided. As the poleward current cools, the water becomes denser and sinks, producing a return flow of deep, cold water heading towards the equator along the western edge of the Atlantic. But according to the latest analysis by researchers in Southampton of ocean water along a cross-section of the sub-tropical north Atlantic, this conveyor-like circulation is slowing. Between 1957 and 1992 there was scarcely any sign of change, but the measurements in 2004 show a 30 per cent decrease in circulation relative to 50 years ago.
The cause of the change is probably a “freshening” of surface ocean water. There is more fresh water being injected into the north Atlantic from rain, rivers and melting ice, probably owing in turn to global warming. Fresh water is less dense than salty water, and so it sinks less readily at the northern extremity of the conveyor. The same thing is thought to have caused the Younger Dryas event: as the ice sheets melted, fresh water shut down the north Atlantic circulation and cut off the heat supply from the tropics.