In our first ice age riddle a couple of days ago we looked at the link between CO2 rise and ice retreat, after the last ice age had reached its max, some 18,000 years ago.

In that article we also mentioned about 12,000 years ago in Europe the cold suddenly returned for another 1300 years: the Younger Dryas. ‘Suddenly’ is to be taken literally here: temperatures went from mild to deep freeze in less than 3 decades, scientists think.

That episode [a ‘stadial’ – mini ace age] deserves a better look for another reason too – as it is the one case study paleoclimatologists think they have that shows an influx of fresh water from Greenland or another ice cap could seriously disrupt the thermohaline circulation – perhaps even stopping the Gulf Stream, thereby isolating the Arctic region, favouring ice-age-like conditions at high latitude northern hemisphere [and through a ‘bipolar seesaw’ milder conditions around Antarctica.]

Megalake full of melt water

The dramatic scenario that set the stage for the cold snap of the Younger Dryas does however seem hard to reproduce. That’s because North America had an odd geographical feature during the milder Allerød insterstadial that led up to the Younger Dryas: Lake Agassiz.

It was an immense fresh water lake that covered much of the Canadian-US border area around the present Great Lakes. It may have been as large as 440,000 square kilometres, much larger than any present lake – and probably it contained a lot of melting water from the dwindling American ice cap.

‘It suddenly drained, the Gulf Stream stopped, cold returned’

The theory goes it was somehow dammed, perhaps by ice, and that dam broke, suddenly releasing the water to the Atlantic, where it mixed with the salty waters of the Gulf Stream, which then failed to sink further north – and came to a stop.

On Monday at the annual conference of the Geological Society of America a new paper was presented on the subject.

Geology Professor Thomas Lowell of the University of Cincinnati has carried out sediment measurements around the fossil lake. From this he concludes the drop in water levels in Lake Agassiz did not coincide with the onset of the Younger Dryas freeze. He even offers additional explanations for why water levels lowered: the lake area increased – and there could have been a response [through precipitation decrease] to already plummeting temperatures, so not the other way around.

Wow. That’s one week – and two well-established ice age hypotheses down.

If it turns out Lowell is right, we no longer have a convenient explanation for why the Gulf Stream stopped [very likely that did still happen]. To us it’s uncomforting. If the thermohaline circulation is capable of shutting down overnight, we should at least now how and why. [In which case: yeah, this was good news.]

© Rolf Schuttenhelm | www.bitsofscience.org