Megaflood 'made Island Britain'

This is a touchy subject that is currently the focus of much research. One explanation is the one involving a thermohaline circulation (THC) shutdown, triggered by a catastrophic discharge of freshwater from Lake Agassiz (figure 2). The consequence is a rapid reduction in northward ocean heat transports, leading to an abrupt cooling over Northern Europe and North America. That is why so much attention is focused on the behavior of the North Atlantic ocean circulation: not only by scientists, but also Hollywood screenplay writers. Bear in mind, however, that the greenhouse world we are creating through fossil fuel burning might not behave at all like the Earth of 12,800 years ago, so that this scenario may be irrelevant for future climate change.

Figure 2: (Left) The outline of Lake Agassiz just before the catastrophic flood. At that time its outlet was to the south into the Mississippi drainage. (Right) The outline after the opening of the eastward outlet. A volume of 9500 cubic kilometers of water was suddenly released to the northern Atlantic through the St. Lawrence Valley (Leverington et al. 2000). (Source: Broecker, 2003)

A problem with this hypothesis is the timing of meltwater pulses that are supposed to have triggered the THC shutdown: it was found that a second meltwater pulse, albeit slightly smaller than the first one, occurred at the end of the YD (Fairbanks, 1989): why didn't it also trigger a similar chain of consequences in the climate system?

An alternate explanation (Clement et al., 2001) invokes the abrupt cessation in the El Nino -Southern Oscillation in response to changes in the orbital parameters of the Earth, although how such a change would impact regions away from the Tropics remains to be explained.

The respective merits of both hypotheses have been laid out by Broecker (2003). The issue is far from being settled, and actively researched at Lamont and elsewhere.

There are many other examples of abrupt climate change in the last 50,000 years, which bear the rather cryptic names of Heinrich, and Dansgaard/Oeschger events.