We may be able to cross one potential disaster off the list (but do it in pencil).
The "whiplash" or "abrupt" climate change scenario was the scientific kernel which lay beneath the Hollywood melodrama of The Day After Tomorrow. The argument was that as warming temperatures cause glacial melting, the resulting fresh water dumped into the North Atlantic would "cut off" the warm water current that keeps Europe temperate; the resulting rapid cooling of Europe could then kick the already shaky climate into a counter-intuitive ice age. One key piece of evidence for this notion came from Greenland ice cores, which appeared to show an extremely rapid -- less than a century -- decline from the last warm "interglacial" period (like the one we're in at present) to the last big ice age, about 117 thousand years ago.
But the two ice core studies were found to have subtle problems in the data older than 105 thousand years, resulting from the ice folding near bedrock. A project to do a new ice core -- the North Greenland Ice Core Project, or NGRIP -- was undertaken in part to check this data. Drilling was completed in July of 2003, and the results were just published in Nature (PDF). The September 11-17 edition of New Scientist has a detailed article about NGRIP; unfortunately, that article is not (yet?) online.
With cleaner data stretching back 123 thousand years, the NGRIP team concluded:
This high resolution NGRIP record reveals a slow decline in temperatures from the warm Eemian isotopic values to cooler, intermediate values over 7,000 yr from 122 to 115 kyr BP. The end of the last interglacial thus does not appear to have started with an abrupt climate change, but with a long and gradual deterioration of climate. Before full glacial values are reached, however, the record does reveal an abrupt cooling [...] Thus it seems difficult to call on melting ice or other large freshwater input to the North Atlantic to trigger this event, although clearly we need more information from this and future ice cores to fully understand this first abrupt climate change of the last glacial.
In short, the transition from warm ("interglacial") climate to ice age was gradual, with an abrupt drop only towards the end of the period, not the beginning. The abrupt climate change scenario, with its rapid transition to abnormal cooling triggering mass migrations, famine and worse,
seems to be a non-starter has lost one of its most important pieces of supporting evidence. While this doesn't in any way lessen the threat from global warming-induced climate disruption, it's good to know that this scenario of environmental disaster is very likely to be found only in the movies.
Added later: Looking back on the whiplash climate change material, it's worth noting that the "Lesser Dryas" cooling event of 8-12 thousand years ago isn't altered by the change in ice core findings, and still appears to have happened in a fairly abrupt fashion. While "Lesser Dryas" was severe, it wasn't a drop into an ice age, which is what the 115 thousand year data previously suggested. So it's probably most fair to say that the "abrupt climate change taking us into an ice age" scenario is likely dead, while the "abrupt climate change accelerating climate disruption by adding severe regional cooling" scenario is still very much alive.
Just because that particular ice age didn't begin with abrupt climate change does not imply that no other ice age in the past begun with, nor that no ice age in the future could begin with, abrupt climate change. This post is simply more disengenuity.
In order to assert that such an abrupt switch from an interglacial to an ice age is possible, it helps to have evidence of it happening in reality. The 115kyr transition was the biggest and most dramatic example of an abrupt transition, as it was a shift to a full ice age (the so-called "Lesser Dryas" event of 8-12kyr ago was a cooling, but not an ice age). But the 115kyr event is now seen to have been a gradual transition, despite the peak temperatures of the interglacial period. This undermines the abrupt change argument.
The strong assertion that the abrupt change scenario was unlikely to happen came from my reading of the (not on the web) New Scientist article, not the Nature article.
I've added some final text making the conclusions more clear.