What was done
The authors review the current status of our knowledge about abrupt climate change and its possible relationship to changes in the thermohaline circulation (THC) of the world's oceans, which changes are believed to be driven by changes in the rate of deep water formation, primarily in the Nordic Seas and secondarily in the Southern Ocean along the Antarctic continental shelf in the Weddell and Ross Seas.  One of the compelling reasons for attempting to better understand these phenomena is that some climate models have predicted global warming could lead to an actual shutdown of the THC, with tremendous negative consequences for Europe - not in terms of dramatic warming, but in terms of the dramatic cooling that would follow quickly on the heels of the disappearance of the Gulf Stream.

What was learned
It is stated without equivocation, and rightly so, that "the palaeoclimate record provides the fundamental basis for evaluating the ability of models to correctly simulate behavior of the THC."  And what does the palaeoclimate record tell us?  First of all, it indicates that abrupt climate change has never been a characteristic of the Holocene, i.e., the interglacial in which we currently live.  Rather, these rapid warmings, which are typically followed by slower coolings, are phenomena of the past, and "were characteristic of the last glaciation."  Second, both real-world data and most climate models "suggest that abrupt climate change during the last glaciation originated through changes in the Atlantic thermohaline circulation in response to small changes in the hydrological cycle."  Nevertheless, the authors state that "our understanding of abrupt climate change remains incomplete," and that "modeling past abrupt climate change remains one of the greatest challenges for palaeoclimate modelers."

What it means
When all is said and done, the authors state that "the palaeoclimate data and the model results also [our italics - and isn't that interesting!] indicate that the stability of the thermohaline circulation depends on the mean climate state."  And since there are no indications of vast readjustments in the thermohaline circulation having ever occurred during this or any other interglacial, and since there are no indications of any abrupt hemispheric or global warming of the type considered in this review having ever occurred during this or any other interglacial, it seems clear to us there is close to zero likelihood we will ever see such phenomena, as long as the planet maintains its current warmth.