What was done
The authors analyzed a number of studies of different oceanic water-movement tracers, including (1) the sum of phosphate and oxygen, (2) radioactive carbon-14, (3) heat content, (4) salt content, and (5) chlorofluorocarbon-11, all of which tell something about the different origins of the mix of water found at various places in the global ocean abyss.

What was learned
The two major locations of oceanic deep water formation, which drives the global thermohaline circulation (which in turn exerts a tremendous influence on global climatic conditions), are the North Atlantic and the Southern Ocean surrounding Antarctica.  Broecker et al. conclude that each of these source regions has been supplying about 15 sverdrups (15 x 10⁶ m³/sec) of new deep water to this oceanic "conveyor belt" system for most of the past 800 years.  Over the last several decades, however, they believe that the contribution of the Southern Ocean has decreased to only about a third of this longer-term 800-year rate.

What it means
Because there is evidence for an antiphasing of Northern and Southern Hemispheric climate change on the order of several hundred years' duration, which seems to be driven by a similar time-scale antiphasing of Northern and Southern Hemispheric deep water formation, Broecker et al. speculate that their analysis may have implications for the occurrence of such climatic phenomena as the Medieval Warm Period, the subsequent Little Ice Age, and the climatic transition of the past century or so, which seems to be returning much of the planet to another warm interval.  Specifically, they suggest that the Little Ice Age was a consequence of more intense deep water formation in the Southern Ocean and reduced deep water formation in the North Atlantic.  And it is not much of a stretch to believe that if that is true, the slowdown in the rate of Southern Ocean deep water formation over the past several decades may well be causally related to the warming of the past hundred years.

These ideas are incredibly important, for they suggest that something other than the historic buildup of carbon dioxide in the atmosphere may be responsible for significant climate changes that recur on multi-century time scales.  Specifically, they suggest that the current warming may be totally unrelated to the concurrent increase in the air's CO2 content; for the atmospheric burden of carbon dioxide varied but little over the entire climatic cycle that brought us into the Medieval Warm Period, out of the Medieval Warm Period, and into the Little Ice Age, whereas there is much evidence that the global thermohaline circulation may have been intimately involved with all three of these climatic transitions.

To more fully determine what role, if any, the ongoing rise in the air's CO2 content may be having on earth's climate at the present time, we must, in the words of Broecker et al., "gain a better understanding of the Little ice Age and its demise."  One of the important implications of this suggestion is that the current wisdom about CO2 and global warming may be way off base.  Clearly, our understanding of several of the key concepts related to this question is still rudimentary.  We can only hope that the work of Broecker and his colleagues will stimulate us to remedy the situation before we launch an assault on global warming with a battle plan designed to subdue an innocent (indeed, even helpful) bystander, i.e., CO2.