Flying above the clouds often presents an interesting picture when there are mountains below.  Out of the sea of undulating white clouds, mountain peaks stick up like islands.  Greenland looks like that, even on a cloudless day – but the great white mass between the occasional punctuations is not a fluffy cloud layer but a massive ice sheet, miles deep.  In places this frozen fresh water descends from the highlands in a wavy staircase, looking far more massive and magisterial than any alpine glacier.

     Twenty thousand years ago a similar ice sheet lay atop the Baltic Sea and the land surrounding it.  Another sat on Hudson Bay, and reached as far west as the foothills of the Rocky Mountains – where it pushed, head to head, against ice coming down from the Rockies.  These northern ice sheets were almost as high as Greenland’s mountains, obstacles sufficient to force even the jet stream to make a detour.

     The ice ages provide a valuable perspective on how climates change, a view of the times in which our brief warm interlude is situated.  Figuring out the mechanisms for ocean circulation modes depends on the evidence from paleoclimate, and on the stabilities seen via the great computer simulations.  Most of the whiplash climate changes of the past were during icy periods that had ice sheets in Canada and Scandinavia.

     Now only Greenland’s ice remains, but the abrupt cooling in the last warm period shows that a flip can occur in situations much like the present one.  What could possibly halt the salt-conveyor belt that brings tropical heat so much farther north and limits the formation of ice sheets?  Oceanographers are busy studying present-day failures of annual flushing, which give some perspective on the catastrophic failures of the past.

What could possibly stop the salty conveyor that brings tropical heat so much further north, and limits the formation of these ice sheets?  It’s not an idle question, and flushing failures are the obvious candidate.  But, just as in medicine where we have to ask what the natural history of a disease is (natural ups and downs can fool you into thinking that a treatment is working when it’s just happenstance), we need to know what the natural history of flushing failure is.

     In the Labrador Sea, flushing failed during the 1970s, was strong again by 1990, and then declined.  In the Greenland Sea over the 1980s salt sinking declined by 80 percent.  Obviously, local failures can occur without catastrophe – it’s a question of how often and how widespread the failures are, and whether they occur simultaneously in both the Greenland Sea and in the Labrador Sea.  Large-scale flushing at both those sites is certainly a highly variable process, and perhaps a somewhat fragile one as well.  And in the absence of a flushing mechanism to sink cooled surface waters and send them southward in the Atlantic, additional warm waters do not flow as far north to replenish the supply.

     There are a few obvious precursors to flushing failure.  One is diminished wind chill, when winds aren’t as strong as usual, or as cold, or as dry – as is the case in the Labrador Sea during the negative phase of the North Atlantic Oscillation.  This decade-scale shift in the atmospheric-circulation pattern over the North Atlantic, from the Azores to Greenland, strongly affects wintertime downwind.  At the same time that the Labrador Sea gets a lessening of the strong winds that aid salt sinking, Europe gets particularly cold winters and diminished rainfall.

     Another precursor is more floating ice than usual, which reduces the amount of ocean surface exposed to the winds, in turn reducing evaporation.  The sea ice in the major flushing sites is only partly indigenous – much comes sailing down from farther north to replace what melts.  Fresher water freezes more easily.  Thus an ice lid is one possible candidate for what occasionally shuts down the highly efficient north-of-sixty-degrees portion of the Gulf Stream circulation.  Arctic cold could thus cause a much more general worldwide cooling, abruptly.

     Yet another precursor, as Henry Stommel suggested in 1961, would be the addition of fresh water to the ocean surface, diluting the salt-heavy surface waters before they became unstable enough to start sinking.  Two mechanisms can be seen when piecing together the past:  icebergs melting in the Heinrich events, and the freshwater floods seen just before the Younger Dryas and the 8,200 year event.  But there is a third way to get enough fresh water layered on the ocean surface:  more rain falling into the North Atlantic.

     Yes, I know that it sounds like carrying coals to Newcastle.  But, at least in certain regions such as the Greenland and Labrador Seas, rain falling into the ocean is a serious problem.  Lots more rain falling is now what is predicted to happen from global warming.  Even that which falls on land at higher latitudes tends to be carried toward the sea on north-flowing rivers, and so into the already somewhat less salty Arctic Ocean.  It can stop salt flushing.  Whereas the Norwegian coast stays ice-free now, an addition of fresh water would allow it to freeze more easily and so cap the oceans, preventing the wind-driven evaporation that makes the sea water dense enough to sink.  

     There is also a great deal of unsalted water in Greenland’s glaciers, just uphill from the major salt sinks. The last time an abrupt cooling occurred was in the midst of global warming.  Many ice sheets had already half melted, dumping a lot of fresh water into the ocean.  The Greenland Sea got a lot of re-freshing from the east side of Greenland, making the out-of-the-Arctic Greenland Current even less salty.  The  Labrador Sea not only got the meltwater from Greenland’s west coast but also from the Hudson’s Strait flow that came off of the great mountain of ice which accumulated atop Hudson Bay and Labrador.

     In the models of greenhouse warming, the gradual dilution of northern oceans by rain eventually slows the flushing until it is about 25 percent below normal.  Further dilution and flushing fails.  And that doesn’t even consider the melting of the ice sheets (or additional perturbations like El Niño or the North Atlantic Oscillation).  Meltwaters could steadily dilute the ocean surface, but years’ worth of melting can also be dumped into the ocean in just one day.

     There are already some indications that the Greenland Sea route for Gulf Stream warming is in trouble.  A fifty-year trend of decline in the deep cold water in one branch of the conveyor’s return loop has just been discovered, suggesting (if not compensated elsewhere) a decline in the warm water surface delivery of more than 20 percent.

The stories scientists tell are not simply bedtime tales.  They place us in the world, and they can force us to alter the way we think and what we do.

- Thomas Levenson, 1989

Persons living in this modern world who do not know the basic facts that determine their very existence, functioning, and surroundings are living in a dream world.  Such persons are, in a very real sense, not sane.  We [scientists] . . . should do what we can, or we shall be pushed out of the common culture.  The lab remains our workplace, but it must not become our hiding place.

- Gerald Holton, 1996

Greenland's Maze of  Fjords