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North Atlantic Deep Water -- Summary
The words "North Atlantic deep water" are often followed by the word "formation," because the North Atlantic Ocean is one of two major places on earth (the other being the Southern Ocean surrounding Antarctica) where cold, saline water sinks to the bottom of the sea.  This phenomenon is believed by many people to provide the impetus for the planet's thermohaline circulation, which operates as a great oceanic "conveyor belt."  In addition to moving water and salt around the planet, this complex system of bottom and surface currents carries prodigious amounts of heat to various parts of the world, significantly influencing the regional climates of many areas and, hence, the global climate as well.  So what's the connection between this phenomenon and the CO2-climate debate?

One linkage is the evolving picture of millennial-scale climate oscillations being driven by the alternate weakening and intensification of this vast oceanic system of energy redistribution.  Keigwin and Boyle (2000) present a brief overview of this phenomenon, discussing the ever-growing body of evidence for a persistent climate oscillation with a return period on the order of 1500 to 2000 years, which is evident in proxy climate data from the Holocene, the last deglaciation and several glacial/interglacial cycles (See Climate Oscillations in our Subject Index).  They also discuss the association of these fluctuating climate changes with concurrent changes in the thermohaline circulation of the North Atlantic Ocean.  The Little Ice Age (LIA) is identified as the most recent cold phase of this recurring cycle, while the Medieval Warm Period (MWP) represents its preceding warm phase.  According to Keigwin and Boyle, "mounting evidence indicates that the LIA was a global event, and that its onset was synchronous within a few years in both Greenland and Antarctica."  Common sense thus suggests that our recent recovery from this global cool spell portends a future return to MWP-like conditions, and that mean global temperature should consequently be in a generally rising mode, both now and in the foreseeable future ... independent of any increase in atmospheric CO2 concentration.

An interesting variation on this theme is that there is also evidence for an anti-phasing of certain climatic anomalies in the Northern and Southern Hemispheres.  Ruhlemann et al. (1999) discuss two such occurrences, one from 16,900 to 15,400 years ago and one from 12,900 to 11,600 years ago.  During both of these Northern Hemispheric cold periods, there were significant reductions in North Atlantic deep water formation, which are believed to have been caused by injections of large volumes of freshwater into the northern North Atlantic.  These enhanced freshwater fluxes supposedly decreased sea-surface salinity there, reducing deep convective overturning of the local ocean waters that feed into the thermohaline circulation.  The resulting slowdown of the oceanic conveyer-belt system was apparently felt throughout the world in both instances; and when Gulf Stream-derived warmth failed to reach Europe, it shivered through the cold climatic events noted above.  With reduced northward heat flow, however, many areas in the south warmed.  Hence, since Ruhlemann et al. note that "atmospheric greenhouse forcing should cause roughly synchronous global temperature changes," their observations of "asynchronous coupling of Arctic and Antarctic temperatures as deduced from the Greenland and Antarctic ice-core records" argues against CO2 having anything to do with the occurrence of these millennial-scale climatic excursions or the transitions between them, such as we may currently be experiencing.

Broecker et al. (1999) looked for more recent signs of this anti-phasing phenomenon in the northern and southern portions of the global thermohaline circulation.  Analyzing a number of different water-movement tracers, they concluded that the North Atlantic and Southern Ocean deep-water sources have each been supplying about 15 sverdrups (15 x 106 m3/sec) of new deep water to the oceanic conveyer belt system for most of the past 800 years.  Over the last several decades, however, they believe the contribution of the Southern Ocean may have decreased to only a third of this rate.  Consequently, since the Little Ice Age appears to be associated with reduced deep water formation in the North Atlantic, the slowdown of the Southern Ocean's contribution to the thermohaline circulation over the last few decades hints at the possible linkage of this phenomenon with the warming of the past century that has rescued us from the global chill of the Little Ice Age, once again pushing CO2 out of the picture.

This scenario is disturbing to climate alarmists, who can't stand to see CO2 relegated to something less than a bit player in the climate change drama.  Hence, as noted in our Editorial of 15 July 1999, they are trying desperately to reformulate the oceanic conveyer belt-climate change hypothesis for their own purposes.  Specifically, they propose that increased atmospheric CO2 concentrations will warm the earth, enhance the planet's hydrologic cycle, and thereby increase continental freshwater runoff into the North Atlantic, which would lessen the density of surface seawater there and largely inhibit it from sinking, thereby destroying much of the driving force for the planetary thermohaline circulation and denying Europe the warmth that comes from the Gulf Stream, somewhat akin to the scenario proposed by Barber et al. (1999) to explain the cold event of 8200 years ago, which is believed to be due to a catastrophic release of more than 1014 m3 of freshwater into the Labrador Sea via the final outburst drainage of glacial lakes Agassiz and Ojibway.  Citing evidence for the great rapidity of certain past climatic changes described by Taylor (1999), the climate alarmists also suggest that an analogous dramatic cooling event could occur in but a decade or so, making it even more horrible.

In addition to the troublesome inconsistency of having to worry simultaneously about catastrophic global warming and devastating continental cooling, a number of peer-reviewed scientific studies raise several other serious questions about this "two-headed monster" scenario.  Latif et al. (2000), for example, used a state-of-the-art global climate model to investigate the sensitivity of the thermohaline circulation to greenhouse warming, finding not a reduced flow but a stabilized flow, due to large-scale air-sea interactions that produce anomalously high salinities in the tropical Atlantic that are subsequently advected into the region of the North Atlantic where deep water formation occurs.

Another serious blow to the climate alarmists' contentions is provided by the study of Rind et al. (2001), who used different versions of the Goddard Institute for Space Studies coupled atmosphere-ocean climate model to perform multiple experiments with sustained and gradual St. Lawrence freshwater inputs to the North Atlantic.  Noting that the freshwater inputs they employed were "similar to that associated with freshening due to the warming climate of the next century," the scientists found that "North Atlantic deep water production decreases linearly with the volume of freshwater added through the St. Lawrance" and that it does so "without any obvious threshold effects," especially those that might induce extremely rapid climate change.  "The effect is not rapid with realistic freshwater inputs," the authors say in reiterating this fact, noting that others such as Schiller et al. (1997) have come to essentially the same conclusion.

Further support for this view is provided by the studies of Haug and Tiedemann (1998) and Driscoll and Haug (1998), who analyzed a number of proxy climate indicators and potential feedback mechanisms in an attempt to understand what triggered the start of the approximate 100,000-year glacial/interglacial cycles that began in the Northern Hemisphere nearly three million years ago.  They concluded that closure of the Central American Seaway by the rise of the Isthmus of Panama enhanced the Gulf Stream's transport of warm surface waters to high northern latitudes, where subsequent increased evaporation provided the moisture needed for ice sheet growth over Europe and Asia while at the same time increasing freshwater delivery to the Arctic via river discharge.  The weakening of the thermohaline circulation caused by this latter phenomenon, however, tends to reduce heat transport to higher latitudes, and would thus have initiated a sort of dynamic tension between the Gulf Stream's ability to both warm and cool the North Atlantic region, perhaps empowering long-term cyclical changes in the orbital parameters of the earth to dictate the subsequent 100,000-year glacial/interglacial cycles of the Northern Hemisphere.

These findings suggest to us that any intensification of the planet's hydrologic cycle that might occur in response to a CO2-induced warming of the globe would also likely enhance the delivery of freshwater continental runoff to the North Atlantic Ocean via fluvial system discharge.  Furthermore, since the work of Driscoll and Haug implies that "the ocean's thermohaline circulation is rather sensitive to even small inputs of fresh water," this phenomenon likely would measurably reduce the rate of formation of North Atlantic deep water and thereby reduce the strength of the thermohaline circulation, creating an impetus for cooling that would tend to counteract the initial impetus for warming ... and in a basically linear fashion, as suggested by the modeling work of Rind et al.  With this dynamic tension at work, therefore, it is possible that the planet's finely-honed climatic balance could be readily tilted in one direction or the other by subtle variations in some external parameter, such as variations in solar activity, but with little probability of a runaway response in either the direction of warming or cooling.

In support of this hypothesis, we cite the impressive work of Bond et al. (2001), who have convincingly demonstrated that "over the last 12,000 years virtually every centennial time-scale increase in drift ice documented in our North Atlantic records was tied to a solar minimum," including "the so-called 'Little Ice Age' and 'Medieval Warm Period'."  They also describe how solar signals "may have been transmitted through the deep ocean as well as through the atmosphere, further contributing to their amplification and global imprint."  Indeed, the scientists unequivocally declare that their results "demonstrate that the earth's climate system is highly sensitive to extremely weak perturbations in the sun's energy output," and that their work "supports the presumption that solar variability will continue to influence climate in the future."

It would appear, therefore, that with the current configuration of the planet's land and water, the sun not only rules in the heavens above, climatically speaking, but on the earth below.  What is more, the moon also plays an important role, as Munk and Wunsch (1998) postulated a few years back, and as Egbert and Ray (2000) have recently demonstrated, which makes the whole situation, in the words of Wunsch (2000), "much more subtle and interesting."

So just how interesting - and complicated! - can it get?  The work of Marchitto et al. (1998) provides one perspective by indicating that "periods of enhanced intermediate-water production alternate with periods of enhanced deep-water formation on both orbital and millennial timescales" and that "analogous dynamics operate in the modern North Atlantic on much shorter (decadal) timescales," just as Bacon (1998) has also reported.  Hence, there are decadal cycles of both climate and thermohaline circulation that are contained within similar centennial cycles that are imbedded within millennial-scale cycles that are nested within 100,000-year cycles; and none of these cyclical climatic and circulation dynamics are significantly perturbed by variations in the air's CO2 content.

Still, the political Juggernaut rolls on, as the nations of the earth are frightened into believing the planet is running an unnatural fever, due to exposure to elevated levels of atmospheric CO2, while they are simultaneously being tricked into fighting the imaginary malady by a high-powered campaign designed to convince them that the only way they can cure this climatic illness is by renouncing their high-CO2 diet of fossil fuels.  Based on the true status of our scientific knowledge, however, we would have to say that the writing of this prescription is little more than the practice of voodoo medicine, as it has no basis in fact.  Nevertheless, "take two Kyotos and call me in the morning" will likely soon be the remedy of the day.

References
Bacon, S.  1998.  Decadal variability in the outflow from the Nordic seas to the deep Atlantic Ocean.  Nature 394: 871-874.

Barber, D.C., Dyke, A., Hillaire-Marcel, C., Jennings, A.E., Andrews, J.T., Kerwin, M.W., Bilodeau, G., McNeely, R., Southon, J., Morehead, M.D. and Gagnon, J.-M.  1999.  Forcing of the cold event of 8,200 years ago by catastrophic drainage of Laurentide lakes.  Nature 400: 344-348.

Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I. and Bonani, G.  2001.  Persistent solar influence on North Atlantic climate during the Holocene.  Science 294: 2130-2136.

Broecker, W.S., Sutherland, S. and Peng, T.-H.  1999.  A possible 20th-century slowdown of Southern Ocean deep water formation.  Science 286: 1132-1135.

Driscoll, N.W. and Haug, G.H.  1998.  A short circuit in thermohaline circulation: A cause for Northern Hemisphere glaciation?  Science 282: 436-438.

Egbert, G.D. and Ray, R.D.  2000.  Significant dissipation of tidal energy in the deep ocean inferred from satellite altimeter data.  Nature 405: 775-778.

Haug, G.H. and Tiedemann, R.  1998.  Effect of the formation of the Isthmus of Panama on Atlantic Ocean thermohaline circulation.  Nature 393: 673-676.

Keigwin, L.D. and Boyle, E.A.  2000.  Detecting Holocene changes in thermohaline circulation.  Proceedings of the National Academy of Sciences USA 97: 1343-1346.

Latif, M., Roeckner, E., Mikolajewicz, U. and Voss, R.  2000.  Tropical stabilization of the thermohaline circulation in a greenhouse warming simulation.  Journal of Climate 13: 1809-1813.

Marchitto Jr., T.M., Curry, W.B. and Oppo, D.W.  1998.  Millennial-scale changes in North Atlantic circulation since the last glaciation.  Nature 393: 557-561.

Munk, W.H. and Wunsch, C.  1998.  Abyssal recipes II: Energetics of tidal and wind mixing.  Deep-Sea Research 45: 1977-2010.

Rind, D., deMenocal, P., Russell, G., Sheth, S., Collins, D., Schmidt, G. and Teller, J.  2001.  Effects of glacial meltwater in the GISS coupled atmosphere-ocean model.  1. North Atlantic Deep Water response.  Journal of Geophysical Research 106: 27,335-27,353.

Ruhlemann, C., Mulitza, S., Muller, P.J., Wefer, G. and Zahn, R.  1999.  Warming of the tropical Atlantic Ocean and slowdown of thermohaline circulation during the last deglaciation.  Nature 402: 511-514.

Schiller, A., Mikolajewicz, U. and Voss, R.  1997.  The stability of the thermohaline circulation in a coupled ocean-atmosphere general circulation model.  Climate Dynamics 13: 325-348.

Taylor, K.  1999.  Rapid climate change.  American Scientist 67: 320-327.

Wunsch, C.  2000.  Moon, tides and climate.  Nature 405: 743-744.