Ocean Conveyor Belt Dismissed
After nearly 50 years of acceptance, the theory that a great ocean “conveyor belt” continuously circulates water around the globe in an orderly fashion has been dismissed by a leading oceanographer. According to a review article in the journal Science, a number of studies conducted over the past few years have challenged this paradigm. Oceanographers have discovered the vital role of ocean eddy currents and the wind in establishing the structure and variability of the ocean’s overturning. In light of these new discoveries, the demise of the conveyor belt model has been become the new majority opinion among the world's oceanographers. According to M. Susan Lozier, of Duke University, “the conveyor-belt model no longer serves the community well.”
The idea that the ocean conveyor belt transports cold, dense water from the subpolar North Atlantic along the “lower limb” of the conveyor belt to the rest of the global ocean, where the waters are upwelled and then transported along the “upper limb” back to deepwater formation sites, has been supported by the majority of oceanographers for decades. This circulating flow was assumed to operate along western boundary currents in the deep ocean and provide a continuous supply of relatively warm surface waters to deepwater formation sites. While it was thought to be vulnerable to changes in deepwater production at high latitudes, with significant injections of fresh water capable of disrupting the smooth operation of the system, under normal conditions the conveyor belt was thought to function constantly and consistently. Now it seems that opinions within the oceanographic community have shifted, and the great ocean conveyor belt model has fallen from grace.
As detailed in an eye opening article by Dr. Lozier, the conveyor belt has been found wanting and dismissed as the dominant ocean overturning paradigm. Lozier is Professor of Physical Oceanography and Chair of the Earth and Ocean Sciences Division at Duke, and is an expert in large-scale ocean circulation, water mass distribution and variability. The article, “Deconstructing the Conveyor Belt,” begins with a short history of the conveyor belt theory's development. According to Lozier, our modern idea of the ocean’s overturning, and our understanding of its importance to Earth's climate, developed as a result of the work of two prominent oceanographers:
Fifty years ago, Henry Stommel theorized that recently ventilated waters of high-latitude origin must be transported equatorward at depth along western-intensified boundary currents. Assuming that water masses formed via deep convection in isolated regions in the northern North Atlantic and near Antarctica essentially fill the abyssal ocean, Stommel surmised that the deep ocean exports these waters via a distributed upwelling to the surface. Furthermore, he suggested that because such upwelling produces a stretching of the water column that induces a loss of angular momentum, the deep interior waters must compensate by flowing poleward toward regions of higher angular momentum. Thus, the equatorward transport of deep water masses was confined to the western boundaries of the basins. Stommel’s theory gave the ocean’s overturning, previously amorphous in its third dimension, a structure: Deep waters are transported equatorward in a steady, continuous deep western-intensified boundary current from their formation sites at high latitudes.
The abyssal flow field, as theorized by Stommel in 1958.
The second important oceanographer was the eminent Wallace S. “Wally” Broecker, Newberry Professor in the Department of Earth and Environmental Sciences at Columbia University and a scientist at Columbia's Lamont-Doherty Earth Observatory. Arguably one of the world’s greatest living geoscientists, for more than half a century, Broecker has investigated the ocean’s role in climate change. He was among the pioneers in using radiocarbon and isotope dating to track historical climate change, and the influence of climate change on polar ice and ocean sediments. It was Broecker who coined the term “ocean conveyor belt.”
According to Lozier, work by Broecker and colleagues suggested that the ocean’s overturning was responsible for the rapid climate fluctuations experienced during Earth’s last glacial period. “Though the importance of the ocean’s overturning to Earth’s climate had previously been understood, Broecker’s work essentially cemented the role of the conveyor belt as an agent of climate change,” states her review. “Thus, just as Stommel’s work gave spatial structure to the overturning, Broecker’s provided a temporal context.” So what has changed oceanography's mindset enough to proclaim the conveyor belt—arguably the most important discovery in the history of oceanography—an idea whose time has past?
Since its proposal, oceanographers have understood that the conveyor model is an oversimplification of the way ocean overturning actually takes place. But it was believed to be a useful simplification, capable of providing an overall model of the ocean's transportation of heat energy, if not the exact details. But now it seems that some major features of the conveyor belt have been called into question. Here is a list of recent discoveries that have shaken the foundation of the conveyor belt theory.
- Most of the subpolar-to-subtropical exchange in the North Atlantic occurs along interior pathways.
- The deep deep western boundary current (DWBC) breaks up into eddies at 11°S.
- There is little meridional coherence in the overturning transport from one gyre to the next .
- Wind forcing, rather than buoyancy forcing, can play a dominant role in changing the transport of the overturning.
- The southward transport of deep waters at 8°S, off the Brazilian coast, was shown to be carried entirely by migrating coherent eddies.
- Floats launched within the DWBC at 53°N do not follow a continuous boundary current, but instead take multiple paths to the subtropics, including interior pathways far removed from the DWBC.
- Two recent studies have found unexpected pathways in the upper ocean.
- A recent study shows that MOC transport in the subtropical North Atlantic is susceptible to variability in the "leakage" of warm and salty water into the South Atlantic.
- Studies showing little to no coherence across gyre boundaries have prompted interest in monitoring the overturning circulation in the South Atlantic and the subpolar North Atlantic.
- The connectivity of the overturning and, more importantly, of the meridional heat transport from one basin to the next can no longer be assumed on interannual time scales.
When all of these observations are combined, they indicate that the conventional conceptual model of ocean overturning needs revamping. As Dr. Lozier put it: “In sum, the impact of eddies on our concept of a continuous lower limb for the ocean’s overturning has evolved from an understanding that eddies can detrain and entrain fluid along the DWBC to the recognition that the DWBC can, at certain locales and perhaps certain times, be a series of migrating eddies, to the realization that eddy-driven flow provides an alternate pathway for deep waters to spread globally.”
In other words, it doesn't work as simply as we thought. Lozier is in a good position to make such a judgment, since it is partly due to her work that scientists are revisiting the conveyor belt model. As noted on this blog in “Conveyor Belt Model Broken,” work by Lozier and Amy Bower of Wood’s Hole, using RAFOS float data, showed that there was something fundamentally wrong with how the ocean's overturning flow was being modeled.
By analyzing the divagating float paths, it was discovered that ocean currents did not behave as expected. Reported back in May of 2009, their discovery had the potential to affect both short term and long term climate change. This is because ocean currents not only redistribute surface warmth, the oceans themselves are a vast reservoir for heat and carbon dioxide. I concluded that this finding invalidated the IPCC's GCM climate model predictions, because the models were based on incorrect behavior of the ocean overturning currents.
At the time, Dr. Lozier took exception to my supposition, stating in an email, “the climate models care first and foremost about the return of the surface waters and our research has no bearing in the slightest on those waters.” I disagreed, saying that the discovery of significant eddies changed the assumptions on how the deep sea currents flow, which must change the boundary conditions between different masses of water. This cannot help but alter the long term reaction of the ocean to the energy flowing through it.
More recently, variations in continuous data measurements from cable-moored instrument arrays identified large and unexpected yearly fluctuations in conveyor flow. As additional discoveries have unfolded, it was also found that there are large reservoirs of CO2 stashed away in the deep ocean, again previously unexpected. As the evidence has piled up, Dr. Lozier has been forced to admit that there are implications for climate change and the way the Earth system is modeled. In her own words:
Added impetus for revamping comes from a recent study revealing a considerable reservoir of anthropogenic CO2 in the deep North Atlantic, surmised to result from the production of high-latitude water masses and their subsequent equatorward spread. Clearly, an improved understanding of the pathways of the upper and lower limbs of the ocean’s overturning will aid assessments of the ocean’s role in the uptake, transport, and storage of heat and CO2, crucial components of Earth’s climate system.
This reinforces the claim that previous climate models—which are highly dependent on the coupling between ocean and atmosphere and, hence, the ocean circulation models they contain—cannot be considered accurate reconstructions of Earth's climate system. I repeat my earlier assertion: if the conveyor belt model is wrong then none of the IPCC's model results can be taken seriously. This point is underscored by recent work that found small changes in high latitude insolation, driven by Earth's orbital cycles, can trigger significant changes in lower latitude ocean and atmospheric circulation. The circulation of Earth's oceans is now known to be much more complex and nuanced than even a decade ago, which has significant implications for climate modeling.
This spate of recent discoveries serves to underline a fundamental tenet of science—that no theory, no matter how elegant or widely believed, is sacrosanct. As the great philosopher of science, Karl Popper, stated, science progresses by moving from one false theory to another, still false theory that is nonetheless closer to the truth. There is nothing wrong with dismissing the conveyor belt model for another, more correct model. In fact, a scientist incapable of realizing that a cherished, comfortable old theory is false and must be discarded is not capable of doing good science at all. Keeping that in mind, here is Lozier's summary of the case against the conveyor belt:
Though appealing in its simplicity, the ocean conveyor-belt paradigm has lost luster over the years, precisely because it has overdistilled the complexity of the ocean’s overturning. This complexity has slowly been revealed as the ocean has increasingly been observed at finer scales in space and time and in places previously only sparsely sampled. As discussed, the ocean’s eddy field, unaccounted for just decades ago and now uncovered by measures at appropriate scales, figures prominently in the dismantling of the conveyor-belt paradigm. Another player in this dismantling is the ocean’s wind field. The traditional assignation of surface ocean gyres to wind-forcing and overturning to buoyancy forcing has ignored the vital impact of winds on overturning pathways and mechanics. As the study of the modern ocean’s role in climate continues apace, the conveyor-belt model no longer serves the community well— not because it is a gross oversimplification but because it ignores crucial structure and mechanics of the ocean’s intricate global overturning.
So, after very logically and methodically making the case for dismissing the conveyor belt model, Dr. Lozier claims we should not discard it because it is a gross oversimplification, but because it “no longer serves the community well.” I would call any theory that ignores the intricate mechanics and crucial structures of the thing is is attempting to describe as worse than a gross oversimplification—I would call it wrong. It seems that Dr. Lozier just cannot bring herself to say the words, “this theory is false.”
Regardless, oceanography is in the process of moving on to new, hopefully less false theories. Some period of grief and denial is probably to be expected from those who literally grew up with the conveyor belt theory. Now, if climate science would only face up to the falseness of the gross oversimplification they have promoted over the past few decades—anthropogenic global warming.
The failure of anthropogenic global warming is not only tied to the recent discoveries in oceanography, but to scores of other scientific advances in biology, geology, atmospheric physics and Earth sciences. The fiction that human generated CO2 is responsible for climate change, and that our continued emission of that greenhouse gas will damage this planet's ecosystem is as gross an oversimplification as has ever been postulated.
While simplicity is generally considered a good thing, and it is no sin for a scientist to invent a theory which proves to be false, it is a great sin to refuse to recognize a theory's falseness. Only by dismissing old false theories can science move on to new, more correct ones. Climate science has damaged its credibility, and the reputation of all science, by clinging to this outdated, failed theory. It is time for the climate science community to prove that they are real scientists by openly pronouncing anthropogenic global warming a false theory.
Be safe, enjoy the interglacial and stay skeptical.