Scientists studying past climate epochs have found times when the oceans' circulation lost its vital character, perhaps even shut down. Analyses of ice cores, deep-sea sediment cores, and other geologic evidence have clearly demonstrated that the Conveyor has abruptly slowed or halted many times in Earth's past. That has caused the North Atlantic region to cool significantly and brought long-term drought conditions to other areas of the Northern Hemisphere—over time spans as short as years to decades. According to climate scientist Thomas F. Stocker,
Evidence from paleoclimatic archives suggests that the ocean atmosphere system has undergone dramatic and abrupt changes with widespread consequences in the past. Climatic changes are most pronounced in the North Atlantic region where annual mean temperatures can change by 10° C and more within a few decades. Climate models are capable of simulating some features of abrupt climate change. These same models also indicate that changes of this type may be triggered by global warming. (Stocker, Knutti, and Plattner 2001, 277)
At the end of the last ice age (between 8,200 and 12,800 years ago), ice-core records from Greenland indicate abrupt temperature declines at about the same time that massive amounts of cold freshwater were released from a huge body of glacial meltwater, which glaciologists today call Lake Agassiz, that reached from today's Canadian prairies eastward to Quebec and southward to Minnesota. Reaching the Atlantic Ocean through the St. Lawrence Valley and Hudson Bay, “such a large amount of low-density fresh water would have reduced the density of the North Atlantic surface water considerably, preventing it from sinking and thus slowing down (or perhaps even shutting off completely) the Gulf Stream,” wrote glaciologist Doug Macdougall in Frozen Earth: The Once and Future Story of Ice Ages (2004, 110).
Evidence from the Irish Sea Basin as recently as 19,000 years ago indicates a large reduction in the strength of North Atlantic deep-water formation that provoked cooling of the North Atlantic at the same time that global sea levels were rising rapidly, a response to melting ice. According to Clark and colleagues, “These responses identify mechanisms responsible for the propagation of deglacial climate signals to the Southern Hemisphere and tropics while maintaining a cold climate in the Northern Hemisphere” (Clark et al. 2004, 1141).
Jochen Erbacher studied an anoxic event “in the restricted basins of the western Tethys and North Atlantic” during the mid-Cretaceous period, about 112 million years ago that appears to have been caused by “increased Thermohaline stratification” (Erbacher et al. 2001, 325). The Western Tethys was a sea formed as continents were separating on the site of the present-day North Atlantic Ocean. “Ocean anoxic events were periods of high carbon burial that led to drawdown of atmospheric carbon dioxide, lowering of bottom-water oxygen concentrations and, in many cases, significant biological extinctions,” commented Erbacher and colleagues (Erbacher et al. 2001, 325). “We suggest that that the partial tectonic isolation of the various basins in the Tethys and Atlantic, a low sea level, and the initiation of warm global climates may be important factors in setting up oceanic stagnation” during this event (Erbacher et al. 2001, 327).
According to research by Helga Kleiven and colleagues in Science,
An outstanding climate anomaly 8200 years before the present (B.P.) in the North Atlantic is commonly postulated to be the result of weakened overturning circulation triggered by a freshwater outburst. New stable isotopic and sedimentological records from a northwest Atlantic sediment core reveal that the most prominent Holocene anomaly in bottom-water chemistry and flow speed in the deep limb of the Atlantic overturning circulation begins at ∼8.38 thousand years B.P., coeval with the catastrophic drainage of Lake Agassiz. The influence of Lower North Atlantic Deep Water was strongly reduced at our site for ∼100 years after the outburst, confirming the ocean's sensitivity to freshwater forcing. The similarities between the timing and duration of the pronounced deep circulation changes and regional climate anomalies support a causal link. (Kleiven et al. 2008, 60)
Ocean circulation is not unlike surface weather. It varies widely over short as well as long periods. Ocean circulation experiences a kind of “weather,” as well as a longer-term “climate.” Scientists have studied past epochs, and found that the deep Atlantic Ocean during the height of the last ice age appears to have been quite different from today. One group reviewed observations that implied the
Atlantic meridional overturning circulation during the Last Glacial Maximum was neither extremely sluggish nor an enhanced version of present-day circulation. The distribution of the decay products of uranium in sediments is consistent with a residence time for deep waters in the Atlantic only slightly greater than today. However, evidence from multiple water-mass tracers supports a different distribution of deep-water properties, including density, which is dynamically linked to circulation. (Lynch-Stieglitz et al. 2007, 66)
North Atlantic Deep Water was warmer during the last interglacial than it is today and probably warmed Antarctic waters, accelerating ice loss and raising sea levels. J. C. Duplessy and colleagues wrote that
Oxygen isotope analysis of benthic foraminifera in deep sea cores from the Atlantic and Southern Oceans shows that during the last interglacial period, North Atlantic Deep Water (NADW) was 0.4° ± 0.2 C warmer than today, whereas Antarctic Bottom Water temperatures were unchanged. Model simulations show that this distribution of deep water temperatures can be explained as a response of the ocean to forcing by high-latitude insolation. The warming of NADW was transferred to the Circumpolar Deep Water, providing additional heat around Antarctica, which may have been responsible for partial melting of the West Antarctic Ice Sheet. (Duplessy, Roche, and Kageyama 2007, 89)
Another study found that
An exceptional analogue for the study of the causes and consequences of global warming occurs at the Palaeocene/Eocene Thermal Maximum, 55 million years ago. A rapid rise of global temperatures during this event accompanied turnovers in both marine and terrestrial biota, as well as significant changes in ocean chemistry and circulation. Here we present evidence for an abrupt shift in deep-ocean circulation using carbon isotope records from fourteen sites. These records indicate that deep-ocean circulation patterns changed from Southern Hemisphere overturning to Northern Hemisphere overturning at the start of the Palaeocene/Eocene Thermal Maximum. This shift in the location of deep-water formation persisted for at least 40,000 years, but eventually recovered to original circulation patterns. These results corroborate climate model inferences that a shift in deep-ocean circulation would deliver relatively warmer waters to the deep sea, thus producing further warming. Greenhouse conditions can thus initiate abrupt deep-ocean circulation changes in less than a few thousand years, but may have lasting effects; in this case taking 100,000 years to revert to background conditions. (Nunes and Norris 2006, 60)
M. Latif and colleagues asked, in the Journal of Climate (2006, 4631-4637), “Is the Thermohaline Circulation Changing?” Their answer is that it has changed considerably during the last century, mainly as a result of natural multidecadal climate variability, and that it may change in coming decades because of melting Arctic ice freshening the Nordic Sea. However, these researchers do believe that “such a weakening will not exceed the range of multi-decadal variability [which they take to be 40 percent] for several decades” (Latif et al. 2006, 4635).
The most prominent cold period during the Holocene, 8200 years ago, spanned several hundred years and was caused by an influx of meltwater into the North Atlantic. Evidence from a North Atlantic deep-sea sediment core reveals that the largest climatic perturbation in the present interglacial, the 8,200-year event, is marked by two distinct cooling events in the subpolar North Atlantic at 8,490 and 8,290 years ago. An associated reduction in deep flow speed provides evidence of a significant change to a major down-welling limb of the Atlantic meridional overturning circulation. The existence of a distinct surface freshening signal during these events strongly suggests that the sequenced surface and deep ocean changes were forced by pulsed meltwater outbursts from a multistep final drainage of the proglacial lakes associated with the decaying Laurentide Ice Sheet margin.
Seeking an analogue to a world dominated by severe global warming, Paul A. Wilson and Richard D. Norris investigated the climate of the Mid-Cretaceous period, “a time of unusually warm polar temperatures, repeated reef-drownings in the tropics, and a series of oceanic anoxic events.… with maximum sea-surface temperatures 3° to 5° C warmer than today” (Wilson and Norris 2001, 425). This was period with considerable greenhouse forcing not unlike the end of the twenty-first century may resemble, except that the forcing was natural, not anthropogenic. Writing in Geophysical Research Letters, Orsi and colleagues found no evidence of a recent reduction in generation of Antarctic Bottom Water (Chin 2001, 575). The researchers did not rule out the possibility, however, that global warming could affect formation rates of North Atlantic Deep Water or Antarctic Bottom Water in the future.
The possibility that water temperatures rose to 15° C or perhaps higher in the Arctic during the Cretaceous period also has been explored by Hugh C. Jenkyns and colleagues in Nature (Jenkyns et al. 2004, 888-892). At the time, atmospheric carbon-dioxide levels may have been three to six times today's levels, a “super greenhouse” caused mainly by volcanic out-gassing. Today's climate models have had a difficult time accommodating such warmth in the Arctic without raising projected temperatures at lower latitudes to levels that would have been intolerable for animals and plants that lived at the time.
A study of North Atlantic ocean circulation published in late 2005 (Bryden et al. 2005, 655) reported a 30 percent reduction in the meridional overturning (thermohaline) circulation at 26.5 degrees north latitude, based on readings taken during 1957, 1981, 1992, and 1998. Harry Bryden at the Southampton Oceanography Centre in the United Kingdom, strung buoys in a line across the Atlantic from the Canary Islands to the Bahamas, “and found that the flow of water north from the Gulf Stream into the North Atlantic had faltered by 30 percent since the 1990s.” Less ocean water was going north on the surface and less was coming south along the bottom (Pearce 2007b, 147). Bryden said at the time that he was not sure whether the change he described was temporary or part of a long-term trend. This analysis was, however, “the first observational evidence that such a decrease of the oceanic overturning circulation is well underway” (Quadfasel 2005, 565).
By 2007, scientific consensus was leaning away from Bryden's analysis, as changes earlier taken to be long-term erosion were being regarded, instead, as part of cyclical variation. Stuart Cunningham of the National Oceanography Centre at Southampton, United Kingdom, and colleagues found that the thermohaline circulation varied by 25 percent in one year (Schiermeier 2007b, 844-845).
The idea that changes in thermohaline circulation might cool Europe substantially as most of the world warms has been losing traction among scientists, even as popular entertainment promotes it. Movies such as The Day after Tomorrow (2004) presented the idea in cinematic fashion. Reality intruded, however, as temperatures rose in Europe, and climate models forecast more of the same. Richard Kerr wrote in Science that the ocean-circulation system is “prone to natural slowdowns and speedups. Furthermore, researchers are finding that even if global warming were slowing the conveyor and reducing the supply of warmth to high latitudes, it would be decades before the change would be noticeable above the noise” (Kerr 2006d, 1064). These findings came from the Rapid Climate Change (RAPID) Program, whose researchers moored 19 light-weight cables with instruments along 26.5 degrees north latitude from West Africa to the Bahamas to measure ocean flows.
“The concern had previously been that we were close to a threshold where the Atlantic circulation system would stop,” said Susan Solomon, a senior scientist at the National Oceanic and Atmospheric Administration. “We now believe we are much farther from that threshold, thanks to improved modeling and ocean measurements. The Gulf Stream and the North Atlantic Current are more stable than previously thought” (Gibbs 2007).
The United Nations Intergovernmental Panel on Climate Change said in its 2007 assessment on the basis of 23 climate models that substantial cooling of Europe is not occurring and that a marked disruption of thermohaline circulation is unlikely during the twenty-first century. The IPCC assessment did say that the circulation probably would weaken by about 25 percent through the year 2100, given increased melting of the Greenland Ice Cap and more rainfall in the Arctic that will cause more freshwater to flow southward. However, any resulting cooling of Europe will probably be overwhelmed by a general worldwide warming trend.
“The bottom line is that the atmosphere is warming up so much that a slowdown of the North Atlantic Current will never be able to cool Europe,” said Helge Drange, a professor at the Nansen Environmental and Remote Sensing Center in Bergen, Norway (Gibbs 2007).
Thermohaline circulation is only one contributor to Europe's mild climate. Prevailing winds play a part. In addition, the Greenland Ice Cap would have to melt quickly to stop the ocean conveyor, according to present theories, “The ocean circulation is a robust feature, and you really need to hit it hard to make it stop,” said Eystein Jansen, a paleoclimatologist who directs the Bjerknes Center for Climate Research, also in Bergen. “The Greenland ice sheet would not only have to melt, but to dynamically disintegrate on a huge scale across the entire sheet” (Gibbs 2007). Any collapse would probably require centuries.
See also: Ocean Circulation, Worldwide
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