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Younger Dryas & Abrupt Climate Change

Posted on: December 25, 2018

 

FIGURE 1: THE LAST GLACIATION

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  1. Warming of the current interglacial (the Holocene) at the end of the Last Glaciation Period proceeded by way of a series of abrupt returns to glacial climate.  The most intense and most studied of these unstable brief glaciation events is the Younger Dryas which drove temperatures at the summit of Greenland to ≈15 °C colder than today. This brief and unstable glaciation event that started abruptly also ended abruptly, over a period of about 50 years.
  2. Figure 1 is video clip from Youtube [LINK] of the Last Glacial Period, also known as the Weichselian, showing glacial growth from ≈115,000 years ago to ≈12,000 years ago, but with a brief and violent return to glaciation and an abrupt end ≈11,000 years ago. The Younger Dryas, serves as a well known example of the violent and unstable nature of the glaciation and interglacial cycles that is apparent in the animation in Figure 1. The paleo data show that almost as soon as the Last Glacial Period had apparently ended, a series of brief but violent return to glaciation conditions intervened.
  3. Some features of the Younger Dryas event play important roles in the way climate science has presented the current warming period and the possible dangerous ways that it may evolve in the future. The specific issue discussed here is the repeated fear of “abrupt climate change” due to changes in the North Atlantic Overturning Circulation caused by AGW. It is feared that freshwater discharge from glacial melt in the current warming period will cause a slowdown of the North Atlantic Overturning Circulation portion of the Thermohaline circulation and cause abrupt and dangerous climate change analogous to the Younger Dryas as described in the papers tagged with ACC in the Bibliography below. Outside of the Younger Dryas, these abrupt climate change scenarios are found only in climate models and with large uncertainty in terms of differences from model to model and for different simulations with the same model. A high level of uncertainty is acknowledged by most authors. It implies a low level of information content in the climate model simulation results. In this post we present the case that the high level of interest in the slowdown caused by meltwater in the current warming period is an attempt to insert Younger Dryas realities into AGW without the assumed correspondence between these two climate events.
  4. Changes to the Thermohaline Circulation due to fresh water discharge from de-glaciation and Abrupt Climate Change: The Younger Dryas events are derived from Greenland ice core data and they were primarily a feature of the North Atlantic region. One interpretation of the extreme rapidity of these changes is that they may have been responses to some kind of trigger in the North Atlantic climate system. These rapid changes in the Younger Dryas may have been the creation of changes in North Atlantic Ocean circulation triggered by very large volumes  of glacial meltwater. During periods of intense deglaciation meltwater discharge rates exceeded 13,000 cubic kilometers per year. These observations serve as the rationale for the hypothesis that meltwater discharge weakens the Atlantic thermohaline circulation (THC) and associated northward heat flux. Concerns about abrupt climate change in the current AGW warming period due to perturbations of the THC are likely derived from its apparent role in the Younger Dryas. The meridional overturning circulation was slowed to a crawl in the North Atlantic region by way of catastrophic iceberg and meltwater discharge. Following these meltwater events, there was a rapid accelerations of the meridional overturning circulation particularly so in the two strongest regional warming events during deglaciation. These results are thought to confirm the significance of variations in the rate of the Atlantic meridional overturning circulation for abrupt climate change“. These fears of abrupt climate change in the current warming period, derived from similar events in the Younger Dryas, is described in seven papers listed in the Bibliography. They are (Rahmstorf 1995), (Manabe 1995), (Clark 2002), (Vellinga 2002), (McManus 2004), (Zhang 2005), (Stouffer 2006). The essence of all seven papers is that coupled ocean-atmosphere climate models appear to indicate an effect of meltwater in the North Atlantic Thermohaline circulation and that therefore AGW warming may turn out to be much more catastrophic than previously thought. An alternative view is presented by Carl Wunsch in a series of papers on the Thermohaline circulation [LINK]  and on the abrupt climate change issue [LINK] .
  5. In the abrupt climate change paper, Professor Wunsch argues that “Suggestions that Dansgaard–Oeschger (D–O) events in Greenland are generated by shifts in the North Atlantic Ocean circulation seem highly implausible, given the weak contribution of the high latitude ocean to the meridional flux of heat. A more likely scenario is that changes in the ocean circulation are a consequence of wind shifts. The disappearance of D–O events in the Holocene coincides with the disappearance also of the Laurentide and Fennoscandian ice sheets. It is thus suggested that D–O events are a consequence of interactions of the windfield with the continental ice sheets and that better understanding of the wind field in the glacial periods is the highest priority. Wind fields are capable of great volatility and very rapid global-scale teleconnections, and they are efficient generators of oceanic circulation changes and (more speculatively) of multiple states relative to great ice sheets. Connection of D–O events to the possibility of modern abrupt climate change rests on a very weak chain of assumptions”.
  6. In the Thermohaline Circulation paper he writes, “The ocean is best viewed as a mechanically driven fluid engine, capable of importing, exporting, and transporting vast quantities of heat and freshwater. Although of very great climate influence, this transport is a nearly passive consequence of the mechanical machinery. In its original form, the term “thermohaline circulation” explicitly provided a source of mechanical energy in the form of mixing devices. These devices disappeared in subsequent discussions and extensions of this influential model. For past or future climates, the quantity of first-order importance is the nature of the wind field. It not only shifts the near-surface wind-driven components of the mass flux, but also changes the turbulence at depth; this turbulence appears to control the deep stratification. The wind field will also, in large part, determine the regions of convective sinking and of the resulting 3D water properties. Fluxes and net exports of properties such as heat and carbon are determined by both the mass flux and spatial distribution of the property, and not by either alone. Tidal motions were different in the past than they are today, owing to lower sea level during glacial epochs, and moving continental geometry in the more remote past. The consequent shifts in tidal flow can result in qualitative changes in the oceanic mixing rates, and hence in the mass and consequent property fluxes. The term “thermohaline circulation” should be reserved for the separate circulations of heat and salt, and not conflated into one vague circulation with unknown or impossible energetics. No shortcut exists for determining property fluxes from the mass circulation without knowledge of the corresponding property distribution. His views on paleo climate in general may be found in a related post at this site [LINK] .

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YOUNGER DRYAS & ABRUPT CLIMATE CHANGE

BIBLIOGRAPHY

Featured Authors

Willi Dansgaard, Richard Fairbanks, Richard Alley, Wally Broecker

 

 

 

  1. 1988: Broecker, Wallace S., et al. “The chronology of the last deglaciation: Implications to the cause of the Younger Dryas event.” Paleoceanography and Paleoclimatology 3.1 (1988): 1-19.  It has long been recognized that the transition from the last glacial to the present interglacial was punctuated by a brief and intense return to cold conditions. This extraordinary event, referred to by European palynologists as the Younger Dryas, was centered in the northern Atlantic basin. Evidence is accumulating that it may have been initiated and terminated by changes in the mode of operation of the northern Atlantic Ocean. Further, it appears that these mode changes may have been triggered by diversions of glacial meltwater between the Mississippi River and the St. Lawrence River drainage systems. We report here Accelerator Mass Spectrometry (AMS) radiocarbon results on two strategically located deep‐sea cores. One provides a chronology for surface water temperatures in the northern Atlantic and the other for the meltwater discharge from the Mississippi River. Our objective in obtaining these results was to strengthen our ability to correlate the air temperature history for the northern Atlantic basin with the meltwater history for the Laurentian ice sheet.
  2. 1989: Dansgaard, W. H. I. T. E., J. W. C. White, and S. J. Johnsen. “The abrupt termination of the Younger Dryas climate event.” Nature 339.6225 (1989): 532.  PREVIOUS studies on two deep Greenland ice cores have shown that a long series of climate oscillations characterized the late Weichselian glaciation in the North Atlantic region1, and that the last glacial cold period, the Younger Dryas, ended abruptly 10,700 years ago2. Here we further focus on this epoch-defining event, and present detailed heavy-isotope and dust-concentration profiles which suggest that, in less than 20 years, the climate in the North Atlantic region turned into a milder and less stormy regime, as a consequence of a rapid retreat of the sea-ice cover. A warming of 7 °C in South Greenland was completed in about 50 years. 
  3. 1989: Fairbanks, Richard G. “A 17,000-year glacio-eustatic sea level record: influence of glacial melting rates on the Younger Dryas event and deep-ocean circulation.” Nature 342.6250 (1989): 637.  Coral reefs drilled offshore of Barbados provide the first continuous and detailed record of sea level change during the last deglaciation. The sea level was 121 ± 5 metres below present level during the last glacial maximum. The deglacial sea level rise was not monotonic; rather, it was marked by two intervals of rapid rise. Varying rates of melt-water discharge to the North Atlantic surface ocean dramatically affected North Atlantic deep-water production and oceanic oxygen isotope chemistry. A global oxygen isotope record for ocean water has been calculated from the Barbados sea level curve, allowing separation of the ice volume component common to all oxygen isotope records measured in deep-sea cores.
  4. 1990: Fairbanks, Richard G. “The age and origin of the “Younger Dryas climate event” in Greenland ice cores.” Paleoceanography and Paleoclimatology 5.6 (1990): 937-948.  230Th/234U and 14C dating of Barbados corals has extended the calibration of 14C years B.P. to calendar years B.P. beyond the 9200 year tree ring series (Bard et al., 1990). This now permits the conversion of 14C chronozones, which delimit major climate shifts in western Europe, to calendar years. The Younger Dryas chronozone, defined as 11,000 to 10,000 14C years B.P., corresponds to 13,000 to 11,700 calendar years B.P. This calibration affects the interpretation of an intensely studied example of the “Younger Dryas climate event,” the δ18O anomaly between 1785 and 1793 m in Dye 3 ice core. The end of the δ18O anomaly in Dye 3 ice core has been dated by measurements of 14C in air bubbles (Andree et al., 1984, 1986) and by annual layer counting (Hammer et al., 1986). The older 14C dates fall out of the range of the tree ring calibration series but can now be calibrated to calendar years using the Barbados 230Th/234U calibration. The 14Ccorrectedage for the end of the δ18O event is 10,300 ± 400 calendar years B.P. compared to the annual layer counting age of 10,720 ± 150 years B.P. Thus, the “Younger Dryas” event in the Dye 3 ice core ends in the Preboreal chronozone (11,700 to 10,000 calendar years B.P.) and is not correlative with the end of the Younger Dryas event identified in pollen records marking European vegetation changes. The end of the Dye 3 δ18O event is, however, correlative with the end of meltwater pulse IB (Fairbanks, 1989), marking a period of intense deglaciation with meltwater discharge rates exceeding 13,000 km³/yr.
  5. 1993: Alley, Richard B., et al. “Abrupt increase in Greenland snow accumulation at the end of the Younger Dryas event.” Nature362.6420 (1993): 527. THE warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate, the best-known of which is the Younger Dryas event1. Despite much study of the causes of this event and the mechanisms by which it ended, many questions remain unresolved1. Oxygen isotope data from Greenland ice cores2–4 suggest that the Younger Dryas ended abruptly, over a period of about 50 years; dust concentrations2,4 in these cores show an even more rapid transition (20 years). This extremely short timescale places severe constraints on the mechanisms underlying the transition. But dust concentrations can reflect subtle changes in atmospheric circulation, which need not be associated with a large change in climate. Here we present results from a new Greenland ice core (GISP2) showing that snow accumulation doubled rapidly from the Younger Dryas event to the subsequent Preboreal interval, possibly in one to three years. We also find that the accumulation-rate change from the Oldest Dryas to the Bø11ing/Allerød warm period was large and abrupt. The extreme rapidity of these changes in a variable that directly represents regional climate implies that thalleye events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the North Atlantic climate system.
  6. 1994: Bard, Edouard, et al. “The North Atlantic atmosphere-sea surface 14C gradient during the Younger Dryas climatic event.” Earth and Planetary Science Letters 126.4 (1994): 275-287. We attempt to quantify the 14C difference between the atmosphere and the North Atlantic surface during a prominent climatic period of the last deglaciation, the Younger Dryas event (YD). Our working hypothesis is that the North Atlantic may have experienced a measurable change in 14C reservoir age due to large changes of the polar front position and variations in the mode and rate of North Atlantic Deep Water (NADW) production. We dated contemporaneous samples of terrestrial plant remains and sea surface carbonates in order to evaluate the past atmosphere-sea surface 14C gradient. We selected terrestrial vegetal macrofossils and planktonic foraminifera (Neogloboquadrina pachyderma left coiling) mixed with the same volcanic tephra (the Vedde Ash Bed) which occurred during the YD and which can be recognized in North European lake sediments and North Atlantic deep-sea sediments. Based on AMS ages from two Norwegian sites, we obtained about 10,300 yr BP for the ‘atmospheric’ 14C age of the volcanic eruption. Foraminifera from four North Atlantic deep-sea cores selected for their high sedimentation rates ( > 10 cm kyr−1) were dated by AMS (21 samples). For each core the raw 14C ages assigned to the ash layer peak is significantly older than the 14C age obtained on land. Part of this discrepancy is due to bioturbation, which is shown by numerical modelling. Nevertheless, after correction of a bioturbation bias, the mean 14C age obtained on the planktonic foraminifera is still about 11,000–11,100 yr BP. The atmosphere-sea surface 14C difference was roughly 700–800 yr during the YD, whereas today it is 400–500 yr. A reduced advection of surface waters to the North Atlantic and the presence of sea ice are identified as potential causes of the high 14C reservoir age during the YD.
  7. ACC 1995: Rahmstorf, Stefan. “Bifurcations of the Atlantic thermohaline circulation in response to changes in the hydrological cycle.” Nature 378.6553 (1995): 145.  The sensitivity of the North Atlantic thermohaline circulation to the input of fresh water is studied using a global ocean circulation model coupled to a simplified model atmosphere. Owing to the nonlinearity of the system, moderate changes in freshwater input can induce transitions between different equilibrium states, leading to substantial changes in regional climate. As even local changes in freshwater flux are capable of triggering convective instability, quite small perturbations to the present hydrological cycle may lead to temperature changes of several degrees on timescales of only a few years.
  8. ACC 1995: Manabe, Syukuro, and Ronald J. Stouffer. “Simulation of abrupt climate change induced by freshwater input to the North Atlantic Ocean.” Nature 378.6553 (1995): 165.  Temperature records from Greenland ice cores1,2 suggest that large and abrupt changes of North Atlantic climate occurred frequently during both glacial and post glacial periods; one example is the Younger Dryas cold event. Broecker3 speculated that these changes result from rapid changes in the thermohaline circulation of the Atlantic Ocean, which were caused by the release of large amounts of melt water from continental ice sheets. Here we describe an attempt to explore this intriguing phenomenon using a coupled ocean–atmosphere model. In response to a massive surface flux of fresh water to the northern North Atlantic of the model, the thermohaline circulation weakens abruptly, intensifies and weakens again, followed by a gradual recovery, generating episodes that resemble the abrupt changes of the ocean–atmosphere system recorded in ice and deep-sea cores4. The associated change of surface air temperature is particularly large in the northern North Atlantic Ocean and its neighbourhood, but is relatively small in the rest of the world.
  9. 1997: Bond, Gerard, et al. “A pervasive millennial-scale cycle in North Atlantic Holocene and glacial climates.” science278.5341 (1997): 1257-1266.  Evidence from North Atlantic deep sea cores reveals that abrupt shifts punctuated what is conventionally thought to have been a relatively stable Holocene climate. During each of these episodes, cool, ice-bearing waters from north of Iceland were advected as far south as the latitude of Britain. At about the same times, the atmospheric circulation above Greenland changed abruptly. Pacings of the Holocene events and of abrupt climate shifts during the last glaciation are statistically the same; together, they make up a series of climate shifts with a cyclicity close to 1470 ± 500 years. The Holocene events, therefore, appear to be the most recent manifestation of a pervasive millennial-scale climate cycle operating independently of the glacial-interglacial climate state. Amplification of the cycle during the last glaciation may have been linked to the North Atlantic’s thermohaline circulation.
  10. 1997: Alley, Richard B., et al. “Holocene climatic instability: A prominent, widespread event 8200 yr ago.” Geology 25.6 (1997): 483-486.  The most prominent Holocene climatic event in Greenland ice-core proxies, with approximately half the amplitude of the Younger Dryas, occurred ∼8000 to 8400 yr ago. This Holocene event affected regions well beyond the North Atlantic basin, as shown by synchronous increases in windblown chemical indicators together with a significant decrease in methane. Widespread proxy records from the tropics to the north polar regions show a short-lived cool, dry, or windy event of similar age. The spatial pattern of terrestrial and marine changes is similar to that of the Younger Dryas event, suggesting a role for North Atlantic thermohaline circulation. Possible forcings identified thus far for this Holocene event are small, consistent with recent model results indicating high sensitivity and strong linkages in the climatic system.
  11. 1998: Severinghaus, Jeffrey P., et al. “Timing of abrupt climate change at the end of the Younger Dryas interval from thermally fractionated gases in polar ice.” Nature 391.6663 (1998): 141.  Rapid temperature change fractionates gas isotopes in unconsolidated snow, producing a signal that is preserved in trapped air bubbles as the snow forms ice. The fractionation of nitrogen and argon isotopes at the end of the Younger Dryas cold interval, recorded in Greenland ice, demonstrates that warming at this time was abrupt. This warming coincides with the onset of a prominent rise in atmospheric methane concentration, indicating that the climate change was synchronous (within a few decades) over a region of at least hemispheric extent, and providing constraints on previously proposed mechanisms of climate change at this time. The depth of the nitrogen-isotope signal relative to the depth of the climate change recorded in the ice matrix indicates that, during the Younger Dryas, the summit of Greenland was 15 ± 3 °C colder than today.
  12. 1997: Broecker, Wallace S. “Thermohaline circulation, the Achilles heel of our climate system: Will man-made CO2 upset the current balance?.” Science 278.5343 (1997): 1582-1588.  During the last glacial period, Earth’s climate underwent frequent large and abrupt global changes. This behavior appears to reflect the ability of the ocean’s thermohaline circulation to assume more than one mode of operation. The record in ancient sedimentary rocks suggests that similar abrupt changes plagued the Earth at other times. The trigger mechanism for these reorganizations may have been the antiphasing of polar insolation associated with orbital cycles. Were the ongoing increase in atmospheric CO2 levels to trigger another such reorganization, it would be bad news for a world striving to feed 11 to 16 billion people.
  13. 1999: Marchal, O., et al. “Modelling the concentration of atmospheric CO2 during the Younger Dryas climate event.” Climate Dynamics 15.5 (1999): 341-354.  The Younger Dryas (YD, dated between 12.7–11.6 ky BP in the GRIP ice core, Central Greenland) is a distinct cold period in the North Atlantic region during the last deglaciation. A popular, but controversial hypothesis to explain the cooling is a reduction of the Atlantic thermohaline circulation (THC) and associated northward heat flux as triggered by glacial meltwater. Recently, a CH4-based synchronization of GRIP δ18O and Byrd CO2 records (West Antarctica) indicated that the concentration of atmospheric CO2 (COatm2) rose steadily during the YD, suggesting a minor influence of the THC on COatm2 at that time. Here we show that the CO2atm change in a zonally averaged, circulation-biogeochemistry ocean model when THC is collapsed by freshwater flux anomaly is consistent with the Byrd record. Cooling in the North Atlantic has a small effect on CO2atm in this model, because it is spatially limited and compensated by far-field changes such as a warming in the Southern Ocean. The modelled Southern Ocean warming is in agreement with the anti-phase evolution of isotopic temperature records from GRIP (Northern Hemisphere) and from Byrd and Vostok (East Antarctica) during the YD. δ13C depletion and PO4 enrichment are predicted at depth in the North Atlantic, but not in the Southern Ocean. This could explain a part of the controversy about the intensity of the THC during the YD. Potential weaknesses in our interpretation of the Byrd CO2 record in terms of THC changes are discussed.
  14. ACC 2002: Clark, Peter U., et al. “The role of the thermohaline circulation in abrupt climate change.” Nature 415.6874 (2002): 863.  The possibility of a reduced Atlantic thermohaline circulation in response to increases in greenhouse-gas concentrations has been demonstrated in a number of simulations with general circulation models of the coupled ocean–atmosphere system. But it remains difficult to assess the likelihood of future changes in the thermohaline circulation, mainly owing to poorly constrained model parameterizations and uncertainties in the response of the climate system to greenhouse warming. Analyses of past abrupt climate changes help to solve these problems. Data and models both suggest that abrupt climate change during the last glaciation originated through changes in the Atlantic thermohaline circulation in response to small changes in the hydrological cycle. Atmospheric and oceanic responses to these changes were then transmitted globally through a number of feedbacks. The palaeoclimate data and the model results also indicate that the stability of the thermohaline circulation depends on the mean climate state.
  15. ACC 2002: Vellinga, Michael, and Richard A. Wood. “Global climatic impacts of a collapse of the Atlantic thermohaline circulation.” Climatic change 54.3 (2002): 251-267.  Part of the uncertainty in predictions by climate models results from limited knowledge of the stability of the thermohaline circulation of the ocean. Here we provide estimates of the response of pre-industrial surface climate variables should the thermohalinecirculation in the Atlantic Ocean collapse. For this we have used HadCM3, an ocean-atmosphere general circulation model that is run without flux adjustments. In this model a temporary collapse was forced by applying a strong initial freshening to the top layers of the NorthAtlantic. In the first five decades after the collapse surface air temperature response is dominated by cooling of much of the Northern Hemisphere (locally up to 8 °C, 1–2 °C on average) and weak warming of the Southern Hemisphere (locally up to 1 °C, 0.2 °C onaverage). Response is strongest around the North Atlantic but significant changes occur over the entire globe and highlight rapid connections. Precipitation is reduced over large parts of the Northern Hemisphere. A southward shift of the Intertropical Convergence Zone over the Atlantic and eastern Pacific creates changes in precipitation that are particularly large in South America and Africa. Colder and drier conditions in much of the Northern Hemisphere reduces oil moisture and net primary productivity of the terrestrial vegetation. This is only partly compensated by more productivity in the Southern Hemisphere.The total global net primary productivity by the vegetation decreases by 5%. It should be noted, however, that in this version of the model the vegetation distribution cannot change, and atmospheric carbon levels are also fixed. After about 100 years the model’s thermohaline circulation has largely recovered, and most climatic anomalies disappear.
  16. 2003: Alley, Richard B., et al. “Abrupt climate change.” science299.5615 (2003): 2005-2010.  Large, abrupt, and widespread climate changes with major impacts have occurred repeatedly in the past, when the Earth system was forced across thresholds. Although abrupt climate changes can occur for many reasons, it is conceivable that human forcing of climate change is increasing the probability of large, abrupt events. Were such an event to recur, the economic and ecological impacts could be large and potentially serious. Unpredictability exhibited near climate thresholds in simple models shows that some uncertainty will always be associated with projections. In light of these uncertainties, policy-makers should consider expanding research into abrupt climate change, improving monitoring systems, and taking actions designed to enhance the adaptability and resilience of ecosystems and economies.
  17. ACC 2004: McManus, Jerry F., et al. “Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes.” Nature 428.6985 (2004): 834. The Atlantic meridional overturning circulation is widely believed to affect climate. Changes in ocean circulation have been inferred from records of the deep water chemical composition derived from sedimentary nutrient proxies1, but their impact on climate is difficult to assess because such reconstructions provide insufficient constraints on the rate of overturning2. Here we report measurements of 231Pa/230Th, a kinematic proxy for the meridional overturning circulation, in a sediment core from the subtropical North Atlantic Ocean. We find that the meridional overturning was nearly, or completely, eliminated during the coldest deglacial interval in the North Atlantic region, beginning with the catastrophic iceberg discharge Heinrich event H1, 17,500 yr ago, and declined sharply but briefly into the Younger Dryas cold event, about 12,700 yr ago. Following these cold events, the 231Pa/230Th record indicates that rapid accelerations of the meridional overturning circulation were concurrent with the two strongest regional warming events during deglaciation. These results confirm the significance of variations in the rate of the Atlantic meridional overturning circulation for abrupt climate changes.
  18. ACC 2005: Zhang, Rong, and Thomas L. Delworth. “Simulated tropical response to a substantial weakening of the Atlantic thermohaline circulation.” Journal of Climate 18.12 (2005): 1853-1860.  In this study, a mechanism is demonstrated whereby a large reduction in the Atlantic thermohaline circulation (THC) can induce global-scale changes in the Tropics that are consistent with paleoevidence of the global synchronization of millennial-scale abrupt climate change. Using GFDL’s newly developed global coupled ocean–atmosphere model (CM2.0), the global response to a sustained addition of freshwater to the model’s North Atlantic is simulated. This freshwater forcing substantially weakens the Atlantic THC, resulting in a southward shift of the intertropical convergence zone over the Atlantic and Pacific, an El Niño–like pattern in the southeastern tropical Pacific, and weakened Indian and Asian summer monsoons through air–sea interactions.
  19. ACC 2006:  Stouffer, Ronald J., et al. “Investigating the causes of the response of the thermohaline circulation to past and future climate changes.” Journal of climate 19.8 (2006): 1365-1387.  The Atlantic thermohaline circulation (THC) is an important part of the earth’s climate system. Previous research has shown large uncertainties in simulating future changes in this critical system. The simulated THC response to idealized freshwater perturbations and the associated climate changes have been intercompared as an activity of World Climate Research Program (WCRP) Coupled Model Intercomparison Project/Paleo-Modeling Intercomparison Project (CMIP/PMIP) committees. This intercomparison among models ranging from the earth system models of intermediate complexity (EMICs) to the fully coupled atmosphere–ocean general circulation models (AOGCMs) seeks to document and improve understanding of the causes of the wide variations in the modeled THC response. The robustness of particular simulation features has been evaluated across the model results. In response to 0.1-Sv (1 Sv ≡ 106 m3 s−1) freshwater input in the northern North Atlantic, the multimodel ensemble mean THC weakens by 30% after 100 yr. All models simulate some weakening of the THC, but no model simulates a complete shutdown of the THC. The multimodel ensemble indicates that the surface air temperature could present a complex anomaly pattern with cooling south of Greenland and warming over the Barents and Nordic Seas. The Atlantic ITCZ tends to shift southward. In response to 1.0-Sv freshwater input, the THC switches off rapidly in all model simulations. A large cooling occurs over the North Atlantic. The annual mean Atlantic ITCZ moves into the Southern Hemisphere. Models disagree in terms of the reversibility of the THC after its shutdown. In general, the EMICs and AOGCMs obtain similar THC responses and climate changes with more pronounced and sharper patterns in the AOGCMs.

 

 

 

 

 

 

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