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Thermohaline circulation

Main Contributors:

Juan Carlos Rocha, Rolands Sadauskis

Other Contributors:

Reinette (Oonsie) Biggs, Garry Peterson


A regime shift may be perceived in Thermohaline circulation (THC) as it might stop due to the increasing depletion of Arctic ice resulting in a catastrophic release of freshwater to the North Atlantic Ocean. This would balance out the differences in water salinity and density between Southern and Northern Atlantic Ocean. Therefore this would lead to collapse of water transport from tropical Atlantic to North Atlantic. Thus the high latitudes of the North Atlantic Ocean are perceived as key regions for triggering the collapse of THC. The regime shift is initiated by the main driver - input of greenhouse gases in atmosphere. The release of CO2 increases the atmospheric temperatures thus leading to a decrease in Arctic ice volumes. The amount of freshwater entering the ocean waters decreases the salinity and density of the water which weakens the density driven water transport. Several essential mechanisms such as freshwater-overturning mechanism, water temperature-density mechanism and evaporation-salinity mechanism are proposed to weaken the THC. Current management strategies primarily relate to the decrease of greenhouse gas emissions on a global scale.


Key direct drivers

  • Global climate change


Ecosystem type

  • Polar

Key Ecosystem Processes

  • Primary production
  • Water cycling


  • Biodiversity

Provisioning services

  • Food crops
  • Livestock
  • Fisheries

Regulating services

  • Climate regulation

Human Well-being

  • Food and nutrition
  • Livelihoods and economic activity
  • Social conflict

Key Attributes

Typical spatial scale

  • Sub-continental/regional
  • Global

Typical time scale

  • Decades
  • Centuries


  • Unknown


  • Models
  • Paleo-observation

Confidence: Existence of RS

  • Speculative – Regime shift has been proposed, but little evidence as yet

Confidence: Mechanism underlying RS

  • Contested – Multiple proposed mechanisms, reasonable evidence both for and against different mechanisms

Links to other regime shifts

Alternate regimes

Thermohaline circulation is complex, but can be summarised as a global scale deep overturning of water masses, with sinking motions occurring in the northern Atlantic and around Antarctica and a transport of relatively warm surface waters towards these sinking regions (Steffen et al. 2004). The term thermohaline circulation thus refers to a particular driving mechanism; it is a physical, not an observational concept.

Strong Thermohaline circulation

THC regime can be characterized by the formation of deep water currents in the high latitudes of both hemispheres; the spreading of deep waters, as deep western boundary currents; upwelling (or upwards movement) of deep waters which is thought to take place mainly in the Antarctic Circumpolar Current region, possibly aided by the wind, and near-surface currents, required to close the flow (Rahmstorf et al. 2006).

Collapse of the Thermohaline circulation

This regime is characterized by weak formation of the dense and saline high latitude deep waters due to the increased influx of freshwater. The decreased salinity of the deep waters is weakening the density southwards driven water movement. The transport of less dense and saline waters from south towards north is decreased causing additional atmospheric temperature increase in tropical areas.

Drivers and causes of the regime shift

Increasing greenhouse gas concentration from anthropogenic sources is predicted to cause a rise in global mean temperatures (Cubasch et al. 2001). One of the most common anthropogenic greenhouse gases is carbon dioxide (CO2). It is considered to be the main driver for this regime shift and is typically operating in global scale due to the global input of CO2 in the atmosphere. Increasing CO2 levels in atmosphere increases the mean atmospheric temperature. This results in weakening ice-albedo mechanism for the initial regime as a result of increased atmospheric temperatures and changes in albedo.

The increasing freshwater influx from sea-ice loss is accelerated by freshwater-overturning and precipitation-river runoff. This leads to an in increased evaporation, river runoff, and changes to water salinity and density. The burning of fossile fuels such as coal and natural gas also drives this regime shift. This occurs in a regional scale but has global impact and is well established in the literature.

How the regime shift works

In the regime of strong THC, the most simple explanation of the main variable that maintains the regime would be the high-latitude cooling. In the deep ocean, the predominant driving force of movement of currents is differences in density, caused by salinity and temperature (the more saline the more denser and the colder the water, the more dense). The ocean's density distribution is itself affected by currents and mixing. Thermohaline and wind-driven currents therefore interact in non-linear ways and cannot be separated by oceanographic measurements. Changing the wind stress will alter the THC; altering thermohaline forcing will also change the wind driven currents.

The collapse of THC occurs due to increasing CO2 levels in atmosphere that slowly enhance the mean atmospheric temperature. As a result the high latitude cooling is weakened, increasing freshwater influx from the loss of ice, both from the Arctic, Greenland and possibly the north of Canada. This strengthens two main mechanisms for the new regime: freshwater-overturning mechanism and the precipitation-river runoff mechanism. Both increase the fresh water inflow into the ocean. Therefore new mechanism such as increased evaporation, river runoff, changes in water salinity and density are activated possibly reinforcing the new regime. Once the system has shifted, the warm conditions in Northern latitudes maintain a weak formation of the dense and saline high latitude deep waters due to the increased influx of freshwater.

Most ocean–atmosphere models predict a reduction of the THC for a warmer climate in the near future. Recent observational evidence provides some support to the model projections that the North Atlantic THC circulation could weaken or shut down later this century (Steffen et al. 2004). Lenton et al. (2008) report that the IPCC models predict the regime shift to happen after 2100, however, their models do not include freshwater runoff which is expected to accelerate the process. The models that project the collapse of THC under greater perturbations identify that the collapsed state can last for centuries and might be irreversible (Delworth et al. 2008). Nevertheless the latest IPCC report shows that it is very unlikely that the THC will undergo a large abrupt transition during the course of the 21st century (IPCC 2007). Some models show a recovery of the THC after the elimination of the freshwater perturbation whereas others do not. The mechanism for this different dynamical behavior of the THC is not completely understood (Stouffer et al. 2006).

Impacts on ecosystem services and human well-being

The weakening of THC will impact global water regulation as water circulations in global scale are influenced by the changes in temperature, salinity and density. In response, climate regulation is altered and changes as the transport of heat is distributed in a different pattern. The water mass and heat circulation will no longer transport warm currents in a northly direction. This could increase the occurence of hurricanes, a southward shift of tropical rainfall belts with resulting agricultural impacts, as well as disruptions to marine ecosystems. Provisioning services such as fisheries, livestock and food crops are therefore potentially affected. Schmittner et al. (2007) show that changes in Atlantic circulation can have large effects on marine ecosystems and biogeochemical cycles, even in areas remote from the Atlantic, such as the Indian and North Pacific Oceans. Although loss of THC could affect Western Europe and result in colder climates. 

Food crisis and emigration are potential threats in the regions that are subjected to this regime shift (Barnett & Adger 2007). In addition, regional changes in sea level are predicted to rise of up to 80 cm in the North Atlantic (Knutti et al. 2002). This could affect the coastlines of the United States, Canada, and Europe causing coastal erosion and affect the security of housing and infrastructure (Delworth et al. 2008). The marine biodiversity of flora and fauna will also be affected. The change in salinity and temperature may facilitate the introduction and spread of invasive species that are able to thrive in the new conditions. This could affect the livelihoods and economic activity of local fishermen as they would have to adapt to new circumstances.  



Management options

This regime shift is mainly driven by the increase in greenhouse gases into the atmosphere at a global scale. Therefore a decrease in CO2 emissions and other greenhouse gases has to be achieved in order to prevent further climate warming leading to large influx of freshwater from the depletion of Arctic ice. In the best-case scenario, it may be possible to avert a regime change in a situation where the underlying driver(s) of change are amenable to fast manipulation through management, and where there are not substantial lags in ecosystem response. In the case of the THC neither of these factors hold: the underlying causes of global warming appear very difficult to address through rapid changes in management, and there are substantial inertias in ecosystem response to changes in the driving factors. Therefore an early warning indicators would be useful in providing advance warning of a substantial coming change in ecosystem conditions, but is unlikely to be useful in averting the regime change.

Key References

  1. Barnett J, Adger WN (2007) Climate change, human security and violent conflict. Journal of Experimental Marine Biology and Ecology 26:639u2013655.
  2. Bitz CM, Gent PR, Woodgate RA, Hall A, Holland MM, and Lindsay R. 2006. The influence of sea ice on ocean heat uptake in response to increasing CO2. J. Clim 20, 2437-2450.
  3. Bond G, Showers W, Elliot M, Evans M, Lotti R, Hajdas I, Bonani G, Johnsen SJ, 1999. The North Atlantic’s 1-2 kyr climate rhythm: Relation to Heinrich events, Dansgaard/Oescheger cycles and the Little Ice Age. In: Clark PU, Webb RS, Keigwin LD (eds) Mechanisms of global climate change at millennial time scales. American Geophysical Union, Washington. USA.35-58.
  4. Broecker WS. 1997. Thermohaline Circulation, the Achilles Heel of Our Climate System: Will Man-Made CO2 Upset the Current Balance? Science 278,1582 – 1588.
  5. Clark PU, Pisias NG, Stocker TF, Weaver AJ. 2002. The role of the thermohaline circulation in abrupt climate change. Nature 415,863-869.
  6. Cubasch U. et al. 2001. Projections of future climate change. Climate Change 2001: The Scientific Basis, Houghton JT, et al., Eds., Cambridge University Press. 525–582.
  7. Delworth TL, Clark PU, Holland M, Johns WE, Kuhlbrodt T, Lynch-Stieglitz J, Morrill C, Seager R, Weaver AJ, and Zhang R. 2008. The potential for abrupt change in the Atlantic Meridional Overturning Circulation. In: Abrupt Climate Change. A report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. U.S. Geological Survey, Reston, VA, pp. 117–162.
  8. Higgins PAT. and Schneider SH. 2005. Long-term potential ecosystem responses to greenhouse gas-induced thermohaline circulation collapse. Global Change Biology 11,699-709.
  9. Holland MM, Bitz CM, and Tremblay B. 2006. Future abrupt reductions in the summer Arctic sea ice. Geophys. Res. Lett. 33.
  10. IPCC 2007. Climate Change 2007. The Physical Science Basis. Cambridge University Press, New York.
  11. Knutti R, Stocker TF. 2002. Limited predictability of the future thermohaline circulation close to an instability threshold. J Climate 15,179-186.
  12. Lenton, T. et al. 2008. Tipping elements in the Earth's climate system. Proceedings of the National Academy of Sciences 105, 1786 .
  13. Lindsay RW, and Zhang J. 2005. The thinning of Arctic sea ice, 1988– 2003: Have we passed a tipping point?, J. Clim., 18,4879– 4894.
  14. Lu RY, and Dong BW. 2008. Response of the Asian summer monsoon to a weakening of Atlantic thermohaline circulation. Adv. in Atmos. Sci. 25,723-736.
  15. Otterå OH, Drange H, Bentsen M, Kvamstø NG, and Jiang D. 2004. Transient response of the Atlantic meridional overturning circulation to enhanced freshwater input to the Nordic Seas-Arctic Ocean in the Bergen Climate Model. Tellus, 56A, 342-361.
  16. Peterson BJ, Holmes RM, McClelland JW, Vorosmarty CJ, Lammers RB, Shiklomanov AI, Shiklomanov IA, Rahmstorf S. 2002. Increasing river discharge to the Arctic Ocean. Science 298,2171-2173.
  17. Rahmstorf S. 2000. The thermohaline ocean circulation—A system with dangerous thresholds?, Clim. Change 46,247–256.
  18. Rahmstorf S. 2006. Thermohaline Ocean Circulation. In: Encyclopedia of Quaternary Sciences, Edited by Elias SA. Elsevier, Amsterdam.
  19. Rigor IG, Wallace JM, and Colony RL. 2002. Response of sea ice to the Arctic Oscillation. J. Climate 15,2648–2668.
  20. Ruhlemann C, Mulitza S, Muller PJ, Wefer G, Zahn R. 1999. Warming of the tropical Atlantic Ocean and slow down thermocline circulation during the last deglaciation. Nature 402,511-514.
  21. Schmittner A, Brook EJ, & Ahn J. 2007. Ocean Circulation: Mechanisms and Impacts - Past and Future Changes of Meridional Overturning (eds Schmittner, A, Chiang JCH, & Hemming SR.) Geophysical Monograph Series, American Geophysical Union 173,209–246.
  22. Steffen W, Sanderson A, Jager J, Tyson PD, Moore B III, et al. 2004. Gloabl change and the Earth system: A Planet Under Pressure. Heidelberg: Springer-Verlag. 239-242.
  23. Stouffer RJ, Yin J, Gregory JM, Dixon KW, Spelman MJ, Hurlin W, Weaver AJ, Eby M, Flato GM, Hasumi H, Hu A, Jungclaus JH, Kamenkovich IV, Levermann A, Montoya M, Murakami S, Nawrath S, Oka A, Peltier WR, Robitaille DY, Sokolov A, Vettoretti G, Weber SL. 2006. Investigating the causes of the response of the thermohaline circulation to past and future climate changes. Journal of Climate 19,1365-1387.
  24. Zhang R, Delworth TL. 2005. Simulated tropical response to a substantial weakening of the Atlantic thermohaline circulation. Journal of Climate 18, 1853-1860.
  25. Zhang X, and Walsh JE. 2006. Towards a seasonally ice-covered Arctic Ocean: Scenarios from the IPCC AR4 simulations, Journal of Climate 19,1730– 1747.
  26. Zickfeld K, Eby M, and Weaver AJ. 2008. Carbon-cycle feedbacks of changes in the Atlantic meridional overturning circulation under future atmospheric CO2, Global Biogeochem. Cycles, 22.


Juan Carlos Rocha, Rolands Sadauskis, Reinette (Oonsie) Biggs, Garry Peterson. Thermohaline circulation. In: Regime Shifts Database, Last revised 2017-02-06 20:18:19 GMT.
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