Earth:Shutdown of thermohaline circulation

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Short description: Effect of global warming on a major ocean circulation.
A summary of the path of the thermohaline circulation. Blue paths represent deep-water currents, while red paths represent surface currents

The Thermohaline Circulation is a pattern of water flow through the world's oceans. Warm water flows along the surface until it reaches one of a few special spots near Greenland or Antarctica. There, the water sinks, and then crawls across the bottom of the ocean, miles/kilometers deep, over hundreds of years, gradually rising in the Pacific and Indian oceans.

The Gulf Stream is part of this circulation, and is part of the reason why northern Europe is warmer than it would normally be; Edinburgh has the same latitude as Moscow. The Thermohaline Circulation influences the climate all over the world.

A shutdown or slowdown of the thermohaline circulation is a hypothesized effect of global warming on a major ocean circulation.

A 2015 study suggested that the Atlantic meridional overturning circulation (AMOC) has weakened by 15-20% in 200 years.[1]

General

Don Chambers from the University of South Florida College of Marine Science mentioned: "The major effect of a slowing AMOC is expected to be cooler winters and summers around the North Atlantic, and small regional increases in sea level on the North American coast."[2] James Hansen and Makiko Sato stated:

AMOC slowdown that causes cooling ~1 °C and perhaps affects weather patterns is very different from an AMOC shutdown that cools the North Atlantic several degrees Celsius; the latter would have dramatic effects on storms and be irreversible on the century time scale.[3]

Downturn of the Atlantic meridional overturning circulation has been tied to extreme regional sea level rise.[4]

A 2017 review concluded that there is strong evidence for past changes in the strength and structure of the AMOC during abrupt climate events such as the Younger Dryas and many of the Heinrich events.[5]

Slowdown

Lohmann and Dima 2010 found a weakening of the AMOC since the late 1930s.[6] Climate scientists Michael Mann of Penn State and Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research suggested that the observed cold pattern during years of temperature records is a sign that the Atlantic Ocean's Meridional overturning circulation (AMOC) may be weakening. They published their findings in 2015, and concluded that the AMOC circulation showed exceptional slowdown in the last century, and that Greenland melt is a possible contributor, with the slowdown of AMOC since the 1970s being unprecedented over the last millennium.[7]

A study published in 2016 found further evidence for a considerable impact from sea level rise for the U.S. East Coast. The study confirms earlier research findings which identified the region as a hotspot for rising seas, with a potential to divert 3–4 times in the rate of rise, compared to the global average. The researchers attribute the possible increase to an ocean circulation mechanism called deep water formation, which is reduced due to AMOC slow down, leading to more warmer water pockets below the surface. Additionally, the study noted, "Our results suggest that higher carbon emission rates also contribute to increased [sea level rise] in this region compared to the global average."[8]

Shutdown

Global warming could, via a shutdown of the thermohaline circulation, trigger cooling in the North Atlantic, Europe, and North America.[9][10] This would particularly affect areas such as the British Isles, France and the Nordic countries, which are warmed by the North Atlantic drift.[11][12] Major consequences, apart from regional cooling, could also include an increase in major floods and storms, a collapse of plankton stocks, warming or rainfall changes in the tropics or Alaska and Antarctica, more frequent and intense El Niño events due to associated shutdowns of the Kuroshio, Leeuwin, and East Australian Currents that are connected to the same thermohaline circulation as the Gulf Stream, or an oceanic anoxic event — oxygen (O2) below surface levels of the stagnant oceans becomes completely depleted – a probable cause of past mass extinction events.[13]

Potential effects: anoxia and euxinia

See also: Anoxia|Anoxic waters|Anoxia

Light penetrates only about 100 meters to 200 meters of the ocean top layer,[14] so this is the layer in which oxygen production by phytoplankton can occur. The thermohaline cycle causes mixing of the deep ocean water (that would be oxygen-free) with the oxygen-rich water from the surface.[15] Thus, the thermohaline cycle brings oxygen into the deep layers of the ocean and allows marine life to breathe, and degradation to happen aerobically. If the thermohaline cycle shut down, it has been proposed that the marine life dies off and sinks to the ocean ground. It has been established that climate change is responsible for the loss of oxygen in the ocean, both because oxygen dissolves worse in warm water, and because of weakening thermohaline circulations.[16]

With too little oxygen, anaerobic digestion through bacteria would create methane and hydrogen sulfide from the biomass.[17][18] The toxic hydrogen sulfide gas could then, when the ocean contains too much, get released into the atmosphere in a so called chemocline upward excursion.[17] Hydrogen sulfide poisoning of the atmosphere is one of the potential causes that might have led to the Permian-Triassic extinction event.[19][18] [20][citation needed]

Effects on weather

Hansen et al. 2015 found that the shutdown or substantial slowdown of the AMOC, besides possibly contributing to extreme end-Eemian events, will cause a more general increase of severe weather. Additional surface cooling from ice melt increases surface and lower tropospheric temperature gradients, and causes in model simulations a large increase of mid-latitude eddy energy throughout the midlatitude troposphere. This in turn leads to an increase of baroclinicity produced by stronger temperature gradients, which provides energy for more severe weather events.

Many of the most memorable and devastating storms in eastern North America and western Europe, popularly known as superstorms, have been winter cyclonic storms, though sometimes occurring in late fall or early spring, that generate near-hurricane-force winds and often large amounts of snowfall. Continued warming of low latitude oceans in coming decades will provide more water vapor to strengthen such storms. If this tropical warming is combined with a cooler North Atlantic Ocean from AMOC slowdown and an increase in midlatitude eddy energy, we can anticipate more severe baroclinic storms.

Hansen et al. results at least imply that strong cooling in the North Atlantic from AMOC shutdown does create higher wind speed. The increment in seasonal mean wind speed of the northeasterlies relative to preindustrial conditions is as much as 10–20%. Such a percentage increase of wind speed in a storm translates into an increase of storm power dissipation by a factor ∼1.4–2, because wind power dissipation is proportional to the cube of wind speed. However, the simulated changes refer to seasonal mean winds averaged over large grid-boxes, not individual storms.[21]

Observations

2010 and earlier

In April 2004, the hypothesis that the Gulf Stream is switching off received a boost when a retrospective analysis of U.S. satellite data seemed to show a slowing of the North Atlantic Gyre, the northern swirl of the Gulf Stream.[22]

In May 2005, Peter Wadhams reported in The Times (London) about the results of investigations in a submarine under the Arctic ice sheet measuring the giant chimneys of cold dense water, in which the cold dense water normally sinks down to the sea bed and is replaced by warm water, forming one of the engines of the North Atlantic Drift. He and his team found the chimneys to have virtually disappeared. Normally there are seven to twelve giant columns, but Wadhams found only two giant columns, both extremely weak.[23][24]

In 2005 a 30% reduction in the warm currents that carry water north from the Gulf Stream was observed from the last such measurement in 1992. The authors noted uncertainties in the measurements.[25] Following media discussions, Detlef Quadfasel pointed out that the uncertainty of the estimates of Bryden et al. is high, but says other factors and observations do support their results, and implications based on palaeoclimate records show drops of air temperature up to 10 °C within decades, linked to abrupt switches of ocean circulation when a certain threshold is reached. He concluded that further observations and modelling are crucial for providing early warning of a possible devastating breakdown of the circulation.[26] In response Quirin Schiermeier concluded that natural variation was the culprit for the observations but highlighted possible implications.[13][27]

In 2008, Vage et al. reported "the return of deep convection to the subpolar gyre in both the Labrador and Irminger seas in the winter of 2007–2008," employing "profiling float data from the Argo program to document deep mixing," and "a variety of in situ, satellite and reanalysis data" to set the context for the phenomenon. This might have a lot to do with the observations of variations in cold water chimney behaviour.[28]

In January 2010, the Gulf Stream briefly connected with the West Greenland Current after fluctuating for a few weeks due to an extreme negative phase of the Arctic oscillation, temporarily diverting it west of Greenland.[29][30]

Post-2020

A study published in Nature Climate Change in August 2021 draws on more than a century of ocean temperature and salinity data and shows significant changes in eight indirect measures of the circulation’s strength.[31]

There is a possibility that the AMOC is a bistable system (which is either "on" or "off") and could collapse suddenly.[32][33]

Thermohaline circulation and fresh water

The red end of the spectrum indicates slowing in this presentation of the trend of velocities derived from NASA Pathfinder altimeter data from May 1992 to June 2002. Source: NASA.

Heat is transported from the equator polewards mostly by the atmosphere but also by ocean currents, with warm water near the surface and cold water at deeper levels. The best known segment of this circulation is the Gulf Stream, a wind-driven gyre, which transports warm water from the Caribbean northwards. A northwards branch of the Gulf Stream, the North Atlantic Drift, is part of the thermohaline circulation (THC), transporting warmth further north to the North Atlantic, where its effect in warming the atmosphere contributes to warming Europe.

The evaporation of ocean water in the North Atlantic increases the salinity of the water as well as cooling it, both actions increasing the density of water at the surface. Formation of sea ice further increases the salinity and density, because salt is ejected into the ocean when sea ice forms.[34] This dense water then sinks and the circulation stream continues in a southerly direction. However, the Atlantic Meridional Overturning Circulation (AMOC) is driven by ocean temperature and salinity differences. But freshwater decreases ocean water salinity, and through this process prevents colder waters sinking. This mechanism possibly caused the cold ocean surface temperature anomaly currently observed near Greenland (Cold blob (North Atlantic)).[35]

Global warming could lead to an increase in freshwater in the northern oceans, by melting glaciers in Greenland, and by increasing precipitation, especially through Siberian rivers.[36][37]

An AMOC shutdown may be able to trigger the type of abrupt massive temperature shifts which occurred during the last glacial period: a series of Dansgaard-Oeschger events – rapid climate fluctuations – may be attributed to freshwater forcing at high latitude interrupting the THC. 2002 model runs in which the THC is forced to shut down do show cooling – locally up to 8 °C (14 °F).[38]

Studies of the Florida Current suggest that the Gulf Stream weakens with cooling, being weakest (by ~10%) during the Little Ice Age.[39]

Subpolar gyre

Recent studies (2017) suggest potential convection collapse (heat transport) of the subpolar gyre in the North Atlantic, resulting in rapid cooling, with implications for economic sectors, agriculture industry, water resources and energy management in Western Europe and the East Coast of the United States.[40] Frajka-Williams et al. 2017 pointed out that recent changes in cooling of the subpolar gyre, warm temperatures in the subtropics and cool anomalies over the tropics, increased the spatial distribution of meridional gradient in sea surface temperatures, which is not captured by the AMO Index.[41]

IPCC models

Based on coupled Atmosphere-Ocean General Circulation Models from 2001, the THC tends to weaken somewhat rather than stop, and the warming effects outweigh the cooling, even over Europe.[42] In the IPCC Fifth Assessment Report, it was reported that it is very unlikely that the AMOC will undergo a rapid transition (high confidence).[43]

In popular culture

The film The Day After Tomorrow exaggerates a scenario related to the AMOC shutdown.

Kim Stanley Robinson's science-fiction novel Fifty Degrees Below, a volume in his Science in the Capital series, depicts a shutdown of thermohaline circulation & mankind's efforts to counteract it by adding great quantities of salt to the ocean.

In Ian Douglas' Star Corpsman novels, an AMOC shutdown triggered an early glacial maximum, covering most of Canada and northern Europe in ice sheets by the mid-22nd century.

See also

References

  1. ↑ Rahmstorf, Stefan; Box, Jason E.; Feulner, Georg; Mann, Michael E.; Robinson, Alexander; Rutherford, Scott; Schaffernicht, Erik J. (2015). "Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation". Nature Climate Change 5 (5): 475–480. doi:10.1038/nclimate2554. ISSN 1758-678X. Bibcode: 2015NatCC...5..475R. https://eprints.ucm.es/32657/1/robinson10postprint.pdf.  closed access PDF in UNEP Document Repository
  2. ↑ University of South Florida (January 22, 2016). "Melting Greenland ice sheet may affect global ocean circulation, future climate". Phys.org. http://phys.org/news/2016-01-greenland-ice-sheet-affect-global.html. 
  3. ↑ Hansen, James; Sato, Makiko (2015). "Predictions Implicit in "Ice Melt" Paper and Global Implications". http://csas.ei.columbia.edu/2015/09/21/predictions-implicit-in-ice-melt-paper-and-global-implications/. 
  4. ↑ Yin, Jianjun; Griffies, Stephen (25 March 2015). "Extreme sea level rise event linked to AMOC downturn". CLIVAR. http://www.usclivar.org/research-highlights/extreme-sea-level-rise-event-linked-amoc-downturn. 
  5. ↑ Jean Lynch-Stieglitz (2017). "The Atlantic Meridional Overturning Circulation and Abrupt Climate Change". Annual Review of Marine Science. doi:10.1146/annurev-marine-010816-060415. Bibcode: 2017ARMS....9...83L. 
  6. ↑ Mihai Dima; Gerrit Lohmann (2010). "Evidence for Two Distinct Modes of Large-Scale Ocean Circulation Changes over the Last Century". Journal of Climate 23 (1): 5–16. doi:10.1175/2009JCLI2867.1. Bibcode: 2010JCli...23....5D. https://epic.awi.de/id/eprint/20790/1/Dim2009a.pdf. 
  7. ↑ Stefan Rahmstorf; Jason E. Box; Georg Feulner; Michael E. Mann; Alexander Robinson; Scott Rutherford; Erik J. Schaffernicht (2015). "Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation". Nature 5 (5): 475–480. doi:10.1038/nclimate2554. Bibcode: 2015NatCC...5..475R. https://eprints.ucm.es/32657/1/robinson10postprint.pdf. 
  8. ↑ Mooney, Chris (February 1, 2016). "Why the U.S. East Coast could be a major 'hotspot' for rising seas". The Washington Post. https://www.washingtonpost.com/news/energy-environment/wp/2016/02/01/why-the-u-s-east-coast-could-be-a-major-hotspot-for-sea-level-rise/. 
  9. ↑ University Of Illinois At Urbana-Champaign (20 December 2004). "Shutdown Of Circulation Pattern Could Be Disastrous, Researchers Say". ScienceDaily. https://www.sciencedaily.com/releases/2004/12/041219153611.htm. 
  10. ↑ Possible Economic Impacts of a Shutdown of the Thermohaline Circulation: an Application of FUND. 
  11. ↑ "Weather Facts: North Atlantic Drift (Gulf Stream) - Weather UK - weatheronline.co.uk". http://www.weatheronline.co.uk/reports/wxfacts/North-Atlantic-Drift-Gulf-Stream.htm. 
  12. ↑ "The North Atlantic Drift Current". http://oceancurrents.rsmas.miami.edu/atlantic/north-atlantic-drift.html. 
  13. ↑ 13.0 13.1 Schiermeier, Quirin (2007). "Ocean circulation noisy, not stalling". Nature 448 (7156): 844–5. doi:10.1038/448844b. PMID 17713489. Bibcode: 2007Natur.448..844S. 
  14. ↑ "How far does light travel in the ocean?". https://oceanservice.noaa.gov/facts/light_travel.html. 
  15. ↑ Körtzinger, Arne; Schimanski, Jens; Send, Uwe; Wallace, Douglas (19 November 2004). "The Ocean Takes a Deep Breath" (in EN). Science 306 (5700): 1337. doi:10.1126/science.1102557. ISSN 0306-1337. PMID 15550662. https://www.science.org/doi/abs/10.1126/science.1102557. 
  16. ↑ Breitburg, Denise; Levin, Lisa A.; Oschlies, Andreas; Grégoire, Marilaure; Chavez, Francisco P.; Conley, Daniel J.; Garçon, Véronique; Gilbert, Denis et al. (5 January 2018). "Declining oxygen in the global ocean and coastal waters" (in EN). Science 359 (6371). doi:10.1126/science.aam7240. PMID 29301986. Bibcode: 2018Sci...359M7240B. 
  17. ↑ 17.0 17.1 Lee R. Kump; Alexander Pavlov; Michael A. Arthur: Massive release of hydrogen sulfide to the surface ocean and atmosphere during intervals of oceanic anoxia. Geology 2005;33;397-400. doi: 10.1130/G21295.1
  18. ↑ 18.0 18.1 Robert A. Berner, Peter D. Ward: Positive Reinforcement, H2S, and the Permo-Triassic Extinction: Comment and Reply. doi: 101130:G22641.1
  19. ↑ Jeffrey T. Kiehl, Christine A. Shields: Climate simulation of the latest Permian: Implications for mass extinction, Geology 33(9) (2005). DOI: 10.1130/G21654.1
  20. ↑ Peter D. Ward: Under a Green Sky: Global Warming, the Mass Extinctions of the Past, and What They Can Tell Us About Our Future Paperback – Illustrated, March 25, 2008. p192
  21. ↑ J. Hansen, M. Sato, P. Hearty, R. Ruedy, M. Kelley, V. Masson-Delmotte, G. Russell, G. Tselioudis, J. Cao, E. Rignot, I. Velicogna, E. Kandiano, K. von Schuckmann, P. Kharecha, A. N. Legrande, M. Bauer, and K.-W. Lo (2015). "Ice melt, sea level rise and superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2 °C global warming is highly dangerous". Atmospheric Chemistry and Physics Discussions 15 (14): 20059–20179. doi:10.5194/acpd-15-20059-2015. Bibcode: 2015ACPD...1520059H. http://www.atmos-chem-phys-discuss.net/acp-2015-432/. 
  22. ↑ Satellites record weakening North Atlantic Current. NASA, 15 April 2004.
  23. ↑ Leake, Jonathan (8 May 2005). "Britain faces big chill as ocean current slows". The Sunday Times. http://www.timesonline.co.uk/article/0,,2087-1602579,00.html. 
  24. ↑ Gulf Stream slowdown? RealClimate.org, 26 May 2005.
  25. ↑ F. Pearce. Failing ocean current raises fears of mini ice age. NewScientist, 30 November 2005
  26. ↑ Quadfasel D (December 2005). "Oceanography: The Atlantic heat conveyor slows". Nature 438 (7068): 565–6. doi:10.1038/438565a. PMID 16319866. Bibcode: 2005Natur.438..565Q. 
  27. ↑ Schiermeier, Quirin (2007). "Climate change: A sea change". Nature 439 (7074): 256–60. doi:10.1038/439256a. PMID 16421539. Bibcode: 2006Natur.439..256S.  (subscription required); see also "Atlantic circulation change summary". RealClimate.org. 19 January 2006. http://www.realclimate.org/index.php?p=187. 
  28. ↑ Våge, Kjetil; Pickart, Robert S.; Thierry, Virginie; Reverdin, Gilles; Lee, Craig M.; Petrie, Brian; Agnew, Tom A.; Wong, Amy et al. (2009). "Surprising return of deep convection to the subpolar North Atlantic Ocean in winter 2007–2008". Nature Geoscience 2 (1): 67–72. doi:10.1038/ngeo382. Bibcode: 2009NatGe...2...67V. https://archimer.ifremer.fr/doc/00000/6415/. 
  29. ↑ Birchard, George (6 January 2010). "Freak Current Takes Gulf Stream to Greenland". Daily Kos. http://www.dailykos.com/story/2010/1/6/822520/-Freak-Current-Takes-Gulf-Stream-to-Greenland. 
  30. ↑ Birchard, George (30 December 2009). "Warm Atlantic Water Rapidly Replacing Arctic Sea Ice". Daily Kos. http://www.dailykos.com/story/2009/12/30/820607/-Warm-Atlantic-Water-Rapidly-Replacing-Arctic-Sea-Ice-. 
  31. ↑ Niklas Boers: Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation. Nature Climate Change volume 11, pages680–688 (2021)
  32. ↑ Hawkins, E.; Smith, R. S.; Allison, L. C.; Gregory, J. M.; Woollings, T. J.; Pohlmann, H.; De Cuevas, B. (2011). "Bistability of the Atlantic overturning circulation in a global climate model and links to ocean freshwater transport". Geophysical Research Letters 38 (10): n/a. doi:10.1029/2011GL047208. Bibcode: 2011GeoRL..3810605H. https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2011GL047208. 
  33. ↑ Knutti, R., and T. F. Stocker (2002), Limited predictability of the future thermohaline circulation close to an instability threshold, J. Clim., 15(2), 179–186.
  34. ↑ "Salinity and Brine". NSIDC. https://nsidc.org/cryosphere/seaice/characteristics/brine_salinity.html. 
  35. ↑ Mooney, Chris (September 30, 2015). "Everything you need to know about the surprisingly cold 'blob' in the North Atlantic ocean". The Washington Post. https://www.washingtonpost.com/news/energy-environment/wp/2015/09/30/everything-you-need-to-know-about-the-cold-blob-in-the-north-atlantic-ocean/. 
  36. ↑ Gierz, Paul (31 August 2015). "Response of Atlantic Overturning to future warming in a coupled atmosphere-ocean-ice sheet model". Geophysical Research Letters 42 (16): 6811–6818. doi:10.1002/2015GL065276. Bibcode: 2015GeoRL..42.6811G. 
  37. ↑ Turrell, B. The Big Chill Transcript of discussion on BBC 2, 13 November 2003
  38. ↑ Vellinga, M.; Wood, R.A. (2002). "Global climatic impacts of a collapse of the Atlantic thermohaline circulation". Climatic Change 54 (3): 251–267. doi:10.1023/A:1016168827653. http://www.ocean.washington.edu/people/faculty/luanne/classes/pcc586/papers/vellingawood_thc2002.pdf. 
  39. ↑ Lund, DC; Lynch-Stieglitz, J; Curry, WB (November 2006). "Gulf Stream density structure and transport during the past millennium". Nature 444 (7119): 601–4. doi:10.1038/nature05277. PMID 17136090. Bibcode: 2006Natur.444..601L. https://authors.library.caltech.edu/56282/5/Fig4.pdf. 
  40. ↑ Sgubin (2017). "Abrupt cooling over the North Atlantic in modern climate models". Nature Communications 8. doi:10.1038/ncomms14375. PMID 28198383. Bibcode: 2017NatCo...8.....S. 
  41. ↑ Eleanor Frajka-Williams; Claudie Beaulieu; Aurelie Duchez (2017). "Emerging negative Atlantic Multidecadal Oscillation index in spite of warm subtropics". Scientific Reports 7 (1): 11224. doi:10.1038/s41598-017-11046-x. PMID 28894211. Bibcode: 2017NatSR...711224F. 
  42. ↑ IPCC TAR WG1 (2001). "9.3.4.3 Thermohaline circulation changes". Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press. ISBN 978-0-521-80767-8. http://www.grida.no/climate/ipcc_tar/wg1/357.htm.  (pb: ISBN:0-521-01495-6)
  43. ↑ "IPCC AR5 WG1". IPCC. p. Table 12.4. http://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_ALL_FINAL.pdf. 

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