Thermohaline circulation is the global movement of ocean water from the surface to the deep ocean, which has a major role in regulating global climate. This movement has been halted in the past due to large inflows of freshwater. A regime shift could occur due to the release of large amounts of freshwater into the North Atlantic Ocean. This would reduce the differences in water salinity and density between Southern and Northern Atlantic Ocean that produce the thermohaline circulation, halting the transport of water from the tropical Atlantic to North Atlantic. 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 global warming, currenlty due to human greenhouse gases emissions. Warming is melting arctic ice, increasing the the amount of freshwater entering the ocean water. Several mechanisms may weaken the thermohaline circulation: freshwater-overturning, water temperature-density, and evaporation-salinity. Reducing the risks of this regime shift primarily require halting global greenhouse gas emissions to halt and reverse human caused climate change.
Key direct drivers
- Global climate change
- Marine & coastal
Key Ecosystem Processes
- Primary production
- Water cycling
- Food crops
- Fuel and fiber crops
- Climate regulation
- Food and nutrition
- Livelihoods and economic activity
- Social conflict
Typical spatial scale
Typical time scale
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
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.
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.
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