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Abrupt Change in the Atlantic Meridional Overturning Circulation




The Atlantic Meridional Overturning Circulation (AMOC) is an important component of the Earth’s climate system, characterized by a northward flow of warm, salty water in the upper layers of the Atlantic, a transformation of water mass properties at higher northern latitudes of the Atlantic in the Nordic and Labrador Seas that induces sinking of surface waters to form deep water, and a southward flow of colder water in the deep Atlantic (Fig. 1.6).  There is also an interhemispheric transport of heat associated with this circulation, with heat transported from the Southern Hemisphere to the Northern Hemisphere.  This ocean current system thus transports a substantial amount of heat from the Tropics and Southern Hemisphere toward the North Atlantic, where the heat is released to the atmosphere (Fig. 1.7).

Changes in the AMOC have a profound impact on many aspects of the global climate system.  There is growing evidence that fluctuations in Atlantic sea surface temperatures, hypothesized to be related to fluctuations in the AMOC, have played a prominent role in significant climate fluctuations around the globe on variety of time scales.  Evidence from the instrumental record (based on the last~130 years) shows pronounced, multidecadal swings in large-scale Atlantic temperature that may be at least partly a consequence of fluctuations in the AMOC.  Recent modeling and observational analyses have shown that these multidecadal shifts in Atlantic temperature exert a substantial influence on the climate system ranging from modulating African and Indian monsoonal rainfall to tropical Atlantic atmospheric circulation conditions of relevance for hurricanes.  Atlantic SSTs also influence summer climate conditions over North America and Western Europe. 
Evidence from paleorecords suggests that there have been large, decadal-scale changes in the AMOC, particularly during glacial times. These abrupt change events have had a profound impact on climate, both locally in the Atlantic and in remote locations around the globe (Fig. 1.1).  Research suggests that these abrupt events were related to discharges of freshwater into the North Atlantic from surrounding land-based ice sheets. Subpolar North Atlantic air temperature changes of more than 10 °C on time scales of a decade or two have been attributed to these abrupt change events.


Uncertainties in Modeling the AMOC

As with any projection of future behavior of the climate system, our understanding of the AMOC in the 21st century and beyond relies on numerical models that simulate the important physical processes governing the overturning circulation. An important test of model skill is to conduct transient simulations of the AMOC in response to the addition of freshwater and compare with paleoclimatic data. Such a test requires accurate, quantitative reconstructions of the freshwater forcing, including its volume, duration, and location, plus the magnitude and duration of the resulting reduction in the AMOC.  This information is not easy to obtain; coupled general circulation model (GCM) simulations of most events have been forced with idealized freshwater pulses and compared with qualitative reconstructions of the AMOC (e.g., Hewitt et al., 2006; Peltier et al., 2006; see also Stouffer et al., 2006).  There is somewhat more information about the freshwater pulse associated with an event 8,200 years ago, but important uncertainties remain (Clarke et al., 2004; Meissner and Clark, 2006).  Thus, simulations of such paleoclimatic events provide important qualitative perspectives on the ability of models to simulate the response of the AMOC to forcing changes, but their ability to provide quantitative assessments is limited.  Improvements in this area would be an important advance, but the difficulty in measuring even the current AMOC makes this task daunting. 
Although numerical models show good skill in reproducing the main features of the AMOC, there are known errors that introduce uncertainty in model results. Some of these model errors, particularly in temperature and heat transport, are related to the representation of western boundary currents and deep-water overflow across the Greenland-Iceland-Scotland ridge.  Increasing the resolution of current coupled ocean-atmosphere models to better address these errors will require an increase in computing power by an order of magnitude. Such higher resolution offers the potential of more realistic and robust treatment of key physical processes, including the representation of deep-water overflows.  Efforts are being made to improve this model deficiency (Willebrand et al., 2001; Thorpe et al., 2004; Tang and Roberts, 2005).  Nevertheless, recent work by Spence et al. (2008) using an Earth-system model of intermediate complexity (EMIC) found that the duration and maximum amplitude of their coupled model response to freshwater forcing showed little sensitivity to increasing resolution.  They concluded that the coarse-resolution model response to boundary layer freshwater forcing remained robust at finer horizontal resolutions.

Future Changes in the AMOC

A particular focus on the AMOC in Chapter 4 of this report is to address the widespread notion, both in the scientific and popular literature, that a major weakening or even complete shutdown of the AMOC may occur in response to global warming. This discussion is driven in part by model results indicating that global warming tends to weaken the AMOC both by warming the upper ocean in the subpolar North Atlantic and through increasing the freshwater input (by more precipitation, more river runoff, and melting inland ice) into the Arctic and North Atlantic. Both processes reduce the density of the upper ocean in the North Atlantic, thereby stabilizing the water column and weakening the AMOC. 
It has been theorized that these processes could cause a weakening or shutdown of the AMOC that could significantly reduce the poleward transport of heat in the Atlantic, thereby possibly leading to regional cooling in the Atlantic and surrounding continental regions, particularly Western Europe. This mechanism can be inferred from paleodata and is reproduced at least qualitatively in the vast majority of climate models (Stouffer et al., 2006). One of the most misunderstood issues concerning the future of the AMOC under anthropogenic climate change, however, is its often-cited potential to cause the onset of the next ice age. As discussed by Berger and Loutre (2002) and Weaver and Hillaire-Marcel (2004), it is not possible for global warming to cause an ice age by this mechanism.
In the past, there was disagreement in determining which of the two processes governing upper-ocean density will dominate under increasing GHG concentrations, but a recent 11-model intercomparison project found that an MOC reduction in response to increasing GHG concentrations was caused more by changes in surface heat flux than by changes in surface freshwater flux (Gregory et al., 2005).  Nevertheless, different climate models show different sensitivities toward an imposed freshwater flux (Gregory et al., 2005).  It is therefore not fully clear to what degree salinity changes will affect the total overturning rate of the AMOC.  In addition, by today’s knowledge, it is hard to assess how large future freshwater fluxes into the North Atlantic might be.  This is due to uncertainties in modeling the hydrological cycle in the atmosphere, in modeling the sea-ice dynamics in the Arctic, as well as in estimating the melting rate of the Greenland ice sheet.  
It is important to distinguish between an AMOC weakening and an AMOC collapse.  Historically, coupled models that eventually lead to a collapse of the AMOC under global warming scenarios have fallen into two categories:  (1) coupled atmosphere-ocean general circulation models (AOGCMs) that required ad hoc adjustments in heat or moisture fluxes to prevent them from drifting away from observations, and (2) intermediate-complexity models with longitudinally averaged ocean components.  Current AOGCMs used in the IPCC AR4 assessment typically do not use flux adjustments and incorporate improved physics and resolution.  When forced with plausible estimates of future changes in greenhouse gases and aerosols, these newer models project a gradual 25-30% weakening of the AMOC, but not an abrupt change or collapse.  Although a transient collapse with climatic impacts on the global scale can always be triggered in models by a large enough freshwater input (e.g., Vellinga and Wood, 2007), the magnitude of the required freshwater forcing is not currently viewed as a plausible estimate of the future.  In addition, many experiments have been conducted with idealized forcing changes, in which atmospheric CO2 concentration is increased at a rate of 1%/year to either two times or four times the preindustrial levels and held fixed thereafter.  In virtually every simulation, the AMOC reduces but recovers to its initial strength when the radiative forcing is stabilized at two times or four times the preindustrial levels. 
Perhaps more important for 21st century climate change, is the possibility for a rapid transition to seasonally ice-free Arctic conditions.  In one climate model simulation, a transition from conditions similar to pre-2007 levels to a near-ice-free Septembe extent occurred in a decade (Holland et al., 2006).  Increasing ocean heat transport was implicated in this simulated rapid ice loss, which ultimately resulted from the interaction of large, intrinsic variability and anthropogenically forced change.  It is notable that climate models are generally conservative in the modeled rate of Arctic ice loss as compared to observations (Stroeve et al., 2007; Figure 1-3), suggesting that future ice retreat could occur even more abruptly than simulated.
This nonlinear response occurs because sea ice has a strong inherent threshold in that its existence depends on the freezing temperature of seawater.  Additionally, strong positive feedbacks associated with sea ice act to accelerate its change.  The most notable of these is the positive surface albedo feedback in which changes in ice cover and surface properties modify the surface reflection of solar radiation.  For example, in a warming climate, reductions in ice cover expose the dark underlying ocean, allowing more solar radiation to be absorbed.  This enhances the warming and leads to further ice melt.  Because the AMOC interacts with the circulation of the Arctic Ocean at its northern boundary, future changes in the AMOC and its attendant heat transport thus have the potential to further influence the future of sea ice.



Our analysis indicates that it is very likely that the strength of the AMOC will decrease over the course of the 21st century.  In models where the AMOC weakens, warming still occurs downsteam over Europe due to the radiative forcing associated with increasing greenhouse gases.  No model under plausible estimates of future forcing exhibits an abrupt collapse of the MOC during the 21st century, even accounting for estimates of accelerated Greenland ice sheet melting.  We conclude that it is very unlikely that the AMOC will abruptly weaken or collapse during the course of the 21st century.  Based on available model simulations and sensitivity analyses, estimates of maximum Greenland ice sheet melting rates, and our understanding of mechanism of abrupt climate change from the paleoclimatic record, we further conclude that it is unlikely that the AMOC will collapse beyond the end of the 21st century as a consequence of global warming, although the possibility cannot be entirely excluded.
The above conclusions depend upon our understanding of the climate system and on the ability of current models to simulate the climate system.  An abrupt collapse of the AMOC in the 21st century would require either a sensitivity of the AMOC to forcing that is far greater than current models suggest or a forcing that greatly exceeds even the most aggressive of current rejections (such as extremely rapid melting of the Greenland ice sheet).  While we view these as very unlikely, we cannot exclude either possibility.  Further, even if a collapse of the AMOC is very unlikely, the large climatic impacts of such an event, coupled with the significant climate impacts that even decadal scale AMOC fluctuations induce, argue for a strong research effort to develop the observations, understanding, and models required to predict more confidently the future evolution of the AMOC.   

NOTE:  The term "forcing” is used throughout this report to indicate any mechanism that causes the climate system to change, or respond. Examples of forcings discussed in this report include freshwater forcing of ocean circulation and changes in sea-surface temperatures and radiative forcing as a forcing of drought. As defined by the IPCC Third Assessment Report (Church et al., 2001), radiative forcing refers to a change in the net radiation at the top of the troposphere caused by a change in the solar radiation, the infrared radiation, or other changes that affect the radiation energy absorbed by the surface (e.g., changes in surface reflection properties), resulting in a radiation imbalance. A positive radiative forcing tends to warm the surface on average, whereas a negative radiative forcing tends to cool it. Changes in GHG concentrations represent a radiative forcing through their absorption and emission of infrared radiation.




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