The meridional overturning circulation (MOC) is part of a global ocean circulation that redistributes heat from Equatorial to Polar regions. In the Atlantic the MOC carries heat northward (the Atlantic Heat Conveyor) which is released to the atmosphere and maintains UK temperatures between 3 to 5°C higher than elsewhere at similar latitudes. However, the present strength and structure of the MOC may not continue. The 2007 IPCC assessment report (IPCC, 2007) suggests that there is less than 10% chance of abrupt changes during the 21^st Century, but that there is greater than 90% chance that MOC will slow by an average of 25% compared to pre-industrial levels, offsetting some of the warming over the European sector of the North Atlantic, and contributing to the rate of sea-level-rise. Daily observations using the RAPID MOC mooring array at 26.5°N are providing a continuous and growing time-series of the MOC strength and structure, but the five year record is at present too short to establish trends in the annual mean MOC. Other observations do not at present provide a coherent Atlantic wide picture of MOC variability, and there is little evidence of any long-term slowing. Ocean assimilation models suggest a slowing over the past decade of around 10%. However, models still have many problems in representing ocean circulation and conclusions of change are very uncertain.

What is already happening: Medium

What could happen: Medium

Our confidence in answering the question 'What is happening now?' has risen, since the 2007-2008 ARC, given the extended length of the RAPID 26°N mooring time series. In fact, confidence in what is happening in the immediate time frame (say the last year or so) is actually high, given the accuracy of the monitoring approach. However, we have assessed here the confidence in what is happening to the longer-term (i.e. decadal) trend, which is of most relevance to society with respect to potential AMOC slowdown, and any possible climatic impacts. Recent projections of future AMOC strength show a consensus of agreement between climate models regarding the sign of change to the AMOC this century. A slow down of between 0 and 50% is considered very likely (>90% chance) by 2100.

The top priority knowledge gaps that need to be addressed in the short term to provide better advice to be given to policy makers are:

Figure 5: Observational programs presently measuring components of the AMOC, and
proposed meridional het flux and AMOC basin-wide, full depth arrays (black dashed).

1. Ocean Observations and Monitoring: Fundamentally we require a set of benchmark observations of the AMOC that can provide the necessary full depth, continent-to-continent dynamical constraints at different latitudes throughout the Atlantic for verifying assimilations, coupled climate model hindcasts and for ocean initialization for climate forecasts (Figure 5) (Cunningham et al., 2009).
2. Ocean Models and Past Reconstruction. Ocean models should be increasingly used to gain a better understanding of past AMOC changes and the present state of the AMOC. In the near future, we can expect further simulation and analysis with eddy-resolving ocean models that achieve ever more realistic pathways, vertical structure and properties (Hecht and Smith, 2009). However, the modeling community must strive to improve key details of model AMOC (e.g., depth of NADW outflow, see Saunders and Cunningham, 2008).
3. Climate Models and Decadal Forecasting. Following the pioneering work of Smith et al., 2007 and Keenlyside et al., 2008, further development of decadal climate forecasting should:
   • place more emphasis on the issue of initializing the AMOC state appropriately
   • be underpinned by continuing research on decadal predictability of the AMOC
   • address different assimilation strategies

Given that almost all climate model projections of future climate incorporate the most probable change to the AMOC this century, i.e. a partial slow down, the majority of socio-economic impacts to the UK and Europe are already summarised through the analysis of changes to primary climatic conditions (i.e. air temperature, precipitation patterns, sea level rise, all of which contain some AMOC-related signal within the broader, and larger, greenhouse gas response).

A broad body of work, however, has addressed impacts relating to the lower-probability, higher-impact, risk of a complete shutdown, some of which attempts to isolate socio-economic consequences.

Initial work in this field, e.g. Higgins and Vellinga (2003), investigated impacts of an AMOC-absent climate system relative to a pre-industrial climate, meaning that any confounding effects on impacts via greenhouse gas warming were unaccounted for. More recently, coupled climate, vegetation and ecosystem modelling experiments have been performed whereby the earth system is allowed to evolve under greenhouse gas emission scenarios, followed by large AMOC perturbation. This approach enables the quantification of AMOC shutdown impacts against the more realistic setting of an unperturbed global warming (called "amoc_ghg"), rather than the pre-industrial climate.

An analysis of the climatic impacts in an amoc_ghg situation using the UK Hadley Centre coupled climate model is given in Vellinga and Wood (2007). A similar experiment, using a different climate model, is summarised by Kuhlbrodt et al., (2009) and is extended to include marine and terrestrial ecosystem model responses. In their experiment, Kuhlbrodt et al. (2009) allow IPCC greenhouse gas emission scenarios to progress and then reduce, assuming co-ordinated political intervention on emissions during the 22^nd century. In one simulation an added perturbation of gradual fresh-water addition is introduced, resulting in an AMOC collapse part way through the 22^nd century: a comparison of the responses in this simulation with the others allows isolation of the additional impacts relating to the AMOC. The key results, likely to be of socio-economic importance, from this study are summarised here:

• Sea-level Rise: An additional sea level rise of ~80cm around European coasts is evident in the AMOC-collapse simulation by 2150. By the end of the 21^st century, the additional AMOC-related sea level rise is 50cm. If this is superimposed upon an approximate estimate of a 'regular' greenhouse gas sea level rise for the same period, ~50cm, the additional financial requirement for European land protection and population relocation would be US$670 million per year, using calculations based on Stern (2007). The sign and magnitude of these sea-level rises are comparable with other investigations into the response of North Atlantic sea level to abrupt changes in the AMOC (e.g. Vellinga and Wood, 2007; Levermann et al., 2005).
• Crops: In all simulations, including the AMOC-collapse scenario, the total area suitable for crop production in Europe increases, despite stresses introduced relating to water shortage. This response can be related to the dominance of carbon fertilisation as the primary forcing agent for crop efficiency and yield, for the region as an average: by 2150, European annual cereal production, estimated by the vegetation model coupled to the climate model, approximately doubles, even when the AMOC collapses.
• Marine Net Primary Production (NPP): By 2150 North Atlantic NPP is seen to markedly decrease in the 'regular' greenhouse gas simulation by around 70% compared with 1990 levels. The response in the AMOC-collapse scenario is similar, but the magnitude of the decrease is only slightly larger, despite the absence of the AMOC. The similarity in responses is related to the reduced tendency for mixing in the upper ocean and the associated depletion of nutrients towards the surface; the larger decrease in NPP for the AMOC-collapse scenario seems related to a reduction in north-eastward nutrient transport through the Faroe-Shetland region associated with a retarded North Atlantic Current.
• Fish: The application of high-resolution regional ocean - ecosystem modules in this study, simulating cod larvae and juvenile survival rates in the prime spawning area north of Norway, demonstrated that a 35% reduction in the AMOC led to a widespread decrease in cod distribution, compared to the present day.  The decrease is thought to be related to local ocean circulation changes, resulting in the advection of cod juveniles and larvae into regions where they are unable to survive. Parameterizing this effect under global warming scenarios shows that whilst in a 'regular' greenhouse warming scenario, with a partial AMOC slow down, cod harvesting remains largely unchanged (due to successful recruitment of new generations), an AMOC collapse super-imposed on this scenario leads to marked reductions in cod harvest as the North Atlantic population diminishes.

Further important socio-economic exposures related to the AMOC do exist which are not addressed in Kuhlbrodt et al., (2009). One prominent such impact is change to energy demand and consumption. With respect to Western Europe, patterns of temperature change under an AMOC collapse within a simulated greenhouse-warming world, suggest that summer temperatures may remain warmer than pre-industrial conditions, although the magnitude of warming is reduced. During winter, however, an AMOC collapse may be sufficient to cool Western Europe below the pre-industrial reference temperature. Vellinga and Wood (2007) demonstrate in one such climate model experiment that the mean winter Central England Temperature in 2049-2059 after a hypothetical AMOC collapse embedded within an IS92a warming scenario is approximately 3°C, 1°C colder than pre-industrial conditions. Further analysis of this scenario also revealed a tripling of the frost season. Such impacts would almost certainly influence energy demands, although as of yet no quantitative assessment has been undertaken.

Robust investigations of what impacts would likely be felt by the UK marine environment per se in the event of abrupt changes to the AMOC are presently lacking in the literature and at best would be speculative given presently-published information. This can be attributed to the relative recentness with which feasible coupled ocean-atmosphere computer experiments can be performed having the necessary geographical resolutions and, perhaps more so, to the immediacy of funding investigations, first, into potential impacts with respect to regional atmospheric and terrestrial climate with their first-order effects upon human population. We hope that as technological resources and funding opportunities develop further this important knowledge deficit can be addressed by the scientific community.

Cunningham, S., R. Marsh., R. Wood, C. Wallace, T. Kuhlbrodt, and S. Dye (2010) Atlantic Heat Conveyor (Atlantic Meridional Overturning Circulation) in MCCIP Annual Report Card 2010-11, MCCIP Science Review, 14pp. www.mccip.org.uk/arc