RAPID-RAPIT: What is the risk of thermohaline circulation collapse?

The aim of this experiment is to assess the risk of AMOC collapse in the coming century.

RAPIT (Risk Assessment, Probability and Impacts Team) is a large project that is part of NERC’s RAPID-WATCH programme. The project involves scientists from the National Oceanography Centre, Durham University, University of Reading, Met Office, University of Oxford, British Antarctic Survey, Imperial College, and London School of Economics.

Motivation

The Atlantic meridional overturning circulation (AMOC), often referred to as the thermohaline circulation, plays a pivotal role in the global climate system. (See the section below for a basic description of the AMOC.) The AMOC transports heat northwards in the Atlantic via the Gulf Stream and North Atlantic Current, making European climate significantly warmer than it would otherwise be. Evidence from palaeoclimate records suggests that this circulation has changed dramatically in the past (e.g., Clark et al, 2002), and there is concern that it could be disrupted in the future. As concentrations of atmospheric greenhouse gases increase and the climate warms, it is expected that there will be increased precipitation in mid-latitudes and less formation of sea ice (Meehl et al, 2007). This would make the surface ocean less salty, which (along with the warming) makes the surface waters less dense, and less likely to sink, meaning that the AMOC would weaken or maybe collapse completely. Previous experiments have shown that if the AMOC were to collapse, the impacts would be felt across the globe, and most severely in the North Atlantic region (see below).

Simple models (Stommel, 1961; Rooth, 1982) and models of intermediate complexity (e.g., Rahmstorf et al, 2005) show that if extra freshwater is added to the North Atlantic (making the surface water less dense), then the AMOC can collapse, and even when the anomalous freshwater forcing is removed, the circulation may not return to its original state; under the same forcing, there can be two stable states for the circulation: “on” and “off”. However, when the AMOC is forced to collapse in more complicated climate models, it often recovers gradually after the forcing is removed, suggesting the off-state may not be stable (e.g., Vellinga et al, 2002; Yin et al, 2006). Also, when these complex models are forced by reasonably realistic future greenhouse gas projections, they tend not to show an AMOC collapse; the models used in the most recent IPCC assessment suggest that the AMOC is likely to gradually weaken over the 21st century, but not collapse abruptly (Meehl et al, 2007).

However, these estimates for the response of the AMOC to future anthropogenic forcing rely on our “best guess” for many of the complex model details, and do not account for uncertainty in the model input parameters. There are many physical processes that climate models are unable to represent explicitly, and their effects must instead be parameterised. These processes include oceanic mixing and eddy activity, atmospheric convection and cloud physics. Parameterisations generally involve choosing the most suitable value for each coefficient, but it is often the case that there is a range of possible values that the coefficients can take. This means that a climate model has a multi-dimensional parameter space, with potentially billions of model versions (theoretically an infinite number), some with more realistic climates than others. Making climate projections using just one version of a model may mean that certain types of behaviour could be missed. This is particularly important for complex nonlinear systems like the AMOC. It is possible that the AMOC may be more or less stable in different parts of a model’s parameter space. To properly assess the risk of AMOC collapse, we must fully explore the possible responses.

Experiment design

This experiment uses HadCM3, a coupled model with fully dynamic atmosphere and ocean components, making it the most comprehensive climate model available on climateprediction.net (it is the higher-resolution counterpart of FAMOUS, which has been used in other CPDN experiments). It is run without flux adjustments which “nudge” the climate towards a realistic state, but have an adverse effect on important ocean processes. HadCM3 has been used extensively for climate research, and was one of the models used in the IPCC fourth assessment report. The experiment consists of a 10,000-member ensemble, covering a wide range of HadCM3’s parameter space. This is the first time that such a large ensemble has ever been carried out using this model. Ordinarily, HadCM3 has to be run on a supercomputer, meaning that it can run fast, but only a few ensemble members are possible (of order tens). With your help, running HadCM3 on climateprediction.net means that we can have a much, much larger ensemble, which is needed to properly explore the model’s capabilities.

Because of the infinite number of possible models living in parameter space, even 10,000 models would not be enough to produce valid risk assessments if taken on their own. We plan to use a state-of-the-art statistical technology called emulation in order make the maximum use of our 10,000-member ensemble. An emulator is a statistical tool that allows us to take any values of the parameters and predict, with associated uncertainty, what our climate model, HadCM3, will do at those parameter settings. The 10,000 runs of HadCM3 will be used to fit and train this statistical model.

Whilst it will take months for a complete run of HadCM3 to finish, the emulator for HadCM3 will take seconds to predict the model output. This means that once the emulator has been built by the RAPIT project statisticians, we can use it to quickly search parameter space for areas of potential AMOC collapse (and then we can run the model there to check). We can also produce a risk assessment that takes into account our uncertainty about what the model will do in any part of parameter space.

The first stage of the experiment involves spinning up each version of the model to be as close to its own equilibrium climate state as possible. Once the models are spun up, we will run them with 20th century forcings, and a variety of idealised future CO2 forcing scenarios to examine how the AMOC responds to changing CO2.

In this experiment, every ensemble member is important! It is likely that many of the model versions will have climates that look wildly unrealistic, and several of them may crash. All of this information is vital for understanding parameter space and for building useful emulators. Somewhere in parameter space there may be a region of non-implausible climate in which the AMOC collapses under CO2 forcing. This region might lie somewhere in between an area in which the climate looks unrealistic and an area in which the climate looks reasonable. If this is the case, then information from the extreme models will be vital for building emulators that can search for the interesting area. Please don’t be discouraged if the model you are running looks crazy – the information it tells us will be very useful!

Extra background information

What is the thermohaline circulation/ AMOC?

In the Atlantic Ocean, there is a net flow of water northwards in the surface layers in the Gulf Stream and its extension, the North Atlantic Current (see schematic). This brings warm, salty water from the tropics to the high northern latitudes. At high latitudes, the ocean releases heat to the atmosphere, making the surface waters cooler, and sea ice is formed, making the surface waters saltier. Both the temperature decrease and the salinity increase make the water denser. This allows the surface waters to sink and return southwards in the deep ocean. This deep water finally returns to the surface, closing the circulation loop; it is thought that one way this happens is through upwelling in the Southern Ocean around Antarctica. The AMOC transports around 20 million cubic metres of water per second, and it transports around 1PW of heat northwards in the Atlantic basin, contributing to the mild climate of Western Europe.

What would happen if the AMOC collapsed?

Model studies suggest that a collapse of the AMOC could lead to a reduction in surface air temperature of around 1-3°C in the North Atlantic region and surrounding land masses, but with local cooling of up to 8°C in areas of increased sea ice (Vellinga and Wood, 2002; Vellinga et al 2002; Manabe and Stouffer; 1997; Jacob et al 2005). A smaller cooling effect would be expected throughout the northern hemisphere, with a slight warming in the southern hemisphere after a few decades (eg., Vellinga and Wood, 2002; Schiller et al, 1997). Several studies suggest that there would be a change in precipitation patterns over the tropics, associated with a southward shift of the intertropical convergence zone (e.g., Vellinga et al 2002; Brayshaw et al, 2009), which could also affect the intensity of the El Nino Southern Oscillation (ENSO) in the Pacific (Timmermann et al, 2007). A collapse of the AMOC may also lead to an intensification of the North Atlantic storm track, with stronger winds over Europe (Vellinga and Wood, 2002; Jacob et al, 2005; Brayshaw et al, 2009). Over a period of years to decades, there would be regional changes in sea level, with a sea level rise in the North Atlantic of up to 80cm (Levermann et al, 2005; Vellinga and Wood, 2008; Kuhlbrodt et al, 2009). Studies also suggest there could be impacts on the carbon cycle (Zickfeld et al, 2008) and on soil moisture and primary productivity of the terrestrial vegetation (Vellinga and Wood, 2002).

Fig. 1: Simple schematic illustration of the AMOC. Surface currents are shown in red, and deep currents are shown in blue. The opaque arrows indicate a warming of the overlying air as the westerly winds cross the Atlantic basin towards Europe.

References

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• Clark, P. U., N. G. Pisias, T. F. Stocker and A. J. Weaver (2002), The role of the thermohaline circulation in abrupt climate change, Nature, 415, 863-869.
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