Tipping Point II - Circulation Change

This week, I will be outlining research and modelling exercises on the potential shutdown of the Atlantic Meridional Overturning Circulation (AMOC), also known as the Thermohaline circulation. 

Thermohaline Circulation (THC)

The thermohaline circulation, popularly called the global ocean conveyer belt, is an integral feature of the present day climate system. It sustains the current climate and is a major contributor to the global heat budget. As illustrated in Fig 1,  global oceanic circulation is driven by density gradients related to the formation of deep water. The density of seawater is governed by temperature and salinity. The THC process is briefly outlined below (Broecker 1997):

1. Warmer, saltier water brought into NE Atlantic, warms the European continent
2. Warm water cools and mixes with cold Arctic water, becomes dense and sinks, forming North Atlantic Deep Water 
3. Further sinking of dense water occurs near Antarctica with cool water sinking from the effect of the circumpolar currents, forming Antarctic Bottom Water 
4. Cold dense water returns to the surface throughout the world's oceans and forms a closed loop of exchanges between warm, surface water and cool, dense deep water

Modes of THC

Driven by density differences, the THC is particularly sensitive to the freshwater budget which will disrupt overturning of deepwater by reducing salinity. Looking in past abrupt changes to the THC, scientists have identified three possible modes (stable states) with fundamentally different climates (Rahmstorf 2000):

1. Warm - interglacial (current) mode where deep water forms in Nordic Seas 
2. Cold - glacial mode where deep water forms south near Greenland, Iceland and Scotland
3. Off - shutdown of THC, no formation of deep water in North Atlantic

Transitions between modes would cause abrupt climate changes. Previous transitions between modes have occurred in the form of Dansgaard-Oeschger cycles and Henrich events where large scale breakdown of N Atlantic icebergs dramatically increases freshwater input (Alley 2000). 

Tipping Point

As the thermohaline circulation is driven by density differences which are particularly sensitive to temperature and salinity, both sufficient heat from continued increase in GHG concentrations and alteration of the freshwater budget from melting ice can lead to fundamental re-organization of ocean circulation and transition to a alternative state (Clark 2002). Classified as being 'low probability with high impacts' by the IPCC, a critical threshold may be observed with hysteresis characteristics from non-linear behaviour. Modelling studies using coupled atmosphere-ocean General Circulation Models have suggested that the THC is particularly sensitive to freshwater infiltration on the order of 0.1 Sv with a transition across critical threshold within range between 0-0.15 Sv (Clark 2002; Rahmstorf 2000).

Modelling responses of the THC to rising CO2 concentrations with warming and melting ice effects, Wood et.al. 1999 proposed that a shutdown of convection in the Labrador Sea would result in a 20-25% reduction in deep water formation by the time CO2 quadruples pre-industrial levels. Salinity in the Nordic seas declined since the 1960s, suggesting a possibility of critical threshold of freshening in this century (Curry and Mauritzen 2005). A 20th century slowing down of the AMOC was witnessed by a region of cooling in Northern Atlantic after 1970 due to increased freshwater input with further uncertainty from melting of the Greenland ice sheet (Rahmstorf et.al. 2015). 

Impacts

The impacts of a THC tipping point has been widely studied with models and experiments. A shutdown of AMOC would lead to cooling effects which may outweigh and reverse current CO2 induced temperature trends whereas a AMOC weakening are dominated by increased CO2 domination (Yin et.al. 2006). Using the Met Office HadCM3 GCM in a modelling experiment, an artificial collapse of the AMOC in 2049 would cause reduction in precipitation in Western Europe and a regional cooling of the Northern Hemisphere by -1.7 degrees with up to -9 degrees cooling locally. Global primary production from vegetation will also decrease by 5% due to temperature and moisture changes. Drying trends will also be noticed in Central America and SE Asia with impacts extending globally within 30 years (Vellinga and Wood 2003; Vellinga and Wood 2008). 

Causing the shutdown of the THC would no doubt constitute as 'dangerous level of interference' (Hansen et.al. 2006), possibly characterising the Anthropocene epoch. The uncertainty in coupled GCMs on the full range of feedback responses and the 'low probability' claim of the IPCC is certainty not an excuse for inaction and a shutdown in the current century should not be ruled out. This also highlights the need for better oceanic monitoring equipment and research to ensure development of early warning systems and proper anticipation of global impacts.

Thank you for reading this post! In the next post, we will look at the possibilities of detecting critical thresholds.