Surpises part 3: A Collapse of the thermohaline circulation?

Given the potential of the  north atlantic thermohaline circulation (NATHC)  to influence climate change as evident from study of abrupt climate changes from paleo-records, the following question is thus posed; will the NATHC collapse due to a rise in greenhouse gas emissions? The concern over the impact can be understood by the logic that an increase in sea surface temperatures due to thermal expansion of the oceans, may in turn reduce the solubility of CO₂ in warmer waters (Stocker et al. (2000 in Seidov et al., 2000). This in turn, may create a positive feedback effect whereby warmer waters hold less inorganic carbon, thereby causing a CO₂ release into the atmosphere (Stocker et al. (2000 in Seidov et al., 2000).

One issue in trying to understand this prediction through modelling is that it is difficult to achieve such a timescale. However, one of the earlier studies; undertaken by Manabe and Stoffer (1994), use a comprehensive atmospheric-ocean general circulation model (AOGCM) to simulate the impact over many centuries. Their results appear to indicate that a critical threshold lies between a doubling and four-fold increase in CO₂ concentrations. However only models of reduced complexity can be used to aid quantification of threshold, as this may allow for long-term simulations.

It has found that in addition to the stabilization level of greenhouse gas concentration in atmosphere, rate of greenhouse gas emission increase may also help determine a threshold. This is illustrated in Figure 1 with a simulation run by Stocker et al. (2000 in Seidov et al., 2000). They set climate sensitivity at 3.7 ° C for doubling of CO₂, and simulations of run concerning the rate of CO₂  increase a in 1 %/yr, 2 % year and 0.5 %/yr scenario (Figure 1a)

Figure 1a) Atmospheric CO₂ simulations for five experiments b) Simulated global mean temperature c) Simulations of maximum meridional overturning of the North Atlantic in Sverdrup (1 Sv= 10⁶ m³/s).

Source: by Stocker et al. (2000 in Seidov et al., 2000)

The global mean surface air temperature in simulations is not dependent on emission history for maximum CO₂ concentration (Figure 1b). However, a bifurcation appears apparent concerning the maximum meridional overturning of the North Atlantic. Reduction is apparent in all scenarios with the level dependent on maximum CO₂ concentration and CO₂ increase according to scenario. The circulation collapses at 750 ppmv with increase at rate of 1%./ yr. It then subsequently, appears to recover and settle to reduce value if increase is slower (0.5 %/yr ) or if the CO₂ level is reduced to 650 ppmv. At a rate of 2 %/yr, the circulation appears to collapse. The simulations appear to indicate that once the THC collapses it may settle a new equilibrium, with irreversible changes occurring, independent of CO₂ concentrations.

However, it must be noted that there is huge degree of complexity in understanding the impact of future changes in the thermohaline circulation due to anthropogenic climate change. These include; a lack of understanding concerning the variety of feedback mechanisms associated with changes in the THC and palaeoclimate modelling. There is a need to quantify components of climate-related components to a signal and test hypotheses regarding science of abrupt climate change. Changes in THC are likely in future and it is known the slowing down of THC moves system closer to thresholds. The uncertainty and importance attached to this as detailed in past blog posts highlight the need for further research.