The role of Thermohaline circulations in abrupt climate change.
Ideally, no one would choose to live in Northern Europe in sub-arctic temperatures. Notably the UK benefits from warmer climates due to Thermohaline circulations (THCs). Without these most of Northern Europe would be upwards of 12.5 degrees celsius colder (Wood, et al, 2003) (Figure 1). THCs are ocean currents of warmer surface water and colder deep waters; typically controlled through horizontal differences in temperature and salinity. This process is very fast moving, shifts vast volumes of waters over thousands of kilometers, all across the globe (see Fig 2).
Figure 1) A map showing temperature change if the North Atlantic THC was to collapse. Courtesy of UK MetOffice and Wood, et al, (2003).
How do Thermohaline Circulations work?
In the North Atlantic, THCs operate in 2 ways. Think of it as a reversible cycle;
- Differential solar heating between the lower and higher latitudes accelerates the warmer surface waters polewards.
- Freshwater enters the higher latitudes, such as Hudson Bay, Canada. This combined with evaporation of surface waters at lower latitudes tends to break the flow of the North Atlantic THC (Clark, et al, 2002).
The former cycle dominates the latter, driving warmer surface waters of the Gulf of Mexico, northward. Upon evaporation of surface waters, concentrations of salt increase, because the water releases heat. This causes deep water currents to form over the North Atlantic. Subsequently, these deep water currents will spread throughout the Atlantic, perpetuating the gradual upwelling, which renews the cycle.
Figure 2) a Map showing the Global system of Thermohaline circulations as well as Salinity (in PSU). Source: NASA/NOAA 2009.
So how has the Atlantic THC changed throughout time?
Developments in linking past abrupt changes in North Atlantic surface-oceans and atmospheric temperatures to changes in deep ocean circulation: confirmed the role that Thermohalines play in abrupt climate change (Clark, et al, 2002). Particularly, the Atlantic THC has gone through periods of On/Off cycles, indicating that to understand Thermohaline circulations is vital to understand abrupt climatic change. A 2017 review suggested that there was strong evidence for changes in both strength and structure of the Atlantic THC during abrupt climate events, such as the Younger Dryas and the Heinrich Events (5 of the last 7 glacial periods) (Lynch-Stieglitz, 2017).
Conclusions?
Results from proxy (paleoclimatic) records (taken from Ice cores in Summit, Greenland) show the global extent of climate variability across the millennium; with varying responses that are consistent with atmospheric and oceanic changes with changes in the Atlantic THC (Clark, Et al, 2002). This proves there is a relationship with increased freshwater flux in the North Atlantic and the decreased relative strength of the THC (Clark, et al, 2001). THC was sensitive to small changes in hydrological cycle during the last glaciation which supports the models generated by Clark’s 2002 paper (Stouffer & Manabe, 1999).
What causes this sensitivity? What are the effects?
Since the paleoclimatic record only provides the fundamentals to assess and critique our models to better improve them we currently have a very limited understanding, Clark’s 2002 paper highlighted several key areas:
- Increase the distribution sites in the Pacific and Southern Oceans
- Develop new scientific tools for greater accuracy of climate data from proxy records
- Further analysis of Carbon-14 dating (proxy records) for the last 50,000 years. As well as developing new tools to synchronise paleoclimatic data and isolating the various relationships inside the THC (Clark, et al, 2002).
We need to consider the recent impact of anthropogenic factors; such as increased Greenhouse gases.
A likely scenario is an increase in mean temperatures would melt ice sheets, decreasing salinity concentrations, increasing freshwater flux in the Atlantic (Huang, 2015). THCs rely on a horizontal salinity gradient to allow for deep water formation through the “sinking” motion that drives the NADW (North Atlantic Deep Water). Predictions suggest; within the next 30-50 years, the Arctic Ocean may be ice free during summer time (Huang, 2015). Such an influx of freshwater may lead to a “halocline catastrophe”, which is why scientists are worried. Ultimately, the fate of THCs and abrupt climate change depend on the THCs response towards anthropogenic climatic change over the next century.
References:
Clark, P. U.,Marshall, S. J.,Clarke, G. K. C.,Hostetler, S. W., Licciardi, J., & Teller, J. T. , (2001).Freshwater Forcing of Abrupt Climate Change During the Last. Science, 293,5528: 283-7
Clark, P. U., Pisias, N. G., Stocker, T. F., & Weaver, A. J., (2002). The role of the thermohaline circulationin abrupt climate change. Nature, 415, 6874.
Lynch-Stieglitz, J. 2017, “The Atlantic Meridional Overturning Circulation and Abrupt Climate Change”, Annual review of marine science, vol. 9, pp. 83. (no available online access)
Osborn, T., Kleinen T., (2008). 7: The thermohaline circulation, Date Accessed 18/05/2020, UEA.
R. A. Wood, M. Vellinga and R. Thorpe (2003): Global Warming and thermohaline circulation stability. Phil. Trans. R. Soc. Lond. A, 361, 1961-1975.
R. X. Huang (2015); “OCEANOGRAPHIC TOPICS | Thermohaline Circulation”, Encyclopedia of Atmospheric Sciences (2nd Edition), Academic Press, 315-328, ISBN: 9780123822253.
Stouffer, R.J. & Manabe, S. 1999, “Response of a Coupled Ocean–Atmosphere Model to Increasing Atmospheric Carbon Dioxide: Sensitivity to the Rate of Increase”, Journal of Climate, vol. 12, no. 8, pp. 2224-2237.
Thank you to Kurzgesagt for use of their video, channel hyperlinked.
Also thank you to mum for helping me write this correctly and not scientific gobblygoop.