What we can learn from the last glaciation

The major threat of abrupt climate change comes from a combination of the three systems discussed in this blog. The major tool for initiating glaciation is the Arctic Meridional Overturning Circulation, a part of the Thermohaline system. Henry et al (2016) found that increasing CO2 levels coincided with the H-stadial reductions in AMOC. This warming then caused the discharge of major icebergs from the Greenland Ice Sheet and a melt in sea which kick-started the cooling the disruption of salinity and temperature in the surface waters of the Arctic. Therefore, inhibiting the circulation and causing global cooling. This extreme swing pattern is not a new concept, James Lovelock’s early ‘Gaia hypothesis’ stipulated that the more extreme we force the global system the more extreme the reaction will be in order to eliminate the forcing. In that case either rapid heating to cause total melt or an onset of glaciation, the latter being the more scientifically sound.

Schmittner characterises the whole process as general warming and cooling in the North Atlantic and the opposite in Antarctica. This characterisation is consistent with disruptions to the interhemispheric heat transport of the thermohaline system. As previously mentioned, warming in the Arctic trigger the cooling through melting of ice causing massive freshwater influxes. Recent evidence reinforces this using deep sea sediment ratios. It transpires that when around a 2degree C differential arises between the North Atlantic and the sub-tropical North Atlantic it corresponds with a sever slowing or even collapse of the AMOC.

What is the likelihood of this happening to us?

Using 4 climate scenarios devised by the IPPCC in their 5^th Assessment Report, Cheng et al (2013) assessed the likelihood of the RCP scenarios causing changes within the AMOC. RCP4.5 assumes an emissions peak of 2040, this is the most conservative estimate given the changing tide of consumers in Asia and an unwillingness to cut emissions in the US. Using this scenario by the year 2100 the AMOC will see a projected weakening of 5-40%, this rises to 15-60% in the RCP8.5 scenarios which assumes continuous rise throughout the 21^st Century. Although most likely an extreme case the RCP8.5 scenario shows that a significant weakening of the system is feasible within the 21^st Century which will incite massive regional climate changes a pose new unplanned problems for the world population. The more conservative RCP4.5 shows a stabilising of the AMOC in the latter half of the 21^st century, however a 40% weakening is still significant enough to bring about noticeable shifts in regional climates, so much so it may generate new climate challenges in new locations such as flooding a drought, or exacerbate those already plaguing the global population. 5-40% is a significant ranger considering the consequences however it does provide a confirmation window for Schneider et al (2007) and Cheng et al (2013) who predicted 25-30% weakening by 2100 and 21% (RCP4.5) and 36% (RCP8.5) which are much more precise observations. Furthermore, Cheng et al concluded through multimodel assessment that North Atlantic SST variability in the late-19^th to early-20^th century is consistent with external forcing implementing aerosol forcing as the main driver of AMOC shifts. This is not to say that CO2 may not result in a similar fate as can be seen from this blog alone a number of academics all agree that the forcing is present and a pressing concern the only ambiguity lies in by how much the AMOC will weaken.