Surprises part 1: The collapse of the Greenland ice sheet

Adopting a paleoclimatic perspective in our study of ocean circulation has highlighted the concern over the stability of the North Atlantic meridional overturning circulation (AMOC) given the discovery of millennial-scale climatic changes. To add to this, further concern must now be raised over the impact of greenhouse gas emissions on the collapse of the Greenland Ice sheet (GIS). It has been well documented that the GIS has been losing mass since the early 1990s, approximately 200 gt net loss (Wu et al., 2011). However, the retreat of this ice mass poses great concern for freshwater balance in the North Atlantic and in turn the AMOC. As can be inferred from the study of abrupt climate change through millennial-scale climate processes, a weakening of the AMOC has implications for global climate.

A vast body on literature exists on projections of the collapse of the Greenland ice sheet on AMOC, However, a study by Wu et al. (2011) is particularly of interest. They use a national centre for atmospheric research community climate system model to assess the influence of a shrinking GIS on AMOC, surface climate and sea level rise. However, the impacts on AMOC are of focus here. The authors use the IPPC A1B scenario, a mid greenhouse gas emissions scenario whereby CO₂ concentrations change from 368.5 ppm at year 1999 to 688.5 ppm at the year 2099, then a constant is assumed from the year 2099 to 2199 in conjunction with three other scenarios to predict the impact on global mean temperature.   This  scenario was used as a baseline which do not included GIS melting and subsequently scenarios were developed where rates of melting were 1 % and 3 % until the year 2099 and then kept at 2099 level until 2199 ( termed 1 % and 3 % exp) and then 7 % till 2050 and kept constant till 2199 ( termed 7 % exp) (Figure 1.)

Figure 1. The global mean surface temperature

Source: Wu et al. (2011).

The projections of the scenario indicate that a weakening of AMOC has a moderate effect on global climate. Mean global climate increases by 2. 43 °C in the last 20 years in the 21^st century, relative to last 20 years of 20^th century and an additional increase of 0.43 °C by end of the 22^nd century is observed in the A1B exp. scenario. Upon incorporating GIS melt-water, global mean temperature is not significantly affected in the first half of the 21^st century in all cases. However, at the end of the 21^st century, small fraction of a degree less is shown in the 7 % exp. scenario and the end of 22^nd century shows warming is 0.25 °C  and 0.50°C in the 3 % and 7% exp. scenarios. The 1 % exp. scenario seems to be in a similar time step to the A1B exp scenario. This has implications for our understanding of the impact of a collapse of the GIS on AMOC. That is that upon melting intensification of the GIS, AMOC weakens, global mean temperature is projected to rise due to increase in greenhouse gas emissions and that a weakened MOC will not reverse the current trend and trigger a global cooling trend.

However, the impact of melting GIS on global sea level means that the issue is still of pressing concern. We now turn to a more complex issue; predicting the impact of a potential collapse in the Antarctic ice sheets..