This week the team look at impacts of climate change on land systems, particularly the cryosphere (icy landmass) and the oceans.
Dr. Damian Mansell takes us first through the impacts on the cryosphere. As we’ve already seen, melting ice will contribute to sea level rise, which comes mainly from thermal expansion of sea water, and the melting of the ice caps (such as the Greenland ice sheet) and mountain glaciers. The Antarctic ice sheet and Greenland together make up 99% of the glacier ice on Earth – equivalent to about 65 metres of sea level rise (this is not a prediction; just an illustration of how much water is locked up in these two ice sheets). To give this some sense of scale, Dr. Mansell tells us that Exeter Cathedral is around 44 metres high.
Greenland has responded more rapidly than originally thought to global warming with ice loss from melting, and from calving glaciers. If ice sheets melt faster than their mass is replaced by fresh snow, they will melt, spread out and lose mass as the edges calve into the ocean – and this is what is happening. In 2012, the Greenland summer experienced up to 97% ice sheet melt lasting 2 months longer than the 1997-2011 mean. Greenland, says Dr. Mansell, may be near a tipping point.
Greenland also has a series of lakes which collect meltwater; these are darker and absorb more heat. The water can escape below the icesheet, warming it up and creating melt-events in which the icesheets move towards the edges (we’ve seen footage of glaciers moving). Also, floating ice can form a ‘butress’ which holds back surface melt loss. If these floating ice shelves melt (as they are doing), they no longer hold back the surface melt to the same extent, and the increased glacier melt (but not the melting floating ice) contribute to sea level rise.
Recent studies have shown an increase in mass loss across Greenland and Antarctica, caused by accelerations in ice flow to the oceans. These observations, combined with the natural processes which affect ice melt such as fluctuations in pressure systems, mean there is a good deal of uncertainty in predicting sea level rise from ice sheet melt. Further research is needed to understand the processes that cause melting of the polar ice sheets.
There is a hypothesis that the Western Antarctica ice sheet could become unstable, because it relies on buttressing ice shelves, and the mass sits low in the water. There is evidence that the ice shelves which make it stable are melting, and it could reach a tipping point. http://www.antarcticglaciers.org/glaciers-and-climate/sea-level-rise-2/marine-ice-sheets/ This is a low probability, but high magnitude event which could contribute around 3.3m to sea level rise (5% probability).
Calving is a natural process caused by many factors and not a direct result of climate change, but could be affected by warming atmosphere and oceans. A very useful blog from John Connett (course participant) here: http://myexclimate.blogspot.co.uk/2014/02/week-5-calving-events.html