Ranyl Rhydwen, a lecturer in CAT’s Graduate school of the Environment on the MSc Sustainability and Adaptation in the Built Environment, drills into the science of sea level rises and looks into the future. First posted by Sustain Magazine:
Humanity has already introduced enough CO2 into the atmosphere to raise the earth’s temperature by 4-6°C. This heat is being added at a rate approximately 300 times faster than when the earth’s ice sheets previously melted; past melt rates are therefore likely to provide low and conservative projections for the future. The earth’s remaining ice sheets contain 70 metres of sea level rise; with 40 metres of that being land locked in the East Antarctic Ice sheet that won’t melt unless CO2 reaches levels of >1000ppm. However the remaining 30m from Greenland, Western Antarctic Ice Sheet and the below sea level EAIS have all previously melted away when CO2 concentration levels were only 400-425ppm (April 2014 level 400ppm). A 30 metre sea level rise involves 50% of humanity, nearly all the world’s mega cities and large swathes of prime agricultural land. Sea levels will take thousands of years to fully rise, however 20 metres is inevitable and 30 metres probable. This needs planning for now as any manmade barrier is very unlikely to be able to cope with a 5 metre rise.
How fast will the melt occur?
Melt rates of up to 4 metres per century have previously occurred and although it is felt it would take the collapse of a major ice sheet to induce this 4 metre rate again, 1-2 metres per century is common, making the IPCC 80 cm projection by 2100 misguided considering the stakes involved. The 4m melt pulses occur due to the collapse of the marine based ice sheets. These ice sheets melt slowly at first as the glaciers get snagged on ocean bed ridges but once free of these ridges, they suddenly (after 200-1000 years) collapse in a process called rapid irreversible marine instability. These ice sheets are particularly vulnerable as they are melted from below by warm deep ocean waters lubricating the glacial flow and due to ocean dynamics warm waters (~3.5°C) currently bathe most of Greenland’s and Antarctica’s marine outlet glaciers.
The discovery that the Amundsen Sea outlet sea glaciers (that drain a third of the WAIS equivalent to 1.2 metres of sea level rise) have developed marine instability (i.e. they will now completely melt away) and are melting at an accelerating rate (30% greater than just 5 years ago) makes 4 metres a century look much more probable. Models suggest that this collapse is irreversible but may take 200-1000 years, however they didn’t account for the inevitable further warming of the melting waters. The last time Greenland, WAIS and parts of EAIS melted (120,000 years ago) melt rates of approximately 2 metres sea level rise per century occurred. The recent finding that the marine based glaciers draining the North East of Greenland (16% of it) have suddenly started rapidly melting and that the Fjords draining Greenland are much wider and extend further inland than previously thought all means that 4 metres in a century is again more likely. Therefore the recent evidence suggests that although 30 metres is the final outcome it is unlikely to occur by 2100, however 1-2 metres is virtually certain, 4-5 metres probable and greater amounts can’t be excluded.
Thus a large proportion of humanity is under direct threat from this sea level rise. The USA military are planning tactical retreat, however moving an army base is not moving a city (London), a state (Florida) or a country (Bangladesh). The first step in adapting to sea level rise is to slow it down and reduce its magnitude and the only way to do that is to remove (bio-sequester) carbon from the atmosphere and getting to 350ppm still means a 20-25 metre sea level rise and require a massive increase in mitigation efforts, which will take a transformation of societal systems to achieve. Adaptation and mitigation therefore need to be considered together. Adapting to sea level rise will mean more than building a sea wall as concrete barriers will have large carbon costs and will be overtopped eventually putting future generations at greater risk.
It seems we need to think again and take the approach of planned retreat, combined with innovative developments that embed humanity’s community into the new ecosystems and create new settlements that are robust to the extreme weather whilst sequestering carbon into the materials used to create them. That radical approach will take a transformation scale of change and the widespread uptake of progressive adaptation planning and is why here at Centre for Alternative Technology, we are putting transformational adaptation into the heart of our sustainability learning and teachings to help understand how to creatively approach the task that sea level rise imposes.