Climate change: Potentially catastrophic sea level rise can still be avoided and Scotland could even see it reduced to zero – Dr Matthew D Palmer
How can such small and insignificant numbers matter? Because they are relentless, aggravated by the passage of time.
Sea levels have risen about 20 centimeters globally since the beginning of the 20th century. The equivalent average elevation in the UK is slightly lower, at around 16cm. These relatively modest increases are already leading to a real and present danger of increased coastal flooding worldwide, with low-lying islands and coastal areas particularly at risk.
During the 20th century, the main contributors to global sea level rise were the expansion of the oceans due to warming and the melting of mountain glaciers.
But things are changing. The contribution to sea level rise from the Earth’s two main ice caps – Greenland to the north and Antarctica to the south – is now four times greater than in the 1990s. The stability of these vast reservoirs of ice in a warming climate poses an existential threat to coastal communities around the world.
The slow response of oceans and ice caps to global warming means that centuries of future sea level rise are already ‘locked in’ by past greenhouse gas emissions.
This behavior is at odds with global surface temperature, which is expected to respond to reductions in greenhouse gas emissions within decades.
As assessed by the Intergovernmental Panel on Climate Change’s latest climate change report, the magnitude of global sea level rise depends on the future peak of surface warming. .
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At four degrees Celsius of maximum warming relative to a pre-industrial climate, we would see between 12 and 16 meters of sea level rise over the next 2,000 years.
If the surface warming peak is limited to 3°C, this range is between four and ten meters. If surface warming is limited to the Paris Agreement target of 2C, the range is two to six meters, and two to three meters for 1.5C.
These are significant numbers that would pose serious challenges to coastal communities around the world. There is also a high degree of uncertainty about how quickly we might reach several meters of sea level rise.
The rate of future sea level rise is highly dependent on the response of the ice sheets. Known “rapid sea ice processes”, especially in Antarctica, could lead to a dramatic acceleration of sea level rise in the coming decades and centuries.
Current scientific knowledge cannot rule out a global sea level rise of about five meters by 2150 and more than 15 meters by 2300 under a scenario of high greenhouse gas emissions if these processes fast ice floes are triggered.
However, we know that this risk can be significantly reduced by limiting future warming through reduced greenhouse gas emissions.
What about the UK? In a useful twist of fate, the UK’s relative proximity to Greenland means that the British Isles will experience almost no sea level rise from the Greenland Ice Sheet.
As the ice is lost, there is a drop in sea level surrounding the ice sheet as the gravitational pull acting on the ocean is reduced. The UK is in the “sweet spot” where the local fall almost exactly balances the global sea level rise due to the loss of ice in Greenland.
The main threat to the UK therefore comes from Antarctica, which is unfortunately where the most ice and the greatest uncertainties lie. In addition, continued land uplift acts to reduce rates of sea level rise in the north of the UK, while land subsidence exacerbates sea level rise in the south.
The origin of this north-south divide can be traced back to Scotland during the last Ice Age. The Scottish landscape tells the story of an ancient environment that was shaped by ice; from the dramatic U-shaped valleys of Strathspey and the western sea lochs, to the high, desolate moorland of Rannoch, which once lay beneath the central dome of the British Ice Cap.
The existence of this ice cap still makes its presence felt in the changing sea level around Scotland today. The Earth’s crust still reacts slowly to ice loss, causing land uplift of up to about 1.5 mm per year.
This neutralizes sea level rise around the Scottish coast and raises a tantalizing possibility; in a low greenhouse gas future, sea level rise could be reduced to zero for much of Scotland’s coastline. This possibility represents a rare ray of sunshine in the generally bleak outlook for climate change.
The science is very clear: the fate of sea level rise, for the UK and the rest of the world, depends on the collective actions of the international community on future greenhouse gas emissions.
If we are able to limit future warming, we can avoid potentially catastrophic future sea level rise that will affect both the UK and countless millions of lives around the world.
For Scotland, there is a very real possibility that future generations will see local sea level rise reduced to zero for centuries to come if the Paris Agreement warming targets are met.
But the window of opportunity to achieve these goals is fast closing and all eyes will be on Egypt in early November to see what progress can be made at the COP27 international climate summit.
Dr Matthew D Palmer is Senior Scientist for Sea Level at the Met Office Hadley Center and Associate Professor at the University of Bristol