David Hume wrote in 1777 that “all inferences from experience suppose … that the future will resemble the past”. This is a natural human tendency and a pretty good rule of thumb, but it has its limitations. Take the example of sea level. It has altered very little in the last 6,000 years, the period during which we have developed our advanced civilisation. But even as Hume wrote those words the sea level was beginning to rise, gradually at first, at the seemingly inconsequential rate of around 1 mm per year. Even this small rise has not been evident where I live in Scotland, where new land is being won from the deep as the land springs back following depression by the great weight of the ice age glaciers. This obscuration of the evidence makes it hard to believe that the sea will inundate the land as the planet warms over the coming decades.
Sea level rise since the last ice age
The rate of rise is already accellerating. Since the 1990s global sea level has been rising by around 3 mm per year. To get an idea of how much sea level can vary we have to go back to the Last Glacial Maximum, about 20,000 years ago, when so much water was locked up in the ice sheets that sea levels were around 125 m lower than they are today. When melting and warming was at its peak, the sea level was rising by an incredible 5 m per century. Get out a map and have a look at the contours, imagine how much land would be lost if the sea was 125 m higher than today’s level.
I give that example purely for illustration, to show how much the world can change. The sea level isn’t going to rise by that amount again, because there isn’t enough ice left. Sea level rise will come from a combination of melting of the polar ice caps and thermal expansion of the water that is already on the sea. The melting Arctic sea ice has grabbed public attention, but the melting of such floating ice does not raise sea level. What will raise sea level is the melting of ice that currently sits on land. Greenland holds about 7 metres of sea level rise equivalent and Antarctica about 60 m. The vast majority of Antarctica’s ice is in the East Antarctic ice sheet, with only about 3 m in the smaller West Antarctic ice sheet.
Past climates provide insight into future sea level rise
So what sea level can we reasonably expect over the coming centuries and millennia? Forecasters at the Intergovernmental Panel on Climate Change (IPCC) have predicted that in the event of a 2°C rise we will see between 0.26 and 0.82 m sea level rise by the end of the century. It is natural to focus on what will happen in our own lifetimes, but the big picture is more important. How much sea level rise will our children and grandchildren see? To answer this question we must look back into the geological past to find periods that were similar to the conditions that we expect to see in the future.
The last time the planet was steadily 2°C warmer than pre-industrial times was during the last interglacial, a warm period that started 124,000 years ago. Sea level rose at rates of up to 3.5 metres per century to peak, after about 2,000 years, at 5 to 10 meters higher than today. But while temperatures were similar to those forecast, CO2 levels were much lower at 280 ppm. To find CO2 levels that match our current level of 400 ppm we have to go back to before the ice age. Three million years ago during the Pliocene the global temperature was 2-3°C above preindustrial levels. The sea level during this period is of great interest, for the layout of the continents, the amount of energy reaching us from the sun and atmospheric composition was very similar to the present time. The sea level during the Pliocene gives an indication of where the sea level might stabilise in the distant future, but exactly how high it was has been the subject of active debate.
How stable are the Antarctic ice sheets?
When I last looked into this a couple of years ago the dominant view was that the East Antarctic ice sheet had been stable for 14 million years. This constrained the amount of extra water in the Pliocene seas to the 10 m that would come from West Antarctica and Greenland. New evidence has emerged since then which shows that the not only did West Antarctica melt, sizable chunks were also taken out of East Antarctica. In total there are almost 23 m sea level equivalent of ice in Antarctica that is grounded below sea level, making it vulnerable to melting. It looks like most of this melted during the Pliocene.
So is there any evidence of a shoreline caused by the 25 m or so of sea level rise that would have occurred due to complete melting of the Greenland ice and partial melting of Antarctica? Indeed there is, the Orangeburg scarp, a fossil shoreline that runs up the east coast of the USA from Florida to North Carolina. Interpretation is complex, because a lot of geology has occurred in the 3 million years since the shoreline formed, uplifting it up to 60 m above present sea level. A reconstruction of these geological processes has concluded that the shoreline was probably formed at 25 m above present sea level. The authors stop short of making a definite statement about the Pliocene sea level, but together with the new information that has emerged about the stability of the Antarctic ice sheet, a picture is emerging that makes the IPCC sea level estimates, of 0.6 to 1 m by 2100, look as if they might prove to be on the conservative side.
25 m of sea level rise in the pipeline?
We show no sign of taking the steps necessary to hit keep the temperature rise below the 2°C target. This means we’re almost certainly in for an instalment of 25 m rather than an instalment of 10 m by the end of the century. We have been softened up to expect a foot or or a metre, which doesn’t sound too bad. A small wall of sand bags could keep that out. Indeed seven thousand square kilometres of the Netherlands is below sea level, with the lowest point a full 7 m below. So engineering solutions may protect us from the rising seas, up to a point, but much will not be protectable, particularly land which is exposed to ocean storm surges.
At some point in the future trillions of pounds worth of sea side and low lying real estate will become uninsurable, then worthless. It will be poignant and sobering to see such an adjustment in human scope, as a society that has settled on a planet with a stable climate and sea level gradually accepts that it is no longer the master of its environment. Human nature being what it is this will be a gradual process, with locations being occupied until the last possible moment before they become inundated, becoming artificial reefs and nesting sites for sea birds. The half-drowned skyscrapers of our great coastal cities will remain as a monument to that 6,000 year period of climatic and sea level stability over which our civilisation flowered and developed, and a poignant reminder of how puny the will of man is relative to the great power of nature.
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