Currently reading Extreme Cities by Ashley Dawson. The projection for sea level rise under the impact of climate change is currently set by the IPCC as a maximum of 1.4 metres above current levels by 2100, assuming a smooth and continuous expansion on existing trends.
The problem is that past rises and falls in sea levels as climate has changed have not been smooth and continuous, but more like earthquakes – long periods of little change punctuated by rapid, potentially catastrophic shifts.
The relevance of this is that the last time there was the current concentration of CO2 in the atmosphere, sea levels were about 20 metres above what they are now. If you want to scare yourself, have a look at this interactive map, which shows what happens as sea levels rise.
http://flood.firetree.net/?ll=52.1335,1.0108&zoom=8&m=20
If you go up to the 20 metre mark it shows that the Thames estuary will start at Windsor and central London will be inundated, taking out most of the systems that the rest of the city needs to function, Cambridge and Peterborough will be North Sea ports, what’s left of Lincolnshire will be an island, York will be like Atlantis and there will be virtually nothing left of Holland.
If we have 20 metres of sea level rise already “baked in” to what we’ve already done, the only question is not what that will look like (we have the map and can see it) but how long it will take.
We tend to think of geological time as slow – moving at the pace of a glacier. But when you think of how rapidly the glaciers are melting it seems that the impact of the Anthropocene could be accelerating in a way that will become unmanageable unless tackled with greater urgency.
Urban ramblings 
Interesting but lacking a crucial fact – when exactly was the last time the water levels were 20 metres higher? A further consideration: if CO2 levels are currently as concentrated as the last time they were at this concentration – why have sea levels failed to rise to that 20 metre figure?
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The bit below is from Wikipedia. We tend to think of shifts in sea levels as very slow – and current projections reflect that. There’s no argument that they are rising. the only question is how quickly it will happen. My worry is that it will be quicker than we think – because the impact of human activity has been so rapid.
Reconstructions show that concentrations of CO2 in the atmosphere have varied from as high as 7,000 parts per million (ppm) during the Cambrian period about 500 million years ago to as low as 180 ppm during the Quaternary glaciation of the last two million years. Global annual mean CO2 concentration has increased by more than 45% since the start of the Industrial Revolution. The concentration was 280 ppm during the 10,000 years up to the mid-18th century,[2] increasing to 407 ppm as of mid-2017.[3][4] The present concentration is the highest in at least the past 800,000 years[5] and likely the highest in the past 20 million years.[6]
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