It is well known that global sea level is rising. During the last ice age sea level was so much lower (approx 400 feet) that Britain was connected to the continent, at its peak 50 square miles of land was lost in the land bridge every year. So it does not come as much surprise to find out that as the world gets hotter still that sea level is rising further.
So what causes sea level rise? This may sound like a stupid question the obvious answer is melting ice, but water also expands as it gets warmer (at least above 4oC). Sea and air currents also cause the sea level not to be uniform, like if you blow on a full saucer of water it will spill on one edge. This joins up to mean that sea level will rise by an average but th rise will not be the same everywhere. Luckily items that float on water do so by displacing their own weight in water so sea ice (unless supported by a bridge or pillar) does not cause rise in sea level as the volume it becomes when melted equals the volume of water displaced.
What do we actually see? Scientists have made measurements of the rate at which sea level is rising and hope to improve the accuracy of these measurements with newly orbited satellites which can now monitor sea level in much greater detail. Unsurprisingly what they have seen is that the rate at which the sea level is rising is going up.
Average increase per year from 1901 to 1970 = 1.3mm
Average increase per year from 1971 to 2005 = 1.9mm
Average increase per year from 2006 to 2018 = 3.7mm
An analysis from NASA for the 12 months Jan 2022 to Jan 2023 quoted a rise of 7.6mm
So when will Knebworth sink beneath the sea?
Luckily for Knebworth most scientists appear to think that there is not enough ice to melt to make that happen. It is a real concern for places like Portsmouth, the Fens and Somerset levels. London too could disappear as could many other major cities around the world. It is clear from above that the rate of increase in sea level is increasing. We can consider easily two scenarios for illustration only.
Rate remains constant from now at 7.6mm per year. Result Knebworth submerged approx 15000 AD, sea level rise by end of 21st Century 1'11" (0.6m), plus any rise from when a datum was taken to 2024.
Rate continues to double every 25 years. Result Knebworth submerged approx 2250 AD, sea level rise by end of 21st century 6'6" (2m)
UN AR6 Predictions
In the IPCC AR6 report several scenarios were reported.
The most optimistic was for temperature increases to remain below 2 degrees above pre-industrial levels. This resulted in a prediction of approx 0.5m (1'8") by 2100. From current trends in measured temperatures, CO2 output and the figure for extrapolating current sea level rises to the end of the century this figure can be considered optimistic.
When predicting for a temperature rise of over 4 degrees, which appears a more likely outcome at the moment they had two predictions. One for the end of the century was approximately 1m (3'4"), the same scenario but including greater pessimism on assumptions on ice sheet instability hit a 1m rise by approx 2070, which is actually a faster rate of sea level rise than the doubling every 25 years assumption.
Sea level rise predictions by the end of the century of between 2 and 7 feet are now common.
Implications:
Both the Thames barrier and the Southsea coastal defense (Portsmouth) have been designed to cope with a 1m rise so it would appear that we have time to raise both even for the most pessimistic of the assumptions taken above. However when you consider our current inability to protect much of the East Anglian and other low lying coastal areas for the rise so far seen, and the length of protection needed it would be a brave man who buys a property in such an zone.
Could it be even worse?
Potentially actually it could be. Global warming does not just give rise to higher sea levels but also to more severe storms, which will result in increased size of storm surges. The IPCC report also assumes that there is no collapse of the north atlantic meridional overturning circulation (AMOC) we know of this as the bit which effects us, the gulf stream. There is now much less confidence in this assumption and collapse of this system could potentially raise levels by a further 6 feet by the end of the century, possibly much faster. The practicality in raising sea defenses 12 feet has to be questioned, even for London, and of course ice is slow to melt so sea level rise will go on long after temperatures cease to rise.
Anything else?
When the weight of ICE above Greenland, and Antarctica goes down so the land actually rises. Not in Britain as we do not have large glaciers. Areas adjacent to retreating glaciers may become more susceptible to earthquakes, Britain should again avoid these although, if the earthquake occurs close to the Atlantic ocean, Britain could be hit by a resulting tsunami. Like Japan, Britain has built its nuclear power stations by the coast.