The map shows St Denys in 1883. The roads had been laid out and some houses built. Over the next decades the area was developed and by 1910 the area was much as it exists today. How has sea level changed since then, and how will it change in future?
What has happened in the past?
Our records of mean sea level are surprisingly short. While it is easy to record how far the sea rises up a quay wall, knowing whether the land that the quay is built on is rising or falling is more difficult. Such land height changes can be regional (the British Isles are still recovering after being pressed down by ice sheets in the last ice age), or local (for example a layer of clay will contract or expand as the water table falls or rises).
It is only over the last 50 years or so that the sea level has been accurately measured at ports along the south coast. In the diagram the different time series have been displaced vertically for clarity. The observations are all consistent with the global increase of sea level during the 20th century of about 1.7 mm/year (IPCC Report) – which is indicated by the red lines.
Since St Denys was built, the global average sea level has risen by about 17 cm.
“But worse flooding occurred in the past”!
People sometimes question the importance of sea level rise and climate change because, they say, events in the past were as bad or worse. Indeed, that is almost certainly true! Very bad floods are rare events which occur when a number of exceptional factors coincide. There is a better chance for that to have occurred within living memory, perhaps the last 80 years or so, than in the most recent years. Going back centuries or more, there is even more chance of a very large event having occurred, for example the Great Storm of 1703. Such a major event might occur once in a few hundred years, and with luck it will not occur during our lifetime. When it does eventually occur, the contribution from recent changes in sea level may not make much difference given such an overwhelming event.
So we should not expect flooding events due to recent sea level rise to be greater than those occasional large events in the past. What sea level rise will cause is flooding from less major events, which will occur more often, and which in the past would not have resulted in floods.
Another factor in limiting flooding is the provision of modern sea defences. For Southampton and the Hampshire coast in general, newspaper articles reported more flooding in the early 20th century than they do today. But in many places where serious flooding used to occur sea defences have since been built. Modern sea defences recently limited flooding along the North Sea coast from conditions which, in 1953, caused severe flooding and loss of life.
In St Denys the decision has been to protect individual properties rather than build a coastal barrier along the River Itchen. However a flood wall is considered necessary for the Itchen bank from Northam Bridge southwards, and a raised wall will become necessary in St Denys by mid-century.
Why is Sea Level increasing?
Global average surface air temperatures have risen by about 0.7K since St Denys was built. Although the expansion of sea water with temperature change is small, there is an awful lot of water available to expand! If the whole depth of the Atlantic (typically 3 to 4km) were to warm by 0.7K, sea level would increase by over half a metre. But it takes a long time to warm that much water and only a fraction of the potential change has occurred. In other words, the ocean is still catching up with air temperature changes which have already happened. That implies that, even if global warming stopped now, the ocean will continue to expand and sea level will continue to rise.
The other main effect is the melting of glaciers and other land ice. While a few mountain glaciers may be growing, on average most are retreating, some at a dramatic rate. How much of the melt water reaches the ocean is hard to determine but probably about half the recent increase in sea level is due to ice melt, the other half being due to the increasing air and sea temperatures.
Will the rate of sea level rise increase?
There is evidence that both the rate of air temperature increase and the rate of land ice melt increased towards the end of the 20 century which implies that the rate of sea level rise will increase in future.
But is sea level really rising faster in St Denys nowadays?
At any one place (like St Denys) these changes in sea level are almost impossible to observe. We are trying to detect changes of a few mm a year. Factors such as changing weather conditions can easily have more effect. For example a series of good summers, with high pressure predominating, will appear to decrease average sea level. Years with lots of storms will appear to increase it. Only by looking at the global mean sea level (shown in the plot) can we clearly see the trend in the rate of mean sea level rise.
Since the mid 1990’s global sea level has been rising faster than previously, at about 3 mm/year. It is true that higher rates were also observed around 1940 after which the rate slowed. These variations can be attributed to complicated effects such as changing patterns of weather, ocean circulation, and global volcanic activity. It is simply too early to be sure whether the increased rate will continue.
However recently we have improved our understanding of why these past variations occurred. Based on that understanding, it seems very likely that the present faster rate of sea level rise will continue into the future or even accelerate. If, as also seems likely, global air temperatures rise even faster in future, then the rate of sea level rise can also be expected to increase.
What does this mean for St Denys?
Whatever the precise rate, there is no doubt that sea level will continue to rise and high tide levels will be higher in future than in the past! For the south coast of England the most likely sea level rise relative to the land over the present century is estimated to be around half a metre but it is possible that it could be 2m or more! Much depends on the rate of air temperature increase and the rate of melting of ice sheets such as the one covering Greenland. For planning purposes for Southampton a value of around 0.7m increase by 2110 is being used, which would seem a sensible compromise. This implies that (using the 1000 year return period value) the highest water levels in the River Itchen will increase by around 16cm between 2010 and 2030. This sort of increase can be mitigated by the use of property level protection measures as promoted under the Belsize Flood Resilience Project.
About the Author: before retirement in 2007, Dr Peter Taylor was a research scientist at the National Oceanography Centre in Southampton and for a number of years was Head of the Ocean Circulation and Climate Research Division. He is a Fellow of the Royal Meteorological Society. Peter lives in Priory Road in one of those few houses in St Denys which are at risk of future flooding. He is happy to do so because he “enjoys riverside living” …but he was the first to have property level flood protection installed just in case!