London Floods
Last Sunday London experienced serious flash floods following torrential downpours. The London Fire Brigade received over 1000 flood related calls. Two hospitals had to close for patient admissions. Many roads and underground stations had to close, as did the Blackwell Tunnel.
But was this just a freak natural event or a symptom of global warming? Is it a portent of things to come?
Global warming means heavier showers
It is difficult to specifically link these flash floods directly to global warming.
However, a warmer climate will mean that showers, when they happen, will tend to be heavier over time. According to the Grantham Institute, warmer air can hold more moisture. That means that an increase in temperature of 1-1.5 degrees centigrade will lead to storms being about 10% stronger than they would otherwise have been.
Since 1880, global temperatures have warmed by a total of around 1oc but over the next century global warming looks like it will increase average temperatures by between 1.5oc and 5.5oc. On that basis we can expect showers to become, typically, 10% – 50% heavier.
So, London could be seeing a lot more flash floods by the end of the century.
Polar melt and rising seas
Climate change brings another threat to London beyond flash flooding from torrential rain. A warmer climate is melting ice in the northern hemisphere, which is causing sea levels to rise.
London is built around the Thames Estuary and, for much of its history, it has served as a major port as well as the nation’s capital. As sea levels rise, the danger of flooding from the Thames becomes more significant.
In the period 1971 to 2009, glacial melt is thought to have occurred at a rate of 226 gigatonnes per year. Over the period 1971-2012 Arctic sea ice has been melting at a rate of 3.5 to 4.1% per decade. Warming has also affected areas like Alaska and Siberia, melting permafrost over time. Average temperatures in these places are thought to be on average 2-3oc higher now than they were back in 1971.
Of course, all this melting ice inevitably means more water in the oceans. During the past centuries it is estimated that sea levels have risen by an average of 1.7mm per year. It is also accelerating and by 2010 it was rising by over 3mm per year.
For a city that has always been vulnerable to flooding, like London, higher sea levels spell trouble ahead.
London’s Defences
The Thames Barrier represents London’s most prominent and important flood defence. These gates can be shut to protect London from North Sea storm surges. As sea levels rise London will need to rely on the barrier more and more and we can expect to see the gates being closed more frequently to prevent surges from flooding the capital.
Water levels in the Thames Estuary are estimated to have risen by around 15cm between 1911 and 2018. So without the Thames Barrier, London would have experienced some significant floods over the past couple of decades. It is estimated that around 16% of London properties lie within the flooding risk zone protected by the Barrier.
Since its construction in the early 80s the gates have been used increasingly over time. So far, 2014 stands out as the most active year (during which it was raised/closed as many as 50 times). During the past couple of decades the barrier has typically been needed about 7-8 times a year.
How long will the barrier hold?
The good news for Londoners is that the Thames Barrier is reasonably future proof. It is likely to continue to protect London well until 2070, although the plan is to start looking for its replacement / upgrade in 2040.
Between 2035 and 2050 it is anticipated that London will need to improve flood defences such as raising flood walls and other smaller barriers and reshaping the riverside.
In terms of local conditions, it is estimated that London might expect 59% more rainfall by the end of the century. By 2100, the water level in the Thames Estuary might be as much as 1 metre or more higher than it is today. The proportion of London properties at risk will increase from 16% to 23% over that time and the Thames Barrier, as it is today, will not be able to protect Londoners any longer under those circumstances.
In less than 20 years from now London will need to start thinking seriously about alternatives.
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