Floods could overwhelm London as sea levels rise - unless Thames Barrier is upgraded

Study concludes there is 1 in 20 chance that existing defences would be unable to cope with extreme storm surge

There is significant risk of London being hit by a devastating storm surge in the Thames estuary by 2100 that could breach existing flood defences and cause immense damage to the capital, a study of global sea-level rise has found.

Melting of polar ice sheets and mountain glaciers could increase sea levels significantly over the coming decades leading to a 1 in 20 risk that the existing Thames Barrier would be unable to cope with an extreme storm surge, the study concluded.

Extreme storm surges that can breach the barrier would in the past have occurred with a frequency of about 1 in 1,000 years, but in a warmer world they could occur as frequently as 1 in every 10 years, scientists said.

The increased threat posed by rising sea levels is one of the reasons why flood defences around the Thames estuary and the barrier itself will be strengthened.

An international panel of glaciologists and climate scientists said there is still huge uncertainty about how sea levels will change in the coming century as a result of climate change and its effect on polar ice sheets and mountain glaciers.

Their best estimate is that the melting ice on its own will contribute between 3.5cm and 36.8cm to mean sea levels, which would come on top of the rise in sea level due to other factors such as the thermal expansion of the warmer oceans.

However, there is a 1 in 20 risk of this being a wild underestimate and that melting polar ice and mountain glaciers alone would contribute more than 84cm to global sea level, which would lead to rises of about a metre around Britain if other factors are taken into account, they said.

"The Thames Barrier was built to provide London with a level of protection that would only be exceeded in about 1 in every 1,000 years. So in any one year the likelihood of exceeding this is about 0.1 per cent," said Professor David Vaughan of the British Antarctic Survey.

"With 50cm of sea level rise we would expect that level of protection to go down from 1 in 1,000 years to about 1 in 100 years, so under that scenario in every year there would be a 1 per cent chance of flooding. If you have a metre rise you go down from 1 in 1,000 years to 1 in 10 years," Professor Vaughan said.

These estimates are based on existing "business as usual" emissions of carbon dioxide, leading to about a 3.5C rise in mean global temperature by 2100. Greater emissions would lead to higher temperatures and faster melting, the scientists said.

How sea-level rise and polar ice sheets will respond to rising temperatures is one of the greatest uncertainties in climate science. The research programme, called Ice 2 Sea, was established by the UN's Intergovernmental Panel on Climate Change to estimate the future contribution to sea level from melting ice.

"There is still extra uncertainty that arises because our models are not complete. There are still processes that we think are important but we haven't been able to include in our models," said Professor Vaughan, the coordinator of the programme.

The scientists carried out "expert elicitation" among themselves to take into account the unknowns that their computer models were unable to include, Professor Vaughan said.

"That has come up with this number: there is less than a 1 in 20 risk of ice sheets and glaciers contributing more than 84cms to sea level rise by 2100. That is trying to capture those climate processes that we suspect are important yet are not fully included in existing models," he said.

Sea levels would rise by varying degrees around the world due to melting ice, and would even decline in areas around Greenland and Antarctica due to the diminished gravitational pull of the dwindling ice sheets. The British coastline would see sea level rises that are slightly below the global average, Professor Vaughan said.

"It is likely that some future ice loss and sea level rise is now unavoidable. But nevertheless, understanding why changes are occurring today and how they could increase in the future is the first step in maintaining the security of our coastal regions for future generations," he said.

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