July 27, 2021

expert reaction to severe weather in parts of the UK over the weekend including flash flooding in some parts of London

Following thunderstorms on Sunday 25th July there have been incidences of flash flooding in parts of London, as well as disruption around the country.

Dr Harvey Rodda, UCL Hazard Centre, Department of Earth Sciences, UCL, said:

“The thunderstorms and intense rainfall which affected London over the weekend were not unseasonal or particularly severe.  They were caused by a small depression originating in the Bay of Biscay and moving north-east bringing moisture laden warm air to the UK.  Such depressions are not uncommon and generally occur during the summer months.  The rainfall totals (listed as around 40mm in 24 hours on some news sites) are also not particularly extreme when looking at rainfall records over the past 150 years in the UK as a whole and also compared the 24-hour totals of up to 150mm which led to the severe flooding in Belgium and Germany on 15th-16th July.

“What is apparent however is the increasing frequency of these events with similar flooding occurring earlier in the summer, and also in recent years, interspersed with warm periods of maximum temperatures above 30 degrees Celsius.

“The drainage infrastructure in London also has a part to play in surface water flooding.  Many surface water sewers are old, have not been designed based on any hydrological studies, and are under-capacity.  Furthermore, local authorities are still accepting drainage designs for new developments which have been based on outdated data and methods from the 1970s rather than following the latest guidelines.”

Comments sent out on Monday 26 July 2021:

Dr Nikos Christidis, Senior Climate Scientist at the Met Office, said:

“Without a specific attribution study it isn’t possible to know whether this event is linked to climate change.  We know a warmer atmosphere can hold more moisture and that therefore rainfall events can become more extreme.  However, natural variations in rainfall mean that this event could be an example of a weather extreme that could have been feasible in the pre-industrial climate that was not affected by human greenhouse gas emissions.”

Dr Liz Stephens, hydrologist and associate professor of climate resilience, University of Reading, said:

“Despite improvements after the summer 2007 floods, when two-thirds of the 57,000 flooded properties were affected by surface water flooding, the UK still has a complicated set of roles and responsibilities for surface water flooding risk.  Lead Local Flood Authorities take responsibility for managing it, the Environment Agency for mapping it, and the Met Office for providing early warning.  This makes it difficult for the public to have a good understanding of their own risk and what can be done.

“A landmark study in 2014 found that the intensity of summer rainfall events in the UK was expected to increase as a result of climate change, but the surface water flood hazard maps for the UK have not been improved since 2013.  These urgently need updating.  The current accuracy of surface water flood maps reflects an investment choice and not what is possible with the state-of-the-art science.  It is deeply concerning that hospital emergency departments have had to close as a result of this flooding, and I hope that this is the wake-up call that the government needs to take this risk of climate change seriously.

“The precise location of the most intense rainfall, and consequently largest water depths, is difficult to forecast far in advance, and this uncertainty is reflected by the weather warnings covering such a large area – such as the Met Office’s amber weather warning covering most of south-east England on Sunday.

“It is difficult to help people prepare for surface water flooding if they don’t know they are at risk, and if they don’t receive precise warnings of the likely impacts when heavy rain is forecast.”

Dr Jess Neumann, Hydrologist, University of Reading, said:

“Summer thunderstorms are not a new occurrence, but it is becoming ever clearer that the worsening impacts of flooding from intense rainfall are having devastating effects here in the UK and across Europe.

“Although it is not possible to attribute a single event to climate change, what we do know is that a warming atmosphere can hold more moisture and we are seeing an increasing number of storm events leading to flooding, with rainfall records being broken time and time again.  The severity and frequency of flooding is a stark warning that we are not prepared to deal with climate change.

“Parts of London have been flooded for the second time in less than a month, affecting people’s property, livelihoods and businesses but also critical infrastructure such hospitals and transport links.  Flash floods can rise quickly with devasting consequences – if you’re a community at risk need, remain alert and heed flood warnings.  If you can, check in on your neighbours, prepare a flood kit, move valuables out of harms way and familiarise yourself with how to turn off electricity supply to your property.

“Flooding from intense summer rainfall is going happen more frequently.  No city, town or village is immune to flooding and we all need to take hard action right now if we are to prevent impacts from getting worse in the future.  There needs to be a complete shift away from all use of fossil fuels to renewable energy sources.  Planning and development need to consider flood risk from all sources (river, groundwater and flash floods) and adapt accordingly – it is not acceptable to keep paving over the land and expect the public to deal with the water when it comes into their homes.”

Prof Richard Allan, climate scientist, University of Reading, said:

“We have seen a spell of extremely hot, dry conditions break down into thunderstorms in parts of the UK, with huge deluges seen in parts of London.  Summer thunderstorms are not unusual, but the intensity of the rainfall and the resulting flash floods are exactly the kind of results we expect as the climate heats up due to a build-up of greenhouse gases in the atmosphere.  Warmer air carries more moisture, so a hotter and more energetic atmosphere will bring more damaging storms.

“Unlike winter rain storms, which tend to sweep in off the Atlantic in a fairly predictable way, summer downpours can be quite localised, and it is therefore more difficult to predict exactly where they will strike.  However, higher-resolution computer simulations of the earth and atmosphere, combined with scientific research into the factors that drive these storms, is improving our ability to predict summer floods.

“As we have seen in London, the summer downpours can strike anywhere.  The impacts can be extremely disruptive, damaging buildings, closing hospitals and schools, and shutting roads.  There is some evidence that cities are at greater risk than the countryside, as the urban heat island effect can give storms an extra boost.”

Declared interests

Dr Jess Neumann: “No conflicts of interest.”

None others received.