It seems likely, on a preliminary analysis by the Met Office in the United Kingdom that extreme rainfall is becoming more frequent in the United Kingdom. Generally extreme rainfall is a very heavy downpour of rain – the kind that we may experience three or four times a year, but the trend shown by observations is that extreme rainfall is now happening five times a year, and the trend of increasing frequency of extreme rainfall has been on a more frequent basis since 1960.
If you look at extreme rainfall events that last between one and two days there has been little change but in the North of England and in Scotland the interval between extreme rainfall events has halved, and the kind of rainfall event that happened every fifty years until 1990, is now happening every eight years in the East of England, every 11 years in the South of England.
Across the Pacific some weather stations have over the like period shown that the proportion of extreme rainfall days recorded has increased over the vast majority of weather stations since 1961. In South East Asia there has also been an increase in the number of hot days and warm nights, with the number of warm days and cool nights decreasing. China and India have also suffered with the number of days when extreme rainfall falls increasing year by year.
The trend is the same in the United States which has shown a steady increase (in most places) in the amount of precipitation from extreme weather events. Many other places are experiencing more frequent rainfall events. If we look for the cause of this the simplest explanation (here I shave with Occam) is the warming of the atmosphere. There is no doubt that the atmosphere has warmed since 1850 and that warming has been more or less continuous, with brief periods when there was no recorded warming of the atmosphere. As air becomes warmer, it holds more moisture, which means that more clouds form and that more precipitation is likely and more frequent precipitation of longer duration is likely.
An atmospheric warming of 0.7⁰C has enable3d the atmosphere to hold 4% more moisture, and that extra moisture will fall as rain when a warm front meets a cold front but because of the extra moisture, we are more likely to receive a downpour more often.
Whatever the cause of the warming is (and I think it is anthropogenic) we have to face the reality that many parts of the world which for millennia have managed with a certain amount of extreme rainfall happening occasionally, now have to manage with more rainfall with more extreme rainfall lasting longer than we were used to.
That means a rethink of flood defence strategies and looking at better ways to build our properties to withstand extreme weather events and bigger sewers and drains. Until recently many thought that the Victorians “over engineered” the drains and sewers that they built. In the weather that we experience today and are likely to experience in future a bit of old fashioned over engineering may well save us billions in the long run.
Filed under: carbon emissions, climate change, global warming Tagged: | climate, drains, environment, extreme rainfall, extreme weather events, flooding flood defences, frequent rainfall, over enegineering, rainfall events, sewers