Guest post: Linking extreme rainfall, flooding and climate change in the recent UK deluges

尼克狐狸是铅对自然科学领域亚慱彩票APPHazards at the UK’s Centre for Ecology & Hydrology, the UK’s foremost authority on flooding science.

December was an extraordinary month for the UK. Battered by storms Desmond, Eva and Frank, most northern and western parts of the country registered more than double their average rainfall. England saw its three highest ever recordedriver flowson the Eden, the Lune and the Tyne.

At the Centre for Ecology & Hydrology, we’ve been working on our latest monthly summary, showing how the storms, rainfall and flooding combined to create such extreme scenes across the UK in December. You can read thefull report, published earlier today, on our website.

Coming so soon after record river flows and catastrophic flooding in 2009 and 2013/14, these events have inevitably prompted speculation that flooding is increasing due to climate change.

It is becoming normal after any major flooding event to ask the scientific community whether or not it was due to manmade climate change or, more correctly, whether the event was more likely because of it.

But this is not a simple question. To answer it, we need to first understand that rainfall and flooding are not the same thing. The media, politicians and others often conflate the two, which leads to confusion over the role of climate change in recent events.

Climate component

我们的出发点是,英国已经变得e significantly warmer over the last few decades, and the link with anthropogenic emissions is unequivocal.

December 2015 in the UK was thewarmestin the record dating back to 1910. It was also thewettestcalendar month in the last 105 years, the Met Office recently announced.

In a warming world, we expect an intensification of rainfall since, put simply, a warmer atmosphere can hold more moisture.

There ismounting evidencefor an increase in heavy rainfall in the UK over the last 40 years or so. In fact, the preliminary results of anattribution studyreleased just yesterday claim to show human activity made the heavy December rainfall 50-75% more likely.

Looking ahead, we can use hydrological models to project whatfuturechanges in rainfall will mean for flooding in the UK. In most cases, these indicate increases in flood flows in northern England and Scotland.

This research, carried out by the Centre for Ecology and Hydrology, has been used to support the development of the Environment Agencyguidelineson climate change and flooding.

But it’s a complicated issue. Increases in rainfall do not necessarily lead to equivalent increases in river flows or flooding. There are a number of factors than can mean the difference between one affected area flooding and another staying dry.

Cause and effect

The response of an individual catchment to changes in rainfall depends on its specific characteristics – things like the catchment size, steepness, soils and geology.

例如,一些集雨可以减少代用t of an increase in rainfall because they have additional storage available. On the other hand, other catchments can accentuate changes rainfall, producing a larger increase in peak river flows.

Because of these extra factors, the link between flooding and climate change is far less clear-cut than for rainfall alone. That’s why it’s important to look at the evidence available.

One way to address the question is to look at long records of rainfall and river flow. A great deal of scientific research has used the data held on theNational River Flow Archiveto assess whether there are long-term trends in flooding in the UK.

This work isreviewedandpublishedby CEH scientists, while anyabo亚博体育app下载of the flow and rainfall maxima was also published by CEH.

In general, the conclusion are counter intuitive. Based on river flows, it suggests there is little compelling evidence for any upward trend in long records of flood magnitude or frequency in the UK that can be attributed to climate change. Why?

The observational record of river flow in the UK is complicated because of the strong impact of natural climate fluctuations. Much of the research using very long records points to pronounced variability between decades, leading to ‘flood rich’ and ‘flood poor’ periods.

For example, the north and the west of the UK have seen an increase in high river flows associated with the positive phase of the North Atlantic Oscillation. The recent flooding has also occurred against the backdrop of a strong El Niño, which in early winter is generally thought to increase the likelihood of a positive NAO. But the link is complicated by lots of other factors.

When it comes to attributing any specific flooding event or cluster of events to anthropogenic climate change, that means it’s very difficult based on observational records of river flow alone. We need to use sophisticated modelling techniques that take into account all the relevant factors in order to undertake attribution studies.

Using this technique, the CEH and our partners publishedsomeresearchpapersfollowing the widespread flooding in 2000, which showed these events were more likely to have occurred because of humans’ influence on the climate.

We are currently working on a similar study for the Winter 2013/14 floods to supplementour reviewof the atmospheric and hydrological drivers that led to this flooding.

These types of study take time. As I write, it is not clear how much other factors contributed to the recent cluster of floods in the north-west, on top of the increased risk of heavy rainfall from climate change.

What we can say is that when it comes to communicating events like we’ve seen in the UK recently, it helps to be clear about the distinction between flooding and the rain that drives it.