UK flooding & climate change – part 2

March 1, 2014
by phil

“We are in danger of losing sight of some glaringly obvious truths about this exceptionally wet and stormy winter.” - Phil Rothwell, Head of Flood Risk Policy at the UK Environment Agency until December 2013

In the last post we looked at the clear link between climate change and the UK flooding. Here we discuss what we know and what we don’t know about the extent to which you can blame (or attribute) the exceptional run of storms that led to the widespread flooding, on (or to) climate change.

All weather events have a climate contribution

As Kevin Trenberth of the National Center for Atmospheric Research argues in Climatic Change, asking whether a particular weather event was “caused” by climate change is the wrong question:

All weather events are affected by climate change because the environment in which they occur is warmer and moister than it used to be (…) no events are ’caused by climate change’ or global warming, but all events have a contribution.

Discerning the size of this contribution is the task of the science of attribution, a science which is still young and can’t tell us very much with certainty. The challenge is discerning how much is natural variability and how much is due to manmade climate change. As the World Meteorological Association’s report 2001-2010: A Decade of Climate Extremes puts it:

Owing to the naturally high internal variability of the climate system, however, it is still difficult to assess in a systematic way the degree and amount of climate-change influence on a single observed event.

Methodologically, the science is complicated and time consuming. The approach that many attribution studies take is to compare a long data sample of real world weather events against the results you get when you run a computer climate model without any human-induced climate change.

Attribution of trends is easier than for events

It’s easier to link climate change to trends than it is to specific events lying outside what is expected. Take for example this extract from an interview with the lead editor on a 2012 collection of studies of extreme events, concerning the record low summer Arctic sea ice in 2012.

Rebecca Lindsey: Of all the events that scientists chose to analyze, the ones that look for a human fingerprint in the record low Arctic sea ice extent in September 2012 might puzzle people. Is there still uncertainty that human-caused warming is the primary cause of Arctic sea ice loss?

Thomas Peterson: There’s still uncertainty about the speed with which Arctic sea ice will be melting in the future and when we will have an ice-free Arctic Ocean during the summer. We can see the long-term trends, and that’s obviously related to climate change. But we also see some major dips, like in 2012, when you have record low event. So what they’re trying to figure out is, even with the long-term trend, what caused that particular year to be extremely low. That’s relevant if you are looking at projections in the future. If this record-low year was due solely to global warming, that would alter one’s expectations of how quickly one should expect an ice-free Arctic Ocean.

RL: So you’re talking about the difference between the influence of human-caused climate change on a long-term trend versus its influence on a specific event?

TP: Yes, there’s attribution of change and attribution of events.

What does attribution science have to say about the UK floods and manmade climate change?

The head of the Met Office’s Climate Monitoring and Attribution team, Peter Stott, has written about what attribution studies are able to say about the link between climate change and the recent UK flooding and the answer is: ‘nothing – there have been no formal attribution studies, but we’ve got some great new models so maybe we’ll have something to tell you in the future.’

As Stott explains, the complicating factors are our lack of understanding about how and why the jet stream is changing and what caused the run of storms to be unusually persistent:

The current exceptionally wet and stormy British winter provides a particularly challenging test case for attribution science. A disturbed and stronger than usual jet stream has brought a sequence of intense storms on a more southerly track than usual.

(…) But an unusual feature of the weather this winter has been the persistence of weather patterns, with storms lining up across the Atlantic to batter the country one after the other.

At present we don’t know how different drivers within the global circulation have affected this sequence of storms, nor do we know for sure if our current generation of climate models are able to calculate the relevant processes reliably enough to make an accurate calculation of the changed risk of such events.

Something that may clear up the issue around the unusual persistence of the storms is research by Jennifer Francis and Stephen Vavrus which has suggested a link between the changes we are seeing in the Arctic (something we explored in this edition of Climate Radio) and an apparent recent pattern of prolonged weather events. This is new science over which there is still some debate. Listen to this conversation between Jennifer Francis and Kevin Trenberth to get an idea of the current state of the debate:

Climate deniers like to pounce on scientific uncertainty as support for their position. Listen to anti-climate action campaigner Nigel Lawson doing precisely this in the context of the UK floods on the agenda-setting BBC Radio 4 Today programme. However, a lack of understanding around a new and emerging situation is simply that. It does nothing to undermine what we already know from modelling, basic physics and observation.

What we know already: “the scale of the impacts will have been exacerbated by climate change”

For now, scientists have made clear statements on how manmade climate change has made the UK flooding worse due to both sea level rise and a greater risk of intense rainfall from increased moisture in the air:

Here’s Tyndall Centre deputy director, Kevin Anderson, on BBC Newsnight on 17th February 2014:

We know that the sea level rise that has already gone on because the oceans have warmed, and we know therefore that some of the impact of Sandy in New York or some of the impacts in Haiyan [in the Philippines], or the impacts we’re seeing now [in the UK] are partly due to the increased sea level rise. So it makes the situation worse even if the overall event is not a climate change event.

and here’s Peter Stott again:

It is clear that global warming has led to an increase in moisture in the atmosphere – with about four per cent more moisture over the oceans than in the 1970s – which means that when conditions are favourable to the formation of storms there is a greater risk of intense rainfall.

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