A perspective on possible drivers of the major winter 2013/14 UK flood events

January 5th, 2015

Dr. Chris Huntingford, UK Centre for Ecology and Hydrology, UK

During and following the extensive flooding that occurred in the United Kingdom last winter there was much discussion by the meteorological and climate modelling community as to the potential causes. Overlaying assessments of cause were the inevitable questions raised in the media as to whether the burning of fossil fuels could have influenced the likelihood of flooding, the intensity of rainfall, or affected the general paths taken by winter storms.

As is so often the case when trying to understand even single attributes of the climate system, the more the issue is analyzed the more complex it becomes, often revealing a vast array of possible drivers. In order to shed light on some of these questions a team of interdisciplinary scientists across the UK began working through the literature and the IPCC reports, as well as initiating discussions between the UK meteorological and climatological modelling communities, in order to try to identify the potential role – if any – of climate change in the recent UK floods. The results were published as a Perspective article in the journal Nature Climate Change and are summarised in this article.1

In the first instance it was important to provide an overview of the large-scale weather events leading up to the storms. Although none of the individual rainfall events was unprecedented, there were noteworthy weather patterns which caused a near-continuous succession of Westerly storms for three months. This had the cumulative effect that for much of the southern UK, the total winter rainfall was record-breaking.2

Preliminary analysis suggests that particularly warm ocean conditions together with heavy rainfall in and around Indonesia triggered wind patterns across the Pacific that travelled northwards, before ultimately drawing cold air down across the USA. This in turn forced an especially strong and persistent Jet Stream across the Atlantic and towards the UK. The Met Office is now analyzing this sequence of events in significantly more detail, and their findings will be interesting to read in future peer-reviewed papers.

Our review looked at how this affected river flow responses in the UK placed within a historical context of river flow measurements. Looking at measurements for the River Thames at Teddington, for example, and mirroring the rainfall drivers, it was found that no peak flow records were broken. However, the results did suggest that the number of consecutive days at high flow levels (e.g. 250 m3s-1, a level which would have caused flooding pre-1947 before flood defenses enhanced) did break records.

With the context set out the next step was to try and consider the potential role of atmospheric greenhouse gases in changing river flows, especially as questions quickly mounted as to whether fossil fuel burning could have had a role. We reviewed the existing research literature for Earth system factors that: (1) may be changing through global warming; and (2) are identified as influences on storm features for the UK. As expected, this demonstrated how remarkably complex and interconnected the climate system is.

Multiple possible UK rainfall drivers were identified that relate to the state of the oceans, the atmosphere, and sea-ice extent. Interestingly the recent rapid decrease in Arctic sea-ice that is widely attributed to global warming,3 for the UK at least, is often portrayed as likely to bring more Easterly winds and colder conditions. The previous three winters had these features for some of the time, in marked contrast to the winter of 2013/14.

Although the precise details of linkages between changing large-scale features of the climate system and UK rainfall intensity are still not fully understood, we are optimistic that our review article does at least present a complete list of such connections such as the possible role of sea surface temperatures. We hope there are no unknown factors lurking out there we have yet to consider – or if there are, that these become known to climate researchers as soon as is possible.

Following the analysis, our Perspective article presented an open debate as to how best the research community should proceed. If we believe that the research community has a good idea of all the drivers expected to affect rainfall, there is a requirement for on-going computer modelling to capture these effects. This, however, raises three challenges:

The need for continued enhancement of physical process representation via ever better parameterized differential equations of the oceans, atmosphere and ice sheets.

Further increasing the numerical grid resolution of climate models, on which these equations are calculated.

Undertaking significantly higher numbers of simulations, all with slightly different initial conditions, creating a large ensemble of projections.

The call for better resolution is due to the fact that some characteristics of storms occur on fine spatial detail, thus needing small spacings between the gridpoints on which calculations are updated. The request for large ensembles of data is because extremes, by definition, are rare events, and so we need to ensure that all heavy rainfall “return times” are fully sampled. All of this needs to occur for pre-industrial and for contemporary levels of atmospheric greenhouse gases, and to aid planning, for a range of future potential scenarios of altered gas concentrations.

During the major flood events affecting much of Southern England from December 2013 to February 2014, multiple questions were asked as to whether fossil fuel burning could have a role. It is always – and correctly – stated that no single observed extreme event can be formally attributed to human-induced changes to atmospheric composition. However, a statistic can be derived that assesses any changing probability of a particular extreme event occurring, a quantity sometimes referred to as “Fractional Attributable Risk”.

Satisfying the three computational challenges mentioned above, should get us nearer to stating if humans are increasing, decreasing, or leaving invariant the chances of rainfall events of the type witnessed. However, even now limitations remain on computer speed and resource, and expenditure on climate research can only ever be finite. Hence a robust debate is now needed as to what constitutes the optimal balance between pursuing these three challenges, in order to get us most quickly towards the required answers.

In any study of meteorological systems, or the full Earth system, it soon becomes apparent how tightly coupled all features of the climate system are. In the recent review, by trying to collate in to a single paper the main factors affecting UK rainfall, this did though provide a timely reminder of such comprehensive interconnections. Many of our findings and identified challenges ahead in the numerical analysis of solving the governing equations of meteorology and climate are generic to other geographical regions. It is currently a very interesting time for on-going climate change research and assessing future impacts.

References:

Huntingford, C. et al. (2014), ‘Potential influences on the United Kingdom’s floods of winter 2013/14’, Nature Climate Change 4, 769-777, doi:10.1038/nclimate2314. Matthews, T. et al. (2014), ‘Stormiest winter on record for Ireland and UK’, Nature Climate Change 4, 738-740, doi:10.1038/nclimate2336. Boe, J. et al. (2009), ‘September sea-ice cover in the Arctic Ocean projected to vanish by 2100’, Nature Geoscience, 2, 341 – 343, doi:10.1038/ngeo467.

Dr Chris Huntingford is a climate modeller at the UK Centre for Ecology and Hydrology. He recently published a multi-author review of possible factors in the UK winter 2013/14 floods which this article is based on. This was accepted for publication as a perspective article in the journal Nature Climate Change and can be accessed at ‘Potential influences on the United Kingdom’s floods of winter 2013/14’ The article also builds on a blog post originally published on the CEH website here.

The views expressed in this article belong to the individual authors and do not represent the views of the Global Water Forum, the UNESCO Chair in Water Economics and Transboundary Water Governance, UNESCO, the Australian National University, or any of the institutions to which the authors are associated. Please see the Global Water Forum terms and conditions here.