Are Shropshire’s floods caused by climate change?

The very high River Severn at Shrewsbury.

Shropshire has been battered by heavy rain and strong winds over the last week, as two storms passed over the county within a few days of each other. Hundreds of trees fell (Shropshire Star, 2014a), including one in my garden. With the ground already saturated thanks to months of wet weather, the already swollen River Severn burst its banks at Shrewsbury and Ironbridge, damaging houses (Shropshire Star, 2014b).  

A fallen tree on Haughmond Hill.

This has been the wettest January on record for Britain (Carrington, 2014), so the recent weather is out of the ordinary. But is it part of the normal variation from year to year that we would expect, or has it been caused by climate change? And if so, are we likely to experience this kind of wet and windy weather more often in the future?

What is climate change?

The climate change we are experiencing now is caused by an increase in greenhouse gases in the atmosphere. These gases absorb heat from the sun, warming up the air, and therefore the land or water below. Burning fossil fuels to heat our homes or generate electricity causes more carbon dioxide – a key greenhouse gas – to be produced, and destroying forests or other vegetation means that less carbon dioxide is removed from the atmosphere through photosynthesis. With more carbon dioxide entering the system than is being removed, the concentration has risen from 278 parts per million (ppm) in the pre-industrial period to 400 ppm today. In turn, the global mean temperature has increased around 0.8°C over the same period (World Bank, 2012).

Climate change and rain

Increasing the temperature of the atmosphere affects rainfall patterns across the world, both in the amount and location of rain. This is because the warmer it is, the more water evaporates from the surface of the earth, and also the more water vapour the air can hold – the water-holding capacity of the atmosphere increases by 7% for every 1 °C increase in temperature (Trenberth, 2011).  It is thought that the net effect of this “speeding up” of the water cycle will be that dry areas of the world will get drier, and wet areas of the world will get wetter (World Bank, 2012). With Britain already famous for its drizzly weather, it seems likely that Shropshire will receive more rain rather than less.

Scientists use climate models to simulate the effect of increasing carbon dioxide levels on precipitation. A study by NASA (2013) using 14 models showed that not only will there be more rain in wet areas across the course of a year, the pattern of rainfall will change so that there are more days with heavy rain and fewer with moderate rain. The latest report from the UN’s Intergovernmental Panel on Climate Change (IPCC, 2013) states that by the late 21^st century, it is very likely that heavy precipitation events will occur more often and more strongly in areas of the world like Europe.

So, in the future our wet weather will come in shorter bursts, making it more difficult for natural drainage systems to cope and river flooding more likely. A study by Ciscar et al (2011) showed that if global average temperatures increased by 2.9 °C above industrial levels, 12,000 people would be affected by river flooding in northern Britain. If temperatures rise by 3.9 °C, this figure rises to 48,000.

Were these storms caused by climate change?

Shrewsbury's Theatre Severn's flood defences held out. 

Climate change is a long process that has many different and overlapping effects. It is therefore very difficult to blame any specific weather event on it, and instead scientists talk about particular aspects of weather being more or less likely in a warmer world. However, this has not stopped politicians and some public figures making statements in the last few weeks either attributing this unusual British weather to climate change, or actively denying it (Shuckman, 2014).

Interestingly, while there is evidence that specific instances of very heavy rain in the last few years are due to climate change, evidence surrounding storms specifically is less clear (Coumou and Rahmstorf, 2012).

Conclusion

Scientific evidence suggests that the unusually high rainfall levels we have experienced in the last few months may become less unusual in the future, as increasing global temperatures make Britain an even rainier place. While we cannot lay definitive blame for the storms of the last week on climate change, as we move forward we may see clearer patterns emerging. After all, the climate is really a collection of all our weather, so we must expect the weather to change unless we reduce carbon emissions drastically.

References

Carrington D (2014) ‘January was England’s wettest winter month in almost 250 years’, The Observer, 01/02/14. Available at www.theguardian.com/uk-news/2014/feb/01/january-uk-wettest-winter-month-250-years

Ciscar J, Iglesias A, Feyen L, Szabó L, Regemorter D V and Amelung B (2011) ‘Physical and economic consequences of climate change in Europe’, Proceedings of the National Academy of Sciences of the United States of America. Available at: http://www.pnas.org/content/108/7/2678.full

Coumou D and Rahmstorf S (2012) ‘A decade of weather extremes’, Nature Climate Change, 2, 491-496. Available at: http://www.nature.com/nclimate/journal/v2/n7/full/nclimate1452.html

IPCC (2013) Climate change 2013 – the physical science basis: Summary for policymakers. Available at: https://www.ipcc.ch/report/ar5/wg1/docs/WG1AR5_SPM_FINAL.pdf

NASA (2013) NASA study projects warming-driven changes in global rainfall. Available at: www.nasa.gov/home/hqnews/2013/may/HQ_13-119_Rainfall_Response.html

Shropshire Star (2014a) ‘Hundreds of trees felled by the storms’, Shropshire Star, 14/02/14. Available at: www.shropshirestar.com/news/2014/02/14/hundreds-of-trees-felled-by-shropshire-storm/

Shropshire Star (2014b) ‘Ironbridge residents urged to move out over flooding’, Shropshire Star, 11/02/14. Available at: www.shropshirestar.com/news/2014/02/11/waters-rising-as-floods-swamp-shropshire/

Shukman D (2014) Barrage over climage change link to floods. Available at: http://www.bbc.co.uk/news/science-environment-26242253

Trenberth K (2011) Changes in precipitation with climate change, Climate Research vol 47: 123-138. Available at: www.int-res.com/articles/cr_oa/c047p123.pdf

World Bank (2012) Turn Down the Heat – Why a 4 °C warmer world must be avoided. Available at: www.worldbank.org/en/news/feature/2012/11/18/Climate-change-report-warns-dramatically-warmer-world-this-century