Understanding the impact of climate change on inland flood risk in the <scp>UK</scp>

Abstract

Flooding is one of the biggest environmental risks facing the United Kingdom. Floods have the potential to cause devastating impacts on livelihoods, the economy and natural ecosystems. Since the widespread floods in 2007, a number of high-profile events (Table 1) have repeatedly led to headlines about record breaking rainfall, the highest water levels on record and ‘unprecedented’ floods. Climate change is expected to lead to an increase in both the frequency and severity of flooding for some parts of the UK. In this briefing we explain these impacts and consider if we are already seeing evidence of this change. Warmer air (owing to climate change) can hold more water and therefore has the potential to increase rainfall intensity and the associated risk of flooding. However, regional weather patterns are also likely to change; some places will be wetter while others will see a reduction in rainfall. Climate change projections over the UK for the twenty-first century show an increased chance of wetter winters. Summers are likely to be drier overall, but an increase in the frequency and intensity of convective rainfall (such as that from thunderstorms) is projected (Met Office, 2019). The effects are projected to vary across the country: in the north and west, where rainfall is driven by the prevailing westerly winds from the Atlantic, the most significant change is the expected increase in winter precipitation. In the drier south and east, there is more concern about the increased risk of summer droughts than flooding (Arnell et al., 2021). Scientists agree that flood risk across the UK is likely to increase due to climate change. As global or regional scale climate models predict results on large grids, one of the big difficulties for hydrologists is understanding the impact of these changes at the scale of individual catchments. UK catchments are varied in terms of size and geology and some are more susceptible to increases in rainfall than others. More rain does not necessarily mean more flooding as changing weather patterns also change the ability of catchments to store water in soils or vegetation. As well as climate change, river flows are affected by changes to land use, farming practices, urbanisation, river engineering, reservoirs and hydropower. Floods only cause significant impacts when people, property, infrastructure or vulnerable ecosystems are located in the path of the flood water. Development on floodplains has therefore played an important role in increasing flood risk regardless of the changing climate. To account for some of these potential feedbacks, scientists use hydrological models that represent the interaction between changes in rainfall, evaporation and land use to predict river flow. These studies consistently show an increase in the chance of river flooding due to climate change of up to 40% by 2100 from long duration winter rainfall events, particularly in western England, Wales and Scotland (Arnell et al., 2021). Flash floods occur in steep-sided catchments or urban areas (sometimes called surface water flooding) during intense storms where the rainfall rate exceeds the rate at which the ground or sewerage system can drain it away (for example in Coverack in 2017, see Table 1). The fast-flowing flood water can be very dangerous. From a hydrological perspective any increase in intense rainfall has the potential to increase the frequency and severity of flash flooding, but predicting these changes is challenging. The type of storms that lead to flash floods are very localised and short-lived, as such, they are hard to represent in climate models. A new generation of ‘convection-permitting’ models, similar to those used for weather forecasting, are enabling scientists to understand more about how future changes to localised intense rainfall events will affect flood risk. Observed changes in rainfall and river flow records (Harrigan et al., 2021) are broadly consistent with climate model projections indicating climate-induced changes are already happening. However, these records can also show changes caused by natural climatic variability. Thus, more years of data will be required before we can be completely confident that any upward trend is a signal of climate change. Similarly, due to the number of factors involved it is difficult to say that any one individual flood event was caused by climate change. But by comparing the driving weather conditions with and without anthropogenic influences on the atmosphere, research has shown that the type of rainfall events which led to widespread flooding in southern England in 2013/2014 (Schaller et al., 2016), and from Storm Desmond in 2015 (Otto et al., 2018) and storms Ciara and Dennis in 2020 (Davies et al., 2021), are more likely to occur in the current climate. Scientists are confident that climate change will alter the rainfall patterns and quantities in the UK. Therefore, there is an urgent need to increase resilience to flooding. Such plans need to be flexible enough to account for the varying risk across the country and the uncertainties around both predicting future rainfall and understanding the impact of climate change on flood risk in individual catchments. This paper was developed in collaboration with the Royal Meteorological Society's Science Engagement Committee. We also thank the reviewers for providing helpful comments to improve the readability of this paper.

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