Future FRM>Claire Barnett, Entec’s climate change expert, explains the link between flooding and climate change, and looks ahead to the UK Climate Impacts Programme’s new climate change scenarios.
A flood of climate change information
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Anyone that reads the papers could be forgiven for being a little confused about the nature of any link between climate change and flood risk. We know that, as the globe warms, it is likely to rain more and that scenarios of climate change for the UK suggest that our winters will become wetter, while summers become drier. This is also the pattern of change seen in rainfall trends measured over the last forty-five years. Flooding events in the summers of 2007 and 2008, along with their record breaking rainfall statistics, seem to contradict this, so how can we make sure that we are resilient to future flooding risks? Understanding the impact of climate change on flood risk is complex but perhaps it is easiest to start by agreeing a few basic concepts? Firstly, there is an important difference between weather and climate. Weather is what we experience every day while climate is the average weather we can expect over a long period of time. For example, the phrase “April showers” comes from the fact that the climate of the UK in April is typically one of showers, however if we get caught in a shower on any particular April’s day, this is weather. The second concept is that there is more than one kind of rain. When computer models are used to generate weather forecasts (or projections of future climate), two classifications of precipitation are used: large scale and convective. Large-scale rainfall is associated significant atmospheric features like weather fronts. These features are much bigger than the resolution of the models and therefore this type of rainfall is generally well simulated, meaning that large scale rainfall can be predicted with some degree of skill. The second type of rainfall is that which comes from convective events like isolated showers and thunderstorms. Although models can represent the type of conditions which lead to convective rainfall, the clouds and resulting showers are very localised and are often smaller in scale than model resolution. This means that models can provide guidance on the area in which showers may occur but not exactly where any one shower may take place. |
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The next concept relates to flooding. There are two generic type of flood events associated with rainfall. Significant volumes of rain falling over a catchment can cause river levels to rise and may result in flood. Typically, this type of flood is the result of a lot of rain over a relatively large area and therefore a weather forecast of either large scale rain or convective activity can be used in combination with catchment models to assess flood risk. Where a risk is identified, a flood risk warning is issued and actions can be taken to protect essential infrastructure, homes, people, etc. The second generic flood type is caused by intense rainfall which falls over a small area in a short period of time. This is the type of rainfall event which can overwhelm drainage systems and leads to localised flooding, particularly in urban environments with limited natural drainage. It comes from the convective events that models can’t easily resolve and, as they are so location dependent, they are very hard to predict. The only way to prepare for this type of event is through appropriate design of landscape and drainage systems.
These concepts lie at the centre of the difficulties in understanding future flood risk as our climate changes. Firstly, climate models do not provide forecasts of the year by year future weather. The output of climate models must be averaged to give a projection of future climate, or used to show levels of variability over a suitably long period. Climate scenarios for the UK show a pattern of wetter winters but drier summers but this does not however mean that a wet summer like that of 2007 will not happen again. The natural variability of our climate means that wet summers are likely to continue to occur, although perhaps with less frequency. Secondly, although climate models can generate projections of frequency of convective events, they cannot resolve the spatial scale of the storms themselves, and as model outputs are averages of daily rates of rainfall, they cannot resolve the intensity of rainfall in these events either. This poses a significant challenge for the design of drainage systems and sustainable urban drainage.
So how can these challenges be tackled? In November 2008 the UK Climate Impacts Programme will release a new set of climate change scenarios of the UK. Based upon the combined output of many climate model runs completed by the Met Office’s Hadley Centre, these will, for the first time, allow probabilities of events to be explored in detail. Tools will allow probability distribution functions to be calculated, showing the probability of an event at any defined location. It will be possible to generate cumulative distribution functions showing the probability of projected changes being below a specific threshold. Plume diagrams will show how rainfall, or any other variable, is estimated to vary through time for any given location. This information will be available at a higher resolution than ever before and for pre-defined aggregated areas (i.e. the administrative regions and river basins).
This new set of climate scenarios will make it possible to assess the changing level of flood risk across catchments over decades to come but more importantly it will allow the changing frequency of extreme events to be explored for the first time. Previous model data for assessment of extreme events was only available for the end of the century, with projections having to rely on extrapolation between the present and the distant future. Most importantly, the way the new data is constructed will allow us to understand more about the uncertainty in climate model projections, and therefore the uncertainty of future flood risk. This means that rather than adding a standardised allowance to account for climate change, we will be able to consider risk to sites for their lifetime, thereby ensuring new and existing developments are cost effectively protected, or made more resilient, taking full allowance of uncertainties in potential future flood depths and frequency. It will be possible to design integrated urban drainage systems with a greater awareness of the potential frequency of extreme rainfall events, allowing essential infrastructure to be better protected. The new scenarios will allow us to make investment decisions with more confidence, meaning that flood management measures can be implemented to a standard which will ensure a UK which is well adapted for the flood events climate change will bring.
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