Scenes such as those in Uckfield in the above video were witnessed all across England and Wales in October and November 2000. It had been the wettest autumn since records began in 1766 - the map below outlines total rainfall and percentage increase above average for the time of year. Over 10,000 properties were flooded and the costs as a result of the damage exceeded £1bn.
In a recent paper published in Science, a group of scientists have been attributing flood risk and anthropogenic climate change as a result of the extreme events in 2000. As mentioned in previous posts, only general explanations are usually offered for any expected increase in flooding. They usually revolve around thermodynamic arguments for precipitation extremes, an example of which can be found here.
In this recent paper a seasonal-forecast-resolution climate model was used to create two scenarios. One realistic scenario whereby actual twentieth century climate was modelled (including atmospheric pollutant concentrations) and a second hypothetical scenario to represent climatic conditions as they might have been had anthropogenic greenhouse gas emissions not occurred. Atmosphere-ocean linkages were taken into account alongside sea surface temperatures and sea ice cover. The scenarios could then be run multiple times and runoff rates could be compared between natural and anthropogenic conditions.
The study outlines that there were sequences of intense weather systems bringing heavy precipitation pulses to catchments multiple times throughout the day in Autumn 2000. This was characterised by the displacement of the North Atlantic jet stream (see image below) bringing more intense systems further into western Europe.
This resulted in many catchments becoming saturated. Daily river runoff was examined using a simple hydrological runoff model and the climate scenarios were compared.
The above diagram is a summary of their results, showing that the fraction of risk attributable to twentieth century greenhouse gases are significant relative to the natural conditions (without anthropogenic impact). The black line is an aggregate relative to the full natural baseline conditions.
Model Results:
In nine out of ten cases, the model results indicate that twentieth century anthropogenic greenhouse gas emissions increased the risk of floods occurring in England and Wales by more than 20%, and in two out of three cases by more than 90%.
These results are yet another step towards providing evidence of how extreme weather events are a direct result of human anthropogenic change. But again I will give a word of caution - changes in built environment (concrete over natural environments) could have played a part in historical model simulations in recent years (affecting both run-off and return time). Furthermore, just because an event-type becomes more likely, does not meant that it will become even more likely in the future - but it does highlight the potential impact of climate change..
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