Sunday, 6 December 2015

Anthropogenic Climate Change and the flooding in Cumbria

Cumbria in North West England has been hit by severe floods as a month's worth of rain falls in one week, the floods are as severe as the floods in 2009, which at the time were claimed to be a 1 in 1000 year event.

Has Anthropogenic (human driven) Climate Change got anything to do with this?

So what has happened? Storm Desmond has entrained a large plume of water vapour from the tropics, this is the proximate cause of the heavy rainfall, which was compounded by the front moving across the region in a band along the west to east line of the front. orographic uplift compounded the problem further by causing the front to dump rain on the Cumbrian mountains.

However the plume of water vapour is notable, from the following animation keep an eye on the Eastern Seaboard of the US at the start of the video.

The Environment Agency currently has 47 severe flood warnings, and 48 flood warnings. The Army has been called out to aid the civilian powers, and over 60,000 homes are without power, with 2 hospitals currently working off back-up generators.

After the 2009 Cumbria flood disaster there were improvements to flood defences to aimed at protecting against a 1 in 100 year flood event (BBC), however yesterday with 1 month's worth of rain falling in 1 day the defences failed to hold. No reasonably attainable amount of defence can protect against such a deluge.

So what of a role for Climate Change?

I don't have daily or regional rainfall data, but I do have access to monthly rainfall data for the UK, England, Wales and Scotland. Taking the overall data for the UK I have previously calculated the average annual rainfall for the UK for the period 1981 to 2010 and used that to calculate the difference from the average for each year (called an anomaly).

So using the available data from 1910 here are the anomalies of annual rainfall.

If we take the 200mm annual anomaly (1250mm annual total) as an unusually wet year from 1910 to 1997 there were two such years, which makes unusually wet years a 1 in 44 year event, since 1998 there have been 6 unusually wet years, which makes unusually wet years a 1 in 2.7 year event.

Note that I have included 2015, but December data is not yet available for 2015, so 2015 is calculated as the rainfall so far (Jan to Nov), and so far we have had rainfall equal to the 1951 to 1980 average. This means that all rainfall in December will go towards exceeding average rainfall. The average December rainfall for 1951 to 1980 is 114.6mm, so 2015 breaking the 200mm mark to fall into the arbitrary 'unusual rainfall' category seems unlikely, however I do not know how much the recent Cumbria flooding and rainfall will add to that in the whole UK data.

There is a very clear increase in precipitation in the UK during the most recent phase of global warming. This fits with expectations from models of an intensification of the hydrological cycle with global warming (e.g. Held and Soden 2008), and evidence of an intensification of the hydrological cycle (e.g. Huntington 2006, and Wild et al 2008). One event cannot necessarily be linked to climate change. However the floods in Cumbria, following as they do a flood event in 2009 of similar magnitude, fit the pattern of increased rainfall that adds to an increase in overall UK rainfall, which is very clear from the data.

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