There's a Storm a Coming

The UK has just experienced its first named storm of Autumn 2016, Storm Angus, which hit the UK on the 20th November. As a result, there were some quite nasty weather conditions and impacts for the south of the UK - flooding, power cuts, ships running aground, and car accidents causing people to be hurt, though there were thankfully no fatalities. Take a look at this nice summary by the Met Office if you are interested in what happened. It is too early to say how much damage was caused by the storm, but this picture of a street in Bristol indicates the sort of issues that might have occurred:

Credit: Lee Gitsham/PA

In light of this recent storm, and following on from the article about jet stream research I posted a few weeks back, I thought it topical this week to explore the relationship between intense weather events and climate change, with a particular focus on the UK. At a global scale, there is consensus from the IPCC that climate change will lead to more extreme rainfall events in the future, but with a higher number of dry days. This is caused by the fact that in a warming world, air can hold more water vapour before it reaches the point where it must fall as precipitation. As such, there will be less rainfall events as a whole (it takes longer for the amount of water needed for there to be rain to build up), but the rainfall events that do occur will be more extreme in both their intensity and duration. There are obvious implications from this sort of climatic arrangement - an increased risk of flooding, damage, and loss of life during extreme weather events.

The discussion has already begun as to whether the recent Storm Angus has been influenced by climate change. The UK has experienced a succession of strong and destructive winter/autumn storms in the past 5 years, leading to widespread debate as to whether climate change is affecting weather patterns in the UK. During the recent severe floods of 2013/14 in southern England, Prime Minister at the time David Cameron told parliament that he 'very much suspected' anthropogenic climate change was associated with the event.  These floods were quite severe, and caused by continual low pressure storm systems moving in from the Atlantic across southern England, with a consistent amount of high level precipitation leading to flooding.

It is very hard to pin a single extreme weather event to climate change; it is simply one event in a chaotic system, and it is hard to tell whether that one event is due to a change in the boundary conditions of the system, or is simply a natural part of the chaotic system. Despite that, attempts have begun to examine whether anthropogenic climate change may have increased the risk of the occurrence of heavy storms and resultant flooding that have been recently been experienced in the UK.

The first piece of such research was published earlier this year by Schaller, et al. (2016), who examined the potential influence anthropogenic climate change had on the floods of 2013/14 in southern England. They conclude (through the use of a variety of climate model simulations) that along with the atmosphere being able to hold more moisture, anthropogenic warming has likely caused a small increase in the number of January days that have a westerly airflow, and a stronger jet stream. In combination, both of these factors increase the risk of the UK experiencing events of extreme precipitation.

The study then feeds this information into a hydrological model, in an attempt to understand how this event affected the River Thames - the area around the Thames is one of the key places flood damage to property occurred during 2013/4. It is found that under these the conditions experienced in 2013/4, the 30 day average of peak river flow in the Thames increased significantly. Combining these results with flood risk mapping, a small increase is found at the number of properties at risk from riverine flooding, though there is great uncertainty in the true value of the number. Despite this, similar events of a greater intensity could lead to even greater flood damage.

All of the aspects of this study have large bounds of uncertainty - the conclusions that have been drawn are simply the best estimates of the data available, reflecting how anthropogenic climate change has so far only had a subtle impact in increasing the risk of intense precipitation events in the UK. Far more attribution studies of this type are needed to begin to understand how climate change is affecting UK weather patterns. However, it is reasonable to suggest from this study alone that it is likely anthropogenic climate change may be beginning to influence extreme weather events in the UK - keep an eye on the weather forecast this winter! The next named storm we experience in the UK will begin with a B, so guesses for the name in the comments please! I'm going for Storm Bertha.