In recent posts here and here we looked into the question of whether the UK’s recent wet weather was unprecedented and concluded that it wasn’t. But is it symptomatic of an upward trend in the intensity and frequency of extreme rainfall events caused by climate change, as some have claimed?
This post addresses this question in two sections. The first summarizes how different organizations define “extreme weather” (or “climate”). The second applies four of these definitions to the UK Met Office’s monthly England & Wales rainfall record to determine a) which months qualify as “extreme” and b) whether there has been an increase in the frequency and intensity of extreme rainfall months since 1766, when the record begins. The conclusions are:
• There is no accepted definition of “extreme weather”. Extreme weather is whatever one wants it to be.
• There has been no statistically significant increase in extreme monthly rainfall in England and Wales for at least the last 250 years regardless of how “extreme” is defined.
• Since England and Wales make up 62% of the surface area of the UK it is likely that this conclusion applies to the UK as a whole, although this can’t be confirmed.
1. Definitions of “Extreme”
We begin with the BBC, which puts the problem in perspective:
Extreme weather is when a weather event is significantly different from the average or usual weather pattern. This may take place over one day or a period of time.
Here the BBC identifies three issues: a) how long should the period of time be, b) how to define “significantly different” and c) how to define “average”. In this review the time period (one month) is fixed by the data and the question of how to define “average” is discussed later. The main question is how to define “significant”, and while there is general agreement that the way to do this is by setting a percentile threshold based on the historical data distribution there are differences of opinion of over an order of magnitude as to where the threshold should be set, as exemplified below:
An extreme weather event is an event that is rare within its statistical reference distribution at a particular place. Definitions of rare vary, but an extreme weather event would normally be as rare as or rarer than the 10th or 90th percentile. By definition, the characteristics of what is called extreme weather may vary from place to place. An extreme climate event is an average of a number of weather events over a certain period of time, an average which is itself extreme (e.g. rainfall over a season).
According to this definition any month above the 90th percentile is rare enough to be considered extreme. I don’t think one month in ten comes close to qualifying as extreme, but I use the IPCC definition anyway. (Note also the uncertainty as to the difference between an extreme “weather” and an extreme “climate” event. Monthly rainfall would appear to fall somewhere in between the two.)
Extreme weather includes unusual, severe or unseasonal weather; weather at the extremes of the historical distribution—the range that has been seen in the past. Often, extreme events are based on a location’s recorded weather history and defined as lying in the most unusual ten percent.
The “most unusual” ten percent includes the dry months at the bottom of the distribution as well as the wet months at the top, so here we are looking at the data above the 95th percentile, not the 90th percentile. But defining one month in twenty as “extreme” maybe still overdoes it a little.
The Met Office rarely refers to “extreme” weather, preferring the term “severe”, which it fails to define. But after some searching I came across this:
The meteorological or statistical definition of extreme weather events is events at the extremes (or edges) of the complete range of weather experienced in the past. Defined in this way, extreme weather events include, but are not limited to, severe events like heatwaves or intense rainfall. UKCP09 (UK Climate Projection 2009) provides probabilistic projections of changes in climate extremes that are based on the 99th percentile, or the value that is exceeded on only 1 in 100 days.
The 99th percentile, or one-event-in-a-hundred, is certainly a more robust definition of “extreme” than the one-in-ten or one-in twenty definitions of the IPCC and NOAA.
The recent decade has seen an exceptional number of extreme heat waves around the world that caused severe damage to society and ecosystems. These events were highly unusual with temperatures typically three standard deviations warmer than the local climatology lasting for several weeks.
This quote is from the paper by Coumou & Robertson linked to in the heading, but more than three standard deviations above the mean, otherwise known as the “three-sigma” limit, seems to be the accepted yardstick for defining extreme heat waves. It defines the top 0.3% of the data, or in this case one extreme month in every 333, which makes it over thirty times more restrictive than the IPCC definition. It’s not clear why it should be applied to extreme heat and not to extreme rainfall, so I have incorporated it in the review.
2. Identifying extreme events in the UK rainfall record
The monthly UKMO rainfall record for England and Wales, which begins in 1766, is plotted in Figure 1 (the UKMO Northern Ireland and Scotland records don’t begin until 1931): The yellow trend line shows only a very minor increase in average precipitation over the last 250 years, but that doesn’t necessarily mean there has been no increase in extreme rainfall events:
Figure 1: UKMO England & Wales monthly rainfall record
Two points should be noted before proceeding:
• Monthly rainfall does not tell us how many floods occurred in the month, although arguably the worst UK flood since 1766 occurred in March 1947 , the month which this review identifies as having had the most extreme precipitation. There are no “monthly number of flood” records for the UK that I am aware of.
• On average the UK receives about 60% more rain in October than in April, so percentile thresholds set relative to mean annual rainfall will tend to make a wet October seem more anomalous than it really is and a wet April less so. Separate “extreme” thresholds are therefore defined for each month using the mean monthly rainfall and standard deviations shown in Figure 2:
Figure 2: Monthly mean rainfall and standard deviation from Figure 1 data.
Now to the results. Figure 3 shows the extreme rainfall months defined by the four thresholds (click to enlarge). The values plotted are the differences in millimeters between observed monthly rainfall and the monthly extreme threshold, i.e. they are a measure of how extreme the monthly rainfall was. Months when rainfall is below the threshold are ignored:
Figure 3: Extreme monthly rainfall events
None of the four plots provides any good evidence for an increase in the intensity of extreme monthly rainfall events since 1766. The trend lines do show shallow positive gradients, but because of the large amount of scatter none of them comes close to being statistically significant. The R squared values of zero or close to it in fact suggest that there has been little if any change in the intensity of extreme monthly rainfall in England and Wales over the last 250 years.
Figure 4, which plots the number of extreme events per decade for the 90th, 95th and 99th percentile thresholds (the three-sigma case does not have enough points to plot), provides information on whether extreme rainfall events are becoming more frequent. The trend lines for the 90th and 95th percentile cases show shallow upward gradients but again at very low R squared values, and the trend line for the 99th percentile case shows a shallow downward gradient. In summary, there is also no good evidence to suggest that extreme rainfall events in England and Wales are any more frequent now than they were in 1766.
Figure 4: Number of extreme monthly rainfall events per decade
In a recent Financial Times article the following prominent scientists are reported as having made the following statements:
Professor Alan Jenkins, Deputy Director of the Centre for Ecology and Hydrology : “We are absolutely convinced that there is weighty scientific evidence that the recent extreme rainfall has been impacted by climate change.”
All climate experts who have analysed this December’s record-breaking temperatures and rainfall in Britain see a significant role for global warming in addition to natural variability in the weather, according to Professor Adam Scaife, head of long-range prediction at the UK Met Office.
Professor Myles Allen, a climate scientist at Oxford university, said predictions of heavier winter rainfall in mid-latitudes as the world warms go back 25 years. “So none of this should really come as a surprise to anyone,” he added. “The hopefully diminishing band of diehards who continue to dismiss climate change concerns as ‘just green crap’ have mud on their hands.”
These gentlemen are invited to point out the flaws in my analysis.