Extreme Rainfall Claims Not Supported By Data
By Paul Homewood
The Guardian, reporting on some recent research from Oxford University, comment:
Climate change caused by humans has made the likelihood of extreme rainfall similar to that seen in England this winter significantly higher, according to analysis seen by the Guardian.
Rainfall events that would previously have occurred only once in a century are now likely to be witnessed once every eighty years in the south of England, the Oxford University work shows.
That will mean far more frequent severe floods for residents of the crowded region, with what were once extremely rare events now happening much more often than the infrastructure of the region is equipped for. The research shows an increase in the rate of such events of about 20 to 25%, which significantly alters the number of homes likely to be vulnerable to flooding.
The first thing to point out is that these results are based on climate models, rather than real world data. As the authors point out:
However, while our findings are statistically robust, the result depends on how man-made climate change is represented in the experiment. We used different climate models to estimate the pattern of global warming which provided a range of possible changes in risk. In several cases, the models gave no change or even a reduction in risk, but overall the simulations showed a small increase in the likelihood of extremely wet winters in the south of England.
They also describe their study thus:
Following preliminary assessments from the Met Office, Oxford University researchers undertook the first scientific experiment to analyse whether the risk of extreme rainfall has changed owing to climate change after the winter deluge between December 2013 and February 2014. Total rainfall in Oxford over the three months was the highest ever recorded by the University’s Radcliffe Observatory since it was set up 200 years ago.
Scientists used the spare capacity on volunteers’ home computers to compare tens of thousands of simulations of possible weather in our present-day climate with tens of thousands of simulations of a hypothetical world without the influence of past greenhouse gas emissions in the atmosphere using the same climate model. Comparing numbers of extremely wet winters between these two groups provides estimates of the influence of climate change on the UK weather. They found that a one-in-100-year winter rainfall event (i.e. a 1% risk of extreme rainfall in the winter of any given year) is now estimated to be a one-in-80 year event (i.e. a 1.25% risk of extreme rainfall in any given winter), so the risk of a very wet winter has increased by around 25%.
Still, let’s see what the real data tells us.
Despite last winter being the wettest on record since 1910 in the South of England, there is no evidence to suggest that wet winters are becoming more common. The wettest 10 years were between 1911 & 1920.
However, we need to recognise that the selection of the narrowly defined period of “December to February” has very little relevance. If there are concerns about “extreme rainfall and flooding”, we should be looking at the months that are traditionally the wettest. In the South of England, these are October to December.
When we analyse these months, we find that the wettest year was 1929, followed by 2000 and 1960. (Oct-Dec rainfall in 1929 totalled 457.7mm, compared to 408.5mm this last winter).
Wet years in 2000 and 2002 pushed the 10-Year trend up to similar levels seen in the 1910’s, but the trend has fallen again since.
Also evident is the well accepted “dry period” of the 1960’s and 70’s. As with the winter trends, there is no evidence that Oct-Dec periods are becoming wetter.
Of course, the study talks about “extreme rainfall”, rather than averages, so is there any evidence that this is becoming more common?
It is important to recognise that the reason why the winter of 2013/14 was so wet was because the wet weather persisted for three months. (It is also worth recalling that the even wetter weather of 1929 persisted for four months, from October to January).
This persistence is a factor of weather and coincidence, rather than having anything to do with a warmer climate. Therefore, to establish whether rainfall, per se, is becoming more extreme and intense, we need to look at monthly figures, rather than seasonal ones.
Figure 3 shows the top 20 wettest months, falling in the October – December period, since 1910 for England South.
Clearly, the wettest months occurred in the early decades. Since 1970, only one month, Oct 2000, has exceeded 160mm, compared with nine such months up to 1970.
Finally, let’s take a look at the Oct-Dec rainfall trends at Oxford itself, where data is available back to 1853. As with England South, the three stand out years are 1929, 1960 and 2000.
In the last decade, we have seen nothing that was not commonplace between 1870 and 1920.
There is no evidence that “extreme rainfall” is becoming more common.
The study’s authors seem to have gone to a lot of bother, trying to establish a link to global warming. They would have saved themselves a lot of work, if they had simply gone and analysed the data.
1) All precipitation data for England South from Met Office
2) Oxford rainfall data
3) Details of the Oxford University study.