by Judith Curry
“If we look at the broader base of evidence then we see things that support the premise that climate change has been making a contribution.” – Dame Julia Slingo
Well, I am home again today – Georgia Tech is closed Tues and Wed in anticipation of a bad ice storm. Looks like they didn’t need to close on Tues, but may well be closed the rest of the week.
Meanwhile, on the other side of the pond, there has been devastating flooding in Somerset (UK), with potentially more to come this weekend.
UK Met Office
The Met Office has published a 29 page document The Recent Storms and Floods in the UK. Excerpts from the Summary:
This winter the UK has been affected very severely by an exceptional run of winter storms, culminating in serious coastal damage and widespread, persistent flooding.
This period of weather has been part of major perturbations to the Pacific and North Atlantic jet streams driven, in part, by persistent rainfall over Indonesia and the tropical West Pacific.
The North Atlantic jet stream has also been unusually strong; this can be linked to exceptional wind patterns in the stratosphere with a very intense polar vortex.
This paper documents the record-breaking weather and flooding, considers the potential drivers and discusses whether climate change contributed to the severity of the weather and its impacts.
Although no individual storm can be regarded as exceptional, the clustering and persistence of the storms is highly unusual. December and January were exceptionally wet. For England and Wales this was one of, if not the most, exceptional periods of winter rainfall in at least 248 years. The two-month total (December + January) of 372.2mm for the southeast and central southern England region is the wettest any 2-month period in the series from 1910.
In a series from 1883, flow rates on the River Thames remained exceptionally high for longer than in any previous flood episode. Correspondingly, floodplain inundations were extensive and protracted.
The severe weather in the UK coincided with exceptionally cold weather in Canada and the USA. These extreme weather events on both sides of the Atlantic were linked to a persistent pattern of perturbations to the jet stream over the Pacific Ocean and North America. There is a strong association with the stormy weather experienced in the UK during December and January and the up-stream perturbations to the jet stream over North America and the North Pacific.
The major changes in the Pacific jet stream were driven by a persistent pattern of enhanced rainfall over Indonesia and the tropical West Pacific associated with higher than normal ocean temperatures in that region.
The North Atlantic jet stream has also been unusually strong; this can be linked to an unusually strong westerly phase of the stratospheric Quasi-Biennial Oscillation (QBO), which in turn has driven a very deep polar vortex and strong polar night jet.
As yet, there is no definitive answer on the possible contribution of climate change to the recent storminess, rainfall amounts and the consequent flooding. This is in part due to the highly variable nature of UK weather and climate.
There is an increasing body of evidence that extreme daily rainfall rates are becoming more intense, and that the rate of increase is consistent with what is expected from fundamental physics. There is no evidence to counter the basic premise that a warmer world will lead to more intense daily and hourly heavy rain events.
More research is urgently needed to deliver robust detection of changes in storminess and daily/hourly rain rates. The attribution of these changes to anthropogenic global warming requires climate models of sufficient resolution to capture storms and their associated rainfall. Such models are now becoming available and should be deployed as soon as possible to provide a solid evidence base for future investments in flood and coastal defences.
The Mirror reports statements from Dame Julia Slingo, UKMO’s Chief Scientist:
Dame Julia said while none of the individual storms had been exceptional, the “clustering and persistence” were extremely unusual.
“We have seen exceptional weather,” she said.
“We cannot say it’s unprecedented, but it is certainly exceptional.
“Is it consistent with what we might expect from climate change?
“As yet there can be no definitive answer on the particular events that we have seen this winter, but if we look at the broader base of evidence then we see things that support the premise that climate change has been making a contribution.”
Paul Homewood has a blog post So what about 1929, Julia? This provides an extensive critique of Dame Slingo’s statements and the Met Office’s analysis. Homewood identifies the winter of 1929/1930 as the wettest winter since 1910. Homewood concludes:
It is somewhat encouraging that Slingo and her colleagues at the Met Office are starting to realise that weather patterns are far too complex to simplistically attribute to “melting Arctic ice”, or “global warming”.
A better understanding of what drives changes in the jet stream, the QBO, and a host of other factors, can only help us in anticipating future weather patterns. To do this, though, needs an acceptance that similar events have happened in the past, and that such events may give us a clue as to what is happening now, and will inevitably happen again in the future.
Where, for instance, is the recognition that 1929/30 had a much longer and wetter spell? And where is the analysis of of what drove the weather then? What, if any, are the similarities between the two years?
It is disappointing then to read the Met Office press release saying:
More research is urgently needed to deliver robust detection of changes in storminess and daily/hourly rain rates and this is an area of active research in the Met Office. The attribution of these changes to anthropogenic global warming requires climate models of sufficient resolution to capture storms and their associated rainfall.
Surely to do science properly, they should be looking to understand what drives weather patterns, regardless of the causes. To start out by attempting to “attribute changes to AGW” is likely to skew results towards their preconceptions, and will undoubtedly lead to a poorer understanding of the Earth’s climate as so many other natural mechanisms will be ignored.
Analysis of the problem and the solution
ScienceMediaCentre has a post Expert reaction to Somerset flooding. Excerpts:
Prof Roger Falconer, CH2M HILL Professor of Water Management at Cardiff University:
“However, I have lectured in hydraulic engineering (in civil engineering) at three universities for over 35 years and have been involved in many environmental impact assessment studies worldwide. Furthermore I am currently President of the International Association of Hydro-environment Engineering and Research. And regrettably I cannot see that dredging would make much impact in alleviating the problems in the Somerset Levels.
“In my view there are two effective solutions to address the real fundamental hydraulics problem: (i) raise the land, or (ii) lower the sea level and create a much larger hydraulic gradient. The first solution is not practical. The second is. There have been a number of proposals in recent years to build a Bridgewater Bay Lagoon to create renewable energy. Such a structure would involve separating the water level in Bridgewater Bay from that in the Bristol Channel.
Ola Holmstrom, UK Head of Water at consultancy firm WSP, said:
“Flood risk management should be based less on focussing on rivers and rather the catchment as a whole. I was present at last week’s Somerset Water Management Partnership and there was much discussion over the ‘catchment-based approach’. However, politically this is seen as inaction and that the dredge would solve immediate issues and answer the communities’ cries. However, given the huge cost of ‘the big dredge’ there must be quantitative proof that this will benefit both the upstream and downstream communities. Catchment based schemes would be significantly cheaper and more sustainable than dredging; they must be given a chance even if the political pressure is on, for the good of the environment and communities throughout Somerset in the future.
WaterPowerMagazine has a very good article by Dr. Colin Clark: Floods on the Somereset Levels: a sad tale of ignorance and neglect. Excerpts:
There was serious flooding in 1854, 1872-3 and 1929-30. During the latter flood at Taunton, from November 1929 to January 1930 537mm were recorded, which is over 70% of the annual average. The floods lasted from December to February. For the same time period in 2013-14 the rainfall in the upper Brue, which drains into the Levels, was 434mm.
An analysis of the highest consecutive three monthly rainfall since 1766 shows that this is the fifth highest, giving it a return period of about 1 in 50 years. This result excludes 1960, when Bedlamgreen in the upper Brue recorded 556mm. Before 1766 possibly the worst flood in historical times was that of 1607.
The additional capacity of the drainage ditches and the 0.5Mm3 per day of general pump capacity would produce an estimated 1.29 (channel enlargement) + 0.5 (pumping) + 16.1 (ditches draining out over 1 day at a rate of 200 l/s per km2) = 17.89Mm3 capacity for excess floodwater. In this way the worst effects of the present floods would be avoided.
If the hedgerow ditches were increased to a cross sectional area of 10m2 then capacity for flood water would increase to 8.19Mm3. If the true area of flooding is about 125km2 and the average depth is 0.5m then the volume of floodwater is 62.5Mm3.
Using the lower volume of flood capacity this gives a time period of 13 days for the flood to take place without any adverse effects. Any shorter time period would cause the system to be overloaded. For the four days from 1-4 January 2014 the average daily rainfall in the upper Brue was 15mm. This produces a volume of rainfall over 908km2 of upland area of 13.62Mm3 which is less than 17.89Mm3 provided by storage, channel improvements and pumping. In the present case the floods have built up since the start of December. The first reports of serious flooding were around Christmas time. Therefore the present proposal should cope. Without the use of the extra storage capacity of ditches a critical time period of 35 days would be needed for the floods to be contained. Clearly pumping and dredging would not have coped in the present case.
As per my twitter feed, the floods are creating quite the political brouhaha in the UK. I hope that sensible policies can be developed to reduce vulnerability to such floods in the future, without the distraction of thinking that reducing carbon emissions would somehow prevent such extreme events.