The connection between global warming and the current floods in the UK

How is the extreme stormy weather we have been getting in the UK related to global warming?

There is a mass of information out there, which I will now try to put it into a brief, comprehensible sequence.

First, sea levels are clearly, undoubtedly rising. Around the UK the change has been 15 cm since 1920 in Newlyn, Cornwall. This process will undoubtedly increase overtopping of sea defences.

Second, there is no doubt that rainfall patterns have changed in line with climate science predictions: warmer air can hold more water vapour, and rainfall is more intense.

Climate science predicts that increased flooding will happen everywhere in the UK, and this prediction is borne out by what we are now observing.

Studies show that the events we are looking at in Somerset are part of a trend.

1. It has been found that there are general trends of increased volume of rainfall and runoff in winter. 
2. Flood risk and vulnerability are increasing with increased frequency of occurrence of extreme events due to climate change &c. 
3. Over the past 100 years the intensity of UK precipitation has increased during winter, and to a lesser extend also during spring and autumn [Abstract]
4. Long term global records show an increase in extreme precipitation

Third, the Jet Stream position and shape is changing

Francis and Vavrus' work suggests that since warming of the Arctic is happening faster than the rest of the world, the temperature gradient between Arctic and the warmer more southerly zone is less, which means that the northern polar Jet Stream is forming meandering loops that get blocked in one position, which explains the extraordinary long series of storms that have hit us in the early part of 2014. It also explains some of the miserable summers we have had recently.

Note also that the recent extreme weather has not been restricted to the UK. There are eleven other major cases of extreme weather going on.

In more detail:

1. Warming causes the height of the troposphere to increase.
2. The trade winds are derived from air lifted up at the Equator that descends again at 30*N. 
3. Trade wind speeds have doubled over the past 20 years, compared to mean speeds over the previous 80. (This comes from the excellent paper by England et al. of the University of New South Wales). Here is a useful summary of the paper.
4. The stronger trade winds have swept warm surface water from the East to the West Pacific, where there is a pool of abnormally warm water, while in the East Pacific there is an upwelling of colder water. (Some of this warmer water is being pushed down, which accounts for a great deal of the hiatus in the increase of global surface warming, as well as the intensity of Typhoon Haiyan).
5. This warm patch of the Pacific feeds moist air into the Jet Stream, according to the Met Office document on the recent storms. 
6. The Pacific Jet stream has become distorted from its usual pattern (Met Office, fig 13). It swings north, then south over North America, bringing Arctic air with it, which caused the recent abnormally cold weather in the USA and Canada.
7. The Jet Stream is speeded up because two yearly oscillation of stratospheric wind around the poles (the QBO) is going in a westerly direction at present.  
8. Also the Polar Night Jet westerlies are twice as strong as usual.
9. Also the warming of the Arctic is thought to be causing the Jet Stream to meander further south (Vavrus hypothesis). The Met Office is still working on evidence for this effect.
10. Also the Atlantic is unusually warm, which adds moisture to the air.
11. Result -an intense, moist series of storms in the UK pushed in our direction by a powerful, too-far-south Jet Stream.

Needless to say, this is an extreme simplification of the science of a vast and highly complex system, but it does show how everything is interconnected on our planetary home, and how warming of the atmosphere can have unexpected effects. 

Every skuleboy kno that Chaos Theory means that the beating of a butterfly's wing on one side of the planet can cause a storm on the other. We have been doing more than flapping a couple of square inches of butterfly. In a couple of hundred years, we have been returning to the atmosphere carbon that was sucked out of it over the course of some sixty million years. 

Time we woke up to what we are doing.