Thursday, January 10, 2013

Extreme Weather? Don't Worry. It's Normal

It was really, really wet in Britain last year, with 1244 mm of precipitation, more than four feet, compared with only 30 inches the year before.

Climate change is to blame, so the Guardian says:
A month's rain fell in a day last week in parts of Britain. There were 140 flood warnings in the north of England, rain forcing the evacuation of Croston and Darwen in Lancashire; elsewhere, it washed out the Isle of Wight festival. Indeed, rainfall over the last three months has broken new records – following two years in which less rain had fallen than at any time since the 1920s.

This is part of a wider pattern. It is not just that world temperatures are on average steadily rising, the weather everywhere is becoming more extreme.
But Met Office data suggest otherwise:

Image source.

What the record shows is that extreme variability in British weather, or at least in rainfall, is not actually extreme, but normal, with little if any long-term trend. Thus although 2012 was very wet, it was not as wet as 1872. Likewise, although 2011 was very dry, it was not as dry as 1788.

Big, extremely violent storms are also quite normal, although they don't happen all the time, obviously, or they would'nt be considered extreme. In 1287, an unusual combination of an extreme low pressure and a very high tide created a storm surge that killed thousands in England and on the other side of the North Sea. Overnight the storm fundamentally changed the geography of the English coast:
 The harbour at Hastings was destroyed, the old town of Winchelsea, which was already under attack from the sea, was abandoned, and the coastline realigned.

Perhaps the most remarkable feature of the damage was that the thriving port of New Romney was turned into a landlocked town. Massive quantities of shingle from Dungeness, along with mud and soil, inundated the town, completely filled the harbour, and left New Romney nearly a mile from the sea.

The river Rother, which ran through the town, was stopped up by the storm and found a new outlet to the sea at Rye, 15 miles away, a course that the river still takes. In New Romney (a Saxon name, so not very new) there is still visible evidence of this extreme event. It is a draw for archaeologists, because the silt and gravel covered and preserved the town.

Visitors to the parish church of St Nicholas, the only surviving building from the period, have to step down into the church. There are still stains on the pillars marking the level of the flood.
Image source
Then there was the grote Mandrenke of 1362:
The Grote Mandrenke (Low Saxon for “Great Drowning of Men”) was the name of a massive southwesterly Atlantic gale which swept across England, the Netherlands, Northern Germany, and Schleswig around January 16, 1362, causing at minimum 25,000 deaths. …An immense storm tide of the North Sea swept far inland from the Netherlands to Denmark, breaking up islands, making parts of the main land into islands, and wiping out entire towns and districts, such as Rungholt on the island of Strand in North Frisia. This storm tide, along with others of like size in the 13th century and 14th century, played a part in the formation of the Zuider Zee.
The chief difference between now and then is that we now know much more than our ancestors would have known about extreme weather events occurring far from home. Thus many people may be under the mistaken impression that extreme weather events are more common now than in the past. Unfortunately, the media rarely put this information in a context that permits rational evaluation of trends and risks.

Thanks to Tallbloke for the info on Medieval storms.

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