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The Ups and Downs of Climate Change

In my first blog in this series I mentioned the Gulf Stream and its relationship with the warming and cooling of the North European climate. It is thought by many climate historians that the cooling effect of the Gulf Stream weakening has happened before, in fairly recent times, during the early part of the Holocene. After the last ice age ended, around 15,000 years ago, there was a period of rapid warming. Ice sheets retreated, sea levels rose – and the Gulf Stream currents, overwhelmed by the melting of the Laurentide ice sheet over Canada, switched off. Almost immediately, in climate terms, global temperatures fell sharply, leading to what is known as the Younger Dryas event, about 12,800 years ago. Ice sheets that had been in retreat across northern Europe re-formed, and global warming went into reverse – for a while. Then the warming trend reanimated and the period we call the Holocene began, usually dated from about 10,000 years ago.

Climate change is so unpredictable, by its very nature (something the IPCC would do well to remember, I sometimes think!) that it isn’t certain that the same consequence would happen this time. And other so-called Dansgaard/Oeschger events that brought rapid climate change have occurred during the Holocene whose causes are more obscure. For example, no one knows why the medieval warm period (a halcyon time in northern Europe during the late twelfth and thirteenth centuries when it was warmer and drier and vines flourished in Yorkshire) arrived, nor why in the fourteenth century temperatures declined again (bringing more rain, and causing the Great Famine in England in 1315--1318, when crops failed for several summers. Even the trigger for the Younger Dryas that I have outlined above is controversial, though it is clear that scientists studying the current system of the North Atlantic believe that if it collapses there could be rapid consequences which would include substantial cooling of north-western Europe.

Which brings me to the whole topic of paleoclimatology – the study of climate in the distant past – which is the subject of my next blog.

 

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