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Well, almost. World leaders have decreed global temperature must not increase by more than 2 degrees over pre-industrial levels.

How realistic this is depends what they mean by ‘pre-industrial.’ Generally that term would be taken to mean the time prior to the industrial revolution. That is, before about 1780. That is, about the time of the Dalton minimum.

The Dalton minimum was a period of low solar activity, low temperatures (one German station recorded a fall of 2 degrees in 20 years), and the ‘year without a Summer’ (1816).

Given that solar activity is at similarly low levels now, and that global mean temperatures have been steady or declining over the last ten years, our beloved world leaders may find they don’t need to do anything at all to achieve their tide holding back ambitions.

However, according to a paper by Wilson, Hendy and Reynolds, published in Nature in 1979 (279, pp315-317), temperatures in New Zealand during the medieval warm period (which is definitely pre-industrial) were about  .75 degrees warmer than the very brief late 20th Century warm period. So we’ve still got 1.25 degrees to go!

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