UPDATE:  At the end of the post, I’ve added  the graph being used as the Feature Image at WUWT.

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Politicians from around the globe gather annually in the UNFCCC meetings so they can propose and fail to come to worthwhile agreements on how to limit global warming and its impacts.  Year in, year out, same thing.  For the results of the most recent failed gathering, see the WattsUpWithThat post GWPF Welcomes Non-Binding And Toothless UN Climate Deal.  One of the primary factors that drive the politicians is an attempt to limit global warming to 2-deg C above preindustrial values, where preindustrial is considered the mid-to-late 1800s.

But where did that 2 deg C limit come from?