Scientists this week warned that the latest IPCC report on global warming could be underestimating the impact of “tipping points”, such as the loss of polar ice caps, which might lead to “runaway warming”. A tipping point is the point at which nothing is the same again. The idea became famous with Malcolm Gladwell’s 2000 eponymous book, which informed excited readers that the tipping point of fax machine usage was the year 1987. But what exactly is being tipped?

If you have in mind the image of something toppling over, you are not wrong. In the 1890s, the tipping point was the point beyond which a listing ship could no longer right itself, or the point at which – in a machine proposed to weigh the water of a flowing stream – a cup being filled with water would tilt and transfer its contents to the next cup down.

The metaphorical usage, though, has an unhappier origin. Dictionaries tell us that in the 1950s the tipping point was the number of African Americans moving into a neighbourhood that caused white people to start moving out. Given the coming mass migrations from regions made uninhabitable, that painful echo might prove unexpectedly apt to global warming, too.