But you simply can't calculate it that way. Since 0 degrees Celsius is an arbitrary point on the range of possible temperatures, the calculation is physically meaningless. Allow me to demonstrate, by instead quoting the temperatures in degrees TristanC, where 0 degrees T is equal to 13 degrees C, and 87 degrees T is 100 degrees C. So pre-industrial temperatures were 1 degree T and they're now 2 degrees T, which by your reckoning means that they've risen by 100%.
To repeat myself: expressing changes in temperature as a percentage only makes sense when you're working on a scale that starts from absolute zero.
