The Climategate emails revealed that the 2C global warming temperature target was entirely arbitrary — “plucked out of thin air,” as Phil Jones put it. Today we can trace this arbitrariness back one step further — to a 1975 paper by economist William Nordhaus.
Check out page 23 of the Nordhaus paper.
So Nordhaus wrote that the natural variation in average global temperature was as much as 2-3C warmer than mid-1970s average global temperature.
While there is no paper trail yet between the Nordhaus paper and the adoption of the 2C target, we can at least see that Nordhaus ran across the idea of the 2C target somewhere as early as 1975.
The other point here is that, according to Nordhaus, the range of natural warmth may extend to and high as 3C higher than the average global temperature of the mid-1970s.
The Earth is, of course, no where near that level of warming — supposedly about 1.1C above preindustrial warming.