I think it stems from this 2005 conference that basically concluded that at below 400ppm, it would be unlikely that temperatures would rise by more than 2°C above pre-industrial levels and most serious consequences of global warming may be avoided.
"Among the conclusions reached, the most significant was a new assessment of the link between the concentration of greenhouse gases in the atmosphere and the increase in global temperature levels. Some researchers have argued that the most serious consequences of global warming might be avoided if global average temperatures rose by no more than 2 °C (3.6 °F) above pre-industrial levels (1.4 °C above present levels)"
"The conference concluded that, at the level of 550 ppm, it was likely that 2 °C would be exceeded, according to the projections of more recent climate models. Stabilising greenhouse gas concentrations at 450 ppm would only result in a 50% likelihood of limiting global warming to 2 °C, and that it would be necessary to achieve stabilisation below 400 ppm to give a relatively high certainty of not exceeding 2 °C."[1]
"Among the conclusions reached, the most significant was a new assessment of the link between the concentration of greenhouse gases in the atmosphere and the increase in global temperature levels. Some researchers have argued that the most serious consequences of global warming might be avoided if global average temperatures rose by no more than 2 °C (3.6 °F) above pre-industrial levels (1.4 °C above present levels)"
"The conference concluded that, at the level of 550 ppm, it was likely that 2 °C would be exceeded, according to the projections of more recent climate models. Stabilising greenhouse gas concentrations at 450 ppm would only result in a 50% likelihood of limiting global warming to 2 °C, and that it would be necessary to achieve stabilisation below 400 ppm to give a relatively high certainty of not exceeding 2 °C."[1]
[1] https://en.wikipedia.org/wiki/Avoiding_Dangerous_Climate_Cha...