In December, the the vast part of the world community has agreed on the tackling of climate change. This was the result of the 2015 United Nations Climate Change Conference, commonly abbreviated to COP 21 (https://en.wikipedia.org/wiki/2015_United_Nations_Climate_Change_Conference).
All media have quoted the goal to stay below 2.0°C and that the world community is even aiming for a global warming change of less than 1.5°C. However, I could not find much information on the reference period. Some time ago I've read about the period 1961 to 1990; Wikipedia states (without reference) that it's compared to pre-industrial levels. Does anyone have further information (with references) on the question, what the reference period for the COP21 agreement is?