A study just out in the Bulletin of the American Meteorological Society tackles this question head on. In this article, lead author Dr. Ed Hawkins from the University of Reading and an international team of colleagues take into account many climate records that date back decades and centuries. Based on their analysis, they determined that the period 1720-1800 is the best selection for “pre-industrial.”[....]
Using three different analysis methods, the authors are able to conclude that from their pre-industrial time to the 1986-2005 period, there was a likely global temperature increase of 0.55–0.80°C (1–1.4°F).What this means is that we passed the 1°C (1.8°F) level by 2015, and by 2016 the 1°C (1.8°F).
[I]t means that we have about a decade less time to act on climate change if we are going to avoid the most serious consequences. It means we simply have no time to waste, and no room for error. It also means that even if we take action right now there will be consequences.I've warned about this before, here and here.