A recent paper published in the Bulletin of the American Meteorological Society concludes that the record high global average temperatures that were recorded in 2015 will be considered average by 2025. The research was done by a group from Australian National University, hub of the Australian Research Council's Centre of Excellence for Climate System Science.
Boosted by one of the strongest El Niño effects on record, much of the planet has experienced record breaking temperatures since 2014. From May 2015 until August 2016, the planet registered an unprecedented 16 months in-a-row in which each month's average temperature was the hottest ever recorded for that month.
The paper provides several projections based on different emission scenarios, and states that under the current trend, the abnormally high temperatures recorded in 2015 will become the norm by 2025. Even under the most optimistic scenario, this "new norm" will still occur no later than 2040. If the record heat in the past few years under El Niño becomes the average, then an abnormally hot year in the future will be beyond anything mankind has experienced before.