As climate scientists predicted as early as late 2015, the final tally shows that last year has once again claimed the unfortunate distinction of being the warmest year on record, based on data going back to 1880. This follows on records set in 2014 and 2015—they're the combined products of long-term human-driven warming and natural variability that has pushed individual years up or down slightly.
The biggest source of year-to-year variability is the oscillation between El Niño and La Niña in the tropical Pacific Ocean. In El Niño years, warm surface water flows eastward to cover up cooler, deep water; La Niñas see rising deep water pushing west against that warmer water. Depending on which conditions predominate, the average global surface temperature sits above or below the long-term trend. It’s a bit like walking up or down a step on a moving escalator.
The record-strength El Niño that developed in the latter half of 2015 carried into 2016 before fading into a weak La Niña by the end of the year. The El Niño was enough to help lift 2016 to its record position. Current forecasts call for the ongoing La Niña to be a short one, with neutral conditions early next year, and possibly another El Niño on the other side.
from Yet another new hottest year on record: 2016