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.

NASA's dataset estimates that 2016 was about 1.2°C (almost 2.2ºF) warmer than the late 1800s, and 0.12°C warmer than 2015. NOAA's dataset, which treats areas of the rapidly warming Arctic without measurements a little more conservatively, eclipses 2015 by 0.04°C. If you're an error bar enthusiast, NOAA gives 2016 a 62 percent chance of beating 2015 for the record while NASA's data gives it a 96 percent chance. The rankings show that the last 16 years account for 16 of the 17 warmest years on record, with only 1998 squeaking into the list.

Every month from May 2015 to August 2016 set or tied global records for warmest on record—a remarkable 16-month streak. NOAA notes that North America set a new record for warmth in 2016, with the contiguous United States coming in second behind 2012 despite cold weather in the east back in January. Europe had its third-warmest year and Australia had its fourth-warmest, while the UK was a bit less extreme with its 11th-warmest year.

All major temperature datasets agree with the global picture. Numbers from the UK Met Office and the Berkeley Earth team—as well as the preliminary estimate from the Japanese Meteorological Agency—all put 2016 at the top. The Remote Sensing Systems satellite datasets for temperatures farther above the surface also show records for 2016, in agreement with the other satellite datasets.

As for 2017, the prediction from the UK Met Office would put it in third place behind 2016 and 2015. And that's without the El Niño boost those years received.

Listing image by NASA