Parts of the Northern Hemisphere might be shivering through a bitterly cold winter right now, but global temperature averages still soared over the last year. NASA and the National Oceanic and Atmospheric Administration (NOAA) have both released reports stating that 2017 was one of the hottest years since records began in 1880. That continues a long-term trend of rapid warming that doesn't bode well for the future.
NASA's yearly analysis uses a baseline mean temperature calculated from the global average each year between 1951 and 1980, and then compares that to the global average temperature of each passing year. The agency determined that the worldwide average in 2017 was 1.62° F (0.9° C) warmer than the baseline, making it the second-hottest year on record behind only 2016.
A separate study from the NOAA determined that 2017's average was 1.51° F (0.84° C) higher than the 20th century average, ranking it the third-warmest year after 2016 and 2015, respectively. That difference is due to a range of factors: Climate science, especially on a global scale, is complex and ever-changing; weather stations move locations; the methods of measurement change over time; and different organizations use different techniques, baselines and sources for their calculations.
Generally though, findings from both NASA and the NOAA are largely in agreement on the wider warming trends. Temperatures have been higher than average for the last 41 consecutive years, and the top five warmest all fall between 2010 and now. Winter chills might lead some skeptics to trash the science, but the numbers trump those concerns – especially when several other studies from around the world all reached very similar conclusions.
Weather phenomena known as El Niño and La Niña can trigger short-term variations in temperature, and a strong El Niño event during 2015 and the first third of 2016 likely played a role in making 2016 the hottest on record. There was no El Niño in 2017 and conversely a La Niña pattern emerged in later months, which would have taken the edge off. In an analysis that cancelled out the effects of El Niño and La Niña, 2017 suddenly jumped to the top of the charts.
While the vast majority of the planet experienced at least some warming, the polar regions continue to be the hardest hit. The Arctic's sea ice levels were the second-smallest ever seen (again after 2016's record low). At the other end of the planet, the Antarctic sea ice extent was the smallest on record, covering an area some 154,000 sq miles (400,000 sq km) smaller than the previous record low.
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