In January 2016, NASA and NOAA made an announcement that surprised almost no one in the climate science community — and alarmed many outside of it: 2015 was the hottest year ever recorded since modern record-keeping began in 1880. And it didn't just break the previous record. It shattered it.
By How Much?
According to NASA's Goddard Institute for Space Studies, globally averaged temperatures in 2015 were 0.87°C (1.57°F) above the 1951-1980 baseline — blowing past 2014's record by 0.13°C (0.23°F). NOAA calculated a slightly different figure: 0.90°C above the 20th-century average, also the largest margin by which an annual record had ever been beaten.
Ten months of 2015 individually tied or broke monthly temperature records. December 2015 alone was more than half a degree Fahrenheit warmer than December 2014. The warming wasn't confined to one region — 2015 was the warmest year on record for Asia and South America, and the second warmest for the continental United States, Africa, and Europe.
Why NASA and NOAA Sometimes Report Different Numbers
One interesting aspect of temperature monitoring is that different agencies can report slightly different numbers for the same year, and both can be correct. This is because they use different baseline periods for comparison, different collections of monitoring stations, and different methods for handling data gaps or adjustments for urban heat islands.
NASA compiles data from roughly 6,300 meteorological stations worldwide, plus ship and buoy measurements of sea surface temperatures, and Antarctic research stations. NOAA uses a similar but not identical network. The Japanese Meteorological Agency and the University of California Berkeley also maintain independent temperature records, and all of them showed 2015 as the warmest year on record.
Satellite measurements paint a slightly different picture: they showed 2015 as only the third hottest year since satellite records began in 1979. Scientists note, however, that satellites measure temperatures higher in the atmosphere rather than at the surface, and come with a larger margin of error.
El Niño and the Long-Term Trend
The 2015 record was boosted by one of the strongest El Niño events in history, which warmed ocean surface temperatures significantly. But as NASA GISS Director Gavin Schmidt noted, "Last year's temperatures had an assist from El Niño, but it is the cumulative effect of the long-term trend that has resulted in the record warming that we are seeing."
That long-term trend is driven primarily by the buildup of carbon dioxide and other greenhouse gases from human activity. At the time of the announcement, 15 of the 16 hottest years ever recorded had occurred since 2001.
The 2015 record brought global average temperatures about halfway to the 2°C ceiling that world leaders had agreed to try to stay under at the Paris Climate Conference just weeks earlier.
Source: NASA / NOAA
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