The two main U.S. government agencies responsible for monitoring climate have made it official: 2015 was earth’s warmest year since modern record-keeping began in 1880. An unusually sharp increase broke the record set in 2014—part of an almost continuous series of new highs set in the last 18 years. The announcement came today from NASA’s Goddard Institute for Space Studies (GISS) and the National Oceanic and Atmospheric Administration (NOAA). Almost simultaneously, the United Kingdom’s Met Office announced that it, too, had determined 2015 was the hottest year in recorded history.
The record is part of a century-plus upward trend driven by human emissions of greenhouse gases. Scientists around the world agree that temperatures will almost certainly continue to rise in the 21st century, and well beyond, unless emissions are curbed.
According to the GISS analysis, the planet’s average 2015 surface temperature exceeded the 2014 record by 0.23 degrees Fahrenheit (0.13 degrees C). It was the second-largest yearly jump from the previous record yet seen. (The largest was in 1998.) The planet’s temperature is now about 1.8 degrees F (1 degree C) above what it was in the late 19th century.
2015 also marked a symbolic milestone: the first time that global temperatures rose to 1 degree C above the level of the late 1800s. At the Paris climate summit last month, all the world’s nations agreed to try and avert a rise of more than 2 degrees C. Negotiators agreed to the 2 degrees target on the idea that this would limit the worst effects of sea-level rise, extreme weather and other challenges presented by rising temperatures. However, many scientists are now starting to argue that the 2 degrees C figure is arbitrary and probably too high.
“2015 was remarkable even in the context of the larger, long-term warming trend,” said GISS director Gavin Schmidt. According to the GISS analysis, most of the warming has happened in the last 35 years. Fifteen of the 16 warmest years have occurred since 2001; the only other was 1998. Due to differing methods of analysis, NOAA’s figures for the exact temperatures each year are slightly different, but the overall trends tracked by both agencies are in close agreement. Yearly reports by other major bodies including the World Meteorological Organization, the Japan Meteorological Agency and the United Kingdom’s Met Office have hewn closely to U.S. figures.
The new high came as no surprise. Last week, a private California-based group, Berkeley Earth, declared 2015 the warmest year; the Japan Meteorological Agency has also released a preliminary estimate saying the same. Scientists from the U.S. government, the World Meteorological Organization and others had been predicting the record since summer. The projections became more certain as temperatures broke all-time records month by month. September 2015 was briefly recorded as showing the greatest monthly above-average temperature anomaly since 1880. But each of the succeeding three months were even greater; December 2015 now holds the all-time monthly record.
The sharp increase in 2015 was driven in part by El Niño, a natural weather cycle in the eastern equatorial Pacific that warms the ocean surface every 2 to 7 years. The latest El Niño started in late 2015 and is expected to last through spring 2016. It is among the strongest ever recorded, but Schmidt and others say it has provided only a temporary assist to the planet’s rising temperature. “We would not have seen the record warming without the long-term trend,” said Schmidt.
The rising heat was accompanied by an extraordinary series of extreme weather events and associated phenomena: heat waves in India and Pakistan that killed thousands; continuing drought and record-low snowpack in California; crop failures raking large parts of Africa; uncontrolled wildfires in Indonesia; record rains and flooding in Texas, Oklahoma and other U.S. states. Scientists have long projected that such events would become more common in a warming world, and an increasing number of studies in the past several years say the projections are now coming true.
Jason Smerdon, a climate scientist at Columbia University’s Lamont-Doherty Earth Observatory said of the new record, “It’s alarming, but it’s not at all surprising. The trend has been predicted for decades, and all the consequences associated with it have been predicted as well. If you know about gravity, it’s not that hard to predict what will happen if a cup falls out of your hand.” NASA scientists first publicly discussed global warming in 1988, which then was the warmest year on record. Now it is the 23rd-warmest year on record.
Researchers are already widely predicting that 2016 will be at least as warm as 2015, or warmer. This is not only because of the continuing presence of El Niño, but the steady upward march of carbon dioxide in the atmosphere. Measurements of the gas also continued to set new monthly records in 2015—for the first time ever, routinely above 400 parts per million. If 2016 does end up setting another temperature record, it will be the first time that new highs have been reached in three consecutive years.
The GISS analysis incorporates surface temperature measurements from some 6,300 land-based weather stations, and ship- and buoy-based observations of the ocean surface. Satellite data is used to cross-check the sea-surface measurements. The raw data is analyzed using algorithms that take into account the varied spacing of stations around the globe, and local effects that could skew calculations, such as the generally higher temperatures seen around urban areas. GISS, based in New York City, is affiliated with Columbia University’s Earth Institute.