All five of the hottest years on record have occurred in the last five years, according to global temperature data released Wednesday by NASA and the National Oceanic and Atmospheric Administration.
While 2018 was slightly cooler than the three prior years, Earth still had its fourth-warmest year since scientists began keeping records in 1880, the federal agencies said. Their separate analyses add to decades of evidence that the burning of fossil fuels, the clearing of forests and other human activities are releasing heat-trapping greenhouse gases into the atmosphere and causing the planet to warm.
"If you smooth out these year-to-year variations and look at the big picture, the overall trend in the past few decades is one of accelerating change," said Alex Hall, who directs the Center for Climate Science at UCLA and was not involved in either government analysis. "We are seeing more and more warming that is happening at a faster and faster rate."
Last year's average global surface temperature was 1.42 degrees Fahrenheit above the 20th-century average, according to NOAA.
The warmest year was 2016, followed by 2017, 2015, 2018 and 2014, according to NASA's rankings.
All five of those years were exceptionally warm, with only slight differences that were driven by natural variations in the weather, including the alternating cool and warm cycles from El Nino and La Nina.
"You get ups and downs -- years that are a little bit warmer, a little bit cooler -- but the long-term underlying trend is very, very clear," said NASA climatologist Gavin Schmidt, who worked on the space agency's analysis. "It's the long-term trends that are having impacts on ice, on severity of droughts, on heat waves, on sea level rise and wildfires."
The combination of rising greenhouse gases and a mild El Nino underway in the Pacific Ocean means it's likely that 2019 will be hotter than 2018. Scientists say there's a very good chance this year will wind up ranking among the top five hottest on record, barring an abrupt planet-cooling event such as a giant volcanic eruption.
NOAA and NASA each analyze temperature measurements from thousands of sites around the world, including weather stations on land and ships and buoys spread across the world's oceans.
The two agencies use much of the same data but perform independent analyses with minor differences in methods that yield slightly different rankings. NASA, for instance, ranked 2015 as the third-warmest year on record while NOAA found it was 2017. But in the long-term, the two agencies strongly agree on the pace and trajectory of global warming.