NASA ranks 2017 the second-hottest year on Earth

US scientists said today that 2017 was among the hottest years recorded on earth.

In their annual global temperature analysis, NASA scientists found 2017 to be the second-warmest year since record keeping began in 1880, second only to 2016. The National Oceanic and Atmospheric Administration, which also conducts global climate analysis, ranked it the third-warmest year, slightly less hot than 2016 and 2015.

The fact that 2017 came in so hot is especially significant because, unlike 2016, 2017 was not an El Niño year. El Niños are atmospheric patterns over the Pacific ocean that typically add significant heat to global average temperatures. In 2016, .12 degrees Celsius of that year’s record-breaking average temperature anomaly was due to El Niño. In 2017, none of the temperature anomaly could be attributed to that natural heat source.

So even an El Niño-free year on earth is now a record-breaking one.

“It really brings it home that the trends we’re seeing are independent of anything that’s happening in terms of variability in the Pacific,” Gavin Schmidt, director of the Goddard Institute for Space Studies, the NASA lab that conducted the analysis, told reporters on Jan. 18.

March 2017 was particularly record-breaking, measuring 1.9 degrees Fahrenheit (1.03°C) above the 20th century average. “This marked the first time the monthly temperature departure from average surpassed 1.8°F (1.0°C) in the absence of an El Niño episode in the tropical Pacific Ocean,” NOAA reported.

But why the difference in rank?

NASA found 2017 was 1.62°F (0.90°C) above the 1951 to 1980 average, while NOAA found it was 1.51°F (0.84°C) above the 20th-century average. Both agencies say they remain in agreement with each other about the overall trend of climate change.

“The overall picture is very, very clear and coherent,” despite the small differences, Schmidt said. “The long term trends are all very clear independent of who is doing these analyses.”

The extremely slight difference is due to different methods for analyzing global temperature, and is a great argument for having more than one group of federal scientists studying climate change—a concept known as scientific redundancy.

The case for scientific redundancy

Both NASA and NOAA independently study global climate change. The slight difference in their results is a good argument for the importance of scientific redundancy. US President Donald Trump has proposed eliminating NASA’s climate change program, arguing that the redundancy between agencies is wasteful. But in reality, redundancy is a basic tenet of high-quality science. If more than one set of data (produced using separate analysis methods) point to the same trend or conclusion, scientists can have more confidence that the overall conclusion is correct.

In this case, the difference was extremely slight, and the agencies remain in agreement on the overall trajectory of global warming.

“Individual ranking of years is not necessarily the most important thing,” Schmidt told the New York Times. “What we’re seeing is an increasing string of years of temperatures more than 1 degree above the pre-industrial era. And we’re not going to go back.”

2018 to be on-trend

Despite 2018 shaping up to likely be a La Niña year—a phenomena that is the opposite of El Niño, which typically cools global average temperatures—NASA predicts 2018 to still rank as one of Earth’s hottest.

“We’d be looking toward quite a similar year — almost certainly a top-5 year,” Schmidt says.

And none of this is a surprise, he said. “We’re warming at the rate we anticipated a decade ago.”

Read this next: Why do all our climate data start in 1880?

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