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Last year shattered 2014’s record to become the hottest year ever recorded, based on data going back to 1880, two U.S. government science agencies announcedWednesday.

“2015 was by far the record year in all of the temperature datasets that are based on the instrumental and surface data,” said Gavin Schmidt, director of the Goddard Institute for Space Studies at NASA, which made the announcement jointly along with NOAA, the National Oceanic and Atmospheric Administration.

“It really underlines the fact that the planet really is still warming, there is no change in the long term global warming rate, and we know why that is,” he said.

Specifically, the year was 0.23 degrees Fahrenheit (0.13 degrees Celsius) hotter than 2014, the prior record year, according to NASA. The measurement recorded by NOAA was slightly worse: 0.29 degrees Fahrenheit (0.16 degrees C) hotter than 2014.

“A lot of times, you actually look at these numbers, when you break a record, you break it by a few hundredths of a degree,” said Thomas Karl, director of NOAA’s National Centers for Environmental Information. “But this record, we literally smashed. It was over a quarter of a degree Fahrenheit, and that’s a lot for the global temperature.”

Overall, NOAA said, 2015 was 1.62 degrees Fahrenheit (0.9 degrees Celsius) above the 20th century average.

NASA and NOAA both keep independent global surface temperature datasets, measuring temperatures over both the land and the oceans using thermometers, ocean buoys and ship readings. The datasets do not always agree perfectly, but they showed relatively little disagreement this year, Schmidt said.

The new record means that 2014 — the previous record year — only officially held that title for one year. 2014 came by its record by a relatively narrow margin — for instance, NASA gave 2014 a 38 percent chance of having been the warmest year on record, still reserving a nontrivial chance that the real warmest year had been 2010 or 2005. (NOAA gave a 48 percent chance that 2014 had, at the time, been the warmest year.)

This year, in contrast, there is little need for citing percentages or a statistical photo finish. Buoyed by a powerful El Niño event, 2015 shattered the 2014 record. NASA’s Schmidt suggests there’s only a 5 percent possibility that any other year on record was actually warmer.

Fifteen of the sixteen hottest years on record have now occurred in this century, according to NASA.

2015’s record heat — enhanced, especially in later months of the year, by a strong El Niño event that released warmth from the Pacific Ocean — was apparent long before the official declaration of the current record, which comes as little surprise to climate scientists.

Every month in 2015 except for January and April was the hottest of that month on record globally, according to NOAA. In other words, September of 2015 was the hottest September in 136 years, as was October of 2015, November of 2015, and so on.

NOAA also announced Wednesday that for December, the “temperature departure from average was also the highest departure among all months in the historical record and the first time a monthly departure has reached 2 degrees F.”

It became almost a kind of ritual as the year wore on: Each month the agency would announce that the prior month had been the hottest June, or July, or August on record for the globe.

From a climate policy perspective, the warmth of 2015 is also highly significant. Global leaders in Paris agreed in December that the planet should not be allowed to warm 2 degrees Celsius above pre-industrial temperatures — and ideally, warming should be limited to 1.5 degrees Celsius if possible. Based on 2015’s temperature record, though, we’re already half way to 2 degrees.

“This is the first year where the record is clearly above 1 degree Celsius above the 19th century,” said NASA’s Schmidt. NOAA’s data also show that the planet is now more than 1 degree Celsius warmer than the average temperature between 1880 and 1899, said the agency’s Karl.

2015’s El Niño enhanced heat was accompanied by dramatic weather events across the globe, including a new record for the number of Category 3 or greater tropical cyclones in the northern hemisphere. That tally includes Hurricane Patricia, the most intense hurricane ever recorded by the National Hurricane Center.

In some ways most ominously of all, 2015 was the year that scientists announced that an entirely new sector of Greenland — one containing over three feet of potential sea level rise — appeared to have been destabilized. The region is centered on the Zachariae and Nioghalvfjerdsfjorden glaciers of northeast Greenland, which together comprise the endpoint of the northeast Greenland ice stream, which drains 12 percent of the vast ice sheet.

2015’s record warmth also included a major anomaly — very cold temperatures in the North Atlantic Ocean to the south of Greenland. Monthly NOAA temperature maps repeatedly showed a blue colored ‘blob’ of cold in this region, a development that is sparking increasing scientific interest, because of the suspicion that it could represent a sign of a change in the overturning circulation of the ocean.

“In the northern North Atlantic, temperatures were colder than normal, and that was really pretty much the only part of the world that had a sizeable area with below average temperatures,” Karl said.

It certainly isn’t the case that the 2015 temperature record can be entirely attributed to the warming of the globe by human greenhouse gas emissions. Climate change has never meant that every successive year will be warmer than the next, and the powerful 2015 El Niño unlocked immense heat from the Pacific Ocean that drove up the global temperature.

But at the same time, 2015 was also considerably hotter than 1998, another major El Niño year that was, at the time, the hottest year on record. Now, in contrast, it’s fifth or sixth on the list, depending on which agency you consult. And that, say experts, is how the warming of the planet makes itself felt.

“It’s breaking the record because we also have this unusually strong El Niño, but at the same time we know the ocean is now absorbing two times more heat than around the last time we had a big El Niño, which is quite a while ago,” said Katharine Hayhoe, a climate scientist at Texas Tech University.

There has been some talk in scientific circles that 2016 could be even hotter overall than 2015 — which would lead to three record years in a row. The reasoning here is that there is usually a significant lag between when El Niño peaks and when the warming of the globe does in its wake. Thus, 1998 was the hottest year of the 1997-1998 El Niño event.

Britain’s Met Office recently forecast that 2016 could be “at least as warm, if not warmer” than 2015, in the words of research fellow Chris Folland.

“In previous El Niño years, they peak in the wintertime . . . [and] the warmest temperatures are in the subsequent year,” said NOAA’s Karl. “If 2016 continues like we’ve seen in the past, that would suggest 2016 is going to be very close to a record or even a new record.”

However, not all scientists agree. “My guess is that 2016 may not be warmer than 2015,” said Kevin Trenberth, a climate change and El Niño expert at the National Center for Atmospheric Research. He thinks the current El Niño may already have begun to peak (or have peaked) and thus that the second half of 2016 may cool down again somewhat.

In 2015, record warm temperatures and a growing focus on addressing global warming seemed in curious sync. It was the year that Pope Francis released his historic encyclical on the environment, Laudato Si, and the year in which the United States moved to regulate greenhouse gas emissions from the generation of electricity, their largest single source.

Most significant, as heat records over the year accumulated, nations of the world assembled in Paris to forge a global climate agreement that will serve as the template for locking in cuts to greenhouse gas emissions in coming decades.

It’s hard to say that 2015’s warmth directly contributed to these human decisions, and yet it’s also hard to entirely separate the two. The stark warming of the globe in 2015 clearly imparted a newfound sense of policy urgency.

“NASA has been talking about the existence of global warming in public since 1988,” said Schmidt. “1988 was also a record warm year for the time. Just so that people understand, it is now 23rd in the rankings.”


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