Update the record books: 2015 was the hottest year on Earth since we first started keeping track, thanks to the combination of global warming and a powerful El Niño.
No other year has even come close:
:format(webp):no_upscale()/cdn.vox-cdn.com/uploads/chorus_asset/file/5919305/Screen%20Shot%202016-01-20%20at%2011.13.48%20AM.png)
The declaration comes from two separate analyses released today by NASA and the National Oceanic and Atmospheric Administration (NOAA), finding that 2015 was the hottest year globally since records first began in 1880.
NASA says this past year was a full 0.9°C (1.62°F) hotter than the 20th-century average, and 0.13°C (0.23°F) hotter than 2014, the previous record-holder.
El Niño is part of the story, but only a part...
So how did this happen? Two things. First, an extraordinarily strong El Niño has been unfolding in the Pacific Ocean. El Niños are periodic phenomena that occur when the trade winds that typically blow east to west in the tropics start to weaken. That ends up disrupting weather patterns all around the world. It also causes some of the heat that was stored in deeper layers of the ocean to rise to the surface, bumping up global temperatures. (See here for our explainer on how El Niño works.)
This year's El Niño was one of the strongest ever seen, reaching its maximum this winter. You can see its telltale signs below: the area in the east tropical Pacific hit record highs this year.
:format(webp):no_upscale()/cdn.vox-cdn.com/uploads/chorus_asset/file/5919315/Screen%20Shot%202016-01-20%20at%2011.18.33%20AM.png)
Still, El Niño can't be the only thing going on. After all, El Niños recur periodically — most recently, there was a massive one in 1997-'98. But 2015 was much hotter than 1998.
The other big story, of course, is global warming.
Global warming is the other big factor: Even El Niño years keep getting hotter
:format(webp):no_upscale()/cdn.vox-cdn.com/uploads/chorus_asset/file/5919319/Screen%20Shot%202016-01-20%20at%2011.20.05%20AM.png)
The basics of global warming are now familiar to most people. As we burn fossil fuels and add more carbon dioxide to the atmosphere, we steadily trap more heat on the Earth's surface.
What's less well-known is that more than 90 percent of that extra heat is absorbed by the oceans. So subtle interactions between the ocean and the atmosphere can cause fluctuations from year to year.
When a strong El Niño forms, more of that heat is transferred up to the surface, warming things up a bit. By contrast, when those Pacific trade winds strengthen and a La Niña forms, more of that heat is trapped below the ocean surface, cooling things down a bit. This cycle can lead to ups and downs over time.
But as we emit more and more CO2, we keep trapping more heat overall. As the chart above shows, El Niño years are getting hotter over time. La Niña years are getting hotter. Normal years are getting hotter. The overall trend is ... hotter.
By the way, scientists tend to think that El Niño has its biggest effect on global surface temperatures three months after peaking. So it's quite possible that 2016 could see a big surge in heat as well. We'll have to wait and see. "If you were going to be betting, it's likely 2016 will be warmer than 2015," said NOAA's Thomas Karl at a press conference.
Similarly, it's entirely likely that the rate of warming will slow again once El Niño ends and La Niña conditions return to the Pacific. Not every future year will set records. But it's a good bet that records will keep recurring.
One lesson here is to not get too caught up in any specific year or single record. El Niño events, other ocean fluctuations, volcanoes, and even cyclical solar activity can cause blips from year to year. The thing to watch is the long-term trend, which is ultimately getting warmer. How hot it ultimately gets is unclear — it depends, to a large extent, on how much CO2 we keep putting into the atmosphere.
Read more: El Niño, explained: why this year's could be one of the strongest on record
