It's now looking exceedingly likely that 2015 will go down as the hottest year on Earth since we first started keeping records. Through the first nine months, no other year has even come close:
According to a report released by the National Oceanic and Atmospheric Administration yesterday, September 2015 was the hottest September, by a large margin, since records first began in 1880. August 2015 was the hottest August. July 2015 was the hottest July. In fact, so far this year, only January and April haven't broken records.
So what's going on? Two things. First, there's a very large El Niño event brewing in the Pacific Ocean right now. 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.)
El Niño hasn't yet reached its predicted maximum strength, but you can already see its telltale footprints below. The area in the east tropical Pacific has been a lot warmer than usual (though it was also unusually warm in Eastern Europe and the Midwestern United States):
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 this year's shaping up to be a lot hotter than 1998.
The other factor, of course, is global warming:
As we burn fossil fuels and add more carbon dioxide to the atmosphere, we trap more heat on the Earth's surface. But 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. But the 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 up in the atmosphere.