Clockwise from top left: A family sleeping on the roof of a house in New Delhi last May; people navigating a flooded street in a canoe in Arnold, Mo., on Dec. 31; tourists in a haze-shrouded Singapore last September; the drought-stricken Molatedi Dam in South Africa in November. Credit Clockwise from top left; Tsering Topgyal/Associated Press, Jeff Roberson/Associated Press, Edgar Su/Reuters, Stuart Graham/Associated Press
Scientists reported Wednesday that 2015 was the hottest year in the historical record by far, breaking a mark set only the year before — a burst of heat that has continued into the new year and is roiling weather patterns all over the world.
In the contiguous United States, the year was the second-warmest on record, punctuated by a December that was both the hottest and the wettest since record-keeping began. One result has been a wave of unusual winter floods coursing down the Mississippi River watershed.
Scientists started predicting a global temperature record months ago, in part because an El Niño weather pattern, one of the largest in a century, is releasing an immense amount of heat from the Pacific Ocean into the atmosphere. But the bulk of the record-setting heat, they say, is a consequence of the long-term planetary warming caused by human emissions of greenhouse gases.
“The whole system is warming up, relentlessly,” said Gerald A. Meehl, a scientist at the National Center for Atmospheric Research in Boulder, Colo.
It will take a few more years to know for certain, but the back-to-back records of 2014 and 2015 may have put the world back onto a trajectory of rapid global warming, after a period of relatively slow warming dating to the last powerful El Niño, in 1998.
Politicians attempting to claim that greenhouse gases are not a problem seized on that slow period to argue that “global warming stopped in 1998,” with these claims and similar statements reappearing recently on the Republican presidential campaign trail.
Statistical analysis suggested all along that the claims were false, and that the slowdown was, at most, a minor blip in an inexorable trend, perhaps caused by a temporary increase in the absorption of heat by the Pacific Ocean.
“Is there any evidence for a pause in the long-term global warming rate?” said Gavin A. Schmidt, head of NASA’s climate-science unit, the Goddard Institute for Space Studies, in Manhattan. “The answer is no. That was true before last year, but it’s much more obvious now.”
The Hottest Year on Record
Globally, 2015 was the warmest year in recorded history.
How far above or below average temperatures were in 2015Compared with the average from 1901 to 2000
Average global surface air temperaturesCompared with the average from 1901 to 2000
Michael E. Mann, a climate scientist at Pennsylvania State University, calculated that if the global climate were not warming, the odds of setting two back-to-back record years would be remote, about one chance in every 1,500 pairs of years. Given the reality that the planet is warming, the odds become far higher, about one chance in 10, according to Dr. Mann’s calculations.
Two American government agencies — NASA, the National Aeronautics and Space Administration, and NOAA, the National Oceanic and Atmospheric Administration — compile separate analyses of the global temperature, based upon thousands of measurements from weather stations, ships and ocean buoys scattered around the world. Meteorological agencies in Britain and Japan do so, as well. The agencies follow slightly different methods to cope with problems in the data, but obtain similar results.
The American agencies released figures on Wednesday showing that 2015 was the warmest year in a global record that began, in their data, in 1880. British scientists released figures showing 2015 as the warmest in a record dating to 1850. The Japan Meteorological Agency had already released preliminary results showing 2015 as the warmest year in a record beginning in 1891.
On Jan. 7, NOAA reported that 2015 was the second-warmest year on record, after 2012, for the lower 48 United States. That land mass covers less than 2 percent of the surface of the Earth, so it is not unusual to have a slight divergence between United States temperatures and those of the planet as a whole.
The end of the year was especially remarkable in the United States, with virtually every state east of the Mississippi River having a record warm December, often accompanied by heavy rains.
A warmer atmosphere can hold more water vapor, and an intensification of rainstorms was one of the fundamental predictions made by climate scientists decades ago as a consequence of human emissions. That prediction has come to pass, with the rains growing more intense across every region of the United States, but especially so in the East.
The term global warming is generally taken to refer to the temperature trend at the surface of the planet, and those are the figures reported by the agencies on Wednesday.
Some additional measurements, of shorter duration, are available for the ocean depths and the atmosphere above the surface, both generally showing an inexorable long-term warming trend.
Most satellite measurements of the lower and middle layers of the atmosphere show 2015 to have been the third- or fourth-warmest year in a 37-year record, and scientists said it was slightly surprising that the huge El Niño had not produced a greater warming there. They added that this could yet happen in 2016.
When temperatures are averaged at a global scale, the differences between years are usually measured in fractions of a degree. In the NOAA data set, 2015 was 0.29 degrees Fahrenheit warmer than 2014, the largest jump ever over a previous record. NASA calculated a slightly smaller figure, but still described it as an unusual one-year increase.
The intense warmth of 2015 contributed to a heat wave in India last spring that turns out to have been the second-worst in that country’s history, killing an estimated 2,500 people. The long-term global warming trend has exacted a severe toll from extreme heat, with eight of the world’s 10 deadliest heat waves occurring since 1997.
Only rough estimates of heat deaths are available, but according to figures from the Center for Research on the Epidemiology of Disasters, in Brussels, the toll over the past two decades is approaching 140,000 people, with most of those deaths occurring during a European heat wave in 2003 and a Russian heat wave in 2010.
The strong El Niño has continued into 2016, raising the possibility that this year will, yet again, set a global temperature record. The El Niño pattern is also disturbing the circulation of the atmosphere, contributing to worldwide weather extremes that include a drought in southern Africa, threatening the food supply of millions.
Over the past year, scientists have been keeping a close eye on an important swath of the Pacific Ocean, just along the equator. When conditions here are just right, an El Niño can form — and wreak havoc on weather patterns across the globe.
And right now, it looks like we’re on pace for a very large El Niño this fall or winter. Quite possibly one of the strongest on record. Based on past experience, that could potentially bring much-needed rain in California, but also drought in Australia, destructive floods in Peru, and so on. A strong El Niño could also help make 2015 and 2016 some of the hottest years ever recorded. It’d be a very big deal.
But El Niño events are often unpredictable and full of surprises. So nothing’s guaranteed just yet. What follows is a guide to how El Niño works, what we know about the 2015 event, and how a potentially massive El Niño could upend the world’s weather later this year.
A very basic definition of El Niño
El Niño is a weather phenomenon that occurs irregularly in the eastern tropical Pacific every two to seven years. When the trade winds that usually blow from east to west weaken, sea surface temperatures start rising, setting off a chain of weather impacts.
El Niños can be strong or weak. Strong events can temporarily disrupt weather patterns around the world, typically making certain regions wetter (Peru or California, say) and others drier (Southeast Asia). Some countries suffer major damage as a result.
El Niños also transfer heat stored in the deeper layers of the ocean to the surface. When combined with global warming, that can lead to record hot years, as in 1998.
“El Niño” got its name in the 1800s from Peruvian fisherman, who first noticed a mysterious warm current that would appear around Christmas. They called it the “little boy” or “Christ child.”
Why this year’s El Niño could be a huge deal
The last truly massive El Niño appeared in 1997-’98 and ended up causing an estimated $35 billion in destruction and 23,000 deaths around the world. (It also inspired that famous Chris Farley sketch.) Now we may be on the verge of a similar-size event:
Early-August status of the 1997 and 2015 El NIño events. Satellite imagery shows the departure from average sea-surface height for a given time of year, which is correlated with warmth in the upper ocean. (NASA/JPL via Weather Underground)
That, in itself, is a surprise. Back in March, NOAA’s Climate Prediction Center announced that a weak El Niño had formed in the Pacific, but many experts initially thought it might just fizzle out in the summer. Instead, El Niño kept getting stronger, with ocean temperatures in the eastern tropical Pacific continuing to soar. Some forecasters now think this could turn into one of the strongest El Niño events in memory when it peaks later this fall or winter.
“We’re predicting this El Niño could be among the strongest El Niños in the historical record, dating back to 1950,” said Mike Halpert, deputy director of the Climate Prediction Center, in a recent press call. We’ll see if this latest forecast holds up.
If it does, countries across the globe will have to brace themselves. In the past, major El Niño events have brought unusually hot, dry weather to Australia that can cramp wheat yields and amp up wildfires. It can bring hotter, drier weather to India that hurts agriculture. It can bring heavy rain and destructive flooding to Peru, washing away houses and spreading cholera. In 1997, El Niño dried out Indonesia so badly that it led to huge forest fires whose smoke disrupted daily life in Singapore.
Yet El Niño isn’t all bad. In the United States, it could potentially bring needed rain this winter to ease California’s drought (though also mudslides and flooding). Historically, El Niño has also served up milder US winters and helped tamp down hurricanes in the Atlantic Ocean.
One important caveat here is that every El Niño is a bit different — and some have unexpected impacts. As NOAA’s Emily Becker points out, strong El Niño events usually bring rain to California (as in 1982-’83), but occasionally they don’t (as in 1965-’66):
Another story to watch is whether a strong El Niño could help make 2015 or 2016 the hottest year on record. This one seems increasingly plausible.
Global temperatures are already going up over time, thanks to all the carbon dioxide we’re adding to the atmosphere. According to NASA, 2014 was already the hottest year on record. But there was no El Niño that year — and El Niño years tend to be a bit hotter than average, as heat gets transferred from the ocean to the surface. So the combination of El Niño and rising CO2 could help 2015 and even 2016 break records:
Bottom line: There’s still a lot of uncertainty here, but El Niño could very well be the biggest weather story of late 2015, with potentially far-reaching impacts.
How El Niño actually works, step by step
To see how El Niño works, it helps to understand what the equatorial Pacific looks like under normal, or “neutral,” conditions:
1) Neutral conditions in the equatorial Pacific Ocean
Normally, the tropical Pacific features strong trade winds that blow warm ocean water from east to west, where it piles up near Indonesia. Meanwhile, back east along South America, frigid water deep down in the ocean gets pulled up closer to the surface, cooling the area around Peru. Here’s a diagram:
(William Kessler/NOAA/PMEL)
As a result, during “neutral” conditions, sea levels are about half a meter higher near Indonesia than they are in Peru. And the surface water near Indonesia is about 8°C warmer (14.4°F) than it is near Peru. That temperature difference creates a convective loop in the atmosphere that, in turn, reinforces the trade winds.
This ends up affecting a lot more than just this stretch of ocean. Because the Pacific is so vast, this system is a major driving force in the global climate. The large, warm pool of water near Indonesia causes the air above it to rise, creating rainfall in the region. And this system shapes the jet streams that guide weather and storms around the world.
That’s how it works under normal conditions, anyway. But things look a little different when El Niño comes along.
2) Now along comes El Niño
Every few years, those prevailing Pacific trade winds that blow east to west can weaken. (Scientists are still debating the nuances of exactly why this happens.)
When the trade winds weaken, all that warm water that was piled up near Indonesia starts sloshing back eastward, pulled back down by gravity. What’s more, the underwater layer known as the thermocline starts sinking. As a result, there’s less cold water rising up from the deep ocean near South America — so the waters near Peru start warming up. Here’s another diagram:
(William Kessler/NOAA/PMEL)
This causes sea surface temperatures in the east and central Pacific to start rising and the trade winds to weaken even further. What’s more, rainfall starts following that warm pool of water as it travels eastward. That’s why El Niño is usually associated with drier weather in places like Indonesia and Australia, as well as heavier rains in places like Peru (or California). The rain is essentially moving east.
Scientists officially declare an El Niño when sea surface temperatures in the equatorial Pacific Ocean (known as the Niño 3.4 region) rise 0.5°C above their historical baseline for three months in a row — and once atmospheric conditions and rainfall patterns shift accordingly.
Again, because the Pacific is so vast, an El Niño can have large ripple effects on weather around the world, especially during the winter months. Here’s a look at the changes that have historically accompanied El Niño events:
Typical effects of an El Niño during the winter:
(NOAA)
A strong El Niño can weaken monsoons in the Indian Ocean, for example. It can also cause the jet stream to start stretching from the Eastern Pacific across the southern United States, bringing rainfall and storms with it. Still, a lot depends on how strong the El Niño actually is — and occasionally there are aberrations and exceptions to the rule. More on that below.
El Niño’s return in 2015 — and why scientists are talking about a “Godzilla” event
Ever since early 2014, scientists have been expecting this latest El Niño to form. But, in a sign of how slippery the system can be, El Niño kept defying predictions and not showing up.
Finally, in March 2015, after a number of false starts, scientists at NOAA’s climate prediction center were ready to declare that a weak El Niño was underway. Specifically, sea surface temperatures in that Niño 3.4 region (roughly in the center of the chart below) had been at least 0.5°C above their baseline since September. And, importantly, atmospheric conditions were responding in turn, with more rain over the central Pacific and less rain over Indonesia:
Sea surface temperature departures from average (based on 1981–2010) at the end of February 2015. NOAA map by Emily Becker, Climate Prediction Center.
At the time, however, NOAA’s forecasters said that this El Niño looked “weak,” with possibly minimal effects on global weather patterns, and only had a 50 to 60 percent chance of lasting through the summer.
Then, somewhat unexpectedly, El Niño got stronger and stronger. By August 2015, sea surface temperatures had soared to more than 1.2°C above baseline in the Niño 3.4 region, and scientists were seeing the resulting telltale atmospheric changes. Here’s a chart from July and August — notice how the anomalous warm area has moved eastward since March:
Sea surface temperatures in the tropical Pacific: departure from the 1981-2010 average. (NOAA)
NOAA’s Climate Prediction Center is now estimating that there’s a 90 percent chance El Niño will persist through the fall/winter. And when it peaks, signs suggest that this could be an extremely strong event, rivaling the strongest El Niños since detailed records began in 1950. Some forecasters have even dubbed this one a potential “Godzilla.”
Over at NOAA’s ENSO blog, Emily Becker offers a more detailed breakdown of why forecasters are betting on a powerful, possibly record-setting, El Niño. Keep in mind that forecasts often go awry, that surprises occur regularly, and we can’t be perfectly certain of how things will turn out. Still, she writes, “We have a relatively confident forecast for a strong event.”
El Niño could bring rain to California — but may not end the drought
As noted above, El Niño tends to be associated with changes in weather patterns around the world, especially during the Northern Hemisphere winter. The most tantalizing possibility is that a strong El Niño could bring rain to California, potentially alleviating the state’s drought.
But even here, nothing is yet assured. El Niño only affects US weather indirectly, by altering atmospheric circulation and shifting the North Pacific jet stream. (See here for a lucid explanation by Columbia University’s Anthony Barnston.) This is an intricate chain of events, and small kinks at certain points can affect the ultimate outcome.
As such, Becker cautions people to think not in terms of certainties but in terms of probabilities. Here’s an example of how El Niño might shift the odds of a wet winter for California (she notes that this isn’t a prediction, just an illustration):
An example of how a strong El Niño could shift the odds for the amount of seasonal precipitation. Official outlooks from the Climate Prediction Center are available here.
In other words, thanks to El Niño, California has a greater chance of more precipitation this winter, but not a 100 percent chance.
What’s more, even if rain does come, that may not be enough to completely erase the massive water deficit that California has built up over the past five years. The state likely needs record precipitation to end the drought, and it also needs the right mix of rain (to recharge the reservoirs) and snow in the Sierra Nevada mountains (to melt during the spring and summer).
Also, be warned: Heavy rain after a drought can bring floods and mudslides. So California needs to be ready for some negative impacts, as well.
El Niño tends to hurt some countries, and benefit others
It’s not quite right to say that El Niño events are “bad” or “good.” They tend to have different impacts on different regions.
One recent study from the University of Cambridge found that on average, El Niño events hurt economic activity in Australia, Chile, Indonesia, India, Japan, New Zealand, and South Africa. The reasons varied: drought and reduced crop yields in Australia and India, forest fires in Indonesia, less-productive fisheries in Peru.
But that study also found that on average, El Niño tends to boost the economies in Argentina, Canada, Mexico, and even the United States, at least in the very short term. Again, many factors were at play: In addition to bringing needed rain to California and Texas, El Niño was associated with less tornado activity in the Midwestern United States and fewer hurricanes in the Atlantic Ocean.
Here’s a table of the estimated economic impacts on a broad selection of countries:
Again, every big El Niño is different and has its own idiosyncrasies. So think of this table as more a rough guide than gospel.
El Niño could help make 2015 or 2016 the hottest years on record
Thanks to global warming, the Earth’s average surface temperature has been going up over time. But there’s a lot of variation from year to year. El Niño years tend to be a bit hotter than average. La Niña years (when those trade winds strengthen rather than weaken) tend to be a bit cooler than average. Like so:
So what’s going on here? As humans load more greenhouse gases in the atmosphere, we’re trapping more and 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 make a big difference for surface temperatures.
When conditions in the Pacific are neutral, more of that heat is trapped beneath the ocean surface. When a strong El Niño forms, more of that heat is transferred to the surface. That’s why the Earth’s average surface temperatures reached new highs in 1998: you had the combination of global warming and an extremely strong El Niño.
What was remarkable about 2014 is that it was likely the hottest year on record even without an El Niño event — a sign that Earth keeps getting warmer overall. Meanwhile, 2015 has so far been on track to be even hotter than 2014.
Now throw a potentially record-setting El Niño into the mix, and we’re looking at a potential shattering of records. Back in January, NASA’s Gavin Schmidt explained at a press conference that temperatures typically peak about three months after an El Niño event. Given that forecasters expect this current El Niño to last until next spring, it’s entirely possible we could see 2015 or 2016 break the temperature record. We’ll have to wait and see.
