Global warming hasn't gone away. 2014 was likely the hottest year on Earth since records began in 1880. That was the conclusion of separate analyses from NASA and the National Oceanic and Atmospheric Administration:
NASA and NOAA: 2014 was a record hot year
The global average temperature in 2014 was roughly 1.24°F (or 0.69°C) warmer than the average during the twentieth century, NOAA said. That included record heat in the western United States, Europe, Australia, parts of Siberia, and much of the Pacific Ocean.
The previous hottest years on record had been 2010, followed by 2005 and 1998. All 10 hottest years have occurred since 1998.
NASA and NOAA each conducted their own independent analyses using satellites and ground readings. But they came to similar conclusions: Technically, 2010 and 2014 were extremely close, and within the margin of error, though both agencies said it was more likely 2014 was hotter overall. NOAA said there was a 48 percent chance that 2014 was the warmest year and a 18 percent chance 2010 was.
What's particularly striking, scientists say, is that 2014 was so hot without the benefit of an El Niño, a cyclical phenomenon in the Pacific Ocean that can periodically bump up global temperatures a bit — as happened in previous record years like 2010 and 2005 and 1998.
All these new records are no coincidence. Climate scientists expect the Earth to get hotter over time so long as humans keep adding greenhouse gases to the atmosphere. There will be short-term variations from year to year — El Niño years are a bit hotter, La Niña years are a bit cooler — so not every year will set records. But the overall trend is up:
"If you are younger than 29 years old, you haven't lived in a month that was cooler than the 20th-century average," noted Marshall Shepherd, a meteorologist at the University of Georgia.
In all, global average temperatures have now risen 1.4°F (0.8°C) since the 19th century. And scientists have warned that temperatures could rise even more drastically — another 5°F (3°C) or more — this century if greenhouse-gas emissions keep rising. That much heat could have sweeping impacts on global sea levels, ice cover, droughts, and agriculture.
It's worth clarifying that 2014 wasn't the hottest year in the planet's entire 4.5 billion year history. The Earth has been hotter in the distant past, before human civilization arose — a result of natural processes and orbital shifts. But Earth's climate had roughly stabilized over the last 10,000 years, until humans began burning fossil fuels and emitting greenhouse gases. Scientists say the rise in temperatures in the 20th century is a sign of man-made warming.
2014 saw record warmth in Europe and the western US
The heat was spread out around the world: Record warmth in 2014 blanketed the western United States, exacerbating a brutal ongoing drought in California. The Pacific Northwest and Alaska also experienced record highs. For the first time in 100 years of measurements, the temperature in Anchorage, Alaska, never fell below 0°F.
By contrast, the Midwest and Northeastern United States were actually a bit cooler than usual in 2014. One reason for this was jet stream patterns that pushed warm tropical air up into Alaska and nudged colder Arctic air down into the continental United States. Global warming isn't always uniform across the entire planet.
The year also saw extremely warm temperatures in Europe — with 19 countries likely experiencing their hottest year ever — as well as record heat over much of the Pacific Ocean, fueling a number of strong storms in the region. Both agencies said that temperatures over the oceans were the warmest ever recorded.
An El Niño could push temperatures to new highs
It's significant that 2014 was a record hot year even without an El Niño. El Niño events, which occur in the Pacific, tend to transfer heat stored beneath the ocean's surface up into the atmosphere. Years that come right on the tail of a major El Niño tend to be hotter than average — indeed, that was a big reason why 1998 was so unusually hot.
Forecasters say there's still some chance that a (weak) El Niño could reappear in the Pacific Ocean this year — which could in theory push temperatures up in 2015 to new highs. (At a press conference, NASA's Gavin Schmidt explained that temperatures typically peak about three months after an El Niño event.) Still, that's not assured.
Is this the end of the global warming "slowdown"?
If 2014 really is the warmest year on record, then it's certainly wrong to say that global warming "stopped" back in 1998 — a favorite line of climate skeptics like Sen. James Inhofe (R-OK).
But then again, the idea that global warming had "stopped" was never very compelling to begin with — and not just because 1998 was a cherry-picked year. Scientists have assembled plenty of evidence that adding more greenhouse gases like carbon dioxide to the atmosphere will heat up the Earth over time. Short-term blips in the temperature record don't affect this broad understanding. It's the long-term trends that count.
What has puzzled some scientists, however, is the fact that global average surface temperatures appear to have risen at a slightly slower rate over the past 16 years than they did in the 20 years before that — despite the fact that greenhouse gases are piling up in the atmosphere at a record pace. This is what's often referred to as the "slowdown." Both NOAA and NASA found some signs of a recent slowdown, though it now appears very slight:
You can see a rundown of possible explanations for the slowdown in the 2000s here — hypotheses include the idea that some of the extra heat trapped by greenhouse gases had been stored temporarily in the ocean, or that an outburst of volcanic activity muted the pace of warming briefly. These details are certainly of some interest to scientists: For instance, if extra heat did go into the ocean because of, say, strong trade winds in the Pacific, that could set the stage for more rapid warming in the years ahead.
But the big picture is the same as it ever was — regardless of year-to-year blips. As we put more carbon dioxide into the atmosphere, the Earth will get hotter. There will be short-term fluctuations here and there. Some years will be record hot years. Others won't. El Niño years will be a bit hotter. La Niña years will be a bit cooler. But over a long enough time horizon, global warming is still with us.
Four different agencies track global temperatures
A side note: NASA and NOAA are only two of four major government agencies worldwide keeping track of global temperature trends, using both satellites and a network of ground-based thermometers, buoys, radar, and other tools. In January, the Japan Meteorological Agency (JMA) released its own preliminary analysis saying that 2014 was the hottest year on record. The Met Office Hadley Centre in the United Kingdom has yet to report.
All four tend to agree that the Earth is warming, though they work with different datasets and come up with slightly different numbers. NASA has the most comprehensive coverage, with its data covering 99 percent of the globe.
There is also a separate satellite-only record, maintained at the University of Alabama-Huntsville, that shows 1998 remains the hottest year, well above 2014 (which is only the ninth-hottest year in this dataset). But scientists say that satellites alone give an incomplete picture and other ground-based measurements are needed for a full analysis.
"You will hear some skeptics say that the satellite-based temperature records don't support these findings, but we also used ground-based instruments like thermometers and rain gauges to validate these measurements," said Shepherd.
Further reading: If you really want to wade into the details, check out this analysis of temperature records from NASA's Gavin Schmidt and James Hansen.
Watch: If our CO2 emissions weren't invisible, here's what they'd look like.