Last year was the hottest on record since 1880, and the 10 warmest years in recorded history have all occurred since 2000, with the exception of just one - 1998. The previous hottest year was 2010, followed by 2005, and 1998.
These were the findings of two NASA and NOAA-led analyses of surface temperature records, which also revealed that over the past 135 years, the average surface temperature of Earth has increased by about 0.8 degrees Celsius (1.4 Fahrenheit). The researchers state that this warming has been driven by an increase in carbon dioxide and other human-caused emissions that continue to accumulate in Earth's atmosphere. And this increase hasn't been consistent across the past century or so - most of the warming has occurred over the past 30 years.
The team predicts that global temperatures could rise by 3 degrees Celsius (4 Fahrenheit) or more, if we don't curb our greenhouse gas emission rates soon.
"This is the latest in a series of warm years, in a series of warm decades," said NASA's Goddard Institute of Space Studies (GISS) director, Gavin Schmidt, in a press release. "While the ranking of individual years can be affected by chaotic weather patterns, the long-term trends are attributable to drivers of climate change that right now are dominated by human emissions of greenhouse gases."
The NASA GISS analysis gathered surface temperature measurements from 6,3000 weather stations, and sea surface temperatures were collected from instruments attached to a number of ships and floating buoys. Measurements were also taken from research stations based in Antarctica, and the information was analysed using an algorithm that accounts for how each of these stations are situated around the world and how the extra heat given off by urban environments could have affected the readings.
The NOAA analysed roughly the same raw data, but used different methods to formulate their own estimates for the global temperature changes, and came up with the same result - 2014 is the hottest so far in recorded history. Earlier this month, the Japan Meteorological Agency (JMA) also released an independent study that came to the same conclusion.
Here's what it looks like:
El Niño and La Niña - natural phenomena that have been occurring over the past 15 years, where changes in the Pacific Ocean and the atmosphere above it cause a warming or a cooling of the central and eastern tropical Pacific region, respectively - are known to cause fluctuations in global temperature. But 2014 was actually what's known as a 'neutral' year, so neither of these events occurred throughout that 12 months. During previous "hottest" years, 2010 and 1998, El Niño was in full swing.
Now, before you say, "No way this is true, last winter in X Home Town was bitter cold!" and someone sends you this tweet by Stephen Colbert in response, the researchers note that regional temperature fluctuations are much more sensitive to the chops and changes of local weather events than the global mean will ever be. The researchers point out that while parts of the Midwest and East Coast of North America were unusually cool during much of 2014, some of the western US states were sweating their way through their hottest ever year on record, according to the NOAA study.
Of course, recorded history is just a blink of the eye when you look at Earth's entire 4.5-billion-year existence, and Earth has certainly been hotter than it is now, millions of years ago, but as Brad Plumer notes at Vox, things had actually settled down over the past 10,000 years, until humans came along and discovered how to burn fossil fuels and release greenhouse gases into the atmosphere.
Being the most recent year on record and also the hottest, it would be tempting to say that global average surface temperatures are getting hotter faster as we spew more gasses into the atmosphere. But the data actually showed that over the past 16 years, the increase has been slightly slower than it was during the two decades that preceded them. Sometimes this 'slowdown' is used by climate change deniers to insist that there is no trend of increasing surface temperatures, but this phenomenon actually appears to be entirely natural.
"You can see a longer 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 has been stored temporarily in the ocean, or that there's been an outburst of volcanic activity that muted the pace of warming in the 2000s. These details are certainly relevant: For instance, if extra heat did go into the ocean because of, say, strong trade winds in the Pacific, that might 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."
The report has garnered its fair share of controversy, as climate change-related things are want to do. Atmospheric scientist John R. Christy from the University of Alabama in the US - who is known for his criticism of the perceived seriousness of global warming - told Justin Gillis at The New York Times that 2014 only surpassed the other "hottest" years by a few hundredths of a degree, and this sits well within the margin of error for global temperature measurements, he adds. "Despite such arguments from a handful of scientists, the vast majority of those who study the climate say the Earth is in a long-term warming trend that is profoundly threatening and caused almost entirely by human activity," says Gillis.
You can read the study online, and access the data set of 2014 surface temperature measurements here. You can also view the methodology used by the NASA team to calculate the mean global surface temperatures here.