“This trend continues a long-term warming of the planet,” the agencies say. They point to Earth’s average surface temperature having warmed by about 1.4 degrees F (0.8 Celsius) since 1980, a trend they attribute largely to increased carbon dioxide and other emissions resulting from human activities. Most of that warming has occurred in the past three decades.

The NASA and NOAA scientists say year-to-year fluctuations in average global temperature are expected to continue, as a result of phenomena such as El Niño and La Niña. They say those phenomena over the tropical Pacific Ocean “are thought to have played a role in the flattening [often referred to as the hiatus or pause] of the long-term warming trend over the past 15 years.”

They also pointed to regional temperature differences which, in contrast with the global mean, are more strongly affected by weather dynamics than the global mean. They pointed to “unusually cool” temperatures in parts of the Midwestern U.S. and along the East Coast, with Alaska, Arizona, California, and Nevada experiencing their warmest year on record.

Gavin Schmidt, NASA GISS director, pointed out that global surface temperatures in 2014 were “not quite” the warmest, but that surface temperatures were offset by the higher ocean temperatures, making the combined global mean average the warmest in the instrumental record.

NOAA’s Tom Karl pointed to comparable findings regarding 2014 reported, or expected soon to be reported, by British and Japanese meteorological agencies. Even considering reasonable uncertainties, Karl said, NOAA and NASA data agree about the 2014 calendar year’s being the warmest.

Responding to a question during a January 16 media briefing on their findings, NASA’s Karl said he does not expect any additional fine-tuning of the agencies’ research will lead to a change in the conclusion of 2014’s being the warmest year in the instrumental record. “The picture is not going to change,” Karl said.

Schmidt responded to a question concerning the projected duration of the current hiatus or “flattening” of the warming trend by saying he expects continued greenhouse gas emission increases virtually assure a return to earlier projections of continued warming. He allowed that “not much” warming has occurred between 1998 and now, but said that looking at data from 1970 to 1997, excluding 1998, and since then, the increased global temperature is about what had been projected prior to the strong 1998 El Niño year. “There’s no evidence the long-term trend is much different from what was expected,” he said, without venturing an estimate of how much longer the hiatus may continue.

Additional resources:

GISS Surface Temperature Analysis (GISTEMP)
GISS Surface Temperature Analysis (Sources)
NCDC Releases 2014 Global Climate Report