The so-called “pause” of global warming continued through 2013, according to NASA, as there was no statistically significant rise in global temperatures last year.

“The trends over the last 10 to 15 years compared to the trends before do appear to be lower than they were,” Gavin Schmidt, climatologist with NASA’s Goddard Institute for Space Studies, told reporters.

“We’ve been looking at this in separate work and partially it seems to be a function of internal variability in the system, so the fact is that we’ve had more La Nina-like conditions over the last few years compared to earlier on in the 2000s or in the late 1990s,” Schmidt added.

According to NASA, 2013 was tied with 2009 and 2006 for the seventh warmest year on record. The National Oceanic and Atmospheric Administration (NOAA) ranks 2013 as the fourth warmest year on record, tying with 2003.

“In summary they both show that the ‘pause’ in global surface temperature that began in 1997, according to some estimates, continues,” wrote David Whitehouse, who holds a doctorate in astrophysics and was the BBC’s science editor. “Statistically speaking there has been no trend in global temperatures over this period.”

“Given that the [Intergovernmental Panel on Climate Change] estimates that the average decadal increase in global surface temperature is 0.2 [degrees Celsius], the world is now 0.3 [degrees Celsius] cooler than it should have been,” Whitehouse added.

In 2013, the average temperature was 58.3 degrees Fahrenheit, which is 1.1 degree Fahrenheit warmer than the mid-20th century baseline temperature. According to NASA, the average global temperature has risen about 1.4 degrees Fahrenheit since 1880.

The government scientists emphasized that weather patterns cause average temperatures to fluctuate and that the increasing levels of greenhouse-gas emissions will raise global temperatures in the long run. Each year may not be warmer than the last, according to NASA, but scientists predict that each decade will be warmer than the last.

“Our expectations for what temperatures should be changing like, they come from our understandings of our forcings of climate change,” Schmidt said, adding that such forcings include greenhouse gases, volcanoes, solar activity and air pollution — for example, aerosols from coal burning, smog and volatile organics.

“Our ability to properly quantify the air pollution around the world … is actually not very good, and we have had historically a problem in defining those aerosol forcings very accurately … and that has not improved,” Schmidt admitted.

“And this is really one of our biggest kind of gaps in the data that we’re producing for the climate,” he continued. “And it means that when we’re looking at relatively short-term trends … the variance in that and the inability to really constrain those aerosol forcings really kind of make it hard for us to say what we should have expected over that time period.”

Climate scientists have struggled to explain why global temperatures have not significantly risen in the last 17 years. Some scientists argue that the excess greenhouse gases have gone into the ocean, negating any warming effect while others contend that natural climate and solar cycles are to blame.

“So the situation that we’re seeing now, there’s some natural variability components,” said Schmidt, “there is some uncertainty in what the trends of the different forcings have been, but we’ve also had slightly more volcanic activity than we anticipated and the sun … has been slightly dimmer than we anticipated 10 years ago.”

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