On Tuesday, scientists released the latest  analysis of global temperatures – which shows 2013 tied with 2003 for the fourth warmest year since 1880. But what can one year’s temperature data tell us about climate change? We asked some climate scientists for their thoughts.

2013 in context

Each year, the United States National Oceanic and Atmospheric Administration (NOAA) releases its analysis of global temperature over the past year – and the official figures for 2013 are in. On Tuesday, a press release accompanying the figures reported:

“The year 2013 ties with 2003 as the fourth warmest year globally since records began in 1880. The annual global combined land and ocean surface temperature was 0.62 degrees Celsius above the 20^th century average of 13.9°C.

Using a slightly different method, the US space agency, NASA, ranked 2013 as the seventh warmest on record, tied with 2009 and 2006.

The question of what a single year’s temperature can tell us about climate change pops up each year when the official figures are released.

This time last year, we asked  Dr Gavin Schmidt, climate scientist at NASA GISS, for his thoughts on 2012 being ranked the ninth warmest on record. Schmidt told us:

“That 2012 is fifth, ninth, or twenty-seventh is not really the point. Instead, it is the fact that the long term trends and the decade-on-decade differences are all up (as has been predicted for decades).”

And the same goes for this year. Rather than whether one year was hotter than the last, scientists look at changes over several decades to see how the climate is changing.

Long term warming

As NASA points out, the data for 2013 continues a long term warming trend. The press release reads:

“With the exception of 1998, the 10 warmest years in the 134-year record all have occurred since 2000, with 2010 and 2005 ranking as the warmest years on record” 

2013 was the 4th warmest year on record since 1880, according to official NOAA data released on Tuesday. Source: Climate Central

The important point, as Professor Phil Jones from the Climatic Research Unit at the University of East Anglia explained to us this time last year, is that the world is warmer than it was a few decades ago.

“What matters is that the 2000s were warmer than the 1990s and they were in turn warmer than the 1980s etc back to the 1960s.”

Or to put it a different way, 2013 was the 37^th consecutive year that the annual global temperature has been above the 20th century average, according to NOAA’s new analysis.

Natural highs and lows

But why do we see differences in global temperature from one year to the next? It’s mainly down to natural oscillations in ocean temperature, as Jones explained:

“Much of the year-to-year variability of global temperature averages is caused by whether we have an El Nino or La Nina or neither type (neutral) occurring…1998 was exceptionally warm…because of the 1997/1998 El Nino event.”

While 2013 was not the hottest year on record, it fits into what scientists know about natural variability.  As the NASA press release explains:

“[W]eather patterns always will cause fluctuations in average temperatures from year to year … Each successive year will not necessarily be warmer than the year before, but with the current level of greenhouse gas emissions, scientists expect each successive decade to be warmer than the previous.”

This is the reason measurements over long periods of time are so important, adds Schmidt:

“While one year or one season can be affected by random weather events, this analysis shows the necessity for continued, long-term monitoring.”

Building evidence

While scientists are cautious about interpreting one year’s worth of temperature data, together with highs in recent years it fits in with scientists’ understanding of climate change. As Professor Ken Caldeira from the Carnegie Institution of Washington, told us recently:

“Each hot year is another piece of accumulating evidence. This accumulating mountain of evidence has confirmed the science of climate change so that now we have as much confidence in the basic climate science as we have in the fundamental science of plate tectonics or biological evolution.”