On Tuesday, NASA released its latest annual analysis of global temperatures, which shows 2012 was the ninth warmest year since records began in 1880. But does one year’s temperature data tell us anything about climate change? We asked some scientists for their thoughts.

2012 in context

In a report accompanying the data, Dr James Hansen, chief scientist at NASA Goddard Institute for Space Studies (GISS), says:

“Global surface temperature in 2012 was +0.56C (1F) warmer than the 1951-1980 base period average…2012 is nominally the ninth warmest year [since 1880].”

It might be tempting to focus on this fact, but it’s not that simple. As Dr Richard Allan, climate scientist at the University of Reading told Carbon Brief, looking at temperature data for a single year is not very informative.

“The fact that 2012 was the ninth warmest year on record does not tell us much about climate change. Global temperatures fluctuate naturally from year to year by as much as half a degree Celsius.”

Or as Dr Gavin Schmidt, climate scientist at NASA GISS 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).”

Long term warming

So rather than whether one year was hotter than the last, scientists look at changes over several decades to see how the climate is changing. As Kathryn Maskell from the Walker Institute for Climate System Research told us:

“We wouldn’t expect to see each year successively warmer than the last because natural factors like changes in ocean circulation mean that there are natural year to year wobbles in temperature on top of the long term human-induced warming trend.”

The important point, as Professor Phil Jones from the Climatic Research Unit at the University of East Anglia explained to us, 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 as Hansen points out in his report:

“…the 10 warmest years in the record all occurred since 1998.”

Natural highs and lows

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

“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 1997-98 El Nino event.”

So while 2012 was not the hottest year on record, it fits into what scientists know about natural variability. Maskell continues:

“2012 was less warm than some other recent years such as 2010, 2005 and 1998, but year to year variations in temperature are completely consistent with scientists’ understanding of how the climate system works.”

Slower temperature rise

So what about the fact that the 2012 data show global temperatures are rising slower than in previous decades? As Hansen explained in his report:

“2012 is nominally the ninth warmest year, but it is indistinguishable in rank with several other years, as shown by the error estimate for comparing nearby years.”

As we’ve written extensively about recently, the slowdown in recent temperature rise shouldn’t be taken as evidence that human-induced global warming has slowed down or stopped. As Prof Sir Brian Hoskins from the Grantham Institute for Climate Change, told us:

“In the last decade or so [natural] variability has given a cooling that has just about balanced the long-term warming. In decades to come we can expect a return to the 1990s style warming again. Global warming has not gone away!”

John Cook, research fellow at the University of Queensland and founder of the Skeptical Science blog, explained to us a bit more about what we can expect:

“[W]hile surface temperature shows much variability from year to year as heat is exchanged between the atmosphere and oceans, the steady increase in heat means we will continue to see even warmer years in the coming decade.”

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 Prof Ken Caldeira from the Carnegie Institution of Washington, told us:

“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.”

Or as a spokesperson from the Climate Monitoring and Attribution group at the Met Office explained to us:

“Whilst 2012 continues a run of temperatures above the long-term global average, it is ultimately just one number amongst a vast quantity of evidence and information which informs us about climate change.”