"Nine of the 10 warmest years on record have occurred since 1995."
A widely reported study last week said 2005 was the warmest on record. But headlines failed to note that the results were not concrete and a new study out this week challenges the findings.
Whatever the outcome, scientists say it is all moot: Last year was surprisingly warm and the record will fall soon enough.
The latest result came Monday from the National Climatic Data Center (NCDC), which is part of the National Oceanic and Atmospheric Administration (NOAA). These are the folks that run the National Weather Service. Their study concludes that the global temperature in 2005 can't be statistically distinguished from the record set in 1998.
Last year was a warm year at Earth's surface, especially considering the lack of a heat-producing El Nino, but for now experts do not agree whether it was a record.
Last week, The Associated Press and others reported that a NASA scientist said 2005 was the warmest year on record, nosing out 1998.
Lost in many of the headlines, however, was this quote from the report's lead researcher, James Hansen, director of NASA's Goddard Institute for Space Studies: "We couldn't say with 100 percent certainty that it's the warmest year, but I'm reasonably confident that it was."
Hansen looked at different data in different ways compared to the NOAA team. The NASA study considered in particular data from the Arctic, which is warming faster than the rest of the planet. And for the latter part of 2005 both reports relied on preliminary data, so the analyses could change.
In an email interview yesterday, Hansen reiterated his caveat.
"I believe that 2005 is the warmest year, because the main source of difference is the Arctic, and I believe it is likely that our estimate there is in the right ballpark even though it is based on some extrapolations," Hansen said. "However, I admit that it could be wrong, in which case 2005 might be slightly cooler than 1998."
In both studies, there are margins of error. Much of the analysis involves satellite data that covers just the past three decades or so. Complicating matters, ground-based temperature-monitoring stations are sparse or nonexistent in many parts of the world, particularly in the Arctic. And a key to the results are satellite data that note sea surface temperatures since 1982. Prior years are gauged by less-precise data from ship logs.
Finally, reliable records for most ground locations go back only about a century, so setting records may not be as surprising as if they broke marks that had been around longer.
So while all leading experts agree the planet has warmed about 1 degree Fahrenheit in the past century (and NOAA says the rate has tripled since 1976) ranking the warmest years is a huge statistical challenge.
In fact the NOAA analysis yielded two results: One data set, in use since the late 1990s, found that 2005 was slightly cooler than 1998, with 2005 being 1.04 degrees Fahrenheit above the 1880-2004 average, while 1998 was 1.12 degrees above that norm.
The other NOAA data set and analysis technique (which will become the primary method used henceforth) puts 2005 slightly warmer than 1998. It has 2005 at 1.12 degrees above the norm and 1998 at 1.06 degrees above the norm. But the report states that "uncertainties associated with the various factors and methodologies used in data set development make 2005 statistically indistinguishable from 1998."
A third study
Still another study, led by John Christy, director of the Earth System Science Center at the University of Alabama in Huntsville, paints a different picture.
Christy said in early January that 2005 tied with 2002 for second place.
But Christy looked at entirely different data, and the results are not conflicting, he said. Christy examined the entire "bulk" troposphere, from the surface up to about 35,000 feet. In that measurement of the atmosphere, 2005 "clearly was not the warmest," he said in a telephone interview yesterday.
Christy said his approach, which relies on observations from satellites and balloons, is more systematic and global than the estimates provided in the surface-temperature studies. On the other hand, it does not incorporate data more than a few decades back in time.
Interestingly, the troposphere as a whole tends to lag behind rising surface temperatures, Christy said. So measurements over the next few months could show an increase in the troposphere.
The bottom line
Regardless where 2005 ends up, this statement from NOAA puts things in perspective: "Nine of the 10 warmest years on record have occurred since 1995."
And beyond the temperature data, there is plenty of stark evidence for significant warming at the surface. Ground in the Northern Hemisphere that's been frozen since the last Ice Age is melting and collapsing. Animals are changing migration and mating habits. And glaciers are melting and shrinking at alarming rates.
Meanwhile, climatologists are impressed with nature's showing in 2005, because by conventional thinking it should not have been first or second on the all-time list. That's because 1998, the previous hottest year, saw temperatures boosted by a strong El Nino, which was not in place during 2005.
"The bottom line: 2005 was very warm," said Richard Heim, who worked on the NOAA report.
"2005 was not an El Nino year, yet we were toying with tying the 1998 El Nino year," Heim said. "If we had had an El Nino, how warm would it have been?"
NASA's Hansen is already looking ahead to years that he and most other experts expect to be warmer.
"We may get a more definitive assessment from additional data, but it also may be that we will never know for sure," he said. "However, it doesn't matter much. I am confident that we will exceed both of these years within the next few years."
Sign up for the Live Science daily newsletter now
Get the world’s most fascinating discoveries delivered straight to your inbox.
Robert is an independent health and science journalist and writer based in Phoenix, Arizona. He is a former editor-in-chief of Live Science with over 20 years of experience as a reporter and editor. He has worked on websites such as Space.com and Tom's Guide, and is a contributor on Medium, covering how we age and how to optimize the mind and body through time. He has a journalism degree from Humboldt State University in California.