As 2005 draws to a close, climate scientists are making their annual pronouncements on how its temperatures compare to historical records. And although this year is among the warmest ever recorded, small differences in the claims highlight the uncertainty of such rankings.

Depending on whom one believes, 2005 will end up just above or below 1998 as the hottest year on record. Most significant, climate scientists say, is that this year's readings occurred without the help of a major El Niño event. “In just seven years, the background global temperature has increased to a level equal to the peak in the 1997–98 El Niño,” says James Hansen, a researcher at NASA's Goddard Institute for Space Studies in New York City.

That record-breaking El Niño slathered the tropical Pacific with anomalously warm sea water. There was no such event this year, but many other regions were notably warm — including the North Atlantic, where an unprecedented number of tropical cyclones formed.

Hansen says that NASA is likely to dub 2005 as the warmest year on record, but a team at the University of East Anglia in Norwich, UK, is poised to rate it as number two, behind 1998. And a preliminary report from the National Oceanic and Atmospheric Administration (NOAA) shows a photo finish between the two years, with 1998 ahead by a nose (see ‘Sources of disagreement’). Final rankings will be released over the next few weeks.

This year's record-breaking temperatures included a devastating heatwave in Pakistan. Credit: M. KHURSHEED/REUTERS

This year's heat was not a total surprise — NASA predicted early in 2005 that it would be one of the warmest years on record. Over the past century, says NASA, Earth's average surface temperature has risen 0.8 °C, with three-quarters of that occurring since the 1970s. Nine of the ten warmest years on record have occurred since 1995.

Hansen, who compiles the annual rankings for NASA, says the recent warming is consistent with the increase in heat-trapping greenhouse gases in the atmosphere. “Climate change is real and should begin to be noticed by real people,” he says.

Although differing rankings for 2005 might puzzle the public, it is less of an issue for the scientists who compile them. Most of the time, the ratings agree. “People sometimes make too much of whether a year is ranked warmest or second warmest,” says Jay Lawrimore, who oversees month-to-month tracking for NOAA.

Scientists hope to put the rankings in better perspective by pointing out uncertainties in them. In 2006, NOAA will shift to an analysis technique that will include uncertainty ranges for the first time. This may reduce the drama of the year-end rankings, but it could also accentuate just how many of the past few years lie at the top of the temperature heap.