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OSLO (Reuters) - This year is on track to be about the 10th warmest globally since records began in 1850 but gaps in Arctic data mean the world may be slightly underestimating global warming, a leading scientist said on Tuesday.
A natural cooling of the Pacific Ocean known as La Nina kept a lid on temperatures in 2008 despite an underlying warming trend, said Phil Jones, director of the Climatic Research Unit at the University of East Anglia in England.
"This year is about 10th," he told Reuters in a telephone interview. "La Nina in the Pacific lasted longer than we envisaged."
Jones's unit is one of the main sources of global climate data for the United Nations.
The warmest year on record was 1998, followed by 2005 and 2003, with other years this century closely bunched. Tenth place would make 2008 the least warm since 1999.
The update marginally cools an estimate from January, when Jones's unit and the British Met Office (Britain's meteorological service) estimated that 2008 would be "another top 10 year," near the bottom of the ranking.
The U.N. Climate Panel says human emissions of greenhouse gases, mainly from burning fossil fuels, are blanketing the planet. Rising temperatures will bring more floods, heatwaves, more powerful storms and rising sea levels, it says.
Jones said temperature records may fractionally underestimate warming because of gaps in measurements in the Arctic for 1961-90, the benchmark years for judging change, and problems in verifying ocean temperatures.
"The world is probably a little warmer than we are measuring," he said.
Arctic sea ice shrank to a record low in summer in 2007 and almost matched the low again in 2008. U.N. studies say the region may be warming twice as fast as the world average.
Ships are traveling more often in the Arctic and "now there are temperature measurements coming back. But we can't use the data because we don't have the 1961-90 averages," he said.
He said scientists suspected that ocean temperature measurements from buoys, widely deployed since about 1990, underestimated temperature rises perhaps by up to a 0.05 Celsius (0.08 Fahrenheit), compared to previous ship-based readings.
"There's nothing wrong with the land measurements but we might be underestimating the oceans," he said.
Scientists were now scouring records of ships over the past 15 years to try to pin down when they were close to buoys. That would let them compare thermometer readings and see if there was a consistent mismatch.
"It's an awkward thing to try to find them when they were close together," Jones said, adding that the hopes the findings will be published in 2009.
Skeptics about a human cause of global warming say climate change has stopped because 1998 was the warmest year. But Jones said 1998 was warmed by a shift in the Pacific Ocean known as El Nino, the opposite of the La Nina effect.
"1998 was the anomalous year. if you take out the El Nino and La Nina effects we are still warming," he said.
Natural variations such as El Nino or volcanic eruptions that dim the sun accounted for swings of about 0.2 C a year, while global warming was adding about that much per decade.
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Editing by Michael Roddy