By Irene Klotz
(Reuters) - The average temperature of Earth maintained its warming trend in 2013, despite seasonal and regional variations that included a shrinking ice cap in the Arctic and a massively growing one in the southern hemisphere, U.S. scientists said on Tuesday.
NASA said the planet's average temperature in 2013 was 58.3 degrees Fahrenheit (14.6 degrees Celsius), tying 2006 and 2009 for the seventh warmest year since 1880 when global climate record-keeping began.
Using the same data but different analysis processes, the U.S. National Oceanic and Atmospheric Administration said 2013's average temperature was 58.12 degrees Fahrenheit, which tied what NOAA considers to be the fourth hottest year on record.
The agencies differ in their analysis techniques. NASA for example uses more temperatures from Antarctica, but said the overall trend remains what has been measured every year since 1976 when global temperatures first surpassed the 20th Century's global average of 57 degrees Fahrenheit (13.9 degrees Celsius).
"The patterns of temperature change are very similar across the different analyses, but rankings and the exact numerical value are a function of some of the small differences that we have in the processing," Gavin Schmidt, deputy director of NASA's Goddard Institute for Space Studies in New York, told reporters on a conference call.
Global temperatures began climbing in the late 1960s, a phenomena that has been tied to heat-trapping greenhouse gases in Earth's atmosphere.
NASA, or the National Aeronautics and Space Administration, said the amount of carbon dioxide in Earth's atmosphere is higher now than at other time in the last 800,000 years.
Carbon dioxide levels were about 285 parts per million in 1880, the first year in the global temperature record. By 1960, levels reached 315 parts per million. In 2013, the amount of carbon dioxide peaked at more than 400 parts per million.
The relationship between greenhouse gases and global temperatures is complicated. In 2013, for example, the continental United States experienced its 42nd warmest temperature on record while Australia had its hottest year ever, NASA and NOAA data shows.
Ice in the polar regions presents another puzzle. The amount of Arctic sea ice in the northern hemisphere continued its ubiquitous and well-documented decline, while sea ice in Antarctica in the southern hemisphere increased a record amount, scientists said.
"The situation in the southern hemisphere is more complicated," Schmidt said, noting that wind patterns are impacted by the region's ozone hole and other factors.
"There's a lot of complicated physics going on," he added. "It's not a clean picture."
Ocean temperatures, including El Nino and La Nina warming and cooling patterns in the equatorial Pacific, also disconnect regional, seasonal and yearly temperatures with overall global trends, the scientists said.
"The long term trends in climate are extremely robust," Schmidt said. "There are times, such as today, when we can have snow, even in a globally warmed world. But the long-term trends are very clear. They are not going to disappear. It isn't an error in our calculations."
A third study on 2013 global temperatures is due to be released later this month by the Met Office Hadley Center in the United Kingdom.
(Editing by Kevin Gray, Bernard Orr)