In descending order, the years with the highest global average annual temperatures were 2005, 1998, 2002, 2003 and 2004, NASA said in a statement.
"It's fair to say that it probably is the warmest since we have modern meteorological records," said Drew Shindell of the NASA institute in New York City.
"Using indirect measurements that go back farther, I think it's even fair to say that it's the warmest in the last several thousand years."
The highest global surface temperature in more than a century of instrumental data was recorded in the 2005 calendar year in the Goddard Institute for Space Studies annual analysis. However, the error bar on the data implies that 2005 is practically in a dead heat with 1998, the warmest previous year.
But Shindell said last year was slightly warmer than 1998, taking into account indirect evidence from the Arctic, even without any extraordinary weather pattern. "That very anomalously warm year (1998) has become the norm," Shindell told reporters.
However, what's significant, regardless of whether 2005 is first or second warmest, is that global warmth has returned to about the level of 1998 without the help of an El Nino, NASA said.
The result indicates that a strong underlying warming trend is continuing. Global warming since the middle 1970s is now about 0.6 degrees Celsius (C) or about 1 degree Fahrenheit (F). Total warming in the past century is about 0.8° C or about 1.4° F.
"The five warmest years over the last century occurred in the last eight years," said James Hansen, director of NASA GISS. Over the past 30 years, Earth has warmed by 1.08 degrees F. Over the past 100 years, it has warmed by 1.44 degrees F.
Shindell, in line with the view held by most scientists, attributed the rise to emissions of greenhouse gases such as carbon dioxide, methane and ozone, with the burning of fossil fuels being the primary source.
The 21st century could see global temperature increases of 6 to 10 degrees F, Shindell said.
"That will really bring us up to the warmest temperatures the world has experienced probably in the last million years," he said.
Source: NASA and Reuters
Charts and graphs: Global Temperature Trends: 2005 Summation
© People & the Planet 2000 - 2006