The US space agency also found that 2009 was the second-warmest year on record since modern temperature measurements began in 1880. Last year was only a small fraction of a degree cooler than 2005, the warmest yet, putting 2009 in a virtual tie with the other hottest years, which have all occurred since 1998.
According to James Hansen, who heads NASA's Goddard Institute for Space Studies, global temperatures change due to variations in ocean heating and cooling.
"When we average temperature over five or 10 years to minimize that variability, we find global warming is continuing unabated," Hansen said in a statement.
A strong La Nina effect that cooled the tropical Pacific Ocean made 2008 the coolest year of the decade, according to the New York-based institute.
In analyzing the data, NASA scientists found a clear warming trend, although a leveling off took place in the 1940s and 1970s.
The records showed that temperatures trended upward by about 0.36 degrees Fahrenheit (0.2 Celsius) per decade over the past 30 years. Average global temperatures have increased a total of about 1.5 degrees Fahrenheit (0.8 Celsius) since 1880.
"That's the important number to keep in mind," said Gavin Schmidt, a climatologist with the institute.
"The difference between the second and sixth warmest years is trivial because the known uncertainty in the temperature measurement is larger than some of the differences between the warmest years."
Last year's near-record temperatures took place despite an unseasonably cool December in much of North America and a warmer-than-normal Arctic, with frigid air from the Arctic rushing into the region while warmer mid-latitude air shifted northward, the institute said.
The analysis was based on weather data from over a thousand meteorological stations worldwide, satellite observations of sea surface temperatures and Antarctic research station measurements.
But the newly released figures were unlikely to quell a heated climate debate.
The so-called "climategate" controversy that exploded last fall on the eve of UN-sponsored climate talks unleashed a furor over whether the planet was heating and, if so, at what pace.
Hundreds of emails intercepted from scientists at Britain's University of East Anglia, a top center for climate research, have been seized upon by skeptics as evidence that experts twisted data in order to dramatize global warming.
World powers agreed at the Copenhagen climate summit last month to seek to prevent average global temperatures from rising beyond 3.6 degrees Fahrenheit (two Celsius) above pre-industrial levels in order to halt the most devastating effects of global warming.
"There's a contradiction between the results shown here and popular perceptions about climate trends," Hansen said. "In the last decade, global warming has not stopped."