Last year was among a handful of the warmest on record since 1880, according to US government figures out Tuesday that provide more evidence that the planet is heating up.

Human-caused pollution and the burning of fossil fuels such as oil and coal have driven up greenhouse gas levels, leading to this long-term rise in temperatures, said the US space agency NASA.

Carbon dioxide is at its highest level in the atmosphere in 800,000 years, having risen from 285 parts per million in 1880 to 400 parts per million last year, NASA said.

Unless current trends change, scientists said the world should expect each of the coming decades to be warmer than the last, said NASA climatologist Gavin Schmidt.

He described the warming of the past few decades as "unusual," and urged people not to judge whether climate change is happening or not based on random weather events like cold snaps."

"The long-term trends in climate are extremely robust," he told reporters.

"People have a very short memory when it comes to their own experience of weather and climate, and the only way that we can have a long-term assessment of what is going on is by looking at the data."

NASA and the National Oceanic and Atmospheric Administration (NOAA) both released their annual global figures on climate, which were independently produced but found similar increases in temperature across the planet.

According to NOAA, the average of combined land and ocean surface temperatures in 2013 was 1.12 degrees Fahrenheit (0.62 Celsius) above the 20th century average of 57 degrees Fahrenheit (13.9 Celsius).

NOAA found that 2013 tied with 2003 as the fourth warmest year since records began in 1880, while NASA said last year ranked seventh.

However, experts said the actual temperature differences between years are very small, and that the overall trend toward a warming planet is clear.

Last year also marked the 37th year in a row with higher than average global temperatures.

All 13 years of the 21st century have been among the warmest on record, NOAA said, with the hottest being 2010, 2005, and 1998.

A key difference between last year and other top years of the past decade is that 2013 had no El Nino effect to warm the equatorial region, a weather phenomenon that would have been expected to cause an uptick in global temperatures.

Forecasters say El Nino could return in 2014, with the potential to make this coming year even hotter than last.

Another concerning effect of global warming is the melting of sea ice in the Arctic, which is expected to cause sea level rises over time that will endanger coastal communities around the world.

"Arctic sea ice is down considerably, especially over the past 10 to 11 years," said Tom Karl, director of NOAA's Climatic Data Center.

Last year marked the sixth smallest sea ice extent in the Arctic on record, while the Antarctic saw the opposite trend, and sea ice was above average.

While most of the world experienced above-average annual temperatures, a few small regions in the central United States, eastern Pacific and South America were cooler than average, according to NOAA.