2013 tied for the world's fourth warmest year on record, according to calculations by the National Oceanic and Atmospheric Administration.

The NOAA's National Climatic Data Center said on Tuesday that last year ties with 2003's ranking, and marks a continuation of a global warming trend.

According to calculations by NASA also released Tuesday, 2013 tied with 2009 and 2006 for the seventh warmest year since records began in 1880.

Both federal agencies agree that 9 of the 10 warmest years have happened since 2000.

"Long-term trends in surface temperatures are unusual and 2013 adds to the evidence for ongoing climate change," said  climatologist Gavin Schmidt of NASA's Goddard Institute for Space Studies.  "While one year or one season can be affected by random weather events, this analysis shows the necessity for continued, long-term monitoring."

A statement from NASA adds that "the level of carbon dioxide in Earth's atmosphere presently is higher than at any time in the last 800,000 years."

In 2013, the world also hit the "sobering milestone" of 400 parts per million (ppm) of CO2 —a first in human history.  "It feels like the inevitable march toward disaster," Maureen E. Raymo, a Columbia University earth scientist, said of hitting the 400 mark.

In one possible emissions scenario, one in which "people pursue personal wealth rather than environmental quality" and current emissions trajectories continue, the world could reach 1,000 ppm in one hundred years.

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