Energy-related carbon emissions reached a record level last year, according to the International Energy Agency (IEA).
The watchdog says emissions rose again after a dip caused by the financial crisis in 2009, and ended 5% up from the previous record in 2008.
China and India account for most of the rise, though emissions have also grown in developed countries.
The increase raises doubts over whether planned curbs on greenhouse emissions will be achieved, the group says.
At a meeting last year in Cancun, Mexico, world leaders agreed that deep cuts were needed to limit the rise in global temperature to 2C above pre-industrial levels.
But according to the IEA's estimate, worldwide CO2 emissions from the energy sector reached a record 30.6 gigatonnes in 2010.
The IEA's Fatih Birol said the finding was "another wake-up call".
"The world has edged incredibly close to the level of emissions that should not be reached until 2020 if the 2C target is to be attained," he added.
"Unless bold and decisive decisions are made very soon, it will be extremely challenging to succeed in achieving this global goal agreed in Cancun."