Energy-related emissions were two percent higher in December 2020 than in the same month a year earlier, driven by economic recovery and a lack of clean energy policies, the IEA said in a report.
"The rebound in global carbon emissions toward the end of last year is a stark warning that not enough is being done to accelerate clean energy transitions worldwide," IEA executive director Fatih Birol said in a statement.
"If governments don’t move quickly with the right energy policies, this could put at risk the world’s historic opportunity to make 2019 the definitive peak in global emissions."
A year ago, the intergovernmental agency called on governments to put clean energy at the heart of economic stimulus plans, but the appeal appears to have fallen on deaf ears for the most part.
"Our numbers show we are returning to carbon-intensive business-as-usual," Birol said.
In China, carbon pollution last year exceeded 2019 levels by more than half a percent despite a draconian, though brief, lockdown to halt the virus' spread.
China -- which accounts for more than a quarter of global CO2 output -- was the only major economy to grow in 2020.
Other countries are also now seeing emissions climb above pre-Covid crisis levels, the report found.
In India, they rose above 2019 levels from September as economic activity increased and Covid restrictions relaxed.
The rebound of road transport in Brazil from May drove a recovery in oil demand, while increases in gas demand toward the end of 2020 pushed emissions above 2019 levels in the final quarter.
US emissions fell by 10 percent in 2020, but by December were approaching levels from the year before.