"The precious Amazon is teetering on the edge of functional destruction and, with it, so are we," the editors of a leading scientific journal wrote.
The leak emitted more of the gas in 20 days than the oil and gas industries of some European nations do in a year.
"It is worth recalling that the last time the Earth experienced a comparable concentration of CO2 was 3-5 million years ago," WMO Secretary-General Petteri Taalas said.
Nitrous oxide is 300 times more potent than carbon dioxide as a greenhouse gas, although it is found in the atmosphere in smaller quantities.
Researchers reconstructed atmospheric carbon dioxide concentrations dating back 2.5 million years.
The new method offers a way to recycle one of the leading greenhouse gases responsible for climate change.
The U.S. benefits from other countries working to curb greenhouse gases and improve air quality.
The report says humans have caused some warming, a view at odds with that of the Trump crew.
Despite the avoidance of millions of tons of carbon dioxide emissions through use of renewable energy, increased efficiency and conservation efforts, the rate of increase of carbon dioxide in the atmosphere remains high.
It's a vicious cycle: As the weather warms, the Earth's permafrost is melting, releasing greenhouse gases that are going to make the planet even hotter.
By bringing emissions down to the Paris climate talk target, the U.S. will improve air pollution to the point that it could save hundreds of thousands of lives
If the people lead, hopefully the so-called leaders will follow.
No. 3 greenhouse gas emitter India faces growing pressure to devise a clear strategy and step out of China's shadow during pivotal global climate talks.