The cause of global warming shows no sign of slowing down: Levels of the three most significant human-caused greenhouse gases – carbon dioxide (CO2), methane and nitrous oxide – continued their steady climb last year, federal scientists reported this month.

In fact, because of the burning of fossil fuels such as coal, oil and natural gas, those three greenhouse gases in our atmosphere have risen to levels not seen in at least 800,000 years − and potentially far longer, perhaps millions of years, the National Oceanic and Atmospheric Administration, said.

"We still have a lot of work to do to make meaningful progress in reducing the amount of greenhouse gases accumulating in the atmosphere," said Vanda Grubišić, director of NOAA's Global Monitoring Laboratory, which released the report.  

Carbon dioxide increased in 2023

The global surface concentration of CO2, averaged across all of 2023, was 419.3 parts per million (ppm), an increase of 2.8 ppm over the prior year. This was the 12th consecutive year CO2 increased by more than 2 ppm, extending the highest sustained rate of CO2 increases on record.

“The 2023 increase is the third-largest in the past decade, likely a result of an ongoing increase of fossil fuel CO2 emissions, coupled with increased fire emissions possibly as a result of the transition from La Niña to El Niño,” said Xin Lan, a University of Colorado Boulder atmospheric scientist who works with NOAA.

The increase in carbon dioxide also coincided with yet another unusually warm year for the planet in 2023: Data from both NASA and NOAA agreed that global average temperatures last year were the warmest on record.

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Methane, nitrous oxide also rose

Methane, which is less abundant than carbon dioxide but more potent at trapping heat in our atmosphere, rose to an average of 1922.6 parts per billion (ppb), according to NOAA. The 2023 methane increase over 2022 was 10.9 ppb. In 2023, levels of nitrous oxide, the third-most significant human-caused greenhouse gas, climbed by 1 ppb to 336.7 ppb.

“Fossil fuel pollution is warming natural systems like wetlands and permafrost," Rob Jackson, who heads the Global Carbon Project, told The Associated Press. "Those ecosystems are releasing even more greenhouse gases as they heat up. We’re caught between a rock and a charred place.”

What causes global warming?

The burning of fossil fuels such as coal, oil and gas releases greenhouse gases such as carbon dioxide and methane, which has caused the temperature of Earth's atmosphere to rise to levels that cannot be explained by natural causes, scientists say. 

Carbon dioxide is called a greenhouse gas because of its ability to trap solar radiation and keep it confined to the atmosphere.

It is invisible, odorless and colorless, yet is responsible for 63% of the warming attributable to all greenhouse gases, according to NOAA's Earth System Research Laboratory in Colorado. 

Atmospheric levels of carbon dioxide are now similar to where they were during the mid-Pliocene epoch, about 4.3 million years ago, NOAA said.