Levels of the three most important human-caused greenhouse gases – carbon dioxide (CO2), methane and nitrous oxide – continued their steady climb during 2023, according to the US National Oceanic and Atmospheric Administration.
While the rise in the three heat-trapping gases recorded in air samples in 2023 was not quite as high as the record jumps observed in recent years, they were in line with the steep increases observed during the past decade.
The global surface concentration of CO2, averaged across all 12 months of 2023, was 419.3 parts per million (ppm), an increase of 2.8 ppm during the year. This was the 12th consecutive year CO2 increased by more than 2 ppm, extending the highest sustained rate of CO2 increases during the 65-year monitoring record. Three consecutive years of CO2 growth of 2 ppm or more had not been seen in NOAA’s monitoring records prior to 2014. Atmospheric CO2 is now more than 50% higher than pre-industrial levels.
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 Nina to El Nino.
Atmospheric methane, less abundant than CO2 but more potent at trapping heat in the atmosphere, rose to an average of 1922.6 parts per billion (ppb). The 2023 methane increase over 2022 was 10.9 ppb, lower than the record growth rates seen in 2020 (15.2 ppb), 2021(18 ppb) and 2022 (13.2 ppb), but still the 5th highest since renewed methane growth started in 2007. Methane levels in the atmosphere are now more than 160% higher than their pre-industrial level.
In 2023, levels of nitrous oxide, the third-most significant human-caused greenhouse gas, climbed by 1 ppb to 336.7 ppb. The two years of highest growth since 2000 occurred in 2020 (1.3 ppb) and 2021 (1.3 ppb). Increases in atmospheric nitrous oxide during recent decades are mainly from use of nitrogen fertilizer and manure from the expansion and intensification of agriculture. Nitrous oxide concentrations are 25% higher than the pre-industrial level of 270 ppb.
By far the most important contributor to climate change is CO2 , which is primarily emitted by burning of fossil fuels. Human-caused CO2 pollution increased from 10.9 billion tons per year in the 1960s – which is when the measurements at the Mauna Loa Observatory in Hawaii began – to about 36.6 billion tons per year in 2023, setting a new record.
The amount of CO2 in the atmosphere today is comparable to where it was around 4.3 million years ago during the mid-Pliocene epoch, when sea level was about 75 feet higher than today, the average temperature was 7 degrees Fahrenheit higher than in pre-industrial times, and large forests occupied areas of the Arctic that are now tundra.
About half of the CO2 emissions from fossil fuels to date have been absorbed at the Earth’s surface, divided roughly equally between oceans and land ecosystems, including grasslands and forests. The CO2 absorbed by the world’s oceans contributes to ocean acidification, which is causing a fundamental change in the chemistry of the ocean, with impacts to marine life and the people who depend on them. The oceans have also absorbed an estimated 90% of the excess heat trapped in the atmosphere by greenhouse gases.
(Adapted from materials provided by NOAA, April 6, 2024)