The World Meteorological Organization (WMO) marks the publication of its 20th Greenhouse Gas Bulletin with a sobering milestone - global atmospheric carbon dioxide (CO2) concentrations have increased 11.4% over the last 20 years. The inaugural 2004 Greenhouse Gas Bulletin reported a CO2 level of 377.1 parts per million (ppm). This figure is now 420.0 ppm.
The latest measurements from the WMO Global Atmosphere Watch (GAW) show record high levels of the Earth’s three major greenhouse gases – CO2, methane (CH4) and nitrous oxide (N2O) - in 2023. The Bulletin authors highlight worrying trends in methane concentrations over the past 5 years – a dominant emission increase from microbial sources signals an ongoing climate feedback, further increasing greenhouse gas emissions from the natural systems.
Fossil fuel emissions, fires and reduced net terrestrial carbon sinks drive CO2 concentrations
In 2023, globally averaged atmospheric concentrations of carbon dioxide reached a record high of 420.0±0.1 ppm. Atmospheric concentrations of carbon dioxide are now 151% higher relative to pre-industrial (before 1750) levels. The Bulletin notes that the 2023 increase of CO2 dioxide in the atmosphere was one of the largest recorded while modern measurements have been running. Increased CO2 emissions are mainly driven by historically large fossil fuel emissions in the 2010s and 2020s.
Fires also contributed to the increase in atmospheric CO2. Carbon monoxide (CO) is a strong indicator of fire emissions, as it results from incomplete combustion. 2023 saw high concentrations of CO registered away from industrial emissions and traffic, indicating enhanced CO2 emissions from fires.
2023 was an exceptionally hot year that saw net ecosystem carbon exchange estimated to be 28% lower in 2023 than in 2021-2022. A reduced uptake could be explained by heat stress potentially reducing carbon uptake by plants.
Methane and nitrous oxide emissions also reach historic heights
2023 saw globally averaged atmospheric concentrations of methane reach 1934±2 ppb (parts per billion). This represents an increase of 265% compared to pre-industrial levels.
Concentrations of atmospheric methane reached a new high in 2023, largely driven through anthropogenic emissions. While roughly 40% of methane emissions come from natural sources like wetlands, 60% of methane emissions are human-induced through processes like rice agriculture, biomass burning and landfills. The Bulletin authors note "a significant increase in CH4 emissions from natural wetlands in response to warmer temperatures and particularly wetter land conditions during the 2020–2022 La Niña."
Globally averaged atmospheric concentrations of nitrous oxide reached 336.9±0.1 ppb in 2023 – a 125% increase over pre-industrial levels. The third major greenhouse gas, nitrous oxide is emitted into the atmosphere from natural (approximately 57%) and anthropogenic sources (approximately 43%) from oceans, soils, biomass burning, fertilizer use, and various industrial processes. The growth in human-induced atmospheric N2O is chiefly driven by nitrogen additions to croplands.
A long history of high-quality observations
“The Bulletin is an important communication tool to make people realise that climate change is real and that greenhouse gas concentrations are continuing to increase at record pace,” says Alex Vermeulen, Director of ICOS Carbon Portal and the main editor of the WMO Greenhouse Gas Bulletin.
Continued record increases of greenhouse gas concentrations put the climate at real risk. “We are turning at the knobs of a climate system by triggering warming through long lived greenhouse gas emissions that can run out of control,” Alex outlines. “We may see tipping points that will accelerate global warming - the more we continue, the bigger this risk will become, and when this happens there is very likely no way back within the first few centuries or thousands of years.”
The WMO Greenhouse Gas Bulletin is one tool that has been informing society of the state of atmospheric greenhouse gas concentrations. “A long history of high-quality observations helps inform society about what is happening with these greenhouse gas levels,” Alex notes. “This is the basis of awareness of the problem and also should be the basis of future policy.”
Read the full WMO Greenhouse Gas Bulletin No. 20 on the WMO website here.