Carbon dioxide is accumulating in the atmosphere faster than at any time experienced during human history, rising by more than 11 per cent in just two decades, according to World Meteorological Organization’s (WMO) annual Greenhouse Gas Bulletin.
Now in its 20th issue, this Bulletin is one of WMO’s flagship publications released to inform the UN Climate Change Conference (COP). In 2004, network of monitoring stations of WMO’s Global Atmosphere Watch recorded carbon dioxide level of 377.1 ppm (parts per million) in the atmosphere. The level has increased by 11.4 per cent to 420.0 ppm in 2023. The 2023’s figure is 151 per cent of the pre-industrial (before 1750) levels. This was calculated on the basis of the long-term observations within the Global Atmosphere Watch network of monitoring stations.
Carbon dioxide is the single most important greenhouse gas in the atmosphere related to human activities, accounting for approximately 64 per cent of the warming effect on the climate, mainly because of fossil fuel combustion and cement production, the Bulletin said.
The long-term carbon dioxide increase is due to fossil fuel combustion, but there are year-to-year variations due to the El Nino–Southern Oscillation, which impacts photosynthetic carbon dioxide uptake, respiratory release, and fires. In May 2023, Earth transitioned from a three-year long La Nina to El Nino.
The Bulletin said that carbon dioxide emissions from large vegetation fires along with a possible reduction in carbon absorption by forests and high fossil fuel carbon dioxide emissions from human and industrial activities contributed to the increase in 2023.
In 2023, global fire carbon emissions were 16 per cent above average, seventh amongst all fire seasons since 2003. Canada experienced its worst wildfire season on record. Australia experienced its driest three-month period on record in 2023 from August to October, with severe bushfires.
The 2023 increase of carbon dioxide in the atmosphere was higher than that of 2022, although lower than that of the three years before that. The annual increase of 2.3 ppm in 2023 marked the 12th consecutive year with an increase greater than 2 ppm.
The Greenhouse Gas Bulletin reports on greenhouse gas concentrations rather than on emission levels. Referring to the data analysis, the Bulletin said just under half of carbon dioxide emissions remain in the atmosphere. Just over one quarter are absorbed by the ocean and just under 30 per cent by land ecosystems – although there is considerable year-to-year variability in this because of naturally occurring phenomena like El Nino and La Nina.
WMO Secretary-General, Celeste Saulo, said: “Another year. Another record. This should set alarm bells ringing among decision makers. We are clearly off track to meet the Paris Agreement goal of limiting global warming to well below 2°C and aiming for 1.5°C above pre-industrial levels. These are more than just statistics. Every part per million and every fraction of a degree temperature increase has a real impact on our lives and our planet.”
Methane
The globally-averaged surface concentration of methane reached 1934 parts per billion (ppb), and nitrous oxide 336.9 parts per billion (ppb) in 2023. These values are 265 per cent, and 125 per cent of pre-industrial levels.
Methane is a powerful greenhouse gas which remains in the atmosphere for about a decade. Methane accounts for about 16 per cent of the warming effect of long-lived greenhouse gases.
Approximately 40 per cent of methane is emitted into the atmosphere by natural sources such as wetlands and termites, and about 60 per cent comes from anthropogenic sources such as ruminants, rice agriculture, fossil fuel exploitation, landfills and biomass burning, the Bulletin said.
An emission increase from sources such as wetlands and agriculture could, at least partly, be due to on-going climate feedback that further increases greenhouse gas emissions from the natural systems.
The growth in atmospheric methane in 2023 was smaller than 2022 – but was record high for the five-year period.
Nitrous oxide
Nitrous oxide, an ozone depleting chemical, accounts for about 6 per cent of the radiative forcing – the warming effect on the climate -- by long-lived greenhouse gases.
Nitrous oxide is emitted into the atmosphere from both natural sources (60 per cent) and anthropogenic sources (40 per cent), including oceans, soils, biomass burning, fertilizer use, and various industrial processes.
Climate change and greenhouse gases
During El Nino years, greenhouse gas levels tend to rise because drier vegetation and forest fires reduce the efficiency of land carbon sinks.
“The Bulletin warns that we face a potential vicious cycle. Natural climate variability plays a big role in carbon cycle. But in the near future, climate change itself could cause ecosystems to become larger sources of greenhouse gases. Wildfires could release more carbon emissions into the atmosphere, whilst the warmer ocean might absorb less carbon dioxide. Consequently, more carbon dioxide could stay in the atmosphere to accelerate global warming. These climate feedbacks are critical concerns to human society,” said WMO Deputy Secretary-General, Ko Barrett.
As long as emissions continue, greenhouse gases will continue accumulating in the atmosphere leading to global temperature rise. Given the extremely long life of carbon dioxide in the atmosphere, the temperature level already observed will persist for several decades even if emissions are rapidly reduced to net zero.
The last time the Earth experienced a comparable concentration of carbon dioxide was 3-5 million years ago, when the temperature was 2-3°C warmer and sea level was 10-20 meters higher than now, it said.