Record-breaking levels of greenhouse gases were recorded in the Earth's atmosphere in 2023 – reaching concentrations not seen for millions of years, the World Meteorological Organisation (WMO) warned on Monday.
In a report released ahead of the Cop29 climate summit in Azerbaijan, the WMO said the peak in greenhouse gas levels already locks in future temperature rises, even if emissions are cut to net zero.
Carbon dioxide climbed to 420 parts per million (ppm), a concentration that has not existed for three to five million years. During that era, temperatures were 3C warmer and sea levels up to 20 metres higher than today.
"Another year. Another record. This should set alarm bells ringing among decision makers," said WMO secretary-general Celeste Saulo. "We are clearly off track to meet the Paris Agreement goal of limiting global warming to well below 2C."
Last year's rise of 2.3ppm in atmospheric CO2 marked the 12th consecutive year with an increase above 2ppm – bringing overall levels to 151 percent higher than pre-industrial times.
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Wildfires and El Nino
Natural phenomena like El Nino, combined with large vegetation fires and persistent fossil fuel emissions, helped drive up CO2 levels, the WMO report found.
The effectiveness of carbon sinks such as forests and oceans has been reduced, creating what scientists call a "vicious cycle".
"Climate change itself could cause ecosystems to become larger sources of greenhouse gases,” said WMO deputy secretary-general Ko Barrett.
“Wildfires could release more carbon emissions into the atmosphere, whilst the warmer ocean might absorb less CO2. Consequently, more CO2 could stay in the atmosphere to accelerate global warming."
Global fire carbon emissions were 16 percent above average in 2023, including record-breaking wildfire seasons in Canada. Australia experienced its driest three-month period on record from August to October, with severe bushfires.
The report showed methane levels reached 1,934 parts per billion – 265 percent above pre-industrial levels. Nitrous oxide hit 336.9 parts per billion, up 125 percent.
Analysis reveals that just under half of CO2 emissions stay in the atmosphere, with about one quarter absorbed by oceans and just under 30 percent by land ecosystems.
However, this varies yearly due to El Nino, which reduces carbon uptake by plants, and La Nina, which can increase it.
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Long-term warming impact
Greenhouse gases are warming the Earth 51.5 percent more than they did in 1990, with CO2 driving four-fifths of that increase, according to the US National Oceanic and Atmospheric Administration.
Scientists emphasised that CO2's extremely long lifetime means temperature increases will persist for decades, even with rapid emissions cuts.
Carbon dioxide accounts for approximately 64 percent of climate warming, mainly from fossil fuel burning and cement production.
With the concentration of CO2 at its highest in millions of years, the WMO emphasised the urgent need for global action.
“Every part per million and every fraction of a degree temperature increase has a real impact on our lives and our planet,” Saulo said.
The WMO's findings serve as a key publication ahead of next month’s discussions at Cop29 in the Azeri capital Baku.
