How big a problem are methane and other non-CO2 greenhouse gases?

Carbon dioxide is not the only greenhouse gas under fire at the COP28 climate summit in Dubai, United Arab Emirates. On 2 December, the UAE, the US and China are convening other countries for a meeting focused on methane and other “non-CO2” greenhouse gases, with announcements expected from the fossil fuel industry on cutting these emissions.

In terms of contribution to climate change, methane is the most significant of these. But other gases, such as nitrous oxide and fluorinated gases made for refrigeration, are also of concern.

How much of a problem are non-CO2 greenhouse gases?

Together, they are responsible for around a third of human-caused warming so far, and the Paris agreement climate targets won’t be reached without slashing these emissions along with CO2, says Mathijs Harmsen at Utrecht University in the Netherlands. “It doesn’t get the attention in line with the climatic impact it has.”

But there is renewed focus at COP28. “We see COP28 as a turning point” for limiting the emissions of these short-lived non-CO2 greenhouse gases, says Martina Otto, who heads the Climate and Clean Air Coalition at the UN.

What are the main non-CO2 greenhouse gases?

Methane is the non-CO2 greenhouse gas that has had the greatest effect on climate change. It is followed by nitrous oxide and manufactured fluorinated gases, or “F-gases”, which are emitted in relatively small amounts but are far more potent than CO2. Sulphur hexafluoride, for instance, which is used as an electrical insulator, has a warming effect more than 23,000 times that of CO2, gram for gram.

Methane has both natural and human-caused sources – the majority of the latter come from fossil fuel production, due to leaks from wells, coal mines, pipelines and ships. Burping cows and other livestock make up the second largest share, followed by landfills, which release methane as food and other organic matter decomposes. Warming temperatures also increase methane emissions from natural sources like wetlands and thawing permafrost. These many sources have made it challenging for researchers to zero in on exactly what has been driving a steady rise in emissions since 2007.

Nitrous oxide also has natural sources, such as wetlands. But human-caused emissions of the gas have risen since the industrial revolution, mainly due to the increasing use of nitrogen fertilisers and livestock manure.

F-gases have been manufactured for refrigeration and other applications since the early 20th century. Chlorofluorocarbons (CFCs) were widely used as a refrigerant until countries agreed in the 1987 Montreal Protocol to phase out their use due to their degrading effect on the ozone. This phase-out is expected to avoid as much as 0.5°C of warming this century, although CFCs are still illegally being produced in some countries. The most common replacement for CFCs, hydrofluorocarbons (HFCs), do not degrade the ozone, but have a warming effect that is hundreds or thousands of times more potent than CO2. In 2016, countries agreed to amend the Montreal Protocol to begin phasing out HFC’s as well.

Which of these non-CO2 gases is the biggest concern?

Methane is responsible for nearly all of the warming caused by non-CO2 gases so far, or about a quarter of overall warming. “It’s really a methane story,” says Harmsen.

Methane sticks around in the atmosphere for only around 12 years, while CO2 can remain for millennia, but the gas is around 30 times more potent than CO2. While long-lasting CO2 emissions increasingly dominate over time, dealing with methane and other non-CO2 gases has a significant near-term effect. Harmsen and his colleagues recently found that keeping global warming within Paris agreement limits would be impossible without steep reductions in these non-CO2 gases.

“You will need to go quite deep with cutting non-CO2 to think about reaching the Paris agreement targets,” says Harmsen.

What are countries doing to reduce non-CO2 greenhouse gas emissions?

At the COP26 climate summit in Glasgow in 2021, the US and the European Union launched the Global Methane Pledge, where countries promised to cut the world’s methane emissions by 30 per cent by 2030. Since then, more than 150 countries have joined the pledge. Approaches to stemming the tide include plugging leaks or stopping releases from fossil fuel production and capturing emissions from landfills, among many others.

Nitrous oxide emissions could be reduced by using fertiliser more efficiently and by adopting more sustainable modes of agriculture, for instance. And thanks to the Montreal Protocol, the F-gases are already being replaced with less harmful alternatives.

What is happening at COP28 now around non-CO2 gases?

Methane is a major focus at COP28 in Dubai. “What we see here is an evolution and sharpening of what started in Glasgow,” says Matt Watson at the Environmental Defense Fund, an environmental advocacy organisation in the US. “The companies generating emissions are being asked to step to the line and make company-specific, meaningful targets.”

A joint statement by the US and China ahead of the meeting committing to include specific targets on methane and other non-CO2 greenhouse gases has also raised hopes for a strong outcome at COP28.

In his opening address to the summit, COP president Sultan Ahmed Al Jaber called on countries to adopt net-zero methane targets. He also hinted at pledges on reducing methane from oil and gas companies expected at the summit. Though he did not name specific firms, he says these will include pledges from the nationally owned oil and gas companies responsible for the majority of emissions. “Zeroing out methane emissions would make a huge impact in the shortest time frame,” he said.