It’s time to look at the other greenhouse gases – methane and nitrous oxide

When it comes to climate change, carbon dioxide is by far the greenhouse gas that has caused the most global warming. However, scientists are beginning to give more attention to methane and nitrous oxide – two other greenhouse gases that, if addressed, could play a significant role in keeping the world within the Paris Agreement goal of less than 1.5 degrees Celsius of warming.

Carbon dioxide accounts for a whopping 76 percent of all greenhouse gas emissions, while methane and nitrous oxide rank second and third, at 16 and 6 percent respectively, according to the Center for Climate and Energy Solutions. Yet, with different heating potentials and lifespans in the atmosphere, these gases punch above their weight in terms of the warming they cause.

Methane has received fresh focus after more than 100 countries signed the Global Methane Pledge, a new agreement to reduce methane emissions by 30 percent by 2030 during the COP26 summit in Glasgow earlier this month, and an ‘observatory’ was launched that tracks its existence in real time. Nitrous oxide, meanwhile, has been the subject of a recent bout of research as well as agricultural policy.

“The strong global support for the [Global Methane] Pledge illustrates growing momentum to swiftly reduce methane emissions – widely regarded as the single most effective strategy to reduce global warming,” reads a press release issued by the European Commission about the launch of the methane agreement.

“Achieving the 2030 goal would prevent over 200,000 premature deaths, hundreds of thousands of asthma-related emergency room visits, and over 20 million tons of crop losses a year by 2030.”

The world has already warmed 1.1 degrees Celsius since the Industrial Revolution and is on track to warm 1.3 degrees more this century, which would lead to increasingly more climate disasters and weather events. As scientists race to find ways to avoid this outcome, the importance of reducing human-caused methane and nitrous oxide emissions is clearer than ever.

Methane

The chemical composition of methane is CH₄, and it is the main ingredient of natural gas. Although it only remains in the atmosphere for about a decade, it has 28 to 36 times the warming potential of carbon dioxide over 100 years. According to the Intergovernmental Panel on Climate Change (IPCC), the concentration of atmospheric methane has grown by 150 percent and might have already contributed to 0.5 degrees Celsius of global warming in the past decade. 

Currently, the greatest sources of human-caused methane emissions are agriculture and the energy sector, particularly fossil fuels. Lowering emissions from the latter is considered a low-hanging fruit, as it can be done cheaply, and technological fixes already exist.

The International Energy Agency estimates that it is technically possible to eliminate three-quarters of methane emissions from the oil and gas industry, with almost half of this avoidable at no overall cost, given that methane can be captured and resold for uses such as for cooking or home heating. Policies that could limit methane emissions from this sector include requiring the repair of detected gas leaks from equipment and limiting the flaring and venting of gas for non-emergency reasons.

Such policies have already been enacted by many local and national governments, such as Canada’s 2018 rules to promote the capturing of methane and avoid venting, or the U.S. Environmental Protection Agency’s proposed rules to curb possibly 41 million tons of methane emissions from the fossil fuel sector by 2035. The E.U. has also formed a strategy to reduce these emissions across the energy, agriculture and waste sectors. In agriculture, raising livestock contributes to nearly a third and rice cultivation around 8 percent of all human-caused methane emissions.

Several technological innovations could lead to less methane produced by livestock, such as by adding synthetic methane inhibitors or seaweed into the animals’ feed, breeding animals that produce less methane or developing a methane vaccine. However, these solutions are still in development. 

Reducing the global demand for animal products could also lead to less methane emitted, although the lack of affordable access to non-animal products could hamper this approach, as well as the importance of livestock for many small-scale food producers in the developing world. 

Aside from the Global Methane Pledge, another promising international development is the launch last month of the International Methane Emissions Observatory by the U.N. Environment Programme and E.U. The initiative will provide a public record of global methane emissions in real time – including their quantity and location – by using company data, satellites and other data streams.

Nitrous oxide

Nitrous oxide, with the chemical composition of N₂O, has an atmosphere-warming potential 265 to 298 times greater than that of carbon dioxide over the course of a century. While its total emissions are a fraction of those of carbon dioxide and methane, it remains a powerful greenhouse gas that lingers in the atmosphere for more than 100 years on average.

According to research published in Nature last year, human-caused nitrous oxide emissions have increased by 30 percent over the past four decades to around 7.3 teragrams per year, with an uptake of nitrogen fertilizer added to crop fields accounting for much of the increase. Agricultural soil has also been found to emit enough nitrous oxide to potentially undo the climate change benefits of storing carbon within the same soil. 

Among the most effective ways to reduce nitrous oxide emissions from agriculture is by using nitrogen fertilizer with more precision and not over-applying it to fields. This can be seen from the E.U.’s Nitrate Directive, which calls for limiting inorganic nitrogen fertilizer and organic manure applications, among other measures. Although the directive was introduced in 1991 mainly to improve water quality, by 2008 several European countries had also witnessed large reductions in nitrous oxide emissions, which dropped by 20 percent in the Netherlands and 12 percent in the U.K.

Climate and soil factors also influence the amount of nitrous oxide that is emitted into the atmosphere, according to a study published last month in Nature Food. This could allow for a more targeted approach, according to the researchers, who found that nearly a third of global nitrous oxide emissions could be avoided without harming food supplies. Focusing these reductions on only one-fifth of the world’s agricultural areas – within countries such as China, the U.S. and India – could achieve two-thirds of this mitigation potential.

“Nitrous oxide emissions will be increasing in the future because of increasing food demand at the business-as-usual scenario [the climate scenario in which no significant changes are made by society to limit greenhouse gas emissions],” said Feng Zhou, an associate professor at Peking University and author of the Nature Food study, in an email. “Reducing agricultural nitrous oxide emissions is conducive not only to achieving low warming targets but also to preventing stratospheric ozone depletion.” 

According to Zhou, though reducing surplus nitrogen use through targeted application remains the most effective way to limit nitrous oxide emissions, this precision may be difficult on a very large scale without multi-national collaboration and investment in countries where this could happen best. Other options include using fertilizers with nitrification inhibitors and high-yield crop varieties that boost the efficiency of any added nitrogen fertilizers.

Taking global action to limit substances that endanger the atmosphere will not be easy, but it has been achieved before. Chlorofluorocarbons, commonly known as CFCs, were once widely used as refrigerants and in aerosol cans until they were found to be highly damaging to the ozone layer. In response, the world gathered in 1987 to create the Montreal Protocol, which calls for ozone-damaging chemicals like CFCs to be phased out of use.

The Montreal Protocol was ratified by every UN member state to save the ozone, and thus the world. Could a similar level of universal action be repeated once again for our current climate crisis?