- Rapid growth in emissions of methane and nitrous oxide — greenhouse gases that are far more powerful than carbon dioxide — are worsening the impact of rising levels of carbon dioxide, putting the world on track for 2.7oC of warming this century.
- Reducing methane emissions is the strongest lever available to slow climate change over the next 25 years: readily available, low-cost measures (see recommendations below) could halve methane emissions by 2030 and must go hand-in-hand with carbon dioxide mitigation and removal efforts to stabilize global temperature in the long term.
- Rapid reductions in aerosol emissions during the COVID-19 pandemic caused a slight warming of the planet, highlighting the fact that cooling aerosols emitted from fossil fuel combustion to date have partly masked warming from greenhouse gas emissions. While declines in aerosol emissions will improve air quality and benefit the health of billions, this will exacerbate climate warming in the short term.
Powerful greenhouse gases besides carbon dioxide must not be overlooked in efforts to limit global warming to 1.5oC. These include methane and nitrous oxide, emissions of which are both showing rapid growth, contributing to a pathway to warming well above 2oC.
Methane is the major component of natural gas and is responsible for about 20% of global warming since the pre-industrial era. Emissions of methane reached a record high in 2020, 6% above levels in the year 2000. Human-induced nitrous oxide emissions have grown by 30% over the past three decades. Emissions of both gases from the agricultural sector are the main cause of this large growth. Waste treatment in landfills and fugitive emissions from fossil fuel extraction are also major sources of methane.
Human emissions of aerosols – tiny particles of sulphur or nitrogen emitted during fossil fuel combustion – have an overall cooling effect on the climate. They have partly masked warming from greenhouse gas emissions to date. Emissions of aerosols are predicted to decline, the extent of which depends on pollution-control policies. This will improve air quality and benefit the health of billions of people worldwide, but will exacerbate climate warming in the short term. This was clearly illustrated during the COVID-19 pandemic: reduced emissions of cooling aerosols during national lockdowns led to a temperature rise of 0.03oC globally and up to 0.3oC at higher northern hemisphere latitudes in May 2020.
Overall, largely due to a growth in methane and nitrous oxide emissions alongside declines in aerosols, non-CO2 factors have increasingly warmed the climate over the past 20 years. Ongoing increases in non-CO2 greenhouse gases and declines in aerosols will reduce the remaining carbon budget.
The good news is that readily available and low-cost measures could reduce more than 45% of projected anthropogenic methane emissions by 2030. Due to the short lifetime of methane in the atmosphere, addressing the sources of methane emissions will have a rapid impact on climate change.
“Low-hanging fruit” options include reducing fossil fuel leaks and improving waste treatment technologies, which alone could avoid 0.3°C of warming by the 2040s. There are also solutions in the food and agricultural sector that could reduce nitrous oxide and methane emissions — a mix of supply and demand options — such as increasing the efficiency of nitrogen use, further improvement and uptake of feedstocks that reduce the methane emissions of ruminants, promotion of healthy low-meat diets and reduction of food waste. These solutions come with additional health and environmental benefits.
Climate warming is driven by human activities that produce both positive and negative climate forcing. Overall, about 21% of current net climate warming is caused by factors other than carbon dioxide. These include emissions of other greenhouse gases, their precursors or warming aerosols such as black carbon. Non-CO2 factors driving climate warming arise primarily from the burning of fossil fuels, and from land-use including agricultural activities (see image for a full breakdown). They have had an increasing warming effect over the past 20 years, largely due to growth in emissions of the powerful greenhouse gases methane and nitrous oxide. This is worsening the impact of rising levels of carbon dioxide. If anthropogenic emissions of methane and nitrous oxide continue to rise, it will reduce the remaining carbon budget, preventing stabilization of global temperature once net-zero emissions of carbon dioxide are reached.
Implications & Recommendations
At national and local levels, decision makers in government and the private sector are urged to:
- reduce fugitive methane emissions from the fossil fuel sector through regulation – for example certification of suppliers, and through investment in new technologies for leak detection and repair, including in production, transmission and distribution systems;
- reduce net emissions from landfill by promoting the separation of waste at source, recycling, incineration with energy recovery and anaerobic digestion with biogas recovery;
- curtail methane emissions from the food and agriculture sectors through a broad portfolio of policies to reduce food waste and improve land and livestock management – for example promoting the use of lower emission feedstock, improving water management in rice cultivation and encouraging healthy, low-meat diets;
- reduce nitrous oxide emissions in agriculture through practices that limit the use of nitrogen fertilizers and promote their efficient use – for example through improved timing of nitrogen application and tillage practice.