Powerful greenhouse gases besides CO₂ must not be overlooked in efforts to limit global warming to 1.5°C. These include CH₄ and N₂O, emissions of which are both showing rapid growth, contributing to a pathway to warming well above 2°C.

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 global 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 0.03°C globally and up to 0.3°C 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 the projected anthropogenic methane emissions by more than 45% to 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.

Global warming is driven by human activities that produce both positive and negative climate forcing. Overall, about 21% of current net global warming is caused by factors other than CO₂. These include emissions of other greenhouse gases, their precursors or warming aerosols such as black carbon. Non-CO₂ factors driving global 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 CO₂. 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 CO₂ are reached.