IPCC report on the physical science basis of climate change: the role of methane and the urgency of cutting emissions

On 9 August 2021 the Intergovernmental Panel on Climate Change (IPCC) published the Working Group I contribution to its Sixth Assessment Report (AR6), entitled "Climate Change 2021: The Physical Science Basis". The report states that the human influence on the warming of the atmosphere, oceans and land is unequivocal, and that the average surface temperature of the Earth in 2011–2020 was about 1.09°C higher than in 1850–1900. For our compendium the methane thread is particularly important.

Key findings

• Concentrations of carbon dioxide are the highest in at least two million years, and those of methane and nitrous oxide in at least 800 thousand years.
• Warming of 1.5°C will be exceeded during the 21st century unless deep emission reductions follow; in most scenarios this level will be reached within the next two decades.
• Methane is responsible for a substantial share of the warming to date – according to the United Nations Environment Programme (UNEP) and the Climate and Clean Air Coalition (CCAC), about 30% of warming since the pre-industrial era; absolute estimates put it at roughly 0.3–0.5°C.
• Strong, rapid and sustained cuts in methane emissions would limit warming and also improve air quality, because methane contributes to the formation of ground-level ozone.

At the same time, the report indicates that limiting methane will not replace CO₂ reductions: to stabilise the temperature, net-zero CO₂ emissions must be reached.

Implications for biogas, agriculture and waste

The main sources of anthropogenic methane are agriculture (livestock farming, manure, rice cultivation), fossil fuels and waste (landfills, wastewater). Reducing them is therefore directly linked to the biogas economy: digesting manure and slurry reduces emissions from storage, capturing landfill gas prevents methane from escaping into the atmosphere, and waste-processing biogas plants treat biowaste that would otherwise decompose in landfills.

It is worth remembering, however, that the climate benefit of a biogas plant depends on the tightness of the installation. Leaks and fugitive emissions from digestate stores can reduce the effect, which is why monitoring and leak control are becoming more important.

Political significance

The report appeared less than three months before the COP26 climate conference in Glasgow (opening on 31 October 2021) and less than a month after the European Commission presented the "Fit for 55" package (14 July 2021). It was an argument for countries demanding faster and deeper reductions. For the European Union it strengthened the case for the planned methane rules in the energy sector, which the Commission had announced in its strategy of October 2020.

What next

Further IPCC working group contributions, on impacts and on climate change mitigation, were due in 2022. For the biogas and waste sector, it will be important how the reports assess the emission-reduction potential of agriculture and waste management and the role of bioenergy in climate-neutrality scenarios.

How to read the report

The Working Group I report concerns climate science, not policy: it describes what is known about the changes, their causes and possible scenarios. Conclusions on mitigation action are contained in later parts of the assessment. For a reader from the biogas industry, the most important distinction is this: methane has a shorter lifetime than CO₂, so reducing it gives a quick effect, but it does not release us from the need to cut carbon dioxide emissions.