Greenhouse gas emissions could be significantly greater than previously thought if the impact of methane is counted over the next 20 years, data from the Global Registry for Fossil Fuels shows. Global Registry projections show the oil, gas and coal industries triggered emissions equivalent to 38.6 billion tons of carbon dioxide (“CO2e”) in 2020 if methane emissions are counted for their impact over the next century. But if that period is shortened to only 20 years, CO2e from fossil fuels jumps to 45.2 gigatons.

The resulting increase would take total anthropogenic emissions from about 52 gigatons to nearly 59 gigatons per year, an increase of 13%, and mean that carbon budgets designed to limit global heating will be used even faster than anticipated. The Global Registry of Fossil Fuels is the first open source database of the world’s fossil fuel assets and their associated emissions, and is designed as a tool to manage carbon budgets. The 2020 projections are based on production data from the US government’s Energy Information Administration, combined with carbon and methane intensity datasets from the International Energy Agency (IEA).

The Intergovernmental Panel on Climate Change maintains “conversion rates” for all greenhouse gases into an equivalence in carbon dioxide, the biggest greenhouse gas, so that all gases can be evaluated together in one metric – carbon dioxide equivalence (CO2e).

For methane, the IPCC has maintained two conversion rates depending on the period of time for which global heating is to be considered. Over 100 years, a ton of methane is rated the same as nearly 30 tons of carbon dioxide. But over the shorter period of 20 years methane is considered nearly 85 times as potent. The rates are different because methane is a much more powerful warming agent than carbon dioxide but, unlike carbon dioxide, breaks down in the atmosphere over time, becoming gradually less potent.

Other greenhouse gases have still higher potencies but are released in much smaller quantities. Methane is the second most impactful greenhouse gas, after carbon dioxide.

Considering methane over the shorter period significantly increases the footprint of fossil fuel industries in the major producing countries. Under the IEA’s Business as Usual scenario, for instance, China’s fossil fuel industries will trigger 8.9 gigatons of CO2e in 2030 using the 100-year factor, (known as Global Warming Potential 100 or GWP100), but this rises to 10.3 gigatons CO2e if the 20-year factor (GWP20) is deployed instead. The United States would produce 6.4 gigatons in 2030 under the same scenario using the 100-year factor, but 7.7 gigatons if the 20-year factor is used.

There is currently no clear consensus among experts about which metric is best. GWP100 has been used by most countries reporting their emissions to the UN Framework Convention on Climate Change. But use of GWP20 has been increasing as global heating is observed to develop faster than models originally predicted, and as the potential for natural feedback loops to kick in over the next couple of decades becomes more integrated into thinking about climate change. The International Methane Observatory, established by the United Nations Environmental Program in 2021 as the foremost methane measuring body, has settled on GWP20 as its default metric.

Emissions from burning fossil fuels, which form the most significant component of total emissions, stay the same under either factor because emissions from burning fossil fuels primarily consist of CO2. But a significant amount of emissions that occur along the supply chain are from fugitive methane emissions. Globally, the fossil fuel supply chain CO2e in 2020 was 4.8 gigatons of CO2e if considered under GWP100, but 11.4 gigatons under GWP20.