Methane (CH4) is the second most important greenhouse gas contributing to global warming after carbon dioxide (CO2). Its concentration in the atmosphere is increasing and understanding Understanding its global budget – where methane is released (sources) and where it is removed from the atmosphere (sinks) – is crucial for predicting future climate developments and designing effective mitigation strategies. The resources provided present a comprehensive assessment of the global methane budget for the period 2000 to 2020.

Methane fluxes are usually expressed in teragrams of CH4 per year (Tg CH4 yr−1), with 1 Tg CH4 yr−1 corresponding to 10^12 grams of CH4 per year. Atmospheric concentrations are measured as volume fractions in dry air, in parts per billion (ppb).

Methane sources

Methane sources are divided into natural/indirect anthropogenic and direct anthropogenic. The main source categories according to bottom-up (BU) and top-down (TD) estimates for the decade 2010–2019 include:

• Combined wetlands and inland freshwaters: This is the largest category of natural/indirect anthropogenic sources. Estimates differ between the BU and TD approaches, with the BU estimates being 248 [159–369] Tg CH4 yr−1 and the TD estimates being 165 [145–214] Tg CH4 yr−1 for the decade 2010–2019. Wetlands themselves are the main contributor in this category.
• Agriculture and waste: This is the most significant category of direct anthropogenic sources. For the decade 2010–2019, it is estimated at 218 [203–241] Tg CH4 yr−1 (BU) and 190 [177–204] Tg CH4 yr−1 (TD) [Table 3]. Within this category, emissions from enteric fermentation and manure, rice cultivation and landfills are significant.
• Fossil fuels: Emissions associated with the extraction, processing and distribution of fossil fuels (oil, gas, coal) constitute another large anthropogenic category. Estimates for the decade 2010–2019 are 118 [113–125] Tg CH4 yr−1 (BU) and 124 [118–131] Tg CH4 yr−1 (TD).
• Combustion of biomass and biofuels: This category contributes to total emissions of 28 [21–39] Tg CH4 yr−1 (BU) and 26 [21–31] Tg CH4 yr−1 (TD) in the decade 2010–2019. Biofuels account for 30–50 % of this amount.

Other natural resources include geological resources, wild animals, termites, and fires.

Methane absorbers

Methane is removed from the atmosphere mainly by chemical reactions (oxidation). The main sink is the reaction with the hydroxyl radical (OH) in the troposphere. For the decade 2010–2019, the loss of CH4 by OH oxidation in the troposphere is estimated at 563 [462–663] Tg CH4 yr−1 (BU) and 514 [508–521] Tg CH4 yr−1 (TD) [20, Table 3].

Another sink is stratospheric loss, mainly through reactions with O(1D), Cl, F and OH. This sink is smaller, estimated at 39 Tg CH4 yr−1 (BU) and 35 Tg CH4 yr−1 (TD) for the decade 2010–2019 [21, Table 3]. Soils also act as a methane sink, although with significant uncertainty between approaches (31 [20–43] Tg CH4 yr−1 BU vs. 4 [2–7] Tg CH4 yr−1 TD for 2010–2019) [Table 3].

Total sinks (sum of tropospheric loss, stratospheric loss and soil removal) are estimated to be 633 [507–796] Tg CH4 yr−1 (BU) and 554 [550–567] Tg CH4 yr−1 (TD) for the decade 2010–2019. It is noteworthy that the TD estimate of total sinks is essentially equal to the TD estimate of total sources for the period.

Atmospheric growth and budget imbalance

The difference between total emissions (sources) and total removals (sinks) determines the atmospheric growth of methane. This “imbalance” in the budget should match the observed atmospheric growth. The observed atmospheric growth rate for the decade 2010–2019 averaged 20.9 [20.1–21.7] ppb yr−1, which, after conversion, corresponds to 57.5 [55.3–59.6] Tg CH4 yr−1. The imbalance derived from the TD budget is 21 [19–33] Tg CH4 yr−1 for the same period. This large discrepancy suggests either an underestimation of sources or an overestimation of sinks in TD models [Section 5.2.2 not available, but implied by the data].

A significantly higher atmospheric methane increase was recorded in 2020 (41.8 [40.7–42.9] ppb yr−1, which is 115 [112–118] Tg CH4 yr−1). The TD imbalance for 2020 was 32 [15–38] Tg CH4 yr−1.

Regional distribution and climate change

Methane emissions are dominated by the tropics (90°S–30°N), which contribute 64 % to total emissions, although it represents only 53 % of global land area. In this region, agriculture and waste are the largest sources, almost as large as the combined emissions from wetlands and inland freshwaters. In the northern mid-latitudes (30–60°N), anthropogenic emissions dominate, mainly from agriculture and waste, closely followed by fossil fuels. The boreal regions (60–90°N) are dominated by emissions from inland freshwaters.

Climate change has a significant impact on the methane budget. Rising temperatures can affect emissions from natural sources, particularly wetlands and permafrost [Section 7]. Uncertainties in the methane budget, particularly in natural fluxes and their response to changing climate conditions, remain a critical area of research. Changes in the major sink, the hydroxyl radical (OH), also affect the lifetime of methane in the atmosphere and are being studied in the context of a changing atmosphere.

Conclusion

The global methane budget is a complex system with many interacting sources and sinks. While the major anthropogenic sources such as agriculture/waste and fossil fuels are better understood, natural sources, especially wetlands, and sinks still present significant uncertainties. The recent increase in atmospheric concentrations and the persistent imbalance in the budget highlight the urgency of improving monitoring and modeling to more accurately determine changes in sources and sinks and their links to climate change. Reducing anthropogenic methane emissions is essential for mitigating near-term climate warming. Spring

The study is published in the journal Earth System Science Data .

Glossary of key terms

• Tg CH4 yr-¹: Teragrams of methane per year. A unit used to express the amount of methane released or removed from the atmosphere over the course of one year (1 Tg = 10¹² grams).
• Bottom-up: Methods for estimating emissions or removals that rely on detailed information about individual sources or removals, such as process-based models, inventories or direct field measurements, and are then aggregated to a higher level (e.g. regional, global).
• Top-down: Emissions or removals estimation methods that rely on atmospheric methane concentration measurements and atmospheric transport and chemistry models to back-calculate surface emissions or removals.
• Chemical sinks: Processes in which methane is removed from the atmosphere by chemical reactions, primarily by reaction with the hydroxyl radical (OH) in the troposphere, but also in the stratosphere and with other oxidants such as tropospheric chlorine (Cl).
• Soil uptake: The process by which methane is removed from the atmosphere by microbial oxidation in aerobic soils.
• Wetlands: Ecosystems with flooded or saturated soils or peatlands where anaerobic conditions lead to CH4 production. Includes peatlands, mineral soil wetlands and seasonal/permanent floodplains.
• Inland freshwater ecosystems: Freshwater systems such as lakes, ponds, reservoirs, streams and rivers, which are sources of CH4 emissions.
• Double counting: A situation where emissions from a single source or process are counted multiple times when aggregating different emission categories, for example by overlapping estimates from wetlands and inland waters.
• Hydrates: Ice-like crystals formed by methane and water under specific temperature and pressure conditions, especially in marine sediments or permafrost, which may contain large reserves of methane.
• Permafrost (Permafrost): Permafrost is frozen soil, sediment, or rock that has maintained a temperature of 0°C or below for at least two consecutive years. Melting permafrost can release methane into the atmosphere.
• Interquartile range (IQR): The difference between the third quartile (75th percentile) and the first quartile (25th percentile) of a data set, which measures the dispersion of the middle 50 % data. It is used to identify outliers.
• Outliers: Values in a data set that lie significantly outside the normal range of other values. In this study, they are defined as values below the first quartile minus 3 times the IQR or above the third quartile plus 3 times the IQR.