The global methane budget is a comprehensive accounting system scientists use to track methane entering and leaving the atmosphere. This ledger records all emissions (sources) and all removal processes (sinks). Because methane is a powerful greenhouse gas, this budget is a foundational tool for understanding and addressing climate change. Its atmospheric concentration has surged by over 150% since the pre-industrial era, making the study of its balance a focus of climate science.
Methane Sources
Methane originates from both human-caused and natural sources. Human activities, or anthropogenic sources, are responsible for at least two-thirds of all methane emissions globally. These emissions have risen by approximately 20% over the last two decades, with an acceleration in the last five years, making them the primary driver of the recent surge in atmospheric methane.
The largest anthropogenic contributor is agriculture, accounting for roughly 40% of human-caused emissions. This includes methane from the digestive processes of livestock and from the anaerobic decomposition of organic material in flooded rice paddies. The fossil fuel industry is another major emitter, responsible for about 34% of the total, through leaks during the extraction and processing of natural gas, oil, and coal. The decomposition of organic matter in landfills and wastewater facilities contributes around 19%.
Natural processes also release substantial quantities of methane. Wetlands are the most significant natural source, functioning like large compost systems where microbes break down organic matter without oxygen, producing methane as a byproduct. Other natural emitters include termites, geological seeps from the Earth’s crust, and releases from oceans. Additional sources like freshwater bodies are an area of active research due to uncertainty in quantifying their contribution.
Methane Sinks
The primary mechanism for methane removal is its destruction by chemical reactions in the atmosphere. Hydroxyl radicals (OH), often described as atmospheric “detergents,” are responsible for breaking down most methane. This process is the largest sink in the global methane budget, transforming methane into carbon dioxide and water vapor.
A smaller sink for atmospheric methane is found in soils, where certain bacteria known as methanotrophs consume it as an energy source. These microbes are most effective in well-aerated soils where they can access and oxidize methane from the atmosphere. While the soil sink is much smaller than chemical destruction by hydroxyl radicals, it is a natural removal pathway.
The total annual removal of methane by these sinks is estimated to be between 554 and 633 teragrams (Tg) per year. However, the efficiency of these sinks is not keeping pace with the current rate of emissions. This leads to an overall increase in its atmospheric concentration, which creates the budget imbalance.
The Budget Imbalance
The global methane budget is in a state of imbalance because the amount of methane emitted exceeds the amount removed by natural sinks. This disparity creates a surplus that steadily accumulates in the atmosphere. The situation can be likened to a bathtub where the faucet is flowing more rapidly than the drain can remove the water. This surplus is a direct consequence of human activities, causing atmospheric methane to reach levels 2.6 times higher than before the industrial revolution.
The imbalance in the methane budget directly impacts the Earth’s energy balance. As a potent greenhouse gas, the increasing concentration of methane traps more heat in the atmosphere, contributing to global warming. Current emission trends highlight a substantial gap between international goals, such as the Global Methane Pledge, and the reality of rising emissions.
Tracking Methane Emissions
Scientists use two primary approaches to quantify the sources and sinks in the global methane budget: top-down and bottom-up methods. These methods provide independent estimates that can be cross-checked to refine our understanding of methane dynamics. This creates a more accurate picture of where methane originates and where it goes.
The top-down approach involves measuring methane concentrations from a distance using instruments on satellites, aircraft, and ground-based stations. Scientists then use atmospheric models to work backward from these concentrations to infer the location and magnitude of emission sources. This method provides a broad, regional-to-global scale view of the methane budget.
Conversely, the bottom-up method calculates emissions by compiling inventories of all known methane-producing activities. Researchers estimate emissions by counting livestock, measuring rice cultivation area, or quantifying leaks from fossil fuel infrastructure. Summing the emissions from all individual sources builds a global total that complements the top-down perspective.