Atmospheric methane

• 1. Introduction

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  Atmospheric methane sits in Earth's air at concentrations not seen in at least 800,000 years. By 2019, global measurements showed 1,866 parts per billion of CH4 in the atmosphere. That number may sound clinical, but it represents a rise of roughly 160 percent since the mid-18th century, and the pace is still climbing. What makes this molecule so consequential? Why does a gas that accounts for only 3 percent of greenhouse gas emissions by mass drive roughly 23 percent of climate forcing? And how did scientists piece together a record stretching back nearly a million years from Antarctic ice? Those questions will carry us from ancient bacteria to Arctic permafrost to a volcano that briefly rewrote the atmosphere.

• 2. A Potent Greenhouse Force

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  Over a 20-year window, methane warms the climate 84 times more powerfully than carbon dioxide per molecule. That gap narrows over a longer horizon. By the 100-year mark, the warming advantage drops to about 28 times that of CO2, partly because methane breaks down faster than CO2 does. Scientists measure that human imprint on the climate using a unit called radiative forcing, expressed in watts per square meter. A 2007 estimate placed methane's direct radiative forcing at 0.5 W/m2 relative to 1750. Later work revised that figure upward. A 2016 paper in Geophysical Research Letters by M. Etminan and colleagues incorporated the shortwave absorption bands of CH4, which earlier IPCC methods had omitted. That revision pushed forcing estimates roughly 20-25 percent higher than what successive IPCC reports had cited. By 2016, methane accounted for approximately 0.62 W/m2 of radiative forcing, or about 20 percent of the total forcing from all long-lived, globally mixed greenhouse gases. William Collins led a separate team whose 2018 findings reinforced those upward revisions. The UNEP and the Climate and Clean Air Coalition cited both studies in their 2021 Global Methane Assessment, concluding that earlier estimates of methane's societal impact had likely been too low. Collins and his colleagues also noted that cutting atmospheric methane before the end of this century could expand the budget of allowable carbon emissions to 2100, making the Paris climate targets more feasible.

• 3. Sources Ancient and Modern

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  Methanogenesis, the anaerobic conversion of organic compounds into methane by microorganisms, is one of the two primary production pathways and is widespread in aquatic ecosystems. The other is ruminant animals. Both processes release methane near the Earth's surface, where all known major sources are concentrated. The Arctic adds a less predictable contribution: thawing permafrost releases methane that has been locked in frozen ground, and the underwater permafrost of regions like the Laptev Sea contains additional stores. Early in Earth's history, before oxygen became a significant part of the atmosphere, ancient bacteria converted hydrogen and carbon dioxide into methane and water. A 2003 article in the journal Geology described those first microbes as contributors to the methane concentration, and noted that without atmospheric oxygen, methane persisted far longer and at far higher levels than it does now. Carbon dioxide from volcanoes and methane from microbes together likely produced a greenhouse effect that allowed Earth's earliest life to appear. Scientists now use carbon isotope analysis to distinguish between modern sources. By measuring the ratio of carbon-12 to carbon-13 in atmospheric methane samples, researchers can tell microbial, thermal, and combustion sources apart. Several recent inversion models find that fossil fuels, agriculture, and waste sources such as landfills and manure management can each account for roughly half of recent atmospheric increases, though substantial uncertainties remain and the debate is ongoing.

• 4. How the Atmosphere Destroys It

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  Hydroxyl radicals, abbreviated OH, are often called the major chemical scavenger of the troposphere. They react with methane molecules in a process that accounts for roughly 90 percent of atmospheric methane removal, and that destruction happens primarily in the lowest layer of the atmosphere, between 4 and 12 miles above the surface. The oxidation reaction produces water vapor and carbon dioxide. Both of those products are themselves greenhouse gases, which complicates the net warming picture. The water vapor produced this way adds approximately 15 percent to methane's own radiative forcing effect. In the stratosphere, which extends from the top of the troposphere to about 31 miles up, methane that escapes higher can last around 120 years before hydroxyl oxidation destroys it there too. A second significant natural sink operates in soils. Methanotrophic bacteria consume methane as an energy source, reacting it with oxygen to yield carbon dioxide and water. Forest soils work especially well as sinks because moisture levels suit methanotroph activity and gas exchange between soil and air is efficient. Wetland soils, by contrast, often flip to becoming sources rather than sinks because a high water table lets methane diffuse upward before the bacteria can intercept it. Methanotrophic bacteria also live in underwater sediments, where they can limit emissions from sources like the permafrost beneath shallow Arctic seas. A complication arises from methane's effect on the OH radicals that remove it: higher methane concentrations consume more OH, which slightly lowers global mean OH levels and extends methane's atmospheric lifetime, originally estimated at around 9.6 years, creating a feedback loop between methane sources and the very sinks designed to neutralize them.

• 5. Reading the Ice Record

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  From 1996 to 2004, researchers in the European Project for Ice Coring in Antarctica drilled through the ice sheet and analyzed gas bubbles trapped inside, reconstructing greenhouse gas concentrations across the past 800,000 years. Their findings showed that before approximately 900,000 years ago, the cycle of ice ages followed by warm periods ran on a roughly 40,000-year rhythm. By 800,000 years ago that rhythm had stretched dramatically to 100,000-year cycles. Greenhouse gas concentrations were low during ice ages and high during the warm intervals. A 2016 EPA illustration compiled data from several Antarctic sites alongside records from Cape Grim, Australia; Mauna Loa, Hawaii; and the Shetland Islands, Scotland, covering a span from roughly 797,446 BCE through 2015 CE. That composite record is what allows scientists to place today's 1,866 ppb figure against the entire deep past and conclude it is the highest in at least 800,000 years. Geologically rapid methane releases have been linked to past warming events as well. The Paleocene-Eocene Thermal Maximum, roughly 55 million years ago, is one such episode. Scientists at NASA's Goddard Institute for Space Studies and Columbia University's Center for Climate Systems Research presented research at the 2001 meeting of the American Geophysical Union arguing that a vast pulse of methane previously held stable by cold temperatures and high pressure beneath the ocean floor was released into the atmosphere, driving rapid global warming. A 2009 article in the journal Science confirmed that methane's contribution to that ancient warming had previously been underestimated. A similar mechanism has been proposed for the Great Dying, another ancient extinction event.

• 6. Measuring a Moving Target

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  Direct measurements of CH4 in the environment date back to the 1970s. Gas chromatography was the standard technique for much of that early period. It could separate and identify chemical compounds reliably, but it was time-consuming, labor-intensive, and less sensitive than what came later. Spectroscopic methods gradually became the preferred approach for atmospheric gas work because of their sensitivity and precision, and because they are the only means of sensing atmospheric gases remotely. Infrared spectroscopy encompasses a range of techniques including differential optical absorption spectroscopy, laser-induced fluorescence, and Fourier-transform infrared. By 2011, cavity ring-down spectroscopy had become the most widely used infrared absorption technique for detecting methane. It is a form of laser absorption spectroscopy capable of determining mole fraction down to the order of parts per trillion. Long-term monitoring by NOAA showed that methane concentrations nearly tripled since pre-industrial times. In 1991 and again in 1998 the annual growth rate of methane roughly doubled relative to the preceding years. The eruption of Mount Pinatubo on the 15th of June 1991, a magnitude VEI-6 event and the second-largest terrestrial eruption of the 20th century, preceded one of those spikes. Scientists reported in 2007 that unusually warm temperatures in 1998, the warmest year on record at the time, may have triggered elevated methane emissions from wetlands, rice fields, and biomass burning. After a decade of near-zero growth, globally averaged atmospheric methane rose by approximately 7 nmol/mol per year during 2007 and 2008, and that increase continued into 2009. From 2015 to 2019 sharp further rises were recorded. The largest single annual increase occurred in 2021, when concentrations reached a record 260 percent of pre-industrial levels. Global tracking now involves more than fifty international research institutions and 100 monitoring stations, coordinated through the Global Carbon Project's Global Methane Budget.