Methane, an odourless gas, comes from several natural and anthropogenic sources. It is an important contributor to global warming because it traps heat in the atmosphere. It is also involved in the ground-level formation of ozone, which is an air pollutant and bad for human health.
Global methane emissions have risen nearly 10% over the past two decades, resulting in record-high atmospheric concentrations of the powerful greenhouse gas.
Global emissions of methane have reached the highest levels on record. Increases are being driven primarily by the growth of emissions from coal mining, oil and natural gas production, cattle and sheep ranching, and landfills.
Roughly one-third of global methane emissions come from bacteria in natural wetlands that produce the gas when decomposing organic material. Agriculture and fossil fuel sources each account for 20–25% of global methane emissions.
In 2017, the last year when complete global methane data are available, Earth’s atmosphere absorbed nearly 600 million tons of the colourless, odourless gas that is 28 times more powerful than carbon dioxide at trapping heat over 100 years.
More than half of all methane emissions now come from human activities. Annual methane emissions are up 9%, or 50 million tons per year, from the early 2000s when methane concentrations in the atmosphere were relatively stable.
Between 2000 and 2017, levels of the potent greenhouse gas barreled up toward pathways that climate models suggest will lead to 3-4 degrees Celsius of warming before the end of this century. This is a dangerous temperature threshold at which scientists warn that natural disasters, including wildfires, droughts and floods, and social disruptions such as famines and mass migrations become almost commonplace. The findings are outlined in two papers published July 14 in Earth System Science Data and Environmental Research Letters by researchers with the Global Carbon Project, an initiative led by Stanford University scientist Rob Jackson.
“We still haven’t turned the corner on methane,” said Jackson, a professor of Earth system science in Stanford’s School of Earth, Energy & Environmental Sciences (Stanford Earth).
Globally, fossil fuel sources and cows are twin engines powering methane’s upward climb. “Emissions from cattle and other ruminants are almost as large as those from the fossil fuel industry for methane,” Jackson said. “People joke about burping cows without realizing how big the source is.”
Throughout the study period, agriculture accounted for roughly two-thirds of all methane emissions related to human activities; fossil fuels contributed most of the remaining third. However, those two sources have contributed in roughly equal measure to the increases seen since the early 2000s.
Methane emissions from agriculture rose to 227 million tons of methane in 2017, up nearly 11% from the 2000-2006 average.
Amid the coronavirus pandemic, carbon emissions plummeted as manufacturing and transportation halted. “There’s no chance that methane emissions dropped as much as carbon dioxide emissions because of the virus,” Jackson said. “We’re still heating our homes and buildings, and agriculture keeps growing.”
The reports find that emissions have increased in most regions, and most markedly in Africa, the Middle East, China, South Asia and Oceania, which includes Australia and many Pacific islands.
Europe stands out as the only region where methane emissions have decreased over the last two decades. “Policies and better management have reduced emissions from landfills, manure, and other sources here in Europe. People are also eating less beef and more poultry and fish,” said Marielle Saunois of the Université de Versailles Saint-Quentin in France, lead author of the paper in Earth System Science Data.
Tropical and temperate regions have seen the biggest jump in methane emissions. Boreal and polar systems have played a lesser role. Despite fears that melting in the Arctic may unlock a burst of methane from thawing permafrost, the researchers found no evidence for increasing methane emissions in the Arctic—at least through 2017.
Human-driven emissions are in many ways easier to pin down than those from natural sources. “We have a surprisingly difficult time identifying where methane is emitted in the tropics and elsewhere because of daily to seasonal changes in how waterlogged soils are,” said Jackson, who also leads a group at Stanford working to map wetlands and waterlogged soils worldwide using satellites, flux towers, and other tools.
“We’ll need to eat less meat and reduce emissions associated with cattle and rice farming,” Jackson said, “and replace oil and natural gas in our cars and homes.”
Feed supplements such as algae may help to reduce methane burps from cows, and rice farming can transition away from permanent waterlogging that maximizes methane production in low-oxygen environments. Aircraft, drones, and satellites show promise for monitoring methane from oil and gas wells. Jackson said, “I’m optimistic that, in the next five years, we’ll make real progress in that area.”
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