Earth's climate crisis is starting to look even worse than scientists had feared — in part because of just how much meat we eat and how we get around.
Global emissions of methane, a potent greenhouse gas, have soared over the past decade, according to two new studies that tracked growing sources of the odorless, colorless gas. The increased methane, combined with carbon dioxide and other greenhouse gases, could warm Earth's atmosphere by 3 to 4 degrees Celsius before the end of this century — significantly above the levels that scientists have warned could be catastrophic for millions of people around the world.
"This completely overshoots our budget to stay below 1.5 to 2 degrees of warming," said Benjamin Poulter, a research scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. Poulter is an author on both studies published Tuesday, one in the journal Earth System Science Data and the other in the journal Environmental Research Letters.
Poulter and his colleagues found that since 2000, the biggest increases in methane emissions came from agricultural activities — particularly from livestock, such as cattle and sheep — and the fossil fuel industry, which includes coal mining as well as oil and gas production.
Human activities account for about 60 percent of global methane emissions, according to the researchers. Agriculture makes up roughly two-thirds of that, with fossil fuel production and use contributing most of the rest.
In the new studies, researchers analyzed methane emissions from 2000 through 2017 — the latest year for which complete global methane figures are available — and found that a record 600 million tons of methane were released into the atmosphere in 2017. Annual emissions of methane have also increased by 9 percent since the early 2000s, a pace that could contribute to more than 2 degrees Celsius of global warming by 2100.
A report released by the Intergovernmental Panel on Climate Change in October 2018 highlighted that the planet has already warmed by 1 degree Celsius since the 19th century; it used 1.5 degrees of warming above pre-industrial levels as a threshold beyond which the effects of climate change, including extreme heat and sea-level rise, become life-threatening for tens of millions of people around the world.
Another author on both studies, Rob Jackson, a professor of Earth system science at Stanford University, said the amount of methane released into the atmosphere since 2000 is roughly equivalent to adding 350 million more cars on the road.
In 2017, methane emissions from agriculture rose by nearly 11 percent from the 2000-06 average, while methane from fossil fuels jumped by nearly 15 percent compared to the early 2000s.
Methane is released into the atmosphere when coal, oil and natural gas are mined and transported, but microbes also emit it in low-oxygen environments.
"Any place where there is little to no oxygen — wetlands, rice paddies, landfills, the gut of a cow — are all sources of methane," Jackson said.
Overall, methane makes up a much smaller percentage of global greenhouse gas emissions than carbon dioxide does, but it's of particular concern to scientists because methane's molecular structure makes it more readily able to absorb thermal radiation.
"Methane doesn't last as long in the atmosphere as carbon dioxide, but it's much more efficient at trapping heat than carbon dioxide," Poulter said, which makes the gas a key factor in global warming.
To curb methane emissions, countries need to reduce their reliance on fossil fuels, in addition to reducing the number of harmful leaks from pipelines and wells, Jackson said.
Scientists are also studying how to minimize methane emissions in agricultural practices, such as altering water levels in rice paddies and experimenting with changes in the diets of cattle and sheep to reduce the amount of methane belched from their digestive systems. Burger King recently announced that it is adding lemongrass to the diet of its cows to reduce methane emissions with a lower-carb feeding regimen.
But slowing greenhouse gas emissions will also require bigger changes in human behavior, Jackson said.
"Diet matters," Jackson said. "Here in the U.S., we have one of the highest rates of red meat consumption in the world. We don't have to stop eating red meat necessarily, but eating less meat or eating more fish and chicken instead of beef will reduce emissions, too."
And while the coronavirus pandemic is expected to result in significant decreases in carbon dioxide emissions in 2020 — primarily from economic slowdowns and lockdowns that sharply reduced air travel and other transportation — similar declines are not anticipated with methane.
"Our farmers are still producing food, oil and gas production hasn't fallen much yet, and methane plays only a tiny part in the transportation sector," Jackson said. "So while we may see a small decrease this year because of the coronavirus, methane emissions over the last decade are marching upward. And at this rate, we won't see peak methane emissions any time soon."