Ice Cores Indicate Even Higher Methane Emissions Than Previously Believed

Ice Cores Indicate Even Higher Methane Emissions Than Previously Believed
Vasilii Petrenko loading an ice core into the melting chamber for extraction of trapped ancient air.
(Xavier Fain/U. Rochester)

Humans are probably contributing more methane to the atmosphere through fossil fuel use and extraction than scientists previously believed, report researchers.

They also find that the risk that warming will trigger methane release from large natural reservoirs of ancient carbon seems to be low.

In 2011 a team of researchers led by Vasilii Petrenko, an assistant professor of earth and environmental sciences at the University of Rochester, spent seven weeks in Antarctica collecting and studying 2,000-pound samples of glacial ice cores that date back nearly 12,000 years.

The ancient air trapped within the ice revealed surprising new data about methane that may help inform policymakers as they consider ways to reduce global warming.

“…anthropogenic (human-made) fossil fuel methane emissions are even larger than previously thought…”

The researchers report their findings in Nature.

“Our results are suggesting that anthropogenic (human-made) fossil fuel methane emissions are even larger than previously thought,” Petrenko says. “This means we have even more leverage to fight global warming by curbing methane emissions from our fossil fuel use.”

Today’s atmosphere contains methane that is emitted naturally—from wetlands, wildfires, or ocean and land seeps—and methane emitted from human activities like fossil fuel extraction and use, raising livestock, and generating landfills, with human-emitted methane accounting for 60 percent or more of the total.

Scientists are able to accurately measure the total methane level in the atmosphere and how this has changed over the last few decades.

The challenge? Breaking down this total into the specific sources.

“We know rather little about how much methane comes from different sources and how these have been changing in response to industrial and agricultural activities or because of climate events like droughts,” says Hinrich Schaefer, an atmospheric scientist at the National Institute of Water and Atmospheric Research (NIWA) in New Zealand, where a key part of the sample processing took place.

“That makes it hard to understand which sources we should target specifically to reduce methane levels,” says Schaefer.

Scientists can use measurements of different isotopes of methane (methane molecules with atoms of slightly different mass) to fingerprint some of the sources. But even this approach doesn’t always work because the isotope “signatures” of some sources can be very similar.

For instance, fossil methane is methane emitted from ancient hydrocarbon deposits, typically found at sites rich in fossil fuels. Fossil methane that leaks naturally from these sites—”geologic methane”—has an isotope signature that is identical to fossil methane emitted when humans drill gas wells.

Separating the natural and anthropogenic sources and estimating how much humans emit has therefore proven difficult.

In order to better understand the natural and anthropogenic components of fossil methane, Petrenko and his team turned to the past.

Petrenko’s lab is dedicated to understanding how both natural and human-made greenhouse gases respond to climate change. They analyze how past climate changes have affected greenhouse gases over time and the ways in which these gases might respond to future warming temperatures.

In this case, Petrenko and collaborators, studied past atmospheric records using ice cores extracted from Taylor Glacier in Antarctica. These cores date back nearly 12,000 years.

Every year that it snows in Antarctica, the current snow layer weighs on the previous layer, compacting over hundreds or thousands of years to eventually form layers of ice. These ice layers contain air bubbles, which are like tiny time capsules; using vacuum pumps and melting chambers, researchers are able to extract the ancient air contained within these bubbles and study the chemical compositions of the ancient atmosphere.

“Going back before any anthropogenic activities—before the Industrial Revolution—simplifies the picture…”

Humans did not begin using fossil fuels as a primary energy source until the Industrial Revolution in the 18th century. Because of this, 12,000-year-old ice cores contain no fossil methane originating from human activities; fossil methane levels are based solely on methane emitted from natural sources.

Natural geologic methane emissions of the past are thought to be comparable to natural emissions today, so studying ice cores allows researchers to very accurately measure these levels, separate from their anthropogenic counterparts.

“Going back before any anthropogenic activities—before the Industrial Revolution—simplifies the picture and allows us to estimate natural geologic sources very accurately,” Petrenko says.

The natural geologic methane levels the research team measured were three to four times lower than previously estimated numbers. If the natural geologic methane emissions are lower than expected, the anthropogenic fossil methane emissions must be higher than expected—Petrenko estimates by 25 percent or more.

The study also suggests that the risk of methane release from natural ancient carbon reservoirs is lower than previously thought. Scientists have raised the possibility that global warming could release methane from very large ancient carbon reservoirs such as permafrost and gas hydrates—ice-like forms of methane in the sediments at the bottom of the ocean. These become less stable as temperatures increase.

If climate change from burning fossil fuels were to trigger large emissions of methane to the atmosphere from these old carbon reservoirs, this would lead to even more warming.

“The ancient air samples reveal that these kinds of scenarios regarding natural methane emissions are not as important to take into account for future planning,” Petrenko says.

“In contrast, anthropogenic fossil fuel emissions seem to be even larger than we previously thought so reducing these levels has more leverage to mitigate global warming,” he says.

The National Science Foundation supported the research.

Source: University of Rochester

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Product Description: This unique compilation provides comprehensive coverage of all aspects of biogas, methane, farm recovery processes, manure digesters and processing, the AgSTAR program, landfill methane gas, and the Global Methane Initiative. AgSTAR is focused on livestock producers (typically swine and dairy farms) for implementing methane recovery systems appropriate for confined livestock facilities that handle liquid or slurry manure. Gas recovery systems and digester technologies may provide enhanced environmental (air and water) and financial performance when compared to traditional waste management systems such as manure storages and lagoons. When livestock manure that is handled as a liquid or slurry decomposes anaerobically (without the presence of oxygen), it produces biogas. In waste management systems that are designed for treatment, such as digesters and anaerobic lagoons, biogas consists of about 60 to 70% methane and 30 to 40% carbon dioxide. When these gases are collected and transmitted to a combustion device, such as an electric generator, boiler, or absorption cooler, energy is produced. The captured biogas, which is 60 to 70 percent methane, can be used to generate electricity or replace fossil fuels for other energy needs. Municipal solid waste (MSW) landfills are the third-largest source of human-related methane emissions in the United States, accounting for approximately 17 percent of these emissions in 2009. At the same time, methane emissions from landfills represent a lost opportunity to capture and use a significant energy resource. Landfill gas (LFG) is created as solid waste decomposes in a landfill. This gas consists of about 50 percent methane (the primary component of natural gas), about 50 percent carbon dioxide (CO2), and a small amount of non–methane organic compounds. Instead of escaping into the air, LFG can be captured, converted, and used as an energy source. Using LFG helps to reduce odors and other hazards associated with LFG emissions, and it helps prevent methane from migrating into the atmosphere and contributing to local smog and global climate change. LFG is extracted from landfills using a series of wells and a blower/flare (or vacuum) system. This system directs the collected gas to a central point where it can be processed and treated depending upon the ultimate use for the gas. From this point, the gas can be flared, used to generate electricity, replace fossil fuels in industrial and manufacturing operations, or upgraded to pipeline–quality gas where the gas may be used directly or processed into an alternative vehicle fuel. The goals of the Global Methane Initiative (GMI), an international public-private partnership, are to reduce global methane emissions to fight climate change, enhance economic growth, strengthen energy security, and improve local environmental quality and industrial safety. Building on experience from the U.S. Environmental Protection Agency's (EPA's) successful domestic methane emission reduction programs, GMI brings together the public and private sectors to develop projects that can reduce emissions from the agriculture, coal mine, landfill, oil and gas systems, and municipal wastewater sectors. GMI was launched in 2010 based on the strong foundation of the accomplishments of the Methane to Markets Partnership, which was formed in 2004.

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Understanding, quantifying, and tracking atmospheric methane and emissions is essential for addressing concerns and informing decisions that affect the climate, economy, and human health and safety. Atmospheric methane is a potent greenhouse gas (GHG) that contributes to global warming. While carbon dioxide is by far the dominant cause of the rise in global average temperatures, methane also plays a significant role because it absorbs more energy per unit mass than carbon dioxide does, giving it a disproportionately large effect on global radiative forcing. In addition to contributing to climate change, methane also affects human health as a precursor to ozone pollution in the lower atmosphere.

Improving Characterization of Anthropogenic Methane Emissions in the United States summarizes the current state of understanding of methane emissions sources and the measurement approaches and evaluates opportunities for methodological and inventory development improvements. This report will inform future research agendas of various U.S. agencies, including NOAA, the EPA, the DOE, NASA, the U.S. Department of Agriculture (USDA), and the National Science Foundation (NSF).

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