Amount of Methane Emitted by Human Activity has been Vastly Underestimated

Methane is known to be a potent greenhouse gas and contributes considerably to global warming. In the past three centuries, emissions of this gas in the air have increased by around 150%.

Despite this fact, researchers have found it difficult to precisely establish the origin of these emissions; heat-trapping gases such as methane can be produced naturally, and also from human activity.

At the University of Rochester, Benjamin Hmiel, a researcher and postdoctoral associate in the laboratory of Vasilii Petrenko, a professor of earth and environmental sciences, and colleagues quantified the levels of methane present in old air samples. They discovered that investigators have been largely underestimating the amount of methane that is emitted by humans into the air through fossil fuels.

In a study published in the Nature journal, the scientists indicated that decreasing the use of fossil fuels is a major target in mitigating climate change.

Placing stricter methane emission regulations on the fossil fuel industry will have the potential to reduce future global warming to a larger extent than previously thought.

Benjamin Hmiel, Researcher and Postdoctoral Associate, University of Rochester

Two Types of Methane

Methane happens to be the second-largest anthropogenic after carbon dioxide (CO₂) gas and originates from human activity. It plays a major role in global warming.

However, methane has a comparatively short shelf-life when compared to CO₂ and other heat-trapping gases. On average, methane persists in the atmosphere only for nine years, while CO₂, for example, can last for approximately 100 years in the air. That renders methane a particularly suitable target for mitigating the levels of emission in a short period of time.

If we stopped emitting all carbon dioxide today, high carbon dioxide levels in the atmosphere would still persist for a long time. Methane is important to study because if we make changes to our current methane emissions, it’s going to reflect more quickly.

Benjamin Hmiel, Researcher and Postdoctoral Associate, University of Rochester

Methane discharged into the air can be divided into two categories, depending on its carbon-14 signature; carbon-14 is a rare radioactive isotope.

Fossil methane in historic hydrocarbon deposits has been sequestered for millions of years and does not contain carbon-14 anymore because this isotope has decomposed. But carbon-14 is present in biological methane, which comes into contact with flora and fauna on the surface of the planet.

Biological methane can be naturally discharged from sources like wetlands or through anthropogenic sources like livestock, rice fields, and landfills.

The focus of Hmiel’s work is fossil methane. This gas can be discharged through natural geologic seeps or due to humans’ extraction and utilization of fossil fuels such as coal, gas, and oil.

Researchers are able to precisely measure the overall amount of methane discharged into the atmosphere every year; however, it is very hard to break down this total amount of gas into its separate components: Which parts are biological and which portions emerge from fossil sources? What is the level of methane emitted naturally, and how much of this gas is discharged by human activity?

As a scientific community we’ve been struggling to understand exactly how much methane we as humans are emitting into the atmosphere. We know that the fossil fuel component is one of our biggest component emissions, but it has been challenging to pin that down because in today’s atmosphere, the natural and anthropogenic components of the fossil emissions look the same, isotopically.

Vasilii Petrenko, Study Co-Author and Professor, Department of Earth and Environmental Sciences, University of Rochester

Turning to the Past

Hmiel and his collaborators turned to the past to more precisely isolate the anthropogenic and natural components. They did this by drilling and obtaining ice cores from Greenland. The samples of ice core behave just like time capsules: they include air bubbles with trace amounts of old air trapped within.

Using a melting chamber, the scientists collected the ancient air from the bubbles and subsequently analyzed its chemical composition.

While Hmiel’s study expands on earlier work performed by Petrenko, it is focused on quantifying the air composition from the early 18^th century—that is, before the beginning of the Industrial Revolution—to the current day.

It was only until the mid-19^th century that humans started to use large amounts of fossil fuels. Quantifying the levels of emission before this time period enables scientists to detect the natural emissions, without the emissions resulting from fossil fuels that exist in the present-day atmosphere. There is no proof to indicate that natural emissions from fossil methane can differ over the duration of a few centuries.

By quantifying the isotopes of carbon-14 in the atmosphere from over two centuries ago, the team discovered that virtually all the methane discharged into the air was biological in nature until around 1870. That is when the component of the fossil began to increase quickly. The timing matches with a sudden increase in the usage of fossil fuels.

The levels of naturally emitted fossil methane are around 10 times lower when compared to earlier studies reported.

Considering the total fossil emissions quantified in the air currently, Hmiel and his collaborators inferred that the artificial fossil component is more than anticipated—25% to 40% higher was discovered by the team.

Climate Change Implications

This information has major implications for climate studies—if emissions of anthropogenic methane constitute a bigger part of the total, then decreasing emissions from human activities, such as the extraction and use of fossil fuels, will have a much larger effect on mitigating upcoming global warming than previously believed by researchers.

That is actually good news for Hmiel. “I don’t want to get too hopeless on this because my data does have a positive implication: most of the methane emissions are anthropogenic, so we have more control. If we can reduce our emissions, it’s going to have more of an impact,” he added.

The research was funded by the U.S. National Science Foundation and the David and Lucille Packard Foundation and is the latest example of the University of Rochester’s initiatives to further interpret the budget of global methane.

Researchers from the Department of Earth and Environmental Sciences of the University of Rochester have carried out field studies in Earth’s oceans, the Great Lakes, Greenland, and Antarctica. They have utilized climate models and machine learning to develop an understanding of the powerful greenhouse gas methane and the ways it impacts climate change and global warming.