Air bubbles trapped in the ice for thousands of years reveal the prevalence of fires during climate change

Research from Oregon State University shows that fire is linked to major climate changes of the past, confirming concerns that global climate change today could cause more damaging burning events.

by Annika Schmidt
Medill Reports, Dec. 22, 2023

Pockets of air trapped in cores from the world’s thickest ice sheets hold time capsules that allow scientists to directly access a preserved bubble of Earth’s atmosphere from thousands of years ago. This ancient air shows that fire was a key feature of major past climate shifts, an  indication that wild fires could increase as humans push the boundaries on future climate change.

Global fire events are traced in history by examining the ratios of different stable isotopes — or atomic variants — of methane, a greenhouse gas that accelerates global warming. 

Oregon State University Ph.D. student Ben Riddell-Young has been studying methane to determine what caused significant changes in the levels of the gas in the atmosphere over time. The chemical formula for methane is CH4 — an atom of carbon and four atoms of hydrogen. But whether the carbon atom is the classic carbon-12 — with six protons and six neutrons — or the isotope carbon-13 — with an extra neutron — changes everything.

“You can think of the stable isotopes as a fingerprint of the sources so different sources have what we call different stable isotopic signatures such as different values we can measure — it’s the carbon-12 to carbon-13 ratio — and each source has a unique ratio,” Riddell-Young said.

By examining the ratios of two carbon isotopes, carbon-12 and carbon-13, in the methane extracted from Antarctica ice cores, Riddell-Young found that fire was present during major climate events when methane in the atmosphere changed dramatically. These events also correlate with other significant climate events, making methane an important topic, especially as the amount of methane in the modern atmosphere has increased dramatically in the past few decades, primarily due to agriculture and fossil fuels, according to the EPA.

 While drought, shifting rainfall and temperature changes are well-established components of climate change, Riddell-Young said his research shows that fire is too. The methodology for mapping the history of natural burning is reliable because fire emissions generate a distinctive atmospheric ratio of carbon-12 and carbon-13, Riddell-Young said.

“These stable isotopes of methane are a really good proxy or tracer of how fire changed in the past because they’re really sensitive to changes in the emissions of methane from fires,” Riddell-Young said. 

Other methods for examining the history of fire are localized to specific areas, Riddell-Young said. The method using methane is stronger because it’s a global signature of fire activity. Methane is a gas well mixed throughout the global atmosphere, which is how it reflects global changes. 

Riddell-Young presented his research at the Comer Climate Conference in southern Wisconsin in fall 2023, where top climate scientists gather annually to share their latest climate research findings. Many contributors are paleoclimatologists interested in reconstructing the history of climate change to better understand the history of the changing climate.

“The possibility that the ice core people are moving toward the ability to track fires around the world from looking in the ice core is very, very interesting,” said Richard Alley, a geosciences professor at Pennsylvania State University. “This is something we worked on a little bit at Penn State a long time ago and now the Oregon State crew is working [on it]. Ben Riddell-Young did this beautiful work since methane is beautifully recorded in ice cores.”

Alley said this research has critical present-day implications.

“The broadest idea is that when you really change the climate, you disturb the ecosystem and things burn,” Alley said. “What are we doing now? We’re changing the climate a lot and creating conditions that tend to favor burning and we’re seeing more burning. We know the level of disruption to people’s lives when houses burn, and we know the disruption to people’s health when all this smoke is going places.”

Riddell-Young has used Antarctic ice core samples for his research from the National Science Foundation Ice Core Facility in Denver. The archive stores and studies ice cores collected from glaciers and ice sheets around the world and allows scientists to use samples for research.

Ben Riddell-Young, second from left, with fellow researchers holding an ice core in Greenland last spring. Photo courtesy of Ben Riddell-Young.

Riddell-Young also collects ice cores with his research team. He said the ice samples for his research are placed in glass chambers and modern air is pumped out before the ice is melted, releasing the ancient air from a specific time in climate history that has been trapped and isolated for thousands of years. This air provides a snapshot of the atmosphere at that point in climate history.

The air is processed to isolate the key ingredient: methane. 

This lab work is how Riddell-Young spent most of the six years of his Ph.D., which he expects to complete this year. He conducted fieldwork for the first time last spring at Summit Station in Greenland. For two months, he explored a separate hypothesis on ice record reliability, drilling a core from 150 meters below the surface of the ice sheet. 

Methane levels in the modern atmosphere are much higher today, Riddell-Young said. Data from the National Oceanic and Atmospheric Administration also shows that methane levels surged in 2020, possibly due to wetlands emissions as well as wild fires, after plateauing in the early 2000s.

“It doesn’t remove the motivation for studying [methane] in the past,” Riddell-Young said, “because we want to learn how it changes and what causes it to change in the natural cyclicality of everything, but it also puts into perspective just how much we’re changing it.”

Photo at top: A research site near Summit Camp in Greenland where researchers are drilling into ice sheets to access ice core samples from hundreds of meters below the surface. Photo courtesy of Ben Riddell-Young.

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