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Benjamin Hmiel dons a heavy winter coat and gloves to step into the walk-in freezer at Vasilii Petrenko鈥檚 Ice Core Lab in Hutchison Hall. The extra layers of clothing are vital lab accoutrements: the freezer is lined with stacks of ice cores that Hmiel, a PhD candidate in earth and environmental sciences; Petrenko; and other scientists in the lab have collected from yearly expeditions to Antarctica and Greenland. Once back in Rochester, the ice cores鈥攃ylinders of ice 10 inches in diameter and weighing up to 80 pounds鈥攁re kept at Arctic-like temperatures of minus 26 degrees Celsius (minus 14.8 degrees Fahrenheit) while researchers use them to study climate change.

鈥淎ntarctica is actually really nice because it鈥檚 super sunny, and your body gets used to the cold,鈥� Hmiel says. 鈥淢ost of the time, the weather in the field isn鈥檛 as miserable as it is in this freezer in Rochester.鈥�

Stepping into the freezer is like stepping into a time machine: contained within the ice cores is air dating back as far as 50,000 years. Because it contains greenhouse gases from a time before human emissions complicated the picture, the ancient air offers scientists clues to future climate patterns.

鈥淭he main goal in our lab is to understand processes in the earth鈥檚 atmosphere that can help us predict the future climate,鈥� says Petrenko, an associate professor of earth and environmental sciences. 鈥淎 lot of what we do is looking in the earth鈥檚 past because this allows us to see how the chemical composition of the atmosphere has been changing over large stretches of time.鈥�

Petrenko鈥檚 most recent research focuses on methane, a powerful greenhouse gas second only to carbon dioxide in its contribution to human-driven global warming. Today鈥檚 atmosphere contains methane that is emitted naturally鈥攆rom wetlands, wildfires, or ocean and land seeps鈥攁nd methane emitted from human activities like fossil fuel extraction and use, rice agriculture, raising livestock, and generating landfills.

In a study released this fall, Petrenko analyzed methane from 12,000 years ago during the last deglaciation, when Earth was transitioning out of the last ice age. At that point in geological history, global surface temperatures were rising naturally with spurts of rapid regional warming in areas like the North Atlantic Ocean. Although climate patterns in the future may not exactly mimic those conditions, the period of warming allowed Petrenko to reveal an important piece of the climate puzzle: natural methane emissions from ancient carbon reservoirs are smaller than researchers previously thought.

The discovery has two crucial implications for the future of climate change. One is that the risk that future global warming will trigger methane release from these large natural reservoirs of old carbon seems to be low. But Petrenko also found that humans appear to be contributing more methane to the atmosphere through fossil fuel use and extraction than scientists previously believed.

鈥淭he good news is, we have more power than we realized to fight global warming,鈥� Petrenko says. 鈥淲e do not need to worry as much about the natural methane seeps into the atmosphere. But we do need to be concerned about man-made methane emissions.鈥�

Scientists have a good idea of how much total methane is in the atmosphere and how the total has changed over the last few decades. But separating the natural and anthropogenic sources and estimating how much humans emit is more difficult.

鈥淲e 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,鈥� says Hinrich Schaefer, an atmospheric scientist at the National Institute of Water and Atmospheric Research (NIWA) in New Zealand, who collaborates with Petrenko. 鈥淭hat makes it hard to understand which sources we should target specifically to reduce methane levels.鈥�

Researchers 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鈥檛 always work because the isotope 鈥渟ignatures鈥� 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鈥斺�済eologic methane鈥濃�攈as an isotope signature that鈥檚 identical to the fossil methane emitted when humans drill gas wells.

For decades, scientists used 鈥渂ottom-up鈥� methods to estimate methane emission levels. That meant, for example, traveling the globe to the various natural emissions sources鈥攕uch as wetlands and land seeps鈥攁nd conducting measurements and calculations of the methane emitted. These methods resulted in uncertainties, in part because it was impossible to measure all of the emissions sites, and scientists were therefore required to make assumptions about the quantity and strength of the sources. That opened up more room for error; Petrenko estimates that previous natural geological methane approximations were way too high, by a factor of at least three.

鈥淥ur measurements are different because they are top-down, measuring the atmosphere instead of the direct sources, at a time 12,000 years ago,鈥� Petrenko says. 鈥淕oing back before any anthropogenic activities鈥攂efore the Industrial Revolution鈥攕implifies the picture and allows us to estimate natural geologic sources more accurately.鈥�

Humans did not begin using fossil fuels as a primary energy source until the Industrial Revolution in the 18th century. Previous studies suggest that natural geologic methane emissions of the past are at least as high as natural emissions today, so studying the ancient ice cores allows researchers to accurately determine the upper limit of geologic emissions, separate from their anthropogenic counterparts.

Antarctica in the south and Greenland in the north have the oldest and biggest ice sheets in the world. Together, these ice sheets offer researchers a comprehensive record of Earth鈥檚 climate history and play an important role in the future of the global climate system. Because there is so much water contained within the ice, as the ice melts, researchers estimate it could cause an alarming sea level rise affecting hundreds of millions of people along global coastlines.

Every year that it snows in Antarctica, the new snow layer weighs on the previous layer, compacting over hundreds or thousands of years to eventually form layers of ice. When enough snow falls, researchers can drill down and determine the time period based on the layers, much like counting rings on a tree. The ice layers contain air bubbles, which are like tiny time capsules; using melting chambers and vacuum pumps in the field, researchers are able to extract the ancient air contained within the bubbles.

Most of the ice cores Petrenko uses in his methane research are from Taylor Glacier. It takes about one metric ton (2,200 pounds) of ice to get one nine-gallon canister of air. And extracting enough ice to get those nine gallons can take four field scientists three full days of work. The researchers ship only a few unprocessed ice cores, plus the air canisters, back to the Ice Core Lab鈥攁s well as other destinations around the world. 鈥淪ince we collaborate with international labs, these air canisters are really well traveled,鈥� Hmiel says.

When the air finally arrives in Rochester, the researchers must further process it in order to study the specific components, such as methane. To separate the air into its components, they run it through an extraction line that removes compounds like water vapor. The air is then directed into a furnace where the methane combusts to carbon dioxide. The carbon dioxide is trapped, purified, and sealed into small glass tubes.

The best way to estimate the magnitude of fossil methane emissions is by using measurements of methane isotopes, such as carbon. Petrenko determined fossil methane levels in his latest study by using the carbon-14 isotope of methane鈥攖he first such study to do so. His results showed that levels of methane were three to four times lower than previous estimates. If the natural geologic methane emissions are lower than previously thought, the anthropogenic fossil methane emissions must be higher than previously thought鈥擯etrenko estimates by 25 percent or more.

Scientists have also raised the possibility that global warming could release methane from very large ancient carbon reservoirs such as permafrost and gas hydrates鈥攊ce-like forms of methane in the sediments at the bottom of the ocean鈥攖hat become less stable as temperatures increase. If climate change were to trigger large emissions of methane to the atmosphere from these old carbon reservoirs, the result would be even more warming. However, Petrenko found that the gradual, natural global warming and rapid regional warming that characterized the deglaciation 12,000 years ago鈥攅vents that were in some aspects comparable to the current human-driven global warming鈥攄id not trigger detectable releases of methane from these reservoirs.

When it comes to predicting the future course of climate change, therefore, 鈥渢hese kinds of scenarios regarding natural methane are not as important to take into account,鈥� he says. 鈥淗owever, this point is to be taken with caution because modern warming is different from warming at the end of the last Ice Age. Our global temperatures right now are not within the natural cycle. We never went anywhere near as high as today at any point during the last deglaciation.鈥�

In November 2018, Petrenko and members of his lab plan to return to Antarctica to collect ice cores. This time, they鈥檒l try to answer the broader question of how the large-scale chemistry of the atmosphere has changed since pre-Industrial times.

The data will be especially useful to colleagues such as Lee Murray, an assistant professor of earth and environmental sciences, who builds computer models to predict future changes in atmospheric chemistry. Petrenko will collaborate with Murray on the project as well as on an additional study of the factors affecting the ability of the atmosphere to cleanse itself.

Hmiel and other members of the lab envision that their research may also help inform public discourse regarding a more sustainable future.

鈥淲e鈥檙e not trying to solve the green energy problem,鈥� Hmiel says. 鈥淏ut our work can be a catalyst for showing people that things like fossil fuel use and greenhouse gas emissions are a problem and that we need to switch to greener sources. Every year that we continue with business as usual, it鈥檚 going to be harder to meet target climate goals. Realistically, we needed to start changing decades ago.鈥�