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Methane and Frozen Ground
Figure 1. Kevin Schaefer is an NSIDC scientist who studies the carbon cycle.
Kevin Schaefer is a permafrost scientist at NSIDC. He studies the carbon cycle, or the processes by which the Earth's carbon moves around: from the air into plants, from plants into the ground, and then back into the air (Figure 2). Dr. Schaefer studies the carbon that is frozen deep in Arctic permafrost. As the Earth warms, scientists worry that some of the carbon in permafrost could escape to the atmosphere as carbon dioxide or methane. Increasing the amount of these gases in the atmosphere could make Earth's climate warm up even more. See Climate and Frozen Ground for more information on greenhouse gases and climate warming.
Here Dr. Schaefer provides some answers to questions about methane and frozen ground.
What is methane?
Methane is a gas made up of one carbon atom and four hydrogen atoms. It's the same natural gas that some people use to heat their homes, and it also exists naturally in the atmosphere. Scientists worry that if methane increases in the atmosphere, it could cause even more warming than carbon dioxide from the burning of fossil fuels. Although there is much less methane in the atmosphere than carbon dioxide, it traps heat about twenty times as efficiently as carbon dioxide.
What are the sources of methane in the Arctic?
There are two potential sources of methane in the Arctic. The first source of methane is called methyl clathrate. Methyl clathrates are molecules of methane that are frozen into ice crystals. They can form deep in the Earth or underwater, but it takes very special conditions, with high pressure and low temperature, to make them. If the temperature or pressure changes, the ice that imprisons the methane will break apart, and the methane will escape. We're not sure how much methane is trapped in methyl clathrates, or how much is in danger of escaping.
The other major source of methane in the Arctic is the organic matter frozen in permafrost. This is the source of methane that I study. The organic matter in permafrost contains a lot of carbon. It is made of dead plants and animals that have been frozen deep in permafrost for thousands of years. As long as this organic matter remains frozen, it will stay in the permafrost. However, if it thaws, it will decay, releasing carbon dioxide or methane into the atmosphere. This is why permafrost carbon is important to climate study.
Figure 2. Carbon moves through the Earth's atmosphere, oceans, and land in a process called the carbon cycle.
—Credit: NSIDC, modified from NASA Earth Science Enterprise
How did this carbon get into permafrost in the first place?
Carbon was buried in permafrost by processes that took thousands of years. During the last ice age, great ice sheets covered most of the continents. As they spread out and then shrunk back, the heavy fields of ice ground up the rock underneath them into a very fine dust called loess or glacial flour. The ice sheets produced a huge amount of this powdered rock, and wind and rain deposited it onto the soil.
As the ice sheets added loess to the soil, the soil got thicker. As the soil built up, the active layer on top stayed the same thickness. The active layer freezes and thaws each year, and plants can grow in it. But underneath the active layer, roots and other organic matter were frozen into the permafrost, where they can't decay.
Most of the organic matter consists of partially decayed roots, whole roots, and other plant material. However, there are also animals and animal material frozen in the ground--sometimes people find entire mastodons or other animals frozen in the permafrost (Figure 3). Significant deposits of carbon-rich permafrost, or yedoma, have been found in Russia.
How much carbon is stored in frozen ground?
There is a huge amount of carbon stored in permafrost. Right now, the Earth's atmosphere contains about 850 gigatons of carbon. (A gigaton is one billion tons—about the weight of one hundred thousand school buses). We estimate that there are about 1,400 gigatons of carbon frozen in permafrost. So the carbon frozen in permafrost is greater than the amount of carbon that is already in the atmosphere today. That doesn't mean that all of the carbon will decay and end up in the atmosphere. The trick is to find out how much of the frozen carbon is going to decay, how fast, and where.
Figure 3. This steppe bison lay frozen in permafrost for 36,000 years before its discovery in 1979. The bison, known as "Babe," is on display at the University of Alaska, Fairbanks Museum of the North.
—Credit: Photo by Bill Schmoker (PolarTREC 2010), Courtesy of ARCUS
What will happen to the frozen carbon if permafrost thaws?
When permafrost thaws, the frozen organic matter inside it will thaw out, too, and begin to decay. It's like taking a bag of frozen broccoli out of the freezer and putting it into the refrigerator. Once it thaws, it will eventually decay and break down.
As organic matter decays, it gets eaten up and digested by microbes. The bacteria that eat it produce either carbon dioxide or methane as waste. If there is oxygen available, the microbes make carbon dioxide. But if there is no oxygen available, they make methane. Most of the places where methane would form are the swamps and wetlands. And there are many miles of wetlands in the Arctic. When you walk around in the Arctic tundra, it's like sloshing through a giant sponge.
When permafrost carbon turns into methane, it bubbles up through soil and water. On the way, other microorganisms eat some of it. But some methane makes it to the surface and escapes into the air.
How will additional methane from permafrost affect global warming?
There are several opposing processes at work, which make this a hard question to answer. Warmer temperatures mean that permafrost can thaw and release methane to the atmosphere. But warming also means that the growing seasons in Arctic latitudes will last longer. When the growing season is longer, plants have more time to suck up carbon from the atmosphere. Since carbon in the air is what plants use to grow, it can also act as a sort of fertilizer under certain conditions. Then plants to grow faster and take up even more carbon. Right now, the Arctic takes up more carbon than it releases. This means that plants take up carbon during the growing season, but do not release as much carbon through decay. So we say that the Arctic acts as a carbon sink.
But if the Earth continues to warm, and a lot of permafrost thaws out, the Arctic could become an overall source of carbon to the atmosphere, instead of a sink. This is what scientists refer to as a "tipping point." We say that something has reached a tipping point when it switches from a relatively stable state to an unstoppable cycle. In this case, the Arctic would change from a carbon sink to a carbon source. If the Arctic permafrost releases more carbon than it absorbs, it would start a cycle where the extra carbon in the atmosphere leads to increased warming. The increased warming means more permafrost thawing and methane release.
What are the questions that scientists are currently studying about permafrost and methane?
The big questions are: How much carbon is currently frozen in permafrost? How much will thaw out in the future and when will it be released into the atmosphere? We also want to know how much carbon could be released as methane, and how much could be released as carbon dioxide. That's related to how much of the land is wetlands, since ponds and lakes and swamps are the main places that will produce methane.
If governments around the world knew how much methane could be released from permafrost, it could help them decide what to do about it. For example, they would know how much we need to reduce fossil fuel emissions from human activities. They would also need to know how much carbon the Earth is emitting on its own.
The good news is that we haven't reached the tipping point yet. People in some areas have reported that some permafrost carbon has already started to decay. But measurements of carbon dioxide in the air show the Arctic is still a carbon sink. So we are studying permafrost to understand more about how it acts. We are also trying to measure how much carbon there is and where is it located. Then scientists can use that information in computer programs that help us better plan for the future.