Dr. Howard Epstein, Department of Environmental Sciences, UVA
In October of 2008 CA-CP interviewed Dr. Epstein, Associate Professor of the Department of Environmental Sciences, University of Virginia. Dr. Epstein is a tundra ecosystem ecologist who studies climate-plant-soil interactions in Arctic environments.
CA-CP: Dr. Epstein, your recent research has involved measuring the amount of soil organic carbon in the North American Arctic region. Can you explain why it is important to understand how much carbon is stored in the tundra of the Arctic and how it relates to climate change? How could it affect people living in lower-latitudes?
Dr. Epstein: It is important to understand the quantity of organic carbon (C) stored in permafrost regions, because it is such a huge pool of carbon, and therefore relatively small changes to this pool can result in large environmental impacts. Schuur et al. (2008) estimate that the total organic carbon stored in areas that have permafrost to a depth of 3 meters is 1024 Gigatonnes of carbon (1 Gt = 10^15 grams); the amount of carbon presently in our atmosphere is approximately 750 Gt, so this is ~36% more carbon than the total in our atmosphere. For just the arctic tundra biome (which is about one-fourth the size of the entire permafrost region), a circumpolar extrapolation from Ping et al. (2008) yields ~160 GtC to a depth of 1 meter, still a sizable amount.
This carbon is essentially old, dead organic matter from plants and other organisms that has not decomposed due to cold or frozen soils, and saturated soil conditions in certain areas of the tundra. Much of this carbon developed in place due to vegetation growth and subsequent death. However, substantial quantities of carbon were also deposited in windblown sediments during glacial periods onto unglaciated areas of the high latitudes. This organic carbon is converted to, and released as, carbon dioxide during decomposition by of the organic material by micro-organisms. Warming of tundra and permafrost-dominated soils would increase the rate at which this organic matter is decomposed and converted to carbon dioxide. Any thawing of permafrost would expose new, previously-frozen, organic matter for decomposition. Many areas of the tundra are wet, and decomposition is limited by saturated soil conditions. One possible outcome of warming is drier tundra soils, which could enhance decomposition and carbon dioxide production in wet areas. This may also reduce decomposition in areas that already somewhat dry.
Schuur et al. (2008) cite a few estimates of carbon release due to thawing permafrost in the range of 50-100 GtC over the next century. This of course has implications for low latitudes because carbon dioxide is rather well-mixed in our atmosphere. A rough conversion suggests an additional 25-50 ppm carbon dioxide in the atmosphere from permafrost soils; we are presently at ~382 ppm.
CA-CP: How has this latest research built upon the previous understanding of the amount of carbon in Arctic tundra?
Dr. Epstein: The most commonly cited numbers for soil organic carbon in northern regions come from a paper published in 1982 (Post et al.). They estimated 21.8 kg of soil organic carbon (SOC) per square meter. The Ping et al. (2008) study found an average of 34.8 kg SOC per square meter to one meter depth, an increase of almost 60% over the prior estimates, with 38% of the carbon found in the permafrost. Schuur et al. (2008) extend the estimates to the entire permafrost region, which is approximately 3-4 times the size of the arctic tundra biome.
CA-CP: Can you explain cryoturbation and how it affects carbon in the tundra?
Dr. Epstein: Cryoturbation is a disturbance associated with the freezing and thawing of soils. In the winter, tundra soils freeze and expand, and in the summer they thaw and settle. In some locations of the tundra this freezing and thawing is relatively homogeneous, and soils expand a few centimeters in the winter as the liquid water changes to the less dense ice. However, in other locations, depending on local conditions (importantly, soil texture, soil moisture levels, and rates of freezing), some areas may expand to a greater extent than others, even 10-20 cm (known as differential frost heave). The processes of cryoturbation and soil expansion, particularly in places with differential frost heave, can move organic carbon from the surface to greater depths in the soil, possibly even being ultimately frozen in the permafrost (Michaelson et al. 1996; Walker et al. 2004, 2008; Bockheim 2007). Cryoturbation can therefore enhance the sequestering of carbon in tundra soils, by moving it from the exposed surface to deep in the soil profile and potentially in the permafrost.
CA-CP: What is the role of Arctic tundra vegetation in all of this? How could it develop and how would it affect the dynamics of the region? What is the role of photosynthesis and increased vegetation?
Dr. Epstein: Arctic tundra vegetation of course plays a crucial part in the arctic carbon cycle. Growing vegetation during the summers sequesters carbon from the atmosphere in the process of photosynthesis. This carbon is temporarily stored in the vegetation, but may ultimately end up in the soil organic carbon after either just certain plant parts (e.g. deciduous leaves) or the entire plant dies. A warming Arctic is likely to lead to increased vegetation with increased photosynthesis and carbon dioxide uptake from the atmosphere. There is already evidence of increasing vegetation from a number of studies (e.g. Sturm et al. 2001, Jia et al. 2003, Goetz et al. 2005, Tape 2006). The tundra vegetation has other important functions - one is to insulate the soils from variable air temperatures. Soils in a highly vegetation location will be warmer during the winter and colder during the summer than soils, for example, with out vegetation. So the presence and quantity of vegetation and its insulative properties has an effect on soil temperatures which in turn affects micro-organism activity, decomposition of soil organic carbon, and carbon dioxide release to the atmosphere.
A major unknown is how the arctic carbon budget will change in response to a changing environment, given that certain processes lead to greater sequestration of carbon in soils, whereas others will lead to increased release of carbon from soils.
View the projects Dr. Epstein is involved in: Greening of the Arctic, The North American Arctic Transect, and Biocomplexity of Arctic Tundra Ecosystems
Visit Dr. Epstein's home page at the University of Virginia to view a list of his recent publications.
View Dr. Epstein's presentation on the New Estimates of Carbon Stores in Arctic Tundra and Permafrost Soils given at the American Meteorological Society's Environmental Science Seminar Series
CA-CP: Dr. Epstein, your recent research has involved measuring the amount of soil organic carbon in the North American Arctic region. Can you explain why it is important to understand how much carbon is stored in the tundra of the Arctic and how it relates to climate change? How could it affect people living in lower-latitudes?
Dr. Epstein: It is important to understand the quantity of organic carbon (C) stored in permafrost regions, because it is such a huge pool of carbon, and therefore relatively small changes to this pool can result in large environmental impacts. Schuur et al. (2008) estimate that the total organic carbon stored in areas that have permafrost to a depth of 3 meters is 1024 Gigatonnes of carbon (1 Gt = 10^15 grams); the amount of carbon presently in our atmosphere is approximately 750 Gt, so this is ~36% more carbon than the total in our atmosphere. For just the arctic tundra biome (which is about one-fourth the size of the entire permafrost region), a circumpolar extrapolation from Ping et al. (2008) yields ~160 GtC to a depth of 1 meter, still a sizable amount.
This carbon is essentially old, dead organic matter from plants and other organisms that has not decomposed due to cold or frozen soils, and saturated soil conditions in certain areas of the tundra. Much of this carbon developed in place due to vegetation growth and subsequent death. However, substantial quantities of carbon were also deposited in windblown sediments during glacial periods onto unglaciated areas of the high latitudes. This organic carbon is converted to, and released as, carbon dioxide during decomposition by of the organic material by micro-organisms. Warming of tundra and permafrost-dominated soils would increase the rate at which this organic matter is decomposed and converted to carbon dioxide. Any thawing of permafrost would expose new, previously-frozen, organic matter for decomposition. Many areas of the tundra are wet, and decomposition is limited by saturated soil conditions. One possible outcome of warming is drier tundra soils, which could enhance decomposition and carbon dioxide production in wet areas. This may also reduce decomposition in areas that already somewhat dry.
Schuur et al. (2008) cite a few estimates of carbon release due to thawing permafrost in the range of 50-100 GtC over the next century. This of course has implications for low latitudes because carbon dioxide is rather well-mixed in our atmosphere. A rough conversion suggests an additional 25-50 ppm carbon dioxide in the atmosphere from permafrost soils; we are presently at ~382 ppm.
CA-CP: How has this latest research built upon the previous understanding of the amount of carbon in Arctic tundra?
Dr. Epstein: The most commonly cited numbers for soil organic carbon in northern regions come from a paper published in 1982 (Post et al.). They estimated 21.8 kg of soil organic carbon (SOC) per square meter. The Ping et al. (2008) study found an average of 34.8 kg SOC per square meter to one meter depth, an increase of almost 60% over the prior estimates, with 38% of the carbon found in the permafrost. Schuur et al. (2008) extend the estimates to the entire permafrost region, which is approximately 3-4 times the size of the arctic tundra biome.
CA-CP: Can you explain cryoturbation and how it affects carbon in the tundra?
Dr. Epstein: Cryoturbation is a disturbance associated with the freezing and thawing of soils. In the winter, tundra soils freeze and expand, and in the summer they thaw and settle. In some locations of the tundra this freezing and thawing is relatively homogeneous, and soils expand a few centimeters in the winter as the liquid water changes to the less dense ice. However, in other locations, depending on local conditions (importantly, soil texture, soil moisture levels, and rates of freezing), some areas may expand to a greater extent than others, even 10-20 cm (known as differential frost heave). The processes of cryoturbation and soil expansion, particularly in places with differential frost heave, can move organic carbon from the surface to greater depths in the soil, possibly even being ultimately frozen in the permafrost (Michaelson et al. 1996; Walker et al. 2004, 2008; Bockheim 2007). Cryoturbation can therefore enhance the sequestering of carbon in tundra soils, by moving it from the exposed surface to deep in the soil profile and potentially in the permafrost.
CA-CP: What is the role of Arctic tundra vegetation in all of this? How could it develop and how would it affect the dynamics of the region? What is the role of photosynthesis and increased vegetation?
Dr. Epstein: Arctic tundra vegetation of course plays a crucial part in the arctic carbon cycle. Growing vegetation during the summers sequesters carbon from the atmosphere in the process of photosynthesis. This carbon is temporarily stored in the vegetation, but may ultimately end up in the soil organic carbon after either just certain plant parts (e.g. deciduous leaves) or the entire plant dies. A warming Arctic is likely to lead to increased vegetation with increased photosynthesis and carbon dioxide uptake from the atmosphere. There is already evidence of increasing vegetation from a number of studies (e.g. Sturm et al. 2001, Jia et al. 2003, Goetz et al. 2005, Tape 2006). The tundra vegetation has other important functions - one is to insulate the soils from variable air temperatures. Soils in a highly vegetation location will be warmer during the winter and colder during the summer than soils, for example, with out vegetation. So the presence and quantity of vegetation and its insulative properties has an effect on soil temperatures which in turn affects micro-organism activity, decomposition of soil organic carbon, and carbon dioxide release to the atmosphere.
A major unknown is how the arctic carbon budget will change in response to a changing environment, given that certain processes lead to greater sequestration of carbon in soils, whereas others will lead to increased release of carbon from soils.
View the projects Dr. Epstein is involved in: Greening of the Arctic, The North American Arctic Transect, and Biocomplexity of Arctic Tundra Ecosystems
Visit Dr. Epstein's home page at the University of Virginia to view a list of his recent publications.
View Dr. Epstein's presentation on the New Estimates of Carbon Stores in Arctic Tundra and Permafrost Soils given at the American Meteorological Society's Environmental Science Seminar Series