Freeze-thaw cycles in permafrost soils and their impacts on the climate: A literature review
by Maya K. Bergmann
A THESIS
submitted to
Oregon State University
Honors College
in partial fulfillment of the requirements for the degree of
Honors Baccalaureate of Science in Environmental Science (Honors Scholar)
Presented May 27, 2020 Commencement June 2021
AN ABSTRACT OF THE THESIS OF
Maya K. Bergmann for the degree of Honors Baccalaureate of Science in Environmental Science presented on May 27, 2020. Title: Freeze-thaw cycles in permafrost soils and their impact on the climate: A literature review.
Abstract approved: ______Rebecca A. Lybrand
Climate change is occurring at an increasingly rapid rate, with impacts heightened in the cold regions of the world including the Arctic. Warming effects are widespread, with one impacted process – freeze- thaw cycles – increasing in frequency and potentially triggering additional changes in permafrost soils that have previously acted as carbon sinks. Increasing freeze-thaw cycles due to rising average temperatures are stimulating microbial activity in these soils, decomposing carbon and releasing greenhouse gases into the atmosphere. The release of greenhouse gases creates a positive feedback loop as temperatures rise with climate change where permafrost experiences more warming and freeze-thaw cycles, releasing more greenhouse gases, and thereby contributing to climate change. This literature review provides information about microbial activity, as well as the greenhouse gas emissions, from permafrost soils that have undergone freeze-thaw cycles. The overarching goal of this review is to compile the most relevant literature on freeze-thaw cycles in permafrost soils to better consider the future changes we are likely to see as climate change continues to take place. These changes include variations in season length, ability of vegetation and plants to continue to grow in affected regions, and threats to human health and safety, all of which point to the importance of considering permafrost in climate change discussions.
Key Words: Permafrost, soil, climate change, freeze-thaw cycles, microbial communities, carbon dioxide, methane, nitrous oxide, future impacts.
Corresponding e-mail address: [email protected]/[email protected]
©Copyright by Maya K. Bergmann May 27, 2020
Freeze-thaw cycles in permafrost soils and their impacts on the climate: A literature review
by Maya K. Bergmann
A THESIS
submitted to
Oregon State University
Honors College
in partial fulfillment of the requirements for the degree of
Honors Baccalaureate of Science in Environmental Science (Honors Scholar)
Presented May 27, 2020 Commencement June 2021
Honors Baccalaureate of Science in Environmental Science project of Maya K. Bergmann presented on May 27, 2020.
APPROVED:
______Rebecca A. Lybrand, Mentor, representing Department of Crop and Soil Science
______Jeffery A. Hatten, Committee Member, representing Department of Forest Engineering, Resources & Management
______Erin C. Rooney, Committee Member, representing Department of Crop and Soil Science
______Toni Doolen, Dean, Oregon State University Honors College
I understand that my project will become part of the permanent collection of Oregon State University, Honors College. My signature below authorizes release of my project to any reader upon request.
______Maya K. Bergmann, Author
Introduction
The ability of soil to store or emit carbon is proving vital in the discussion of climate change as atmospheric greenhouse gas concentrations increase (Ping et al., 2015; Turetsky et al., 2020;
Chadburn et al., 2017). Microbial activity increases with temperature, resulting in a shift from carbon storage to emission as permafrost soils, known as Gelisols, warm (Mu et al., 2017).
Permafrost (any ground remaining below 0°C for two or more consecutive years) is estimated to contain about 1,600 billion tons of carbon (Turetsky et al., 2019). As permafrost thaws, ancient greenhouse gases including carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4) are released from the soil, resulting in greater concentrations of greenhouse gases in the atmosphere and hastening climate warming in a positive feedback loop (Mu et al., 2018; Turetsky et al.,
2019). Higher levels of greenhouse gas emissions are related to the stimulation of microbial activity in the soil due to thaw (Müller et al., 2018). A cycle between freezing and thawing, or the freeze-thaw cycle, occurs regularly in locations with permafrost, though freeze-thaw only occurs in the active layer overlying the permafrost layer instead of in the permafrost itself.
Freeze-thaw cycles impact the soil aggregate structure, which can disrupt the organic matter and carbon stored within the soil (Liu et al., 2017). The goal of this literature review is to gain an understanding of the key concepts and features of permafrost soils, and the changes seen when undergoing freeze-thaw, specifically features associated with morphology, microbial communities, and gas concentrations. Literature ranging from the 1980s to present will be reviewed, looking at permafrost found as close as Alaska, and as far as Norway. All permafrost will be considered, though it is important to note that Arctic permafrost contains higher levels of carbon than other permafrost regions.
A Definition and Understanding of Permafrost
With the growing body of work surrounding permafrost, a decisive, clear definition of permafrost is critical, especially as many publications use the term permafrost in a way that can be confusing or unclear (Dobiński, 2011). While the most commonly used definition of permafrost is: any ground remaining below 0°C for two or more consecutive years (Soil Survey
Staff, 2014), the definition of permafrost is actually quite complicated and has undergone several iterations in recent years. The International Permafrost Association approved the following updated definition of the term permafrost in 1998: ground (soil or rock and including ice or organic material) that remains at or below 0°C for at least two consecutive years (van Everdigen,
1998). This definition did not cover all aspects of permafrost, excluding many of the physical aspects of permafrost soil that classify it as permafrost. A newer, more inclusive definition has been posed more recently that incorporates the different aspects of permafrost and its definition as it has continued to change and become broader over the years. This definition consists of two parts, with the first stating that a frozen soil can be characterized as permafrost, even if the permafrost does not contain ice/water. Permafrost that does not contain ice is considered a dry permafrost, as the permafrost only consists of solids, such as ice, soil and rock (Dobiński, 2011).
The second part of this definition is that if the permafrost does contain water, it can be frozen, partially frozen, or in a liquid state. This seems contradictory to the simplest definition of permafrost, but there are many well-known permafrost examples that are actually unfrozen. The point is that the water is still in a liquid state at 0°C, due to the phase shift that takes place at 0°C, or salts/particulates making the freezing point of soil water lower than 0°C (Dobiński, 2011).
There are two types of permafrost: old permafrost and young, “ecosystem-driven permafrost.”
Old permafrost is a remnant of paleoclimate, and is often characterized as being polygenetic, with syngenetic permafrost (continuous upward growth of deposits) having developed on epigenetic permafrost (growths which developed on old existing deposits) (Ping et al., 2015).
The younger “ecosystem-driven permafrost” formed during the Holocene as a result of deglaciation and vegetative succession (Ping et al., 2015). Though both old and young permafrost were formed under different climate conditions, and are of drastically different ages, both are impacted by similar physical biological processes, and therefore show many of the same physical attributes. Gelisol profiles are generally comprised of three different layers: the active layer, the transient layer, and the intermediate layer. The active layer is the topmost layer of the profile, which is above the permafrost, and is subject to seasonal thaw. The transient layer is the upper part of the permafrost, which has distinct layering of cryostructures due to fluctuations in the permafrost table. The intermediate layer is below the transient layer, which is characterized by ice-rich cryostructures associated with soil climate changes. Below these three layers is the
“true” permafrost, which is the part of the profile that historically has not been impacted by climate changes or freeze-thaw cycles (Ping et al., 2015).
Cryostructures are ice structures or patterns that are present within the soil, usually due to freeze- thaw processes. Cryostructures are seen as ice crystals, lenses or layers within the matrix of the soil, and are found in the active layer (Ping et al., 2015). Cryoturbation, or movement within the soil due to frost action, is where we see some of these structures occur, and is one of the dominant soil processes in regions with permafrost (Bockheim and Tarnocai, 1998). With cryoturbation we see cryoturbated horizons, which are irregular or broken, where mixing and
sorting of material from other horizons has taken place (Tarnocai, 2009). Cryoturbation is associated frost churning within a horizon, and specifically is associated with displacement of horizon material due to ice segregation and thermal cracking. In cryoturbated soils, organic matter, rocks, and ice are mixed into a broken horizon where these materials would usually be absent. Along with cryoturbation being a major soil process in regions with permafrost, cryoturbation-associated features are also often used for taxonomic identification of Gelisols in many parts of the world, making an understanding of cryoturbation vital to understand permafrost (Bockheim and Tarnocai, 1998; Jelinski, 2013).
Figure 1. Example of cryoturbation, with broken/contorted horizons, and organic matter intrusion (location unknown) (Bockheim and Tarnocai, 1998)
Permafrost soils, which encompass a range of ages and physical properties, are also defined by where on Earth they are found. Regions of permafrost are generally found at high altitudes and in cold climates (Margesin, 2009). Areas within the Arctic region that contain permafrost include
Canada, Russia, the United States (specifically Alaska), Greenland, and Scandinavia. This high latitude region is characterized by its long, extremely cold winters, and short, mild summers
(Tarnocai, 2009). Permafrost is also found in Antarctica, making up 35% of the continent. This high percentage of permafrost is partially due to the climate of Antarctica, which has some of the coldest annual temperatures on Earth (Campbell and Claridge, 2009). The third classification of permafrost is found in mountain ranges, which even if they are located in another permafrost region (i.e., the Arctic), are defined separately from that region (Gruver and Haeberli, 2009).
These permafrost soils are not defined by locational difference seen in comparison to Arctic or
Antarctic permafrost, but by the influence the mountain topography has had on the properties of the permafrost, as there is much more spatial variation seen within these soils than others categorized as permafrost (Gruber and Haeberli, 2009).
Figure 2. Examples of permafrost-affected soils from different regions: a. Ellef Ringes Island, Nunavut, Arctic Canada; b. Mould Bay, Arctic Canada; c. Mys Chukochi, Northern Yakutia, Russia; d. Arctic Alaska; e. northern-central Qinghai-Tibet Plateau, China; f. Qinghai-Tibet Plateau, China; g. northern Alaska; h. central Alaska, and i. central Alaska (Ping et al., 2015)
As the climate has changed, the global average temperature has increased, causing permafrost that had previously been frozen for hundreds to thousands of years to begin to thaw while experiencing seasonal freeze-thaw cycles as part of the deepening active layer. Freeze-thaw cycles are disruptive, altering biogeochemical and physical processes within the soil and leading to the release of greenhouse gases, such as carbon dioxide, nitrous oxides, and methane
(Chadburn et al., 2017). With these increased emissions, we expect to see a further increase in global temperature, which could lead to further thawing within permafrost. If this positive feedback loop between freeze-thaw cycles and climate change continues, a substantial loss of permafrost is predicted, with a calculated 40% loss by 2100 if warming halts at 2°C above pre- industrial levels (Chadburn et al., 2017). With these changes in climate, we are likely to see both the loss of permafrost and the continued loss of biodiversity, a negative impact on human health and wellbeing, and overall, more pressure on the Earth to continue to support our growing population (Davis, 2001). Though the topic of permafrost in climate change discussions is becoming more common, there are still many who are unaware of the importance of permafrost, and the implications of its loss. In order to fully understand these implications, we must understand the impacts of freeze-thaw cycles.
The Process of Freeze-Thaw and Its Physical Impacts
As we now know, permafrost traditionally would not experience freeze-thaw cycles regardless of the geologic time scale being examined. However, over the last 50 to 100 years, the
climate began to change, and we are now seeing evidence of freeze-thaw cycles occurring in a variety of permafrost locations (Davis, 2001). Climate change has placed us in a time of transition, necessitating predictions for future greenhouse gas emissions. As freeze-thaw cycles are a major part of this process, it is vital to understand how they work, and what impact freeze- thaw cycles have on permafrost and the active layer. The active layer experiences freeze-thaw cycles multiple times within a season, with the majority of cycles occurring in Fall and Spring
(Burn, 1998). The transient, intermediate, and true permafrost normally do not experience this seasonal thaw. The thaw associated with freeze-thaw cycles occurs when the belowground environment rises above 0°C, with the rise in temperature travelling downward from the surface of the profile (Davis, 2001). This thawing will take place for as long as the below ground environment is above 0°C, which can range from several hours to several weeks, depending on the season and day length of the area (Davis, 2001).
Freeze-thaw cycles have a variety of physical and morphological impacts on the active layer and underlying permafrost. One of the most discernable physical changes that takes place due to freeze-thaw cycles triggered by warming is the rise in ground temperature, and subsequent thawing of the top of the permafrost layer, thus increasing the thickness of the active layer. If the ground temperature moves above 0°C throughout the entire profile, the permafrost may begin to thaw at both the top and bottom, thus greatly altering the depth at which the permafrost can be found (Davis, 2001). Warming of the soil and thickening of the active layer often promotes increased freeze-thaw events, which triggers changes in the physical properties of the soil.
Depending on the soil environment, profile structure and vegetation present, the active layer can either thicken or thin with longer periods of thaw and more freeze-thaw cycles. Generally, in
areas where the active layer is right above the true permafrost (and therefore an absence of the transient and intermediate layers), a longer period of thaw associated with a rise in temperature will lead to a thickening of the active layer (Davis, 2001). Therefore, in areas with a distinct distance between the active layer and permafrost, or where the profile contains a transient and intermediate layer, a rise in temperature will lead to a decrease in the thickness of the active layer. However, it is also important to understand that along with the distance between the active layer and permafrost, snowfall within the area, and vegetation present can also alter the major physical changes seen in permafrost soils as the climate changes (Davis, 2001).
Along with large scale physical changes within permafrost soils undergoing freeze-thaw cycles, there are also a variety of microscopic changes that take place within the profile, including a change in water holding capacity and pore and aggregate stability. Permafrost soils that are experiencing warming generally hold more water, as frozen soils only hold the ice that is within the profile, while warming and thawed soils have a higher hydraulic conductivity and lower stability, and therefore are able to hold more water within their pore and aggregate structures
(Davis, 2001). With the increase in temperature, thawing landscapes lose the structural stability once provided by underlying ice (Davis, 2001). The process of freeze-thaw also impacts the strength and structure of soil aggregates. Freeze-thaw cycles decrease the stability of soil aggregates, with each freeze-thaw cycle leading to a further decrease in aggregate stability. This decrease in stability is seen at a higher magnitude in moist soils, as these unfrozen soils tend to hold more moisture. As a result of sequential and subsequent freeze-thaw cycles, permafrost soils become moister, and more unstable (Henry, 2007; Davis, 2001). While freeze-thaw cycles have impacts on the physical aspects of permafrost and permafrost soils, they also play a vital
role in the microbial communities of these soils, how they store greenhouse gases as these cycles occur, and how these gases are released.
Microbial Communities in Relation to Freeze-Thaw Cycles
Microbial communities play a major role in soil processes, maintaining the levels of water and air within the soil, cycling nutrients, and impacting carbon storage (Crawford et al., 2011).
Microbial communities are impacted by freeze-thaw cycles, and are responsible for much of the gas release, as their activity is what dictates if permafrost is acting as a carbon source or sink
(Graham et al., 2011). Before getting into the specifics of gas release due to microbial activity in freeze-thaw cycle impacted permafrost, we first must discuss the presence of microbes within the profile as a whole, where within the soil profile these microbes are located, and how they are affected by freeze-thaw cycles. The microbial population of any soil is large, on the order of 11 x
107 to 11 x 108 microbes within the top six inches of a profile (Hoorman and Islam, 2010). With permafrost and permafrost soils this is no exception, though the state of these organisms is slightly different than what we might expect in more temperate soils.
Microbial distribution within the soil profile, and community type within a given location is dependent on a variety of factors. The distribution of soil microbes within a profile reflects the processes that form and change the soil. For example, freeze-thaw triggered cryoturbation leads to buried material, including soil organic matter (SOM) that soil microbes feed on, and thus can greatly impact where large clusters of microbial communities are located throughout the soil profile (Ping et al., 2015; Jelinksi et al., 2013), with previous studies associating large clusters of microbes with SOM (Gittel et al., 2014). Within permafrost that has experienced cryoturbation,
the profile as a whole is low in microbial activity, but within cryoturbated zones soil microbial activity is high. These pockets of microbial communities consist of prokaryotes, with mainly bacterial and archaeal microbes (Gittel et al., 2014). Another factor that impacts the distribution of microbes is where the permafrost is located globally, as the microbial community of permafrost in the Arctic differs from that in Antarctic permafrost. The survival rate and diversity of microbes in these areas differ with permafrost age. The older the permafrost is, the less diverse the microbial communities are, and the less likely microbes are to be viable (Steven et al., 2009).
With warming increasing the frequency of freeze-thaw cycles, it is vital to consider how microbial communities will respond and how that response will impact climate change. The simple answer is that with the thawing of permafrost, whether that be thawing as part of freeze- thaw cycles, or a complete thaw with no re-freeze, the microbial communities once again become active, therefore beginning to break down the soil organic matter within the profile and releasing greenhouse gases (GHG) (Schimel and Clein, 1996; Waldrop et al., 2021). Several processes take place within the microbial community when permafrost thaw occurs. First, microbes become active once they are no longer frozen, as they are able to function and interact with the thawed material around them. Second, thaw triggers changes in functional gene abundances and pathways of microbial evolution, including genes that are involved in the cycling of nitrogen and carbon (Mackelprang et al., 2011). With the first freeze-thaw cycle, we see a great microbial response, specifically within the first 24 hours after each thaw (Schimel and
Clein, 1996). With each subsequent thaw, there is variation in microbial activity, as the microbial communities change with freeze-thaw cycles, generally adapting to the freeze-thaw cycle
processes. Though there is change within the microbial community, microbial respiration, and correlating GHG release, continues (Schimel and Clein, 1996; Ivarson and Sowden, 1970). The initial pulse of activity, and subsequent pulses that lead to the release of GHG are of major concern, because with each freeze-thaw cycle induced by a warming climate, more GHG are released, and the positive feedback loop associated with permafrost thaw and climate change is fueled.
The Release of Carbon Dioxide
When discussing climate change, the scientific community generally focuses on greenhouse gases. The most well-known, and most commonly discussed GHG is carbon dioxide (CO2). With permafrost soils containing about 1,600 billion tons of carbon, CO2 is one of the main GHG that is being released during freeze-thaw cycles (Turetsky et al., 2019). The amount of CO2 being released is expected to continue to grow if nothing is done to slow and stop the positive feedback loop between climate change and freeze-thaw cycles in permafrost (Tarnocai et al., 2009). To understand the release of carbon dioxide, we need to examine carbon storage within permafrost soils, the process by which carbon is decomposed by microbes within the soil, and how biproducts of that decomposition are released into the atmosphere. Understanding the different stages of carbon within the soil will illuminate why permafrost thaw in association with climate change is a major concern, and why permafrost must be brought into the global political, scientific, and public discussion of climate change.
Within all soils, soil organic matter (or the source of carbon within any given soil profile) accumulates much in the same fashion: through the deposition of organic matter such as surface
litter and rhizodeposits into the profile (Ping et al., 2015). Permafrost soils are no exception to these deposition processes. However, permafrost soils differ from most other soils in terms of volume of carbon stored (Ping et al., 2015). Within unfrozen soils, organic matter generally moves quickly through the profile, being broken down and taken up by plants rapidly. Within permafrost soils, the SOM generally is deposited within the profile, and then accumulates within the active layer and frozen portions of the profile rather than being taken up by plants or moving through the profile and leaching out. The accumulation of SOM is enhanced by two factors: low temperature and anaerobic conditions (Ping et al., 2015). Low temperatures slow the SOM decomposition processes that take place rapidly within thawed permafrost soils, while anaerobic conditions within the permafrost soils also slow the process of SOM decomposition by inhibiting oxygen availability and requiring the use of alternate terminal electron acceptors by microbes, and subsequent GHG release.
Knowing how carbon accumulation takes place is only part of the process for understanding how
CO2 is released into the atmosphere during freeze-thaw cycles. When SOM accumulates in the soil, the SOM is not immediately available to the microbes for breakdown and release into the atmosphere. Freeze-thaw cycles are a vital part of SOM availability to microbes. Freeze-thaw cycles impact the stability of soil aggregates, causing destabilization and increasing the availability of previously protected carbon to be broken down and released by microbes within the soil (Bailey et al., 2019). With the thawing of permafrost, we see an increased carbon stock from which microbes can pull and break down carbon. Soil microbes break down and alter soil carbon to pull energy from that carbon for the purpose of growth. Generally, microbes use the organic carbon that is available within the soil carbon pool, process the carbon in order to extract
energy for their own growth, oxidizing the carbon and transforming it into an inorganic form of carbon that will then be released into the atmosphere, usually as either CO2 or methane (CH4)
(Schuur et al., 2008).
The rate of release of greenhouse gases, as well as the estimated output into the atmosphere within the next few decades must be considered, as the rates of GHG emissions will greatly impact the rate of the positive feedback loop. It has been found that generally, in aerobic (oxygen rich) decomposition, the release of GHG into the atmosphere is much greater than the same processes in anaerobic (oxygen poor) decomposition. Aerobic CO2 emissions are an order of magnitude, or ten times higher, than the emission rate of CO2 in anaerobic conditions (Schuur et al., 2008). Since we cannot go out into the field and measure the emission rates from all the remote permafrost soils in the world, lab experiments have been used to evaluate the expected emission rates based on current warming trends. Permafrost zones are estimated to emit roughly
0.5 to 1 Pg (5.51 x 108 to 1.1 x 109 US tons) of carbon into the atmosphere per year (Canadell et al., 2007). For Canadian permafrost, scientists predicted that 48 Pg (5.29 x 1010 US tons) of carbon could be released into the atmosphere in this century, given a temperature increase of 4°C
(Tarnocai, 2006). For Alaska and surrounding regions, it is predicted that by the end of the century between 50 to 100 Pg (5.51 x 1010 to 1.1 x 1011 US tons) of carbon will be emitted into the environment (Stieglitz et al., 2003; Zhuang et al., 2006). It is estimated that if we experience
11 the emission of 167.2 Pg (1.84 x 10 US tons) of CO2 from permafrost by 2100, the global temperature will raise an additional 0.7°C (NASA, 2011). With the current research-based estimates for CO2 emissions out of global permafrost, this emission value is likely to be met, if not exceeded.
The Release of Other Gases into the Atmosphere
While carbon dioxide is the main greenhouse gas discussed in relation to global climate change, there are several other greenhouse gases emitted during permafrost thaw: methane (CH4), the other main carbon-based gas that is released during thaw, as well as nitrous oxide (N2O). The release of CH4 from permafrost is closely related to the release of CO2, though the final step within the process of making soil carbon into an inorganic form through breakdown for energy by microbes is slightly different (Walz et al., 2017). When it comes to CH4 production, there are specific methane-producing microbes that generate and release CH4 out of the soil, known as methanogens. CH4 production is higher within anaerobic conditions than CO2 production is, as the lack of oxygen is favorable to CH4 production. Since CO2 requires oxygen in order to be produced, this high proportion of CH4 to CO2 production in anaerobic conditions is expected
(Walz et al., 2017).
Figure 3. Examples of carbon cycling within the atmosphere. a. The relationship between microbial activity and permafrost thaw in conjunction to permafrost thawing; b. the “microbial priming effect” which stimulates the decomposition of old soil carbon, and c. the interaction between the carbon and nitrogen cycles (Heimann and Reichstien, 2008).
Methane, along with having a higher production rate within anaerobic conditions than carbon dioxide, also has a greater heating potential than carbon dioxide. CH4 has a heating potential between 28 and 34 times higher than CO2, meaning that even if the same amount of CH4 and
CO2 were released in a given location, the CH4 would have a much greater impact on the climate and climate change (Walz et al., 2017). Methane oxidation, which leads to the release of methane, increases with permafrost thaw and is linear throughout thaw stages (Perryman et al.,
2020). Though predictions of the exact values of CH4 emissions in the future are not easily determined, there are a variety of physical changes that are likely to be seen within methane- abundant areas. Permafrost peatlands, which are large sources of methane, are likely to emit higher levels of methane as thaw continues (Perryman et al., 2020). One of the major changes
that is likely to occur in relation to methane emissions, both within the permafrost peatlands as well as the permafrost regions in general, is changes in emission rates, as well as total ecosystem and landscape changes (Schuur et al., 2015).
Nitrous oxide, or N2O, is another greenhouse gas that is of great concern when it comes to permafrost experiencing freeze-thaw cycles in relation to climate change. N2O is processed and released from the soil in a two-step process. The first step is nitrification, in which organic nitrogen in the soil is mineralized, then converted into a nitrogen-based molecule, such as
NH2OH or NO2-. From here, the second step of the process, known as denitrification, will take place, which involves converting NH2OH directly to N2O, or NO2- to NO that is then converted to N2O (Signor and Cerri, 2013). N2O release during freeze-thaw cycles has been observed to be the highest during the first thaw, with declining emission rates during subsequent freeze-thaw cycles (Priemé and Christensen, 2001). Along with this, it has been determined that certain processes have an impact on N2O release that lead to higher emissions, and thawing is one of these processes (Priemé and Christensen, 2001). N2O, like CO2 and CH4, is vital to consider when thinking about greenhouse gas emissions in relation to climate change, as N2O has a warming potential that is 25 to 298 times greater than that of CO2 (Signor and Cerri, 2013). The release of nitrous oxide into the atmosphere must be considered when thinking about the future impacts of climate change and freeze-thaw cycles.
Expected Future Changes
Scientists, policy makers, and citizens must consider what future changes are likely to occur because of climate change as well as permafrost thaw. The amplification of climate change
through a positive feedback loop is likely to occur if significant climate actions are not taken in the near future. There is often the misconception that because a majority of the Earth is warming, another major part of the Earth must be cooling; the idea that “Cold places get cooler, and warm places get warmer” (Kennedy, 2020). Permafrost can be found in some of the coldest places on
Earth, and unfortunately warming is taking place at a higher rate within these regions compared to the global average (Biskaborn et al., 2019). Permafrost temperatures rose by about 0.19°C between 2007 and 2016. In the continuous permafrost zone, the annual temperature of permafrost rose by 0.39°C, and in the discontinuous permafrost region, the annual temperature rose by 0.20°C (Biskaborn et al., 2019). With the sensitivity of permafrost to climate change, and particularly rising air temperatures, the positive feedback loop between atmosphere and thawing ground is of major concern, as the emission rates of GHG from permafrost will be high, and unfortunately the feedback loop is most likely already in effect with amplified climate change imminent (Koven et al., 2011).
Along with an amplification in climate change, we are also likely to see changes across physical landscapes in permafrost regions, hazards to human health, and threats to the globe as a whole, with threats of rising temperatures pushing species out of their range of tolerance, and the rise of oceans forcing humans and species to relocate (Yalcin, 2020). The physical landscape of any given area is always changing, whether that be from plants growing or dying, extreme weather events, or changes in the soil environment due to shifts in microbial communities. In permafrost regions this is no exception, however with freeze-thaw cycles in the permafrost and permafrost soils, there are a variety of detrimental landscape and soil process changes that may occur.
Soil erosion is one of the physical changes that is being seen in permafrost regions experiencing freeze-thaw. While soil erosion in association with spring thaw is normal, it has been determined that erosion is amplified greatly after freeze-thaw cycles in permafrost soils, especially if the soil has a high moisture content (Ferrick and Gatto, 2005). Soil erosion is a major concern, as it negatively impacts the soil environment as a whole through removing active soil from the profile, exposing underlying permafrost to additional thaw, while also impacting bodies of water and organisms reliant on those water bodies through the deposition of sediment. Along with these smaller, harder to observe changes, there are also larger, visible changes that will likely occur if freeze-thaw cycles and total permafrost thaw occurs. These changes include collapses/cave-ins along shorelines, and trees submerges in and along thermokarst lakes which result from ground subsidence following permafrost thaw (Davis, 2001; Murton, 2009). Along with changes in natural landscapes, there are also changes being seen in engineered ground, such as in roadway and building-containing ground. The ground can become unstable, with changes in the level of the roadways and pathways, as well as flooding and water pooling, impacting the safety of those using them, and their usability as a whole (Davis, 2001; Turetsky et al., 2019).
These changes also have a cost, as it is expensive to rebuild homes and whole roads due to changes in ground stability.
Figure 4. Areas of roadway and pathway that have been impacted by freeze-thaw cycles in permafrost. Left: An abandoned portion of a Highway outside of Fairbanks, Alaska, and 2: a bike trial in Goldstream Valley near Fairbanks, Alaska (Davis, 2001)
Along with climate change and permafrost thaw impacting human infrastructure, there is also a negative impact on human health globally outside of permafrost regions experiencing thaw. One of the first health-related issues associated with climate change is heat stroke resulting from heat waves, and an overall rise in global temperature. Along with heat-stroke related deaths, deaths due to malnourishment are likely to increase, as greater average atmospheric temperatures lead to a higher rate of crop loss, and therefore less food to feed those in need (Patz et al., 2005). In addition to contributing to heat related deaths, climate change will also lead to higher rates of infectious diseases within the population. Because disease carrying agents tend to reproduce more rapidly in warmer temperatures, the rising atmospheric temperature will lead to higher rates of infectious diseases, including Malaria, Dengue fever, and even the plague (Patz et al., 2005).
As of 2004, it was believed that by 2030, the mortality risk associated with climate change was likely to double. Given current trends and continued change in our climate this prediction is more than likely to become a reality (McMichael et al., 2004).
Another negative aspect of climate change is its impact on plant life and ecosystems globally. As stated previously, crop production has been, and is likely to continue to be impacted by climate change, and plants outside of the crop system are likely to be impacted much in the same way
(Patz et al., 2004). Changes in temperature is likely to push plants, as well as many animals, out of their range of tolerance, forcing them to either move to a cooler location, or causing them to go extinct. Other plants are likely to prosper from a warmer climate, specifically in the tundra areas, which would then lead to great absorption of solar radiation in those areas, and an increase in regional temperatures (Schuur et al., 2008). The impacts of climate change are visible in both terrestrial environments and aquatic environments. We see that glaciers are currently retreating and melting at great rates, leading to sea level rise. Along with sea level rise threatening ecosystems and coastal infrastructure, we are also seeing increases in oceanic temperatures. This temperature increase is leading to many species becoming threatened or extinct, and changing the processes of whole ocean ecosystems (Yalcin, 2020). Climate change is a serious concern, and the addition of permafrost experiencing freeze-thaw cycles is only going to intensify these changes if no action is taken.
Conclusion
Globally, we are at a tipping point in the fight to address climate change, as decisions we make daily will impact the climate for years to come. It is becoming clear that permafrost is an important factor in the climate change discussion that must be considered as we move forward with making change to slow, and hopefully stop, climate change through reducing greenhouse gas emissions and turning to more sustainable daily practices. Permafrost experiencing freeze- thaw cycles have a larger impact than just changing the landscape where they are located; these
cycles also pose a threat to the processes associated with cycling nutrients and greenhouse gases, the structures and functions of ecosystems, and to human health. By bringing permafrost into the discussion of climate change, and thinking about the impacts of freeze-thaw cycles within these soils, we will be able to broaden our discussion by considering a major part of the carbon cycle, thinking about the impacts that we have had, and continue to have on the environment, while also working towards ways to preserve both permafrost regions, as well as global ecosystems and the climate.
Acknowledgements
I would like to thank my mentor Dr. Rebecca Lybrand, who met with me over two years ago, and saw potential in my ideas, even though I had only ever taken a single soil science course.
Along with Dr. Lybrand, I would like to thank my graduate student mentor Erin Rooney, who has provided me with endless support throughout the entire thesis process. I would also like to thank Dr. Jeff Hatten, who joined my thesis committee happily and openly. Along with those who worked directly with my on my thesis, I would also like to thank my friends and family, who supported me through this process, and were more than willing to listen to me talk about soil. Finally, I would like to thank Derek MacDicken, Julieanne Quigley and Sharon Barnes, who originally introduced me to science, and showed me what an amazing and far-reaching topic it is.
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