Arctic Permafrost Melt and Relative Soil Carbon Release: A Literature Review
Third Place
Rebekah Watkins is a sophomore at Mat-Su College currently pursuing a degree in Environmental Sciences, focusing on Climate Change Biology. She is passionate about wildlife preservation and fighting the effects of climate change on all areas of Alaska’s diverse economy. When not working or studying, she enjoys hiking, skiing, making music, and petting cats.
Abstract
Northern permafrost soils are melting at an alarming pace, concerning scientists studying carbon emissions related to this rapid meltdown. As cryospheric regions contain the largest terrestrial carbon pool on Earth, it is prudent to consider this matter a global crisis and continue research with urgency. Literature regarding this topic highlights several recurring themes, specifically the balance of microbial communities and fluctuation of soil carbon release during “shoulder seasons.” As these themes present themselves over the wide range of this topic, so, too, do some significant research gaps; scientists such as Serikova et al. (2019), Mitzscherling et al. (2019), and Arndt et al. (2020) delineate an overall need of improved biogeochemical models, further analysis of organic matter quality, further analysis of chemical composition of permafrost dissolved organic matter (PDOM), and integration of lake C emission estimates to accurately predict permafrost feedback to a warming climate.Arctic Permafrost Melt and Relative Soil Carbon Release: A Literature Review
Climate change has long been the subject of global attention; however, research and preventative action has faced significant resistance from fossil-fuel industries as well as the United States government. Only recently has it been recognized as an issue requiring immediate action and treated as a global crisis. This delayed response has had serious consequences, the most important being gaps in knowledge specifically concerning the responses of arctic ecosystems to rapid climate change. It is commonly known that man-made carbon emissions are a significant threat to the stability of the atmosphere, but little focus is placed on natural gas release in cryospheric regions. This is disturbing, as arctic tundra methane emissions account for approximately 45% of all arctic methane production and around 7% worldwide (Bao et al., 2020). Several major factors can be attributed to this, the most important being that the Arctic is warming at nearly double the global rate (Bao et al., 2020), triggering a chain of events that are upsetting the balance of microbial communities responsible for organic carbon matter breakdown and leading to the formation of carbon-rich lakes, called thermokarst lakes. Bao et al. (2020) note that these emissions are more apparent during shoulder seasons, specifically autumn freeze, and highlights the importance of studying methane fluctuations during these seasons. This review provides an in-depth look at increased carbon emissions due to climate change-related rapid warming of permafrost soils in cryospheric regions.Links Between Microbial Communities and Soil Carbon Release
Microbial communities are responsible for the breakdown of organic matter in soil, which contribute to a significant amount of terrestrial carbon (see Figure 1); this is most noticeably released during natural spring thaw and autumn freeze shoulder seasons (Yongliang et al., 2020). However, due to rapid melting of perennially frozen permafrost soils, large amounts of harmful carbon are being released into the atmosphere at an uncontrolled pace (Arndt et al., 2020; Bao et al., 2020; Chen et al., 2020; Katey et al., 2018; Kuhn et al., 2018; Natali et al., 2019; Turetsky, 2019; Wang et al., 2018). It was previously known that microbial respiration exponentially increases with rising temperature, meaning that respiration rates are higher during the growing season than the non-growing season (Arndt et al., 2020). Adequate research has been done during this time of year, but the cold season respiration rates have been generally overlooked (considering perennially frozen soils have previously been successful in containing gasses). Consequently, it has been assumed that the cold season is irrelevant to the balance of carbon emissions; however, recent data has shown it accounts for over half of yearly arctic emissions and has the potential to offset summer photosynthesis carbon uptake (Arndt et al., 2020), or the absorption of carbon by plants. Essentially, more CO2 is being released than can be naturally processed, leading to an unprecedented leakage into the atmosphere. It is important to note these tendencies to accurately account for soil microbial composition as this has the potential to predict annual carbon and methane release. (See Figure 2 on methane production.) Without an accurate picture of future patterns, it will be incredibly difficult to adequately prepare or compensate for possible repercussions.Note. Adapted from Trends in atmospheric concentrations of CO2 (ppm), CH4 (ppb) and N2O (ppb), between 1800 and 2017 by the European Environment Agency, 2019, https://www.eea.europa.eu/ and also from Trends in Atmospheric CH4 by Ed Dlugokencky, 2021, NOAA/GML https://gml.noaa.gov/ccgg/trends_ch4/
Note. Adapted from Trends in atmospheric concentrations of CO2 (ppm), CH4 (ppb) and N2O (ppb), between 1800 and 2017 by the European Environment Agency, 2019, https://www.eea.europa.eu/ and also from Trends in Atmospheric CH4 by Ed Dlugokencky, 2021, NOAA/GML https://gml.noaa.gov/ccgg/trends_ch4/
Considering the breakdown of soil organic material belongs to soil microorganisms, it is imperative to understand the community responses to deep-level permafrost melt (Yongliang et al., 2020). Naturally, the functional processes of microbial communities have been significantly affected by the increase/decrease of ground temperature; according to a study conducted by Kwon et al. (2021), heterotrophic respiration rates showed a marked increase during the summer season in control-drainage sites versus control-wet sites. Conversely, methanogenetic substance abundance saw a decrease following rapid drainage, indicating a strong link between groundwater levels and community composition. It seems that the rapid drainage of water eliminated organisms incapable of autotrophy and led to an overabundance of more resilient organisms, whereas the controlled submersion of water encouraged community taxonomy. Kwon et al. (2020) examined taxonomy at both a surface and subsurface level and saw the same results; they note that exponential fungal abundance in these drained sites may have contributed to increased respiration in topsoil. This growth in fungi indicates an increase in breakdown of organic matter and thus carbon dioxide emissions (Kwon et al., 2021). Unfortunately, there is difficulty in tracking functional genes that produce carbon dioxide, as the majority respire and therefore add to emissions (Kwon et al., 2021). Yongliang et al. (2020) emphasize microbial functioning genes, however, hypothesizing instead that the response of these genes to permafrost thaw has more influence on the moderation of the carbon cycle than the taxonomy of these communities. Overall, there is a common pattern of decrease in taxonomy and increase in gene sequencing (Kwon et al., 2021, Yongliang et al., 2020); Yongliang et al. hypothesize the following reasons for this. Hydrological changes associated with permafrost melt could easily destroy rare microbial communities, the less competitive nature of these rare microbes may be overrun by more abundant species, and the fragile relic DNA they are composed of can be rapidly decomposed after losing the protection of soil minerals and low temperatures (Yongliang et al., 2020; Balhausen et al., 2020).
That brings up another important factor to consider when examining microbial communities. According to Oberbauer et al. (2007), Oechel et al. (1998), and Shaver et al. (2006), meltwater plays an incredibly significant role in tundra carbon exchange because of the microbial functions of soil moisture. With future threats of continued warming, increased precipitation and topographical changes due to permafrost collapse threaten to decimate these sensitive communities. It is prudent to consider the effect this rapid melt is having on animal species in these regions as well, as they also play a part in the balance of carbon loss. Habitats are shrinking due to the transition of tundra to wetlands. This has already affected lichen species, which are the food of larger species such as reindeer and lemming. With the possible disappearance of these omnivores, wolves and scavengers can be affected as well (not to mention animals dependent on sea ice). Hydrological changes should be considered one of the largest dangers associated with climate warming and pose a significant threat to arctic ecosystems (Gockede et al., 2019).
Thermokarst Lakes
One of the most notable hydrological issues affecting these communities is the development of thermokarst lakes. Thermokarst represents the largest spread of permafrost thaw in the world and refers to the rapid warming of ground soil leading to ice melt and ground collapse (Katey et al., 2018). As the permafrost melts, it triggers the breakdown of organic matter and pools groundwater into small lakes that, in turn, quickly melt surrounding permafrost soils. Any major hydrological shift upsets the balance of affected microbial communities and accelerates CO2 production, and, considering the rapid pace at which these lakes have recently been developing, this is indicative of large-scale compositional changes and has garnered significant attention from scientists such as Serikova et al. (2019) and Katey et al. (2018, 2021). In Serikova et al.’s (2019) examination of thermokarst lakes in northern Siberia, water temperature seemed less relevant to escape of C gasses as depth and types of ponds (Kuhn et al., 2008); however, colder water (prevalent in the Arctic) tended to contain more C gasses. Shallower ponds on average contained less C, most likely because the water and sediment surface stayed warmer and more oxidized. Vegetation is also presumed to play an important role in C uptake; in deeper lakes with little to no vegetation, more gasses are contained in the water itself and released through ebullition (attributed to approximately 71% of the CH4 flux; Kuhn et al., 2008). With the decline of tundra plant life, less carbon is able to be properly cycled during the normal spring thaw season (when permafrost melt increases), and adds to the amount being released through these lakes. The thermokarst process is rapidly expediting this carbon loss and destroying arctic tundra, prompting a climactic shift towards wetlands which will ultimately lead to the possible extinction of fragile species that are not able to adapt or migrate quickly enough. Katey et al. (2018, 2021) also discuss the need to examine thaw conduits beneath arctic lakes to assess the potential for positive feedback to climate change. Previous data has focused on near-surface thawing and neglected to examine deeper, subsurface thaw patterns, although according to Katey et al.’s findings (2018), emissions from beneath thermokarst lakes “more than double radiative forcing from circumpolar permafrost-soil carbon fluxes this century” (para. 1). This refers to the amount of energy being radiated from the earth back to the sun and returning as heat; to claim that a singular phenomenon such as thermokarst is responsible for more than double the entire carbon loss of the arctic fluxes is incredibly significant and needs to be immediately addressed. If ignored, future warming rates truly may be uncontrollable.Emissions During Shoulder Seasons
Significant research has been done concerning C emissions during the summer growing season; however, cold season emissions have been generally overlooked in the past per the idea that frozen soils effectively trap these gasses below the surface. While this was previously correct, recent research has indicated that non-growing season emissions are more significant than the growing season, due to thermokarst ground collapse and the development of pockmarks/vents allowing the escape of these fossil C gasses (Katey et al., 2021). According to Arndt et al. (2020), most arctic carbon loss occurs during the winter season. It is difficult to pinpoint exact causes for this, however Natali et al. (2019) note that winter respiration rates are largely influenced by these factors: vegetation type and density, availability of labile carbon substrates, soil moisture, microbial community composition and function capability, and snow depth. They also point out a gap in data in these areas due to inadequate equipment and inaccurate large-scale models, not to mention the incredibly complex and ever-changing relationships between these factors. It is difficult to monitor soil activity when it is covered in snow, especially across such vast and diverse regions, and regional soil data collection is necessary to begin to map out patterns. It can be said with certainty that the carbon sink that occurs during the long winters offsets the photosynthetic uptake of the growing season (Arndt et al., 2020); however, rapid (burst) emissions during shoulder seasons still represent a significant amount of yearly carbon release and needs to be monitored (Arndt et al., 2020; Bao et al., 2020; Wang et al., 2018).It is important to have an understanding of when emissions are highest and when they seem to decline so that accurate models can be created to represent these seasonal fluctuations. Research has shown that even with current knowledge of shoulder seasons, it is still very difficult to hypothesize past and future emission rates, mainly due to burst emissions that typically occur in the summer and lack of large-scale models. Bao et al. (2020), in their examination of shoulder season emissions, state that they contribute to approximately a quarter of annual total CH4 emissions. Another relevant source of emissions occur during what Arndt et al. (2020) refer to as the “zero-curtain phenomenon” (para. 3), when ground soil temperatures even out at around 0 degrees Celsius but continue to emit steady amounts of methane and carbon dioxide. In fact, this is attributed to much of the global annual carbon balance. Arndt et al. (2020) suggest that the zero-curtain is responsible for the buildup of gasses leading to these bursts and highlights the importance of examining soil freezing processes. Burst emissions during this season have been the subject of significant attention since they have raised the fall CH4 emissions from 37% to 92% of spring emissions (Arndt et al., 2020) (para 3). Future research into spring burst emissions is necessary to better understand total annual carbon release, which is now known to offset around 46% of summer uptake (Arndt et al., 2020) (para 4). Wang et al. (2018) note that although there have been many studies done on CO2 uptake during summer months, the methodology does not translate well to large-scale regional models and, therefore, cannot be considered completely accurate. This is significant because land surface models are vital to understanding and anticipating the responses of natural processes to global change (Fisher & Koven, 2020). One possible mitigation, according to Holmes at al., (2004), is widespread soil data collection along with remotely sensed land-cover data. This provides an opportunity for data gathered from a variety of regional soil conditions to be directly quantified, predicting large-scale environmental patterns (Holmes et al., 2004).
Research Limitations
It is apparent that net carbon exchange depends on the balance between carbon uptake and carbon losses. Liue et al. (2019) describes it as such:Essentially, it most likely will not be possible to replicate the responses of community reactions to rapid melt because there is such variance in topography, plant life, rate of melting, and amount of labile carbon substrates across the span of the Arctic. It important that data samples are taken from all across these regions during all seasons to ensure the accuracy of future climate models. Thus, there is an urgent need for year-round regional studies focusing on soil community composition, soil moisture content, lake C estimates, and improved large-scale biogeochemical and land surface models (Arndt et al, 2020).The rates of these processes will increase with warming but it remains unclear which will dominate the net carbon balance in the future due to complex interactions among permafrost thaw-induced subsidence, hydrology, and nutrients, which leads to inconsistencies between model simulations and field data (section 4.1, para. 1).
Conclusion
History has provided many examples of the destructive consequences of rapid climate change. For example, it is relevant to mention the Paleocene-Eocene Thermal Maximum (PETM), in which a temperature rise of around 5-8 degrees Celsius occurred and released around 3,000 gigatons of CO2 into the atmosphere. This resulted in the extinction of several marine protozoans, overgrowth of specie regimes, and dwarfing of several species of mammals. The rate at which this occurred is an estimated 0.025 degrees Celsius per hundred years versus current estimated warming rates of 1-4 degrees Celsius, nearly ten times faster than PETM (Stiepani, 2016). The PETM is comparable to our current climate model (albeit slower paced) and should provoke deep concern, considering future consequences are likely to be more drastic if preventative measures are not quickly taken.While it will be impossible to have a completely accurate picture of future warming, it is imperative that as much data as possible is collected on this topic to monitor C uptake, focusing on microbial community composition, hydrology, and seasonal emissions. Neglecting research into this intricate process will surely risk a repeat of a climate emergency such as PETM and the complete offset of atmospheric balance and collapse of arctic ecosystems. The entire community, from microbial to human, depends on stable temperatures, and there is a consequent need for increased awareness in affected areas. Grim outcomes should be expected if this pattern of rapid melt continues, including habitat destruction, wildfires, glacial melt, flooding, hillside collapse, extinction of native wild and plant life, and uncontrollable large-scale rapid temperature fluxes, which can be especially devastating to already arid climates. In essence, it is necessary to understand the crucial role arctic ecosystems play in regulating carbon cycles to better protect and prepare for a warming world.
References
Arndt, K. A., Lipson, D. A., Hashemi, J., Oechel, W. C., & Zona, D. (2020). Snow melt stimulates ecosystem respiration in Arctic ecosystems. Global Change Biology, 26(9), 5042–5051. https://doi.org/10.1111/gcb.15193
Ballhausen, M. B., Hewitt, R., & Rillig, M. C. (2020). Mimicking climate warming effects on Alaskan soil microbial communities via gradual temperature increase. Scientific Reports, 10(1). https://doi.org/10.1038/s41598-020-65329-x
Bao, T., Xu, X., Jia, G., Billesbach, D. P., & Sullivan, R. C. (2020). Much stronger tundra methane emissions during autumn freeze than spring thaw. Global Change Biology, 27(2), 376–387. https://doi.org/10.1111/gcb.15421
Chen, Y., Liu, F., Kang, L., Zhang, D., Kou, D., Mao, C., Qin, S., Zhang, Q., & Yang, Y. (2021). Large‐scale evidence for microbial response and associated carbon release after permafrost thaw. Global Change Biology, 27(14), 3218–3229. https://doi.org/10.1111/gcb.15487
Fisher, R. A., & Koven, C. D. (2020). Perspectives on the Future of Land Surface Models and the Challenges of Representing Complex Terrestrial Systems. Journal of Advances in Modeling Earth Systems, 12(4). https://doi.org/10.1029/2018ms001453
Holmes, K. W., Roberts, D. A., Sweeney, S., Numata, I., Matricardi, E., Biggs, T. W., Batista, G., & Chadwick, O. A. (2004). Soil databases and the problem of establishing regional biogeochemical trends. Global Change Biology, 10(5), 796–814. https://doi.org/10.1111/j.1529-8817.2003.00753.x
Kramshøj, M., Albers, C. N., Svendsen, S. H., Björkman, M. P., Lindwall, F., Björk, R. G., & Rinnan, R. (2019). Volatile emissions from thawing permafrost soils are influenced by meltwater drainage conditions. Global Change Biology, 25(5), 1704–1716. https://doi.org/10.1111/gcb.14582
Kuhn, M., Lundin, E. J., Giesler, R., Johansson, M., & Karlsson, J. (2018). Emissions from thaw ponds largely offset the carbon sink of northern permafrost wetlands. Scientific Reports, 8(1). https://doi.org/10.1038/s41598-018-27770-x
Liu, Z., Kimball, J. S., Parazoo, N. C., Ballantyne, A. P., Wang, W. J., Madani, N., Pan, C. G., Watts, J. D., Reichle, R. H., Sonnentag, O., Marsh, P., Hurkuck, M., Helbig, M., Quinton, W. L., Zona, D., Ueyama, M., Kobayashi, H., & Euskirchen, E. S. (2019). Increased high‐latitude photosynthetic carbon gain offset by respiration carbon loss during an anomalous warm winter to spring transition. Global Change Biology, 26(2), 682–696. https://doi.org/10.1111/gcb.14863
Mitzscherling, J., Horn, F., Winterfeld, M., Mahler, L., Kallmeyer, J., Overduin, P. P., Schirrmeister, L., Winkel, M., Grigoriev, M. N., Wagner, D., & Liebner, S. (2019). Microbial community composition and abundance after millennia of submarine permafrost warming. Biogeosciences, 16(19), 3941–3958. https://doi.org/10.5194/bg-16- 3941-2019
Natali, S. M., Watts, J. D., Rogers, B. M., Potter, S., Ludwig, S. M., Selbmann, A. K., Sullivan, P. F., Abbott, B. W., Arndt, K. A., Birch, L., Björkman, M. P., Bloom, A. A., Celis, G., Christensen, T. R., Christiansen, C. T., Commane, R., Cooper, E. J., Crill, P., Czimczik, C., . . . Zona, D. (2019). Large loss of CO2 in winter observed across the northern permafrost region. Nature Climate Change, 9(11), 852–857. https://doi.org/10.1038/s41558-019-0592-8
Serikova, S., Pokrovsky, O. S., Laudon, H., Krickov, I. V., Lim, A. G., Manasypov, R. M., & Karlsson, J. (2019). High carbon emissions from thermokarst lakes of Western Siberia. Nature Communications, 10(1). https://doi.org/10.1038/s41467-019-09592-1
Stiepani, J. (2016). Paleoclimatology: Examples of Ecological Impacts from Prehistoric Climate Change. Climate Institute. https://climate.org/paleoclimatology-examples-of-ecological impacts-from-prehistoric-climate-change/
Sullivan, T. D., Parsekian, A. D., Sharp, J., Hanke, P. J., Thalasso, F., Shapley, M., Engram, M., & Walter Anthony, K. (2021). Influence of permafrost thaw on an extreme geologic methane seep. Permafrost and Periglacial Processes, 32(3), 484–502. https://doi.org/10.1002/ppp.2114
Turetsky, M. R., Abbott, B. W., Jones, M. C., Walter Anthony, K., Olefeldt, D., Schuur, E. A. G., Koven, C., McGuire, A. D., Grosse, G., Kuhry, P., Hugelius, G., Lawrence, D. M., Gibson, C., & Sannel, A. B. K. (2019). Permafrost collapse is accelerating carbon release. Nature, 569(7754), 32–34. https://doi.org/10.1038/d41586-019-01313-4
Walter Anthony, K., Schneider Von Deimling, T., Nitze, I., Frolking, S., Emond, A., Daanen, R., Anthony, P., Lindgren, P., Jones, B., & Grosse, G. (2018). 21st-century modeled permafrost carbon emissions accelerated by abrupt thaw beneath lakes. Nature Communications, 9(1). https://doi.org/10.1038/s41467-018-05738-9
Wang, T., Liu, D., Piao, S., Wang, Y., Wang, X., Guo, H., Lian, X., Burkhart, J. F., Ciais, P., Huang, M., Janssens, I., Li, Y., Liu, Y., Peñuelas, J., Peng, S., Yang, H., Yao, Y., Yin, Y., & Zhao, Y. (2018). Emerging negative impact of warming on summer carbon uptake in northern ecosystems. Nature Communications, 9(1). https://doi.org/10.1038/s41467- 018-07813-7
Yi, Y., Kimball, J. S., Watts, J. D., Natali, S. M., Zona, D., Liu, J., Ueyama, M., Kobayashi, H., Oechel, W., & Miller, C. E. (2020). Investigating the sensitivity of soil heterotrophic respiration to recent snow cover changes in Alaska using a satellite-based permafrost carbon model. Biogeosciences, 17(22), 5861–5882. https://doi.org/10.5194/bg-17-5861- 2020
Zhang, L., Xia, X., Liu, S., Zhang, S., Li, S., Wang, J., Wang, G., Gao, H., Zhang, Z., Wang, Q., Wen, W., Liu, R., Yang, Z., Stanley, E. H., & Raymond, P. A. (2020). Significant methane ebullition from alpine permafrost rivers on the East Qinghai–Tibet Plateau. Nature Geoscience, 13(5), 349–354. https://doi.org/10.1038/s41561-020-0571-8