Climate change is an existential threat to the vast global permafrost domain. The diverse human cultures, ecological communities, and biogeochemical cycles of this tenth of the planet depend on the ...persistence of frozen conditions. The complexity, immensity, and remoteness of permafrost ecosystems make it difficult to grasp how quickly things are changing and what can be done about it. Here, we summarize terrestrial and marine changes in the permafrost domain with an eye toward global policy. While many questions remain, we know that continued fossil fuel burning is incompatible with the continued existence of the permafrost domain as we know it. If we fail to protect permafrost ecosystems, the consequences for human rights, biosphere integrity, and global climate will be severe. The policy implications are clear: the faster we reduce human emissions and draw down atmospheric CO
2
, the more of the permafrost domain we can save. Emissions reduction targets must be strengthened and accompanied by support for local peoples to protect intact ecological communities and natural carbon sinks within the permafrost domain. Some proposed geoengineering interventions such as solar shading, surface albedo modification, and vegetation manipulations are unproven and may exacerbate environmental injustice without providing lasting protection. Conversely, astounding advances in renewable energy have reopened viable pathways to halve human greenhouse gas emissions by 2030 and effectively stop them well before 2050. We call on leaders, corporations, researchers, and citizens everywhere to acknowledge the global importance of the permafrost domain and work towards climate restoration and empowerment of Indigenous and immigrant communities in these regions.
Nonstructural carbohydrates (NSC) are the most important C reserves in the tissues of deciduous and evergreen tree species. Besides NSC, cell-wall hemicelluloses as the second most abundant ...polysaccharides in plants have often been discussed to serve as additional mobile carbon (C) reserves during periods of enhanced carbon-sink activities. To assess the significance of hemicelluloses as mobile carbon reserves, branches of two deciduous (Carpinus betulus L. and Fagus sylvatica L.) and two evergreen (Picea abies L. and Pinus sylvestris L.) tree species were sampled in a mature mixed forest stand in short intervals before and during bud break to assess NSC and hemicellulose concentrations in response to the increased carbon demand during bud break. Starch concentrations in branch sapwood of deciduous trees strongly decreased immediately before bud break and increased after bud break. In both evergreen species, only small changes of NSC were found in branch sapwood. However, 1-year-old needles exhibited a significant increase in starch concentration shortly before bud break which declined again after flushing. Hemicellulose concentrations (on an NSC-free dry matter basis) in branch sapwood of Carpinus decreased significantly shortly before bud break, but increased again after bud break. Contrarily, in Fagus branch sapwood, hemicellulose concentrations remained constant during bud break. Moderate increases of total hemicellulose concentrations before bud break were found in 1-year-old needles of both conifers, which could be explained by an accumulation of glucose units within the hemicellulose fraction. Overall, cell-wall hemicelluloses appeared to respond in a species-specific manner to the enhanced carbon demand during bud break. Hemicelluloses in branch sapwood of Carpinus and in 1-year-old needles of conifers likely act as additional carbon reserves similar to starch.
Predicting vegetation phenology in response to changing environmental factors is key in understanding feedbacks between the biosphere and the climate system. Experimental approaches extending the ...temperature range beyond historic climate variability provide a unique opportunity to identify model structures that are best suited to predicting phenological changes under future climate scenarios. Here, we model spring and autumn phenological transition dates obtained from digital repeat photography in a boreal Picea‐Sphagnum bog in response to a gradient of whole ecosystem warming manipulations of up to +9°C, using five years of observational data. In spring, seven equally best‐performing models for Larix utilized the accumulation of growing degree days as a common driver for temperature forcing. For Picea, the best two models were sequential models requiring winter chilling before spring forcing temperature is accumulated. In shrub, parallel models with chilling and forcing requirements occurring simultaneously were identified as the best models. Autumn models were substantially improved when a CO2 parameter was included. Overall, the combination of experimental manipulations and multiple years of observations combined with variation in weather provided the framework to rule out a large number of candidate models and to identify best spring and autumn models for each plant functional type.
Here, we leverage multiyear interannual variability in climate with an experimental whole‐ecosystem warming approach to identify model structures that are best suited to predicting phenological changes and future climate scenarios. In spring, seven equally best‐performing models for Larix utilized the accumulation of growing degree days as a common driver for temperature forcing. For Picea, the best two models were sequential models requiring winter chilling before spring forcing temperature is accumulated. In shrub, parallel models with chilling and forcing requirements occurring simultaneously were identified as the best models. Autumn models were substantially improved when a CO2 parameter was included.
Warming of the Arctic can stimulate microbial decomposition and release of permafrost soil carbon (C) as greenhouse gases, and thus has the potential to influence climate change. At the same time, ...plant growth can be stimulated and offset C release. This study presents a 15‐year time series comprising chamber and eddy covariance measurements of net ecosystem C exchange in a tundra ecosystem in Alaska where permafrost has been degrading due to regional warming. The site was a carbon dioxide source to the atmosphere with a cumulative total loss of 781.6 g C m−2 over the study period. Both gross primary productivity (GPP) and ecosystem respiration (Reco) were already likely higher than historical levels such that increases in Reco losses overwhelmed GPP gains in most years. This shift to a net C source to the atmosphere likely started in the early 1990s when permafrost was observed to warm and thaw at the site. Shifts in the plant community occur more slowly and are likely to constrain future GPP increases as compared to more rapid shifts in the microbial community that contribute to increased Reco. Observed rates suggest that cumulative net soil C loss of 4.18–10.00 kg C m−2—8%–20% of the current active layer soil C pool—could occur from 2020 to the end of the century. This amount of permafrost C loss to the atmosphere represents a significant accelerating feedback to climate change if it were to occur at a similar magnitude across the permafrost region.
Plain Language Summary
The Arctic is warming at twice the global average. Shifting environmental conditions including the degradation of permafrost affect the storage of carbon in plants and soils of tundra ecosystems. Carbon uptake by plant growth and carbon release by microbial respiration of soil organic matter both appear to have increased prior to and over 15 years of measurements in a tundra ecosystem in Alaska where permafrost has been degrading. But, carbon release to the atmosphere overwhelmed uptake on average, which leads to net release of carbon to the atmosphere and accelerates climate change.
Key Points
Fifteen years of measurements reveal tundra to be a persistent annual net source of carbon to the atmosphere where permafrost is degrading
Plant and microbial activity increased from historical levels such that respiration losses in most years overwhelmed productivity gains
The longer successional dynamics of plants suggests that respiration may outpace productivity for decades as an accelerating feedback to climate change
The Expanding Footprint of Rapid Arctic Change Moon, Twila A.; Overeem, Irina; Druckenmiller, Matt ...
Earth's future,
March 2019, 2019-03-00, 20190301, Letnik:
7, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Arctic land ice is melting, sea ice is decreasing, and permafrost is thawing. Changes in these Arctic elements are interconnected, and most interactions accelerate the rate of change. The changes ...affect infrastructure, economics, and cultures of people inside and outside of the Arctic, including in temperate and tropical regions, through sea level rise, worsening storm and hurricane impacts, and enhanced warming. Coastal communities worldwide are already experiencing more regular flooding, drinking water contamination, and coastal erosion. We describe and summarize the nature of change for Arctic permafrost, land ice, and sea ice, and its influences on lower latitudes, particularly the United States. We emphasize that impacts will worsen in the future unless individuals, businesses, communities, and policy makers proactively engage in mitigation and adaptation activities to reduce the effects of Arctic changes and safeguard people and society.
Key Points
Rapid changes in the Arctic physical environment have substantial impacts in low and midlatitudes
Loss of sea ice, land ice, and permafrost is accelerating, and these losses are further exacerbating climate change
Effects of Arctic change include rising sea level, increased coastal erosion, greater storm impacts, and ocean and atmospheric warming
This study was aimed to assess the decomposition temperature sensitivity (Q10) of C fractions cycling from yearly through decades' and up to centennial timescales using a data assimilation approach. ...A three-pool C-cycling model was optimally fitted with previously-published data from a 588-day long soil incubation experiment conducted at two temperatures (25 and 35 °C) for 12 soils collected from six sites arrayed across a mean annual temperature gradient from 2.0 to 25.6 °C. Three sets of key parameters of the model, which are initial C pool fractions, decomposition rates and Q10 of individual pools, were estimated with a Markov chain, Monte Carlo technique. Initial C pool fractions were well constrained with pool 1 (the most labile pool), pool 2 (more recalcitrant pool) and pool 3 (the most recalcitrant pool) accounting for 4.7% ± 2.6% (mean ± SD), 22.4% ± 16.1% and 72.9% ± 17.6%, respectively, of the total initial C pools. Mean residence time (MRT) was 0.19 ± 0.17, 2.71 ± 2.34 and 80.15 ± 61.14 years for pool 1, pool 2 and pool 3, respectively. Q10 values increased from pool 1 to pool 3 for individual soils or across all the soils. When Q10 values were plotted against MRT after the data were log-transformed, Q10 for the three pools formed three clusters and increased with MRT. Higher Q10 for decades-old C fractions implies that a major portion of soil C may become a source of atmospheric CO2 under global warming in the 21st century.
•Data assimilation was used to analyze temperature sensitivity of SOC decomposition.•Decomposition temperature sensitivity of different C fractions was analyzed.•The constrained C fractions cycle from yearly to centennial in timescale.•Decades old C fraction has the greatest temperature sensitivity.
Current and future warming of high‐latitude ecosystems will play an important role in climate change through feedbacks to the global carbon cycle. This study compares 6 years of CO2 flux measurements ...in moist acidic tundra using autochambers and eddy covariance (Tower) approaches. We found that the tundra was an annual source of CO2 to the atmosphere as indicated by net ecosystem exchange using both methods with a combined mean of 105 ± 17 g CO2 C m−2 y−1 across methods and years (Tower 87 ± 17 and Autochamber 123 ± 14). The difference between methods was largest early in the observation period, with Autochambers indicated a greater CO2 source to the atmosphere. This discrepancy diminished through time, and in the final year the Autochambers measured a greater sink strength than tower. Active layer thickness was a significant driver of net ecosystem carbon exchange, gross ecosystem primary productivity, and Reco and could account for differences between Autochamber and Tower. The stronger source initially attributed lower summer season gross primary production (GPP) during the first 3 years, coupled with lower ecosystem respiration (Reco) during the first year. The combined suppression of GPP and Reco in the first year of Autochamber measurements could be the result of the experimental setup. Root damage associated with Autochamber soil collar installation may have lowered the plant community's capacity to fix C, but recovered within 3 years. While this ecosystem was a consistent CO2 sink during the summer, CO2 emissions during the nonsummer months offset summer CO2 uptake each year.
Key Points
Arctic tundra ecosystem is an annual net source of CO2
Winter CO2 release offsets growing season CO2 gain
Active layer thickness is a significant driver of NEE, GPP, and Reco
We examined the seasonality of photosynthesis in 46 evergreen needleleaf (evergreen needleleaf forests (ENF)) and deciduous broadleaf (deciduous broadleaf forests (DBF)) forests across North America ...and Eurasia. We quantified the onset and end (StartGPP and EndGPP) of photosynthesis in spring and autumn based on the response of net ecosystem exchange of CO2 to sunlight. To test the hypothesis that snowmelt is required for photosynthesis to begin, these were compared with end of snowmelt derived from soil temperature. ENF forests achieved 10% of summer photosynthetic capacity ∼3 weeks before end of snowmelt, while DBF forests achieved that capacity ∼4 weeks afterward. DBF forests increased photosynthetic capacity in spring faster (1.95% d−1) than ENF (1.10% d−1), and their active season length (EndGPP–StartGPP) was ∼50 days shorter. We hypothesized that warming has influenced timing of the photosynthesis season. We found minimal evidence for long‐term change in StartGPP, EndGPP, or air temperature, but their interannual anomalies were significantly correlated. Warmer weather was associated with earlier StartGPP (1.3–2.5 days °C−1) or later EndGPP (1.5–1.8 days °C−1, depending on forest type and month). Finally, we tested whether existing phenological models could predict StartGPP and EndGPP. For ENF forests, air temperature‐ and daylength‐based models provided best predictions for StartGPP, while a chilling‐degree‐day model was best for EndGPP. The root mean square errors (RMSE) between predicted and observed StartGPP and EndGPP were 11.7 and 11.3 days, respectively. For DBF forests, temperature‐ and daylength‐based models yielded the best results (RMSE 6.3 and 10.5 days).
Plain Language Summary
We used records of forest‐atmosphere carbon dioxide exchange and weather to determine when photosynthesis begins and ends each year in 46 northern hemisphere forests. We used observations of soil temperature to determine the timing of the end of the snowmelt period. We found that evergreen needleleaf forests began photosynthesis ∼3 weeks before snowmelt ended, while deciduous broadleaf forests (DBF) waited until ∼4 weeks after snowmelt ended. The DBF type ramped up photosynthesis in spring, and ramped down in autumn, faster than the ENF, and the length of the photosynthesis (or “growing”) season was ∼50 days shorter for DBF forests. Abundant evidence suggests that spring is occurring earlier in recent decades. We checked whether these forests are starting photosynthesis earlier by looking at forests with long‐term records. We found minimal support for changes in photosynthetic phenology over time, but very strong connections between temperature and the timing of spring and autumn transitions. We tested 19 models that use weather data to predict plant phenological events. We used gridded weather data to drive the models, and the best models were able to predict the spring and autumn photosynthetic transitions to within ∼10 days.
Key Points
Evergreen forests began photosynthesis in spring ∼3 weeks before end of snowmelt, deciduous forests ∼4 weeks after end of snowmelt
There is little evidence for lengthening of the photosynthetic season in the northern hemisphere forest flux tower record
Interannual variation in onset and end of photosynthesis was related to air temperature
Rapid Arctic environmental change affects the entire Earth system as thawing permafrost ecosystems release greenhouse gases to the atmosphere. Understanding how much permafrost carbon will be ...released, over what time frame, and what the relative emissions of carbon dioxide and methane will be is key for understanding the impact on global climate. In addition, the response of vegetation in a warming climate has the potential to offset at least some of the accelerating feedback to the climate from permafrost carbon. Temperature, organic carbon, and ground ice are key regulators for determining the impact of permafrost ecosystems on the global carbon cycle. Together, these encompass services of permafrost relevant to global society as well as to the people living in the region and help to determine the landscape-level response of this region to a changing climate.
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