High methane emissions from thermokarst lakes on the Tibetan Plateau are largely attributed to ebullition fluxes

Ebullition has been shown to be an important pathway for methane (CH₄) emissions from inland waters. However, the CH₄ fluxes and their magnitudes in thermokarst lakes remain unclear due to limited research data, especially on the Tibetan Plateau (TP). The magnitude and regulation of two CH₄ pathways, ebullition and diffusion, were investigated in 32 thermokarst lakes on the TP during the summer of 2020. CH₄ emissions from thermokarst lakes on the TP showed significant spatiotemporal heterogeneity. Diffusion fluxes in lakes averaged 2.6 mmol m^-2 d^-1 (ranging from 0.003 to 48.4 mmol m^-2 d^-1), and ebullition fluxes in lakes averaged 6.6 mmol CH₄ m^-2 d^-1 (ranging from 0.002 to 140.0 mmol m^-2 d^-1). Together, these ebullition fluxes contributed 66.1 ± 24.9% (ranging 5.4 to 100.0%) to the total (diffusion + ebullition) CH₄ emissions, indicating the importance of ebullition as a major CH₄ transport mechanism on the TP. In general, thermokarst lakes with higher CH₄ diffusion fluxes and ebullition fluxes occurred in alpine meadows (2.5 ± 5.3 mmol m^-2 d^-1; 8.2 ± 20.6 mmol m^-2 d^-1), followed by alpine steppes (0.6 ± 5.3 mmol m^-2 d^-1; 0.7 ± 10.8 mmol m^-2 d^-1) and desert steppes (0.2 ± 0.2 mmol m^-2 d^-1; 0.6 ± 0.8 mmol m^-2 d^-1). The organic matter contents in water and sediment were found to be important factors influencing the seasonal variations in CH₄ diffusion fluxes. However, the ebullition CH₄ fluxes did not show a clear seasonal variation pattern. Our findings highlight the importance of considering the large spatiotemporal variations in ebullition CH₄ fluxes to improve the accuracy of large-scale estimations of CH₄ fluxes in thermokarst lakes on the TP. Greater insight into these aspects will increase the understanding of CH₄ dynamics in thermokarst lakes on the TP, which is essential for forecasting and climate impact assessments and to better constrain feedback to climate warming.

An Object-Based Classification Method to Detect Methane Ebullition Bubbles in Early Winter Lake Ice

Thermokarst lakes in the Arctic and Subarctic release carbon from thawing permafrost in the form of methane and carbon dioxide with important implications for regional and global carbon cycles. Lake ice impedes the release of gas during the winter. For instance, bubbles released from lake sediments become trapped in downward growing lake ice, resulting in vertically-oriented bubble columns in the ice that are visible on the lake surface. We here describe a classification technique using an object-based image analysis (OBIA) framework to successfully map ebullition bubbles in airborne imagery of early winter ice on an interior Alaska thermokarst lake. Ebullition bubbles appear as white patches in high-resolution optical remote sensing images of snow-free lake ice acquired in early winter and, thus, can be mapped across whole lake areas. We used high-resolution (9–11 cm) aerial images acquired two and four days following freeze-up in the years 2011 and 2012, respectively. The design of multiresolution segmentation and region-specific classification rulesets allowed the identification of bubble features and separation from other confounding factors such as snow, submerged and floating vegetation, shadows, and open water. The OBIA technique had an accuracy of >95% for mapping ebullition bubble patches in early winter lake ice. Overall, we mapped 1195 and 1860 ebullition bubble patches in the 2011 and 2012 images, respectively. The percent surface area of lake ice covered with ebullition bubble patches for 2011 was 2.14% and for 2012 was 2.67%, representing a conservative whole lake estimate of bubble patches compared to ground surveys usually conducted on thicker ice 10 or more days after freeze-up. Our findings suggest that the information derived from high-resolution optical images of lake ice can supplement spatially limited field sampling methods to better estimate methane flux from individual lakes. The method can also be used to improve estimates of methane ebullition from numerous lakes within larger regions.

The role of lake size and local phenomena for monitoring ground-fast lake ice

In this study, we assess the effect of the lake size on the accuracy of a threshold-based classification of ground-fast and floating lake ice from Sentinel-1 Synthetic Aperture Radar (SAR) imagery. For that purpose, two new methods (flood-fill and watershed method) are introduced and the results between the three classification approaches are compared regarding different lake size classes for a study area covering most of the Yamal Peninsula in Western Siberia. The focus is on April, the stage of maximum lake ice thickness, for the years 2016 and 2017. The results indicate that the largest lakes are likely most prone to errors by the threshold classification. The newly introduced methods seem to improve classification results. The results also show differences in fractions of ground-fast lake ice between 2016 and 2017, which might reflect differences in temperatures between the winters with severe impact on wildlife and freshwater fish resources in the region. Patterns of low backscatter responsible for the classification errors in the centre of the lakes were investigated and compared to the optical Sentinel-2 imagery of late-winter. Strong similarities between some patterns in the optical and SAR data were identified. They might be zones of thin ice, but further research is required for clarification of this phenomenon and its causes.

Monitoring the multi-year carbon balance of a subarctic palsa mire with micrometeorological techniques

This article reports a dataset on 8 years of monitoring carbon fluxes in a subarctic palsa mire based on micrometeorological eddy covariance measurements. The mire is a complex with wet minerotrophic areas and elevated dry palsa as well as intermediate sub-ecosystems. The measurements document primarily the emission originating from the wet parts of the mire dominated by a rather homogenous cover of Eriophorum angustifolium. The CO(2)/CH(4) flux measurements performed during the years 2001-2008 showed that the areas represented in the measurements were a relatively stable sink of carbon with an average annual rate of uptake amounting to on average -46 g C m(-2) y(-1) including an equally stable loss through CH(4) emissions (18-22 g CH(4)-C m(-2) y(-1)). This consistent carbon sink combined with substantial CH(4) emissions is most likely what is to be expected as the permafrost under palsa mires degrades in response to climate warming.

Environmental monitoring and research in the Abisko area-an overview

This article gives an overview of the studies on the environment surrounding the Abisko Scientific Research Station in Swedish Lapland. The long-term monitoring of the Station on processes related to the climate, and to the physical, biotic, and chemical environmental conditions is particularly addressed. Some variables are recorded since more than 100 years. The obtained data in combination with results from short-term studies and manipulation experiments are important to understand past and future conditions of the ecosystems. This has practical applications for the planning of tourism, transports, reindeer herding, and for societal purposes.