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| 2. |
- Kropp, Heather, et al.
(författare)
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Shallow soils are warmer under trees and tall shrubs across Arctic and Boreal ecosystems
- 2021
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Ingår i: Environmental Research Letters. - : IOP Publishing. - 1748-9326. ; 16:1
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Tidskriftsartikel (refereegranskat)abstract
- Soils are warming as air temperatures rise across the Arctic and Boreal region concurrent with the expansion of tall-statured shrubs and trees in the tundra. Changes in vegetation structure and function are expected to alter soil thermal regimes, thereby modifying climate feedbacks related to permafrost thaw and carbon cycling. However, current understanding of vegetation impacts on soil temperature is limited to local or regional scales and lacks the generality necessary to predict soil warming and permafrost stability on a pan-Arctic scale. Here we synthesize shallow soil and air temperature observations with broad spatial and temporal coverage collected across 106 sites representing nine different vegetation types in the permafrost region. We showed ecosystems with tall-statured shrubs and trees (>40 cm) have warmer shallow soils than those with short-statured tundra vegetation when normalized to a constant air temperature. In tree and tall shrub vegetation types, cooler temperatures in the warm season do not lead to cooler mean annual soil temperature indicating that ground thermal regimes in the cold-season rather than the warm-season are most critical for predicting soil warming in ecosystems underlain by permafrost. Our results suggest that the expansion of tall shrubs and trees into tundra regions can amplify shallow soil warming, and could increase the potential for increased seasonal thaw depth and increase soil carbon cycling rates and lead to increased carbon dioxide loss and further permafrost thaw.
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| 3. |
- Anderson, N. John, et al.
(författare)
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The Arctic in the Twenty-First Century : Changing Biogeochemical Linkages across a Paraglacial Landscape of Greenland
- 2017
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Ingår i: BioScience. - : Oxford University Press. - 0006-3568 .- 1525-3244. ; 67:2, s. 118-133
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Tidskriftsartikel (refereegranskat)abstract
- The Kangerlussuaq area of southwest Greenland encompasses diverse ecological, geomorphic, and climate gradients that function over a range of spatial and temporal scales. Ecosystems range from the microbial communities on the ice sheet and moisture-stressed terrestrial vegetation (and their associated herbivores) to freshwater and oligosaline lakes. These ecosystems are linked by a dynamic glacio-fluvial-aeolian geomorphic system that transports water, geological material, organic carbon and nutrients from the glacier surface to adjacent terrestrial and aquatic systems. This paraglacial system is now subject to substantial change because of rapid regional warming since 2000. Here, we describe changes in the eco-and geomorphic systems at a range of timescales and explore rapid future change in the links that integrate these systems. We highlight the importance of cross-system subsidies at the landscape scale and, importantly, how these might change in the near future as the Arctic is expected to continue to warm.
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| 4. |
- Balliu, Enkeleda, PhD student, et al.
(författare)
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High-efficiency radiation-balanced Yb-doped silica fiber laser with 200-mW output
- 2024
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Ingår i: Optics Letters. - : Optica Publishing Group. - 0146-9592 .- 1539-4794. ; 49:8, s. 2021-2024
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Tidskriftsartikel (refereegranskat)abstract
- The focus of this study was the development of a second generation of fiber lasers internally cooled by anti-Stokes fluorescence. The laser consisted of a length of a single-mode fiber spliced to fiber Bragg gratings to form the optical resonator. The fiber was single-moded at the pump (1040 nm) and signal (1064 nm) wavelengths. Its core was heavily doped with Yb, in the initial form of CaF2 nanoparticles, and co-doped with Al to reduce quenching and improve the cooling efficiency. After optimizing the fiber length (4.1 m) and output-coupler reflectivity (3.3%), the fiber laser exhibited a threshold of 160 mW, an optical efficiency of 56.8%, and a radiation-balanced output power (no net heat generation) of 192 mW. On all three metrics, this performance is significantly better than the only previously reported radiation-balanced fiber laser, which is even more meaningful given that the small size of the single-mode fiber core (7.8-µm diameter). At the maximum output power (∼2 W), the average fiber temperature was still barely above room temperature (428 mK). This work demonstrates that with anti-Stokes pumping, it is possible to induce significant gain and energy storage in a small-core Yb-doped fiber while keeping the fiber cool.
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| 5. |
- Frey, B. N., et al.
(författare)
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Cooling Yb-Doped Silica Fibers and Fiber Lasers with Anti-Stokes Pumping
- 2023
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE - International Society for Optical Engineering. - 9781510659797
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Konferensbidrag (refereegranskat)abstract
- Optical cooling in Yb-doped silica fibers using anti-Stokes fluorescence has become a subject of great interest in the fiber laser community. This paper provides an update on the development of silica fibers designed specifically to enhance their cooling properties. This growing list includes a new, nearly single-mode fiber with a borophosphosilicate core that produced –65 mK of cooling with only 260 mW of 1040-nm pump power. The silica compositions that have now been successfully cooled at atmospheric pressure by anti-Stokes fluorescence by our team include aluminosilicate, aluminofluorosilicate, borophosphosilicate, and aluminosilicate doped with one of three different alkali-earth nanoparticles (Ba, Sr, and Ca). By fitting the measured temperature dependence of the cooled fiber on pump power, two key parameters that control the degree of cooling are inferred, namely the critical quenching concentration and the absorptive loss due to impurities. The inferred values compiled for the fibers that cooled indicate that the extracted heat is highest when the Yb concentration is 2 wt.% or more (to maximize heat extraction), the Al concentration is ~0.8 wt.% or greater (to reduce quenching), and the absorptive loss is below approximately 15 dB/km, and ideally below 5 dB/km (to minimize heating due to pump absorption). Only two of the reported fibers, an LaF3-doped and an LuF3-doped nanoparticle fiber, did not cool, because their Yb and Al concentrations were not sufficiently high. This analysis shows that through careful composition control (especially the Al and Yb concentrations) and minimization of the OH contamination, a new generation of Yb-doped silica fibers is emerging with higher Yb concentrations, greater resistance to quenching, and lower residual loss than commercial Yb-doped fibers. They can be expected to have a significant impact not only on optically cooled devices but also on a much broader range of fiber lasers and amplifiers.
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