Doctoral Theses / Doktorsavhandlingar Institutionen för geovetenskaper
https://hdl.handle.net/2077/17671
2024-03-28T18:20:25ZTrophic interactions in the tundra: Impacts of large mammal herbivory on carbon processes and fungal communities
https://hdl.handle.net/2077/79197
Trophic interactions in the tundra: Impacts of large mammal herbivory on carbon processes and fungal communities
Brachmann, Cole
Plant productivity is generally increasing in the Arctic as a consequence of accelerated climate change. The change in plant communities may coincide with a loss of carbon from Arctic soils due to increased decomposition and respiration. Herbivores can mediate these changes through preferential foraging on highly productive plant species, trampling, and waste deposition. Soil fungi are also a major component in these interactions and are controlled by plant community and soil conditions. Soil fungi have large impacts on the cycling of carbon in soil and its subsequent release to the atmosphere. Understanding of the effects of large mammalian herbivores on carbon processes, such as respiration and decomposition, and fungal communities is important for understanding the context of future changes in carbon storage in tundra soils.
I investigated the effect of herbivory on trace gas fluxes, decomposition and stabilization of organic matter, and soil fungal communities through the use of herbivore exclusion fences in tundra communities. Herbivory reduced ecosystem respiration in a meadow community, reduced stabilization under a deciduous shrub in a heath community, reduced arbuscular mycorrhizal fungi across the Arctic, and reduced ectomycorrhizal fungi locally in Swedish tundra. The presence of herbivores on the landscape can have complex effects on carbon in tundra habitats by reducing respiration rates and limiting fast cycling arbuscular mycorrhizal fungi, while simultaneously reducing the stability of organic matter as it decomposes and locally limiting slower cycling ectomycorrhizal fungi. The relative contribution of each of these processes to carbon cycling will determine the net effect of herbivores on tundra soils.
Herbivory impacts are context dependent and the net effect on soil carbon is likely related to the proportion of different tundra community types on the landscape.
2023-12-11T00:00:00ZClimate-associated human health effects
https://hdl.handle.net/2077/78191
Climate-associated human health effects
Chen, Tzu Tung
The intensifying impacts of climate change on human health represent a significant and pressing global health threat of the current century. This encompasses both short and long-term effects on human health, as well as ecosystem changes linked to rapid shifts in climate, and the subsequent spread of vector-borne diseases. The complex interplay between climatic factors, socioeconomic variables, and health outcomes poses significant challenges for contemporary studies. Moreover, investigations into climate-associated health burdens in historical times are constrained by the paucity of extensive datasets.
This dissertation explores the spatial and temporal patterns of health impacts in response to climatic variability during the late pre-industrial era (here, 1749-1859) in the Nordic region including Denmark, Sweden, and Finland. The primary dataset used in this dissertation comprises malaria data (cases and deaths) and all-cause deaths obtained from parish-level vital statistics for the period of 1749 to 1859. To investigate the association between climate and malaria, the distributed lag non-linear model (DLNM) was applied to capture nonlinearity and lag effects of time-varying environmental exposures on malaria risk in Denmark, Sweden, and Finland. Additionally, a spatiotemporal analysis was conducted to explore the climate-mortality relationship in Sweden. This analysis involved comparing excess mortality with gridded climate datasets.
Results from this dissertation present historical evidence of the significant impacts of climate-related changes on both malaria and mortality in temperate regions like the Nordic countries. The analysis demonstrates the influence of temperature, precipitation, and sea-level change on malaria risk, as well as the seasonal association between climate and mortality levels. Moreover, this work identifies diverse lag effects of climate impacts on mortality across different geographical areas in Sweden. These findings underscore the relevance of climate factors in comprehending infectious diseases and mortality dynamics during the pre-industrial era in the Nordic region. By enhancing our understanding of the historical association between climate and health outcomes, this dissertation contributes valuable insights to inform future strategies for mitigating the current and future health risks associated with climate change.
2023-08-11T00:00:00ZTowards an improved understanding of precipitation variations over the Tibetan Plateau
https://hdl.handle.net/2077/75595
Towards an improved understanding of precipitation variations over the Tibetan Plateau
Lai, Hui-Wen
The Tibetan Plateau (TP) and surrounding regions are known as the ``Third Pole'' because of their polar-like environment and large reservoirs of fresh water. Due to the latter aspect, the TP is also considered the ``Water Tower of Asia'', providing essential water resources to the surrounding regions. The most important supply to this water tower is atmospheric precipitation, which is affected by both local processes, such as convection and orographic lifting, and remote mechanisms including atmospheric circulation patterns, such as dynamics of monsoon systems and the westerly jets. However, how precipitation over the TP has changed spatially and temporally and the key mechanisms behind these changes remain to be fully understood. This dissertation addresses the following research questions related to TP precipitation: 1) How does the seasonality of TP precipitation vary spatially, and what roles do large-scale atmospheric circulation patterns over and around the TP play in determining precipitation seasonality? 2) Can fine-resolution regional atmospheric models simulate precipitation over the TP more realistically than state-of-the-art global reanalysis datasets?
The dissertation aims to advance our understanding of precipitation and related large-scale to mesoscale processes over the TP using ground-based and remote sensing observations, reanalyses, and high-resolution atmospheric modeling. Specifically, the dissertation focuses on the seasonality and interannual variability of regional precipitation, as well as long-term changes in atmospheric large-scale circulations. The work also evaluates the capability of long-integration high-resolution atmospheric modeling and explores the model uncertainties from different models, domain sizes, and physical parameterizations of subscale processes in the model.
The first part of the key findings is the spatial variations in the seasonality of precipitation over the TP. Three distinct precipitation regimes were found: winter peak in the western TP, early summer peak in the eastern TP, and late summer peak mainly in the southwestern TP. The winter peak regime is the most robust region with a relatively constant extent in time, while the boundary between the other two regimes varies on an interannual time scale, especially in the central TP. The variations in the boundary are associated with combined changes in summer monsoons, westerly jets, and other large-scale systems. In terms of changes in large-scale circulations, there is a positive trend in the occurrence of summer-type wind patterns. Additionally, while westerly jets were previously thought to mainly influence the winter precipitation, we found that variations in the jets can have a dominant role in changing the precipitation patterns during the transitions seasons as well. Another finding is that a long-term integration of a high-resolution meteorological model improves upon the state-of-the-art global ERA5 reanalysis, with significantly reduced wet bias over the TP by simulating weaker low-level southerly winds.
The findings in this dissertation enhance our understanding of the TP precipitation patterns and associated atmospheric processes, particularly large-scale circulations. Moreover, the positive evaluation demonstrates the potential of high-resolution modeling in simulating precipitation and related atmospheric processes, thereby providing valuable insights into what control the precipitation changes over the TP in the past and potentially in the future.
2023-05-09T00:00:00ZObserving and Modeling Precipitation in the Tibetan Plateau region - from large-scale processes to convective storms
https://hdl.handle.net/2077/75117
Observing and Modeling Precipitation in the Tibetan Plateau region - from large-scale processes to convective storms
Kukulies, Julia
Climate change in mountain regions has far-reaching societal impacts such as increased risks for natural hazards and water scarcity that may affect billions of people in the downstream regions. Precipitation changes play a critical role in these impacts due to their effects on river runoff and flooding. However, these changes remain hard to predict due to uncertainties in
climate models and a lack of reliable observations.
This dissertation aims to enhance the understanding of precipitation and its underlying large-scale and mesoscale processes in the Tibetan Plateau (TP) region, one of the most extensive and vulnerable mountain regions in the world. More specifically, the dissertation combines gauge measurements, satellite observations, reanalysis data, and high-resolution model simulations to investigate present-climate characteristics of clouds and precipitation over TP and its downstream regions.
A key outcome is a data set of large storms, so-called mesoscale convective systems (MCSs), based on two decades of high-resolution satellite observations of clouds and precipitation. This data set is used to study MCS characteristics and their relation to large-scale atmospheric circulation systems and water vapor transport. Satellite observations reveal that MCSs are important precipitation producers in the river basins surrounding the TP, while convection over the TP occurs in a more scattered manner with significantly less precipitation. In addition, satellite observations are used to evaluate kilometer-scale simulations of MCSs. The model simulations capture the general spatial pattern and magnitude of MCS-associated precipitation but show also systematic biases in MCS frequencies in some regions south and east of the TP.
It was found that interactions between large- and mesoscale processes affect the formation
and evolution of MCSs over the TP and its downstream regions. The results identify several
processes, e.g. interactions between the TP and the mid-latitude westerly circulation, that
may drive future precipitation changes and need to be realistically represented in future
climate model projections. As such, this dissertation constitutes a step towards reliable
projections of climate change in the TP region.
2023-02-20T00:00:00Z