Dissertations / Theses on the topic 'Physical Geography|Climate Change'

With more than half of the world's population living in urban areas, it is important that the relationships between the urban environment and climate are better understood. The current research aims to continue the effort in assessing and understanding the urban environment through the use of a global climate model (GCM). Given the relative newness of the presence of an urban land type and model in a GCM, there are many more facets of the urban-climate relationship to be investigated. By comparing thirty-year ensembles of CAM4 coupled with CLM4 both with (U) and without (U_n) the inclusion of the urban land type, the sensitivity of the atmospheric model to urban land cover is assessed. As expected, largest differences tend to be in the Northern Hemisphere due to the location of most of the globe's densest and expansive cities. Significant differences in the basic climate variables of temperature and precipitation are present at annual, seasonal, and monthly scales in some regions. Seasonality to the urban influence also exists with the transition months of Spring and Fall having the largest difference in temperatures. Of the eleven regions defined by Oleson (2012), three were most impacted by the presence of urban land cover in the model—Europe, Central Asia, and East Asia.

Since urban attributes can vary greatly within one world continent, the sensitivity of regional climates to the urban type parameters is also explored. By setting all urban land cover to only one urban density type, the importance of city composition on climate, even within the same city, is highlighted. While preserving the distinct urban regional characteristics and the geographical distribution of urbanized areas, the model is run with homogeneous urban types: high density and tall building district. As with the default urban and excluded urban runs, a strong seasonality to the differences between the solo-high-density simulation and default urban (U_HD – U) and solo-tall-building-district-density simulation and default urban (U_TBD – U) exists. Overall, the transition and winter months are most sensitive to changes in urban density type.

Natural disasters can be very devastating to the public during their impact phase. After a natural disaster impacts a region, the response and recovery phases begin immediately. Weather conditions may interrupt emergency response and recovery in the days following the disaster. This study analyzes the climatology of weather conditions during the response and recovery phases of hurricanes and tornadoes to understand how weather may impact both environment and societal needs. Using specific criteria, 66 tornadoes and 16 hurricane cases were defined. National Weather Service (NWS) recognized weather stations were used to provide temperature, precipitation, snowfall, relative humidity, wind speed, and wind direction data. Regional and temporal groups were defined for tornado cases, but only one group was defined for hurricanes. By applying statistical analysis to weather observations taken in the week following the disasters, a climatology was developed for the response and recovery phase. Tornado and hurricane post-disaster climate closely mimicked their synoptic climatology with cooler and drier weather prevailing in days 2-4 after a disaster until the next weather system arrived about 5 days later. Winter tornadoes trended to occur in the Southeast and were associated with more extreme temperature differences than in other regions and season. The results of this study may help governmental and non-governmental organizations prepare for weather conditions during the post-disaster phase.

The objective of the thesis was to explore the potential of diatoms (Bacillariophyceae) as indicators of Holocene climate and environmental change in northern Sweden (Abisko region, 68°21'N, 18°49'E). A modern surface-sediment calibration set including 100 lakes was developed and lake-water pH, sedimentary organic content (assessed by loss-on-ignition) and temperature were identified as most powerful environmental variables explaining the variance within the diatom assemblages. Transfer functions based on unimodal species response models (WA-PLS) were developed for lake-water pH and mean July air temperature (July T), yielding coefficients of determination of 0.77 and 0.70, and prediction errors based on leave-one-out cross-validation of 0.19 pH units and 0.96 °C for lake-water pH and July T, respectively. The transfer functions were validated with monitoring data covering two open-water seasons (lake-water pH) and meteorological records covering the 20th century (July T). The good agreement between diatom-based inferences and measured monitoring data confirmed the prediction ability of the developed transfer functions.

Analysing a Holocene sediment core from a lake nearby Abisko (Vuoskkujávri), diatoms infer a linearly decreasing July T trend (1.5 °C) since 6,000 cal. BP, which compares well with inferences based on chironomids and pollen from the same sediment core. The lake-water pH inference shows a pattern of moderate natural acidification (c. 0.5 pH units) since the early Holocene, reaching present-day pH values at c. 5,000 cal. BP. By fitting fossil diatom samples to the modern calibration set by means of residual distance assessment within canonical correspondence analysis (CCA), the early Holocene (between 10,600 and 6,000 cal. BP) was identified as a problematic time-period for diatom-based inferences and, consequently, reconstructions during this period are tentative. Pollen-based inferences also show 'poor' fit between 10,600 and 7,500 cal. BP and chironomids probably provide the most reliable July T reconstruction at Vuoskkujávri, with 'poor' fit only during the initial part of the Holocene (between 10,600 and 10,250 cal. BP).

Possible factors confounding diatom-based July T inferences were investigated. Using detrended CCA (DCCA), Holocene sediment sequences from five lakes indicate that during the early Holocene, mainly physical factors such as high minerogenic erosion rates, high temperature and low light availability may have regulated diatom assemblages, favouring Fragilaria species. In all five lakes, diatom assemblages developed in a directional manner, but timing and scale of development differed substantially between lakes. The differences are attributed primarily to the geological properties of the lake catchments (with strong effects on lake-water pH), but other factors such as climatic change, vegetation, hydrologic setting and in-lake processes appear to regulate diatom communities in each lake differently. The influence of long-term natural acidification on diatom assemblages progressively declined during the Holocene with corresponding increase of the influence of climatic factors.

Since their development in the 1990s, gridded reanalysis data sets have proven quite useful for a broad range of synoptic climatological analyses, especially those utilizing a map pattern classification approach. However, their use in broad-scale, surface weather typing classifications and applications have not yet been explored. This research details the development of such a gridded weather typing classification (GWTC) scheme using North American Regional Reanalysis data for 1979-2010 for the continental United States.

Utilizing eight-times daily observations of temperature, dew point, pressure, cloud cover, u-wind and v-wind components, the GWTC categorizes the daily surface weather of 2,070 locations into one of 11 discrete weather types, nine core types and two transitional types, that remain consistent throughout the domain. Due to the use of an automated deseasonalized z-score initial typing procedure, the character of each type is both geographically and seasonally relative, allowing each core weather type to occur at every location, at any time of the year. Diagnostic statistics reveal a high degree of spatial cohesion among the weather types classified at neighboring locations, along with an effective partitioning of the climate variability of individual locations (via a Variability Skill Score metric) into these 11 weather types. Daily maps of the spatial distribution of GWTC weather types across the United States correspond well to traditional surface weather maps, and comparisons of the GWTC with the Spatial Synoptic Classification are also favorable.

While the potential future utility of the classification is expected to be primarily for the resultant calendars of daily weather types at specific locations, the automation of the methodology allows the classification to be easily repeatable, and therefore, easily transportable to other locations, atmospheric levels, and data sets (including output from gridded general circulation models). Further, the enhanced spatial resolution of the GWTC may also allow for new applications of surface weather typing classifications in mountainous and rural areas not well represented by airport weather stations.

In the boreal forests of the Northern Hemisphere, time series of tree-ring width (TRW) and maximum density in the latewood (MXD) are highly correlated to local instrumental summer-temperature data and are thus widely used as proxies in high-resolution climate reconstructions. Hence, much of our present knowledge about climatic variability in the last millennium is based on tree-rings. However, many tree-ring records have a lack of data in the most recent decades, which severely hampers our ability to place the recent temperature increase in a longer-timescale perspective of natural variability.

The main objective of this thesis is to update and extend the Torneträsk TRW and MXD records in northern Sweden. Local instrumental climate-data is used to calibrate the new tree-ring records. The results show that TRW is mainly forced by temperature in the early growing season (June/July) while MXD has a wider response window (June – August) and has a higher correlation to temperature. Two reconstructions of summer temperature are made for (i) the last 7,400 years based on TRW, and (ii) the last 1,500 years based on a combination of TRW and MXD. The reconstructions show natural variability on timescales from years to several centuries. The 20th century does not stand out as a notably warm period in the long timescale perspective. A medieval period from AD 900 – 1100 is markedly warmer than the 20th century.

The environmental impact from a large explosive volcanic eruption in 1628/1627 BC is analysed in the tree rings of 14C-dated bog pines in south-central Sweden and in absolutely-dated subfossil pines from Torneträsk. The results show evidence of an impact in the southern site at approximately this time but no detectable impact in the North.

Subfossil trees of Fitzroya cupressoides in southern Chile were 14C-dated to approx. 50,000 years BP and amalgamated into a 1,229-year TRW chronology. This tree-ring record is the oldest in the world. The variability in this Last-glacial chronology is similar to the variability in present-day living trees of the same species. These results suggest that the growth–forcing mechanisms 50,000 years ago were similar to those at present.

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 1: Manuscript. Paper 3: Manuscript.

This thesis contributes to the understanding of glacier response to climate change by ice dynamical studies on Storglaciären, Sweden, and Bonnevie-Svendsenbreen, Kibergbreen and Plogbreen in Dronning Maud Land, Antarctica. Ice surface velocities, ice geometry and temperature information is fed through a force budget model to calculate ice mass outflux of these glacial systems via three-dimensional stress distributions for a flux-gate.

Field data were collected through repeated DGPS and GPR observations on Storglaciären between July 2000 to September 2001 and on Kibergbreen and Plobreen during the SWEDARP 2002/03 expedition to Antarctica. The work was strongly supported by remotely-sensed information.

The results from Storglaciären show a strength in the force budget model to discern both spatial and temporal variability in ice dynamical patterns. It highlights the influence of seasonality and bedrock topography upon glacier flow. A modeling experiment on Bonnevie-Svendsenbreen suggested that ice temperature increases substantially under conditions of high stress (≥0.4 MPa) due to strain-heating. This provides a positive feedback loop, increasing ice deformation, as long as it overcomes the advection of cool ice from the surface. These results explain, to some extent, the mechanism behind fast flowing ice streams. Mass flux caclulations from Bonnevie-Svendsenbreen suggest that the outflux given from force budget calculations can be used as a gauge for influx assuming steady state conditions. Plogbreen receives an influx of 0.48±0.1 km³ a^-1 and expedites a discharge volume of 0.55±0.05 km³ a^-1. This indicative negative mass balance is explained by a falling trend in upstream accumulation and the recent rise in global sea level, as it is likely to induce glacier acceleration due to a reduction in resistive forces at the site of the gate. This result is comparable with other Antarctic studies reporting negative mass balances, e.g. from WAIS, as caused by changes in the global atmospheric circulation pattern.

Climate warming is occurring at an unprecedented rate in the Arctic, seriously impacting sensitive environments, and triggering land cover change. These changes are compounded by localized human influences. This work classifies land cover change for the Lower Yenisei River, identifies those changes that were climate- and anthropogenic- induced, and discusses the implications for the underlying permafrost system. This is accomplished using a modified version of the “Landsat dense time stacking” methodology for three time periods spanning 29 years that are representative of Russian socio-economic transitions during the mid- to late-1980s (1985-1987), the early 2000s (2000-2002), and the contemporary 2010s (2012-2014). The classified area includes three cities indicative of different post-Soviet socio-economic situations, including continued population and infrastructure decline (Igarka), a relatively stable community (Dudinka), and a community receiving local reinvestment (Norilsk). The land cover classification, in tandem with regional climate reanalysis data, enabled climate- and anthropogenic- induced changes to be identified, characterized, and quantified. Climatic changes within the natural environments have produced a steady greening effect throughout the study area, as well as an increase in large lake abundance, indicative of permafrost degradation. Pollution, in close proximity to heavy industrial activity, caused a secondary plant succession process. The results of this work provide both map products that can be applied to future research in this region, as well as insights into the impacts of the warming climate and human presence on sensitive Arctic environments.

Degree project in the master program: Hydrology, hydrogeology and water resources.

Examensarbete inom masterprogrammet: Hydrologi, hydrogeologi och vattenresurser.