Hydroclimate in the United States (US) is climatologically divided by the 100th meridian. The semi-arid western US has experienced high-amplitude multidecadal swings in drought and soil moisture variability over the last millennium, culminating in anthropogenic warming-driven drying into the early part of the 21st century. In sharp contrast, the climatologically humid eastern US has experienced century-long increases in precipitation and soil moisture, and generally less warming than in the west, creating a fascinating wetting east – drying west contrast over North America. In eastern North America, a large proportion of the annual precipitation trend was driven by fall-season increases in the southeastern US (SE-US). A rigorous examination of this region would lead to greater insight into the broader causes of hydroclimatic change across North America. The objectives of this dissertation are to (1) identify the large-scale drivers of increased fall precipitation in the SE-US and (2) contextualize and evaluate the causes of regional-to-continental scale changes in soil moisture availability across North America.The first three research chapters of my dissertation focus on my first objective to address the causes of the 20th-century fall precipitation trend. In my first research chapter, I identify and describe fall-season precipitation increases in the SE-US. I show that fall precipitation in the SE-US has increased by nearly 40% during 1895-2016 due to increased circulation-driven moisture transport from the Gulf of Mexico into the SE-US, likely associated with a strengthening or relocation of the North Atlantic Subtropical High (NASH). The NASH is a semipermanent high pressure system located over the North Atlantic that directs moisture transport into the SE-US. Using atmospheric general circulation models forced by sea-surface temperatures (SSTs) and anthropogenic emissions, I demonstrate that models have the capability to simulate a precipitation response to the NASH, but the observed precipitation trend was extremely unlikely in both forced and unforced scenarios. This indicates that the fall precipitation trend was likely caused by processes not well represented in these models, suggesting more work is needed to address why models are unable to simulate observed circulation and SE-US precipitation trends. SST-forced simulations do simulate an enhanced, although displaced to the northwest, NASH and greenhouse gases appear to weakly increase the likelihood of fall wetting. In the first research chapter, I evaluated the proximate drivers of the SE-US fall precipitation variability and trends, working towards the goal of identifying the ultimate driver of observed NASH intensification and SE-US wetting. As a next step, it is important to understand how the increases in precipitation have been delivered, particularly given that fall overlaps with the peak of Atlantic hurricane season. In the second research chapter, I complete a daily-scale decomposition of storm types and precipitation intensity in the SE-US to understand how different precipitation events influenced the fall precipitation increase. I show that increases in SE-US fall precipitation occurred largely as a result of highest-intensity non-tropical (mostly frontal) precipitation days (72% contribution to the fall precipitation trend). In contrast, precipitation from tropical cyclones, a major contributor to extreme fall precipitation, demonstrated a nominal but positive contribution to the trend (13%). Nearly all of the precipitation was delivered on the most extreme (top 5%) intensity precipitation days. These results suggest the observed increase in SE-US fall precipitation has critical implications for flash flood risk from high-intensity rainfall events should the trend continue through the 21st century. Once I identified the types and intensity of storms that influence the fall precipitation trend, I sought to diagnose the physical causes of increased circulation into the SE-US and resultant increases in fall precipitation in the third research chapter. I find that fall precipitation was facilitated by an increase in zonal sea-level pressure (SLP) gradient over the Gulf of Mexico, almost entirely driven by increased SLP along the western edge of the NASH. The zonal SLP gradient was linked to an upper-tropospheric wave train over the North Pacific and North America, leading to increased circulation into the SE-US from the Gulf of Mexico. SST-forced simulations are capable of simulating the spatial features of the NASH and wave train but lack the circulation trends that lead to increased zonal SLP gradient and fall precipitation. The models simulated an enhanced tropical-to-subtropical wave train which increased subsidence and SLP over the subtropical Atlantic Ocean and North America and led to a stronger, more expansive modeled NASH intensification relative to reanalyses, suggesting there exists a stronger atmospheric response to tropical SSTs in models. Due to these discrepancies between models and reanalyses, we can anticipate limitations when using atmospheric models forced by observed SSTs to assess regional climate change in the North Atlantic basin. More research will be needed to understand the physical processes that influence this divergence. The ultimate cause of increased fall moisture transports into the SE-US and resulting precipitation increases remains elusive, but this work improves our understanding of the succession of climatic events that contribute to increased fall precipitation and identify key areas of research needed to reduce uncertainty in SST-forced models. In the final research chapter, I address my second dissertation objective and broaden the focus to all of North America to investigate and contextualize the recent increase in the contrast between soil moisture anomalies in eastern and western North America, termed the east-west North American aridity gradient. Positive aridity gradient values refer to periods during which soil moisture anomalies are more positive in eastern relative to western North America. Using observed and tree-ring reconstructed summer soil moisture anomalies, I show that the 2001-2020 aridity gradient was more positive than any 20-year period since 1400 CE, which followed the most negative aridity gradient during 1976-1995. Using hydrologic models, I find the 2001-2020 aridity gradient anomaly was predominantly driven by century-long summer precipitation increases in the East and to a lesser degree by annual temperature and humidity trends and spring precipitation decreases in the West. Model-simulated anthropogenic trends have minimal effects on the aridity gradient trend due to high inter-model spread in modeled precipitation trends and larger warming effects in the East relative to the West. My findings reveal significant uncertainty in how human and natural systems will be impacted by changes in future water resource availability and provide a benchmark for evaluating North American hydroclimatic change in a warming world through the end of the 21st century.

A warming Southern Ocean (SO), due to climate change and global warming, has many implications on the sub-Antarctic Islands in the SO. Due to the distance away from continental land these islands experience an oceanic climate, making them the perfect sentinels to climate change in this sector of the Southern Ocean. Studies have proposed that climate changes reported at the Prince Edward Islands (PEIs) correspond in time to a southward shift of the Antarctic Circumpolar Current (ACC) particularly the Subantarctic Front (SAF). While other studies have shown distinctive trends in ocean and atmospheric parameters such as sea surface temperature (SST), air temperature, sunshine, rainfall, air sea level pressure and wind speed and direction from the 1950s to the early 2000s, the aim of this study is to update those studies to a more recent time with updated time series. Among the changes recorded is an increase in SST and air temperature, which is a strong indication of the changing local and global climate. Using linear regression, this study showed that the rates of increase from 1949 to 2018 of the SST (0.022°C/year), minimum (0.0072°C/year) and maximum air temperatures (0.016°C/year) are smaller than estimated in previous studies. The increasing trend in SST and air temperature reported by previous papers has actually stopped since the 2000s, which reduces the formerly reported trend (0.028°C/year). Although the in-situ measured SST data had gaps, a good correlation with in-situ SST and large scale satellite derived Reynolds SST help to corroborate the covariation between SST, in-situ SST and air temperature giving weight to the hypothesis of a reversal of the positive temperature trends reported by others. The change in decadal variability a decrease in air pressure of 4 hPa since the late 1990s to late 2000s, which coincided with a decrease in minimum and maximum air temperatures of 1°C over the same period; decrease in westerly wind and an increase in the northerly component of the wind, which would explain the decrease of inshore sea surface temperature a while thereafter. This study further corroborates previous findings of a continued decrease in rainfall, while the sunshine has largely remained the same. The seasonal cycle of the air pressure is significantly associated with that of rainfall, showing that the bimodal high air pressure signature resulting from the Semi-annual Oscillation (SAO) is associated with a decrease in rainfall. The Southern Annual Mode (SAM) was significantly yet weakly correlated with the SST (0.24), rainfall (-0.25) and air pressure (0.16), indicating that it does have an impact at the PEIs but not as strong as previously speculated. The El Niño Southern Oscillation (ENSO) has very weak and insignificant relationships with the parameters examined except for a weak relationship with in-situ SST, sunshine and air pressure. These new insights, especially at the decadal timescale, could further our insight on how subAntarctic islands have responded to climatic changes.

The responsiveness of South African fauna to climate change events is poorly documented and not routinely incorporated into regional conservation planning exercises. The lack of detailed distribution data for appreciable numbers of taxa demands a modeling solution. We develop a climate envelope model to predict potential distribution range changes. The model can be used to interpolate the distributions of poorly sampled taxa as well as predict responses to a changing climate. It is predicted that species from the more arid western parts of South Africa will be subject to severe range contraction and range shifts whereas the species from the more mesic eastern parts will experience range contraction with limited range shift. Species that could act as climate change indicator taxa are identified based on their predicted extreme range change responses to climate change. Red-data and vulnerable species were more likely to display range change than less threatened species. Without mitigatory action, conservation areas are likely to lose species. The likelihood of successful range shifts will be affected by the nature of novel communities, habitat suitability and the degree of land transformation encountered. Given the extent of the predicted spatial responses, conservation planners can no longer afford to ignore future climate impacts on species distribution patterns. Disease risk profiles are also expected to change with climate; currently, susceptible forestry plantations exist in areas which may be invaded by an economically important pathogen. Resistant clones should be planted in these future high-risk areas. A decrease in precipitation is an important feature of a future climate. This decrease is expected to impact on the agricultural sector by reducing total employment as producers switch to a more extensive production pattern. The total decline in welfare, therefore, will fall disproportionately on the poor. Climate change presents a significant treat to the South African biodiversity estate, and our ability to manage this transition in the face of changing and competing land uses. Adaptation and mitigation options do exist but they are hampered by a lack of definitive analyses, and ultimately, political will to prioritise the threat of climate change. / Thesis (PhD(Zoology))--University of Pretoria, 2006. / Zoology and Entomology / unrestricted

Understanding the past sensitivity of ice sheets and sea level rise in a warmer climate is essential to future coastal planning under the threat of climate change, as accurately modeling impending scenarios depends primarily on data from the past. Extreme warm events during the Quaternary and Neogene periods hold much of the information needed to predict future global climate conditions due to anthropogenic and natural forcings, and may provide unique glimpses of how much future sea level rise can be expected on both short- and long-term timescales. Constraining global mean sea level (GMSL) during past warm periods becomes increasingly difficult the further back in time one goes, especially as precise dating of globally distributed paleoshorelines, along with long-term vertical displacement rates, is essential for establishing GMSL and ice volume history. However, placing chronological constraints on shorelines beyond the limit of U-series radiometric dating (~600 kyr), or at high latitude sites lacking coral, has remained elusive. Even relatively recent warm periods, such as the Last Interglacial (~117-129 ka) has proved challenging for reconstructing GMSL, primarily due to uncertainties in long-term vertical deformation rates and timing of when the highstand occurred. The first two chapters of this thesis address the dating of carbonate shorelines older than ~500 kyr through refinement of the strontium isotope stratigraphy dating methodology. I apply these techniques to a well-known location with numerous uplifted fossil shorelines (Cape Range, Western Australia) to provide the first geochemically derived ages on three fossil shorelines spanning the Pleistocene to the Miocene. Accurate dating and mapping at this location allows correction of long-term vertical displacement. In the last chapter, I use these rates of uplift, in conjunction with twenty new 230Th/U-ages on corals from Western Australia, to refine the timing and peak elevation of the Last Interglacial sea level highstand. Chapter 1 re-evaluates strontium isotope stratigraphy dating techniques for chronologically constraining fossil shorelines from ~0.5 to >30 Ma. Using marine terraces from South Africa, Western Australia, and the Eastern United States as examples, this chapter presents a refined sampling and dating methodology to overcome limitations on diagenetically altered samples, which are ubiquitous in older carbonate shorelines. Discussion on best practices for constraining maximum or minimum ages includes a novel scoring methodology for alteration and a sequential leaching procedure that is specifically suited for shallow-water biogenic carbonate fauna. In Chapter 2, I apply the revised strontium isotope stratigraphy dating methodology to three previously unknown aged terraces in Cape Range, Western Australia. The results obtained show Late-Miocene, Late-Pliocene and Mid-Pleistocene shorelines, which I then use to reconstruct the vertical uplift history of the anticlinal structure and relative rates of deformation. This study is the first to directly date the three terraces, and provides the deformation history necessary for constraining Last Interglacial sea level at Cape Range. In addition, we are able to place maximum relative sea level constraints on all three of these older shorelines. Chapter 3 builds upon the previous chapter by focusing on the Last Interglacial sea level history along ~300 km of coastline in Western Australia (Cape Range and Quobba). This chapter presents new U-series ages on multiple coral heads that are among the highest in-situ corals ever dated in Western Australia, with ages spanning from ~125.3 – 122.6 ka. Detailed geomorphic analysis, particularly at Cape Range, constrains the relative sea level highstand to 6.9 ± 0.4 m. When glacial isostatic adjustment models are applied to the age and elevation data, the resulting Eemian GMSL highstand occurred between 125.5-123.0 ka and reached an elevation between 4.9 and 6.7 m. This is later in the Interglacial and lower in elevation than many recent studies suggest. This dissertation focuses on refining sea level highstands from the Last Interglacial to the Late Miocene in a relatively small (but historically important) region of Western Australia. However, the methodologies presented here provide a powerful multi-proxy dating and mapping approach, which, when applied to regions with multiple marine terraces, can greatly improve the reliability of younger shoreline elevations by reducing neotectonic and dynamic topography uncertainties. The carbonate screening techniques and 87Sr/86Sr stratigraphy dating described here are applicable to a wide range of marine carbonates, with the ability to place accurate chronologic constraints on shorelines from 0.5 to >30 Ma. As I show in chapter 3, when combined with 230Th/U-dating on Late Pleistocene coral in places where multiple marine terraces exist, valuable long-term vertical deformation constraints can allow for far more accurate analysis of sea level in younger paleo shorelines (i.e. Last Interglacial).

Thesis. 1977. Ph.D.--Massachusetts Institute of Technology. Dept. of Meteorology. / Microfiche copy available in Archives and Science. / Vita. / Bibliography : leaves 206-218. / Ph.D.

M.Sc., Faculty of Science, University of the Witwatersrand, 2011 / Global climate change, often referred to as „global warming‟ is possibly one of the most serious environmental challenges facing the world this century (DEAT, 2004; IPCC, 2007). There have been several studies (e.g. Viner and Agnew, 1999; Higham and Hall, 2005; IPCC, 2007; Midgley et al., 2008) on the potential impacts of climate change on the tourism sector and the likely effects are shown to be extremely wide ranging and may have far-reaching consequences for the tourism sector in many regions and areas of the world. From a review of the literature it was evident that there was limited literature on the response to climate change by the tourism industry in terms of mitigation, adaptation and long- term strategic planning to manage future anticipated climate change impacts. Given this background, this research explores the tourism industry with regard to game and nature reserves in South Africa and probes the perceptions of climate change amongst park managers and tourism operators to understand their awareness regarding the projected impacts of climate change. Mitigation and adaptation strategies that were in place or being developed in the management of the game and nature reserves are identified and examined. Challenges that were experienced by the tourism managers/operators in promoting effective mitigation and adaptation strategies in the nature based tourism sector in South Africa are highlighted and discussed and recommendations are provided. Purposeful sampling was employed in the research and the stakeholders were identified according to their important roles in the South African Tourism Industry with regard to game and nature reserve management. These included the Department of Environmental Affairs and Tourism, South African National Parks (SANParks), South African National Botanical Institute (SANBI) and the Provincial Park Managers which comprises the Eastern Cape Parks, Gauteng Department of Agriculture and Rural Development (GDARD), Ezemvelo KZN Wildlife, Limpopo Tourism and Parks Board, Mpumalanga Parks Board, North West Parks and Tourism Board and Cape Nature. The research data was collected using open-ended questionnaires and interviews with the stakeholders. Results of this research showed that there was a basic understanding of climate change and its associated impacts on tourism consistent with what is being established in the scientific literature. Despite this awareness among relevant stakeholders, there was however not much formal long-term strategic planning or mitigation and adaptation plans in place to manage or „manage‟ the suggested projected impacts of climate change on the tourism industry. The research results also highlighted many challenges experienced by the nature- based tourism sector.

A research report submitted to the Faculty of Science, in partial fulfilment of the requirements for the degree of Master of Science by Coursework and Research Report, University of the Witwatersrand, Johannesburg, March 2015. / Climate change is considered a dominant factor that controls species distribution at a large spatial scale. Changes in climate conditions are expected to have a significant impact on the distribution of maize in South Africa in the coming years. Determining the potential changes in maize distributions is important, as it is a staple crop for the majority of South Africans and contributes significantly to the country’s economy. The specific objectives of the study were to 1) determine potential distribution of maize under current and predicted climate scenarios using Maxent, 2) determine how the environmental factors change between current and predicted climatic habitat distributions and their influence on maize distributions in South Africa, and 3) statistically compare present and future distributions of maize to see how current and predicted climate habitats differed. Distribution models for high and low maize producing areas were built in Maxent using Bioclim variables from Worldclim. Predicted changes in distributions were then projected using predicted 2050 climate. Two emissions scenarios, RCP2.6 (low emission) and RCP8.5 (high emission), from HadGEM2ES model were used to predict the climate suitability of maize. Model evaluations showed that models had adequate predictability for maize under different climate scenarios (AUC values ≥ 0.7). Precipitation of warmest quarter (Bio18), precipitation of wettest quarter (Bio16), annual precipitation (Bio12), and maximum temperature of warmest month (Bio5) variables contributed the most to model predictions. The models showed a decrease in suitable areas for maize growth in the Highveld region. Present range area for maize as climate changes from low (RCP2.6) and high (RCP8.5) emission scenarios showed a contraction. Predictive models suggest that the most affected areas under future scenarios is the western part of the Highveld region, which is currently characterized by relatively low mean annual precipitation. However, there was an increase in suitability in the Eastern Cape province. Statistical comparisons of current and predicted climatic niches for maize showed that there was little difference, this indicates that climate suitability of maize will not change significantly due to climate change, but that the geographic ranges where these climatic habitats are found will change dramatically. The capacity to develop strategies that will enable maize to adapt to climate change will be vital for South Africa’s agro-ecosystem and food security. The results from this study highlight the possible imposition of climate change on maize distribution and could be useful for future work to minimize the potential negative impacts of climate change on food production. Keywords: Climate change, food security, maize, Maxent, niche quantification, South Africa.

Asia, one of the most densely populated regions in the world, receives 50%-80% of annual rainfall during the summer monsoon season. While agricultural yield and water resource over this region benefit greatly from the summer rainfall, human lives and infrastructures are, at the same time, threatened by the frequently occurring heavy downpour. Although large efforts have been devoted to delineate the characteristics and variations of Asian monsoon extreme rainfall, its dynamical triggers and the physical mechanisms underlying the past and future changes remain poorly understood. To address the knowledge gap, this thesis aims to provide a process-oriented perspective on monsoon rainfall extremes with special attention given to the heavy-rain producing weather systems, namely the monsoon low-pressure systems (LPSs). In Chapter 1, an objective feature-tracking algorithm is adopted to compile the observed trajectories of monsoon LPSs over the East Asia monsoon region during the post-1979 satellite era. Two types of LPS are identified. One forms near the downwind side of the Tibetan Plateau (i.e., southwestern China) and travels northeastward toward north-central China. The other forms over the western North Pacific Ocean and migrates along the southern and western peripheries of the subtropical high. The two types of LPS together account for approximately half of the rainfall extremes. The terrestrial LPSs are responsible for a great majority of extreme rainfall over inland areas, whereas the influences of marine LPSs are primarily confined to the coastal regions where they frequently make landfall. The observed long-term change in extreme rainfall, featured by a “south flood-north drought” pattern, aligns well with the change in LPS activity. The decreasing number of northeastward-moving terrestrial LPSs leads to an extreme rainfall dipole with negative trends in north-central China and positive trends in southern China, while the increasing number of northward-recurving marine LPSs enhances the extreme rainfall along the southeastern China coast. These trends are driven by the weakening of the monsoonal southwesterlies and the eastward retreat of the subtropical high. Despite the great importance of terrestrial LPSs in modulating extreme rainfall over East Asia, these storms have so far received limited attention in research community because of the lack of a track archive. Chapter 2 further investigates the dynamical processes fueling the different evolution regimes of individual terrestrial LPSs and explores the environmental factors controlling their evolution. Chapters 3 and 4 concentrate on the South Asian monsoon region, where the long-term trend of LPS activity remains debatable owing to the potential errors arising from the manual and subjective identification of LPSs from weather charts. Using two different tracking algorithms, in Chapter 3 we find that the trends of extreme rainfall and LPS activity indeed exhibit a strong coherence. Over time, the LPSs propagate preferentially through south-central India rather than north-central India, imparting a corresponding dipole footprint in rainfall extremes. In agreement with previous studies that the LPS propagation is a combined effect of the northwestward-propagating component due to horizontal nonlinear adiabatic advection and the southwestward-propagating component due to diabatic heating, the LPSs traveling through south-central India have stronger updrafts on their west-southwestern flank than those passing through north-central India. The increased frequency of LPSs propagating through south-central India is likely due to a strengthened cross-equatorial moisture transport over the Arabian Sea, which favors more vigorous storm convection through the conditional instability of second kind mechanism. Chapter 4 then focuses specifically on the role of LPS in triggering the record-breaking Pakistan flood during summer 2022, when most of the South Asian LPSs were able to propagate into Pakistan with intensity and longevity far exceeding historical records. The abnormal LPS activity was fueled by a historically-high cross-equatorial moisture transport, which is in agreement with the fingerprint of anthropogenic warming in the Coupled Model Intercomparison Project - Phase 6 (CMIP6) models. The last chapter of this thesis proceeds to evaluate the performance of CMIP6 models in simulating the monsoon rainfall extremes and to explore whether the performance is affected by the degree to which the models could realistically capture the LPS activity. The modelled precipitation often occurs more frequently and the extreme events are commonly less intense than in observations. A robust improvement of model performance in simulating monsoon rainfall extremes as resolution increases is seen across most models, both in terms of spatial distribution and intensity. The dry biases get improved in the regions with high exposure to monsoon LPSs, such as central India, southern China, and western North Pacific. The improvement is associated with a better representation of LPSs, which become more frequent and stronger at finer resolution.

As a temperature-delineated boundary, Arctic treeline is predicted to shift poleward and tree growth is expected to increase in response to rapid warming. The massive scale of the Arctic treeline magnifies these changes to impact energy balance, carbon balance, and climate-related feedbacks at local, regional, and global scales. Yet, not all sections of the Arctic treeline are reporting growth, suggesting factors other than temperature may be becoming more limiting as the climate continues to change. This dissertation investigates how water availability and tree size may modify the response to climate change of a dominant conifer species (white spruce, Picea glauca) growing at an Arctic treeline site in the Brooks Range, Alaska, USA. The first chapter examines the influence of temperature and water availability on population regeneration and individual tree growth during the 20th century. A climatic shift towards a warmer and drier climate after 1975 caused divergent responses of sapling regeneration and mature tree growth, suggesting that, while individuals have grown, this section of treeline has remained relatively stationary. The second chapter explores the present-day relationships between tree size, temperature, moisture availability, and tree growth by examining the response of intra-annual radial stem growth rate to changing environmental conditions at the Arctic treeline. Tree size and water availability play important roles in moderating the growth response to increasing temperature. Finally, in the third chapter, the environmental cues which trigger the onset of radial stem growth in spring are identified. The results suggest a combination of winter chilling and subsequent spring heat accumulation initiates onset, like trees growing at lower latitudes. However, the chilling and heating thresholds at this Arctic treeline site were far colder than those identified at lower latitudes, suggesting local adaptation to harsh Arctic winters and springs. Through these new findings, this dissertation advances our understanding of Arctic treeline dynamics and will help to predict the future of the Arctic treeline more accurately in a rapidly changing climate.

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