<p dir="ltr">Robust and actionable information regarding how heat stress will change as climate warms is essential for informing impact assessments and heat mitigation and adaptation strategies. In meeting this demand, this dissertation has two mutually reinforcing goals: to improve heat stress prediction through a more comprehensive account of human heat stress, and to advance our understanding of the driving mechanisms of model-predicted heat stress changes.</p><p dir="ltr">As the initial step in achieving the first goal, we adopt the wet-bulb globe temperature (WBGT) as our preferred metric for heat stress. Then we (i) develop a fast, scalable Python implementation of the “gold standard” physics-based WBGT model, (ii) devise a straightforward, yet effective statistical bias-correction approach, and (iii) generate a global dataset of bias-corrected heat stress prediction at fine spatial and temporal resolutions based on a CMIP6 model ensemble. </p><p dir="ltr">To achieve our second goal of understanding the driving mechanisms of WBGT changes, we take advantage of the underlying physical relationship between WBGT and the simpler, wet-bulb temperature to gain insights into WBGT by first (i) investigating the soil moisture control of wet-bub temperature under present conditions and (ii) using CMIP6 results to understand future changes of wet-bulb temperature. Then, (iii) we develop a linear sensitivity framework that is used to disentangle WBGT changes into contributions from changes in temperature, humidity, wind, solar radiation and surface pressure. This disentanglement enables us to leverage existing theories and methods to understand the driving mechanism of WBGT changes.</p><p dir="ltr">Through this work we find several noteworthy conclusions, which is explained in depth in the rest of the dissertation, but we briefly summarize here. Wide-spread positive coupling between soil moisture and wet-bulb temperature are found over previously identified land-atmosphere coupling hotspots due to the effective control of soil moisture variations on surface energy partition and boundary layer dynamics. This implies that drying-induced amplified warming may be counteracted by relative humidity reductions, and a potential mismatch between relative hotspots of warming and intensifying heat stress. We confirm this hypothesis by showing distinctly different scaling patterns (with global warming) between dry-bulb temperature and WBGT based on a CMIP6 model ensemble. Regionally amplified warming in northern hemisphere mid-latitudes and the Amazon correspond to muted increases in WBGT. The central Sahel emerges as a strong local hotspot of WBGT scaling.</p><p dir="ltr">The sensitivity framework predicts close similarity between the scaling of black globe and natural wet-bulb temperature (two major components of WBGT) and that of dry- and wet-bulb temperature, if wind speed and solar radiation changes have a minor impact. This is confirmed to be the case in a CMIP6 model ensemble, with WBGT scaling primarily influenced by temperature and humidity changes. </p><p dir="ltr">Combining these results together holistically, we reach the following conclusions. Although the widely used and empirically well validated WBGT heat stress metric is a complex function of four environmental variables, as climate changes, the changes in WBGT predicted by climate models can be mostly understood in terms of changes in near-surface air temperature and humidity. Furthermore, the linear sensitivity framework and scaling analyses developed here allow us to partially attribute the WBGT scaling pattern to regional drying or wetting trends, and associated changes in surface energy balance and boundary layer dynamics. Thus, accurate prediction of WBGT changes is to first order largely a matter of getting those temperature and humidity correct and improvements to theories and models for those fields will directly translate to improvements in WBGT prediction as well. </p>

<p></p><p></p><p>Land use alteration and climate change are major contributors to the hydrological cycle within watersheds. They can influence the quantity and quality of water resources, the ecosystem and environmental sustainability. Urban areas have expanded in recent decades, accompanied by a noticeable increase in energy and water use. Such changes in land use have many implications for humans to meet the increasing share of the planet’s resources and water issues. Hence, distinguishing the effects of land use change from concurrent climate variability is a particular challenge for studies on operational management processes. In this work, some shortcomings related to climate variability and land use change have been addressed, as applied to land surface temperature (LST) and groundwater resources. Thus, the main goal of this study is to evaluate the impacts of land use change on surface temperature and the impact of urbanization and climate variation on hydrology. The research methodology included modeling approaches that were used to estimate the land surface temperature and the responses of hydrology to climate change and urbanization.</p> <p>Land use maps derived from Landsat datasets were analyzed using several classification techniques to evaluate the intensity and pattern of urbanization and land surface temperature in the Greater Cairo Region (GCR), Egypt. Accuracy of Landsat derived land use data were relatively high and up to 96.5%. Findings indicated that the GCR land use alteration was dynamic and that vegetation loss was the main contributor to urban expansion in the GCR. Consequently, this led to increased LST and modified urban microclimate. The results showed that vegetation cover decreased by 7.73% within a 26-year timespan (1990-2016).</p> <p>Land use alteration impacted not only land surface temperature, but also, combined with variation in climate, affected watershed hydrology, specifically streamflow and baseflow. Changes in streamflow and filtered baseflow in three watersheds: Little Eagle Creek (LEC), Upper West Branch DuPage River (UWBDR) and Walzem Creek watershed, from 1980 to 2017, caused by climate alteration and land use change were separated and accessed using the SWAT (Soil and Water Assessment Tool) model. Results showed that SWAT performed well in capturing the streamflow and baseflow in urban catchments. SWAT model calibration and validation was within acceptable levels for streamflow and baseflow. About 30%, 30% and 12% of the LEC, UWBDR and Walzem Creek watershed areas changed from agricultural to urban areas. Findings for the LEC watershed indicated that the variability in the baseflow and streamflow appeared to be heavily driven by the response to climate change in comparison to the variability due to altered land use. The contribution of both land use alteration and climate variability on the flow variation was higher in the UWBDR watershed. In Walzem Creek, the alteration in streamflow and baseflow appeared to be driven by the effect of climate variability more than that of urbanization.</p> <p>Finally, the impacts of basin lithology and physical properties on baseflow were examined using multiple regression models. Results suggest that the baseflow index (BFI) can be predicted using the basin’s physical and geological characteristics. This included different land uses and climate variables with high accuracy and low relative errors. BFI was found to be highly driven by precipitation and fractional areas of different lithologies in the basins in various regions. These could be estimated with a high accuracy, as opposed to evapotranspiration that caused lower model accuracy.</p> <p>Information gleaned from these outputs can help in understanding the dynamics of land use change and climate variation, in order to help policy-makers predict and plan for future expansion in developing countries and across the globe, in achieving long-term sustainability of soil and water resources and their impact on climate change. Increasing efforts to prevent further urbanization and vegetation loss should be regarded as a practical management strategy and are of vital significance to many communities. In addition, the regression models developed in this study can be easily exploited in other areas with poor hydrological data quality and ungauged sites in order to estimate the amount of groundwater discharge.</p><p></p><p></p>

<p>This thesis is a collection of three research articles to quantify carbon fluxes and isotopic signature changes across global terrestrial ecosystems. Chapter 2, the first article of this thesis, focuses on the importance of an under-estimated methane soil sink for contemporary and future methane budgets in the pan-Arctic region. Methane emissions from organic-rich soils in the Arctic have been extensively studied due to their potential to increase the atmospheric methane burden as permafrost thaws. However, this methane source might have been overestimated without considering high affinity methanotrophs (HAM, methane oxidizing bacteria) recently identified in Arctic mineral soils. From this study, we find that HAM dynamics double the upland methane sink (~5.5 TgCH₄yr^-1) north of 50°N in simulations from 2000 to 2016 by integrating the dynamics of HAM and methanogens into a biogeochemistry model that includes permafrost soil organic carbon (SOC) dynamics. The increase is equivalent to at least half of the difference in net methane emissions estimated between process-based models and observation-based inversions, and the revised estimates better match site-level and regional observations. The new model projects double wetland methane emissions between 2017-2100 due to more accessible permafrost carbon. However, most of the increase in wetland emissions is offset by a concordant increase in the upland sink, leading to only an 18% increase in net methane emission (from 29 to 35 TgCH₄yr^-1). The projected net methane emissions may decrease further due to different physiological responses between HAM and methanogens in response to increasing temperature. This article was published in <i>Nature Climate Change</i> in March 2020.</p> <p>In Chapter 3, the second article of this thesis, I develop and validate the first biogeochemistry model to simulate carbon isotopic signatures (δ¹³C) of methane emitted from global wetlands, and examined the importance of the wetland carbon isotope map for studying the global methane cycle. I incorporated a carbon isotope-enabled module into an extant biogeochemistry model to mechanistically simulate the spatial and temporal variability of global wetland δ¹³C-CH₄. The new model explicitly considers isotopic fractionation during methane production, oxidation, and transport processes. I estimate a mean global wetland δ¹³C-CH₄ of -60.78‰ with its seasonal and inter-annual variability. I find that the new model matches field chamber observations 35% better in terms of root mean square estimates compared to an empirical static wetland δ¹³C-CH₄ map. The model also reasonably reproduces the regional heterogeneity of wetland δ¹³C-CH₄ in Alaska, consistent with vertical profiles of δ¹³C-CH₄ from NOAA aircraft measurements. Furthermore, I show that the latitudinal gradient of atmospheric δ¹³C-CH₄ simulated by a chemical transport model using the new wetland δ¹³C-CH₄ map reproduces the observed latitudinal gradient based on NOAA/INSTAAR global flask-air measurements. I believe this study is the first process-based biogeochemistry model to map the global distribution of wetland δ¹³C-CH₄, which will significantly help atmospheric chemistry transport models partition global methane emissions. This article is in preparation for submission to <i>Nature Geoscience</i>.</p> <p>Chapter 4 of this thesis, the third article, investigates the importance of leaf carbon allocation for seasonal leaf carbon isotopic signature changes and water use efficiency in temperate forests. Temperate deciduous trees remobilize stored carbon early in the growing season to produce new leaves and xylem vessels. The use of remobilized carbon for building leaf tissue dampens the link between environmental stomatal response and inferred intrinsic water use efficiency (iWUE) using leaf carbon isotopic signatures (δ¹³C). So far, few studies consider carbon allocation processes in interpreting leaf δ¹³C signals. To understand effects of carbon allocation on δ¹³C and iWUE estimates, we analyzed and modeled the seasonal leaf δ¹³C of four temperate deciduous species (<i>Acer saccharum, Liriodendron tulipifera, Sassafras albidum, </i>and <i>Quercus alba</i>) and compared the iWUE estimates from different methods, species, and drought conditions. At the start of the growing season, leaf δ¹³C values were more enriched, due to remobilized carbon during leaf-out. The bias towards enriched leaf δ¹³C values explains the higher iWUE from leaf isotopic methods compared with iWUE from leaf gas exchange measurements. I further showed that the discrepancy of iWUE estimates between methods may be species-specific and drought sensitive. The use of δ¹³C of plant tissues as a proxy for stomatal response to environmental processes, through iWUE, is complicated due to carbon allocation and care must be taken when interpreting estimates to avoid proxy bias. This article is in review for publication in <i>New Phytologist</i>.</p> <p> </p>

<p>Urban areas are warmer than surrounding rural areas. During the cold of winter, warming increases surrounding host temperature and may improve the overwintering survival of marginally hardy insects like evergreen bagworms. Similarly, during the summer, it has the potential to increase the fecundity and abundance of sap feeding insect pests such as scale insects in ways that change the capacity of their natural enemies to regulate their populations. </p> <p>Although in parts of Indiana winters can be cold enough to kill bagworm eggs, they thrive in cities. I conducted field experiments to determine the extent to which impervious surface near an infestation could keep temperatures warm enough to affect bagworm survival during cold of winter. My results suggest that the percentage of live eggs inside overwintering pupae decreased as ambient temperature drops. This response was moderated by the presence of impervious surface around an infested plant. Eggs found in bagworms collected from host trees surrounded by more impervious surface had a higher chance of survival than those collected from trees with low levels of hardscape. However, impervious surface has its limit such that egg mortality was not buffered by impervious surfaces at temperatures at or below -21.67°C. Similarly, I also conducted field experiments with sap feeding insects on honeylocust trees, a commonly planted tree in cities. Hot sites had a mean daily temperature more than 1.5 °C warmer than cool sites and scale insects were more abundant and fecund on trees in the hottest part of Indianapolis compared to cooler areas. No differences were observed in rates of parasitism on the scale insect. However, I found strong density dependence relation between parasitoids and scales abundance at scale density at or below the levels present in cool sites. The top-down regulation was prevalent at or below a critical density of scale hosts. Conversely, bottom-up regulation was prevalent above this host density as pests benefit from bottom-up factors. This suggests that urban habitats helped the scales to escape biological control by resident natural enemies above critical density of scale hosts. </p> <p>My findings can be useful to landscape designers to design landscapes that are less prone to insect pests. My finding adds to a growing body of evidence that suggests that planting urban trees with lesser amount of impervious surface can help reducing the urban warming effect and increase the regulation from natural enemies. </p>