Browsing by Author "Denning, Scott, committee member"
Now showing 1 - 15 of 15
Results Per Page
Sort Options
Item Open Access A study of low cloud climate feedbacks using a generalized higher-order closure subgrid model(Colorado State University. Libraries, 2013) Firl, Grant J., author; Randall, David A., advisor; Denning, Scott, committee member; Johnson, Richard, committee member; Evangelista, Paul, committee memberOne of the biggest uncertainties in projections of future climate is whether and how low cloudiness will change and whether that change will feed back on the climate system. Much of the uncertainty revolves around the difference in scales between the processes that govern low cloudiness and the processes that can be resolved in climate models, a fact that relegates shallow convection to the parameterization realm with varying levels of success. A new subgrid-scale parameterization, named THOR, has been developed in an effort to improve the representation of low cloudiness via parameterization in climate models. THOR uses the higher-order closure approach to determine the statistics describing subgrid-scale processes. These statistics are used to determine a trivariate double-Gaussian PDF among vertical velocity, ice-liquid water potential temperature, and total water specific humidity. With this information, one can diagnose what portion of the grid cell is cloudy, subgrid-scale cloud water content, and subgrid-scale vertical cloud water flux. In addition, samples are drawn from the trivariate PDF in order to drive the microphysics and radiation schemes. Although schemes similar to THOR have been developed over the past decade, THOR includes several novel concepts, like the generalization of the saturation curve to include condensation over both ice and liquid substrates, the determination of the PDF parameters from the given turbulence statistics, the introduction of a stochastic parcel entrainment process for the turbulence length scale, and a sub-column approach for calculating radiative transfer using the PDF. The new model is validated by simulating five test cases spanning a wide range of boundary layer cloud types, from stratocumulus to cumulus and the transition between the two. The results are compared to an ensemble of LES models running the same cases, with particular attention paid to turbulence statistics and cloud structure. For all cloud types tested, THOR produces results that are generally within the range of LES results, indicating that the single-column THOR is able to reproduce the gross characteristics of boundary layer clouds nearly as well as three-dimensional LES. Sensitivity to vertical grid spacing, diagnostic/prognostic third- order moments, choice of turbulence length scale entrainment process, and whether or not PDF sampling is used to drive the microphysics and radiation schemes is assessed for all test cases. Simulation of the cumulus regime was degraded when vertical grid spacing exceeded 200 m, when more third-order moments were predicted, when higher parcel entrainment rates were assumed, and when PDF sampling for the microphysics scheme was omitted. Simulation of stratocumulus was degraded with grid spacing larger than 100 m, when PDF sampling for microphysics was omitted, and when PDF sampling for radiation was included. Lastly, THOR is used to study low cloud climate feedbacks in the northeastern Pacific Ocean in the context of the CGILS project. Initial conditions and forcings are supplied at 13 points along the GPCI cross-section that spans from the ITCZ northeast to the coast of California transecting regions of shallow cumuli and stratocumuli, for both the current climate and a climate with a +2K SST perturbation. A change in net cloud radiative forcing of 0-8 W/m2 was simulated along the cross-section for the perturbed climate, representing neutral to weak positive feedback. The responsible mechanism appeared to be increased boundary layer entrainment and stratocumulus decoupling leading to reduced maximum cloud cover in the cumulus regime and reduced liquid water path in the stratocumulus regime.Item Open Access Climate change education in rural Colorado schools(Colorado State University. Libraries, 2021) Scheer, Madison, author; Balgopal, Meena, advisor; Denning, Scott, committee member; Enns, Kellie, committee memberEarth system science education is becoming more timely as our understanding of climate change (CC) and the effects across our social ecology systems increase. Climate literacy, however, is threatened by hyper-political discourse regarding the anthropogenic causes of climate change, which is especially heightened in rural spaces, where residents' livelihoods are often the target of scrutiny by media and scientists. In this study, rural Colorado teachers' (n=9) explanations and perceptions of their climate change education (CCE) instructional choices were examined using instrumental case study methodology. This study was written with the intention for submission to the Journal of Environmental Education or Research in Science Education. Analysis of multiple data sources (interviews, observations, curricular artifacts, student assessments, school websites) resulted in the identification of three cases defined by the teachers' acceptance of anthropogenic causes of climate change, their use of Claim, Evidence, and Reasoning (CER) instructional model, and their sense of belonging. Participants were grouped into one of three cases: (1) Accepts and teaches CC using CER, (2) Accepts CC but does not use CER, and (3) Does not accept nor teach CC. Teachers' competence and confidence with climate science (knowing) as well as their sense of belonging in their rural school and community (belonging) affected how they framed (Scientific uncertainty or Conflict/Strategy) climate change to their students. By learning how rural science teachers communicate CC in their classrooms, science education experts and climate scientists can collaborate to (1) design meaningful and effective professional development workshops and (2) collate curricular resources, including empirical evidence for CC, so rural science teachers feel prepared to teach CC as a socioscientific issue. Moreover, with access to empirical evidence, rural science teachers are better positioned to model scientific argumentation using the CER model in their lessons. Research focused on climate literacy is fundamental to creating an informed generation capable of making conservation, land stewardship, and natural resource management decisions. Rural teachers and students must be included in endeavors to increase climate literacy.Item Open Access Evaluation of OCO-2 small-scale XCO2 variability using lidar retrievals from the ACT-America flight campaign(Colorado State University. Libraries, 2018) Bell, Emily, author; Kummerow, Christian, advisor; O'Dell, Christopher, advisor; Denning, Scott, committee member; Cooley, Daniel, committee memberWith eight 1.25 x 3 kilometer footprints across its swath and nearly 1 million observations of column-mean carbon dioxide concentration (XCO2) per day, the Orbiting Carbon Observatory (OCO-2) presents exciting possibilities for monitoring the global carbon cycle, including the detection of small-scale column CO2 variations. While the global OCO-2 dataset has been shown to be quite robust, and case studies have shown successful observation of CO2 plumes from power plants and cities, the validation of XCO2 gradients on small spatial scales remains challenging: ground-based measurements, while extremely precise, are sparsely scattered and often geographically stationary. In this work, we investigate the use of an integrated path differential absorption (IPDA) lidar as a source for OCO-2 small-scale validation. As part of NASA's ACT-America project, several campaigns over North America have included a number of direct underflights of OCO-2 tracks with the Multi-Functional Fiber Laser Lidar (MFLL), as well as a set of in situ instruments, to provide a precisely collocated, high-resolution validation dataset. We explore the challenges involved in comparing the MFLL and OCO-2 datasets, from instrument principles to retrieval differences, and develop a method of correcting for some of these differences. After nine underflights, a combination of lidar data and a novel in situ-derived CO2 "curtain" have helped us to identify systematic spurious small-scale features in the OCO-2 dataset due to both surface and cloud effects. We show that though real XCO2 features on scales of tens of kilometers remain challenging to observe and validate, the lidar and OCO-2 generally have comparable spatial gradients on synoptic scales.Item Open Access GWP* of U.S. beef and dairy systems(Colorado State University. Libraries, 2023) Molina Plaza, Erick, author; Dillon, Jasmine, advisor; Archibeque, Shawn, committee member; Denning, Scott, committee member; Sheehan, John, committee memberGlobal warming potential (GWP) is used to quantify the impact that greenhouse gases (GHG) have on the warming of the Earth's atmosphere relative to carbon dioxide (CO2). GWP* is a metric that is used to better quantify short-lived climate pollutants (SLCP) such as methane, hydrofluorocarbons, and sulfur dioxide. GWP* allows SLCP to be more consistently expressed by equating a change in the emission of the SLCP to a one-off pulse emission of CO2. Therefore, GWP* can be positive or negative. The objective of this work was to compare the GWP* and GWP100 for U.S. beef and dairy systems using livestock methane emissions data from the Food and Agriculture Organization (FAO) and the Environmental Protection Agency (EPA). Total methane emissions for this study are the sum of enteric and manure methane emissions. GWP100 was greater than GWP* for both beef and dairy systems using both datasets, with the exception GWP* for dairy using the EPA data. Dairy GWP* calculated using the EPA data was lower than GWP100 from 1990–2000, after which point on it became greater than GWP100 and continued increasing annually, because the emission factors used by the EPA increased annually, and the difference between weighted emissions from that year and the weighted emissions from 20 years prior surpassed the current emissions used in GWP100. Overall, the GWP* of EPA dairy increased by 507% from 1990–2020. The primary drivers of the differences in GWP* and GWP100 with the EPA dataset are the use of methane emission factors for manure methane, which increase yearly, and the use of a larger dairy population estimate than FAO. The EPA emission factors increase yearly based on the trend towards larger farm sizes managing more liquid manure, therefore produce more manure methane emissions. The dairy GWP* using EPA data was greater than the beef GWP* every year, despite greater total methane emissions for beef than for dairy, because the average rate of change for dairy (29.8 kt of CH4/yr) was greater than the average rate of change for beef (9.4 kt of CH4/yr). Accounting methods play a key role in the amount of methane emissions that are calculated, and thus how GWP100 and GWP* are calculated. The EPA larger population estimate and annual increase in manure methane emission factors led to greater GWP* and GWP100 values for the EPA data than for the FAO data for both beef and dairy systems. Data source is critical to the policy implications of GWP* and GWP100 for livestock systems, as evidenced by the differences in GWP* and GWP100 results between datasets.Item Open Access Improving radiation data quality of USDA UV-B monitoring and research program and evaluating UV decomposition in DayCent and its ecological impacts(Colorado State University. Libraries, 2015) Chen, Maosi, author; Gao, Wei, advisor; Davis, John, committee member; Moore, John, committee member; Conant, Rich, committee member; Denning, Scott, committee memberSolar radiation impacts many aspects of the Earth's atmosphere and biosphere. The total solar radiation impacts the atmospheric temperature profile and the Earth's surface radiative energy budget. The solar visible (VIS) radiation is the energy source of photosynthesis. The solar ultraviolet (UV) radiation impacts plant’s physiology, microbial activities, and human and animal health. Recent studies found that solar UV significantly shifts the mass loss and nitrogen patterns of plant litter decomposition in semi-arid and arid ecosystems. The potential mechanisms include the production of labile materials from direct and indirect photolysis of complex organic matters, the facilitation of microbial decomposition with more labile materials, and the UV inhibition of microbes population. However, the mechanisms behind UV decomposition and its ecological impacts are still uncertain. Accurate and reliable ground solar radiation measurements help us better retrieve the atmosphere composition, validate satellite radiation products, and simulate ecosystem processes. Incorporating the UV decomposition into the DayCent biogeochemical model helps to better understand long-term ecological impacts. Improving the accuracy of UV irradiance data is the goal of the first part of this research and examining the importance of UV radiation in the biogeochemical model DayCent is the goal of the second part of the work. Thus, although the dissertation is separated into two parts, accurate UV irradiance measurement links them in what follows. In part one of this work the accuracy and reliability of the current operational calibration method for the (UV-) Multi-Filter Rotating Shadowband Radiometer (MFRSR), which is used by the U.S. Department of Agriculture UV-B Monitoring and Research Program (UVMRP), is improved. The UVMRP has monitored solar radiation in the 14 narrowband UV and VIS spectral channels at 37 sites across U.S. since 1992. The improvements in the quality of the data result from an improved cloud screening algorithm that utilizes an iterative rejection of cloudy points based on a decreasing tolerance of unstable optical depth behavior when calibration information is unknown. A MODTRAN radiative transfer model simulation showed the new cloud screening algorithm was capable of screening cloudy points while retaining clear-sky points. The comparison results showed that the cloud-free points determined by the new cloud screening algorithm generated significantly (56%) more and unbiased Langley offset voltages (VLOs) for both partly cloudy days and sunny days at two testing sites, Hawaii and Florida. The VLOs are proportional to the radiometric sensitivity. The stability of the calibration is also improved by the development of a two-stage reference channel calibration method for collocated UV-MFRSR and MFRSR instruments. Special channels where aerosol is the only contributor to total optical depth (TOD) variation (e.g. 368-nm channel) were selected and the radiative transfer model (MODTRAN) used to calculate direct normal and diffuse horizontal ratios which were used to evaluate the stability of TOD in cloud-free points. The spectral dependence of atmospheric constituents' optical properties and previously calibrated channels were used to find stable TOD points and perform Langley calibration at spectrally adjacent channels. The test of this method on the UV-B program site at Homestead, Florida (FL02) showed that the new method generated more clustered and abundant VLOs at all (UV-) MFRSR channels and potentially improved the accuracy by 2-4% at most channels and over 10% at 300-nm and 305-nm channels. In the second major part of this work, I calibrated the DayCent-UV model with ecosystem variables (e.g. soil water, live biomass), allowed maximum photodecay rate to vary with litter's initial lignin fraction in the model, and validated the optimized model with LIDET observation of remaining carbon and nitrogen at three semi-arid sites. I also explored the ecological impacts of UV decomposition with the optimized DayCent-UV model. The DayCent-UV model showed significant better performance compared to models without UV decomposition in simulating the observed linear carbon loss pattern and the persistent net nitrogen mineralization in the 10-year LIDET experiment at the three sites. The DayCent-UV equilibrium model runs showed that UV decomposition increased aboveground and belowground plant production, surface net nitrogen mineralization, and surface litter nitrogen pool, while decreased surface litter carbon, soil net nitrogen mineralization and mineral soil carbon and nitrogen. In addition, UV decomposition showed minimal impacts (i.e. less than 1% change) on trace gases emission and biotic decomposition rates. Overall, my dissertation provided a comprehensive solution to improve the calibration accuracy and reliability of MFRSR and therefore the quality of radiation products. My dissertation also improved the understanding of UV decomposition and its long-term ecological impacts.Item Open Access Maximizing the utility of available root zone soil moisture data for drought monitoring purposes in the Upper Colorado River Basin and western High Plains, and assessing the interregional importance of root zone soil moisture on warm season water balance(Colorado State University. Libraries, 2016) Goble, Peter, author; Schumacher, Russ, advisor; Denning, Scott, committee member; Chávez, José, committee memberTo view the abstract, please see the full text of the document.Item Open Access Measuring and modeling transpiration and photosynthesis in Zea mays and Helianthus annuus: leaf-level and sap flow studies(Colorado State University. Libraries, 2016) Miner, Grace Susanna Lloyd, author; Bauerle, William, advisor; Ham, Jay, advisor; Denning, Scott, committee member; Shaner, Dale, committee memberTo view the abstract, please see the full text of the document.Item Open Access Neotectonic effects of glacial erosion and deglaciation on the Sangre de Cristo Mountains, southern Colorado(Colorado State University. Libraries, 2023) Hurtado, Cecilia, author; Gallen, Sean, advisor; Singleton, John, committee member; McGrath, Daniel, committee member; Denning, Scott, committee memberInterrelations between climate and tectonics are important to the development of active mountain belts, but rarely are there natural examples that lend themselves to studying the effects of climate on tectonics. The Sangre de Cristo Mountains in southern Colorado provide an optimal natural laboratory to explore the effects of alpine valley glaciation on surface uplift of the footwall and on the active extensional normal fault system in the northern Rio Grande rift. This region has experienced changes in surface loads associated with long-term glacial erosion and sedimentation over the course of the Quaternary, as well as shorter-term deglaciation after the Last Glacial Maximum. These changing loads correspond with stress changes that affect the flexural isostatic response of the lithosphere, and further act as clamping or unclamping stresses on the Sangre de Cristo fault that bounds the western margin of the mountain range. This work quantifies the masses and spatial distributions of these various loads and models the associated flexural isostatic response to estimate potential uplift and subsidence patterns in the study area that could be attributed to climate-driven mechanisms. The glacially-scoured footwall material was estimated by using remnants of the fluvial reaches downstream of glaciated drainage basins, reconstructing the paleofluvial topography, and subtracting it from the modern topography. The quantification of the deposited sediment in the San Luis Basin was measured from an interpolated surface tethered by existing drill cores, geophysical data, and geologic maps. Lastly, the glacial extents and thicknesses were constructed using a simple numerical modeling tool, GlaRe, constrained by preserved depositional and erosional evidence of glaciers. Isostatic responses were calculated using a flexure model with two effective elastic thickness (Te) values, 2 km and 5 km, and stress changes on faults at depth were calculated using an analytical line load model. The results estimate ~29 m of footwall uplift and ~47 m of subsidence in the hanging wall for a realistic Te of 5 km, and footwall uplift of ~48 m and a hanging wall subsidence of ~80 m for an independently calibrated Te of 2 km. Importantly, while topographic reconstructions indicate an ~50 m reduction in mean footwall elevations, isostatic rebound pushes mountain peaks upward by tens of meters. Footwall uplift due to deglaciation has a response of 4 m and 6 m, for a Te of 5 km and a Te of 2 km, respectively. The Sangre de Cristo fault trace was mapped to quantify the offset of fault scarps on Quaternary alluvial fans to determine the spatial and temporal patterns of offset along-strike of the fault. Fault offset magnitudes correlate with glacial domains, and fault slip rates correlate with the post-glacial spatial pattern of isostatic uplift, indicating an unambiguous link between deglaciation and elevated fault activity. This work demonstrates that (1) differential glacial erosion reduces mean footwall elevations, but the associated isostatic response drives surface uplift of mountain peaks, and (2) seismicity along normal faults could be amplified by load changes associated with climate-driven mechanisms, which will become increasingly important as we continue in a period of anthropogenic warming and deglaciation.Item Open Access Paleo-feedbacks in the hydrological and energy cycles in the Community Climate System Model 3(Colorado State University. Libraries, 2008) Burt, Melissa A., author; Randall, David A., advisor; Denning, Scott, committee member; Wohl, Ellen E., 1962-, committee member; Otto-Bliesner, Bette, committee memberThis research focuses on the joint variability of the hydrological and energy cycles for the atmosphere and lower boundary and climate feedbacks associated with these changes at the Last Glacial Maximum. The LGM simulated climate experiences a global cooling of 4.9 K compared to the PI climate, with greatest cooling in the high latitudes of both hemispheres. Additional cooling also exists over the continental ice sheets in North America, Northern Europe, and Antarctica. Precipitation and evaporation are reduced by 10%, and precipitable water by 20%, compared to conditions at PI. Overall, from LGM to PI the changes in clouds are weak. The water vapor, ice-albedo, and cloud feedbacks act to amplify the climate change from LGM to PI. The positive water vapor and ice-albedo feedbacks account for 5.04 W m-2 K-1 and 2.38 W m-2 K-1, respectively of the climate change. The cloud feedbacks produces -2.83 of the change. An interesting and unexpected result was that the sign of the ice-albedo feedback changed regionally and is driven by changes in ocean basin size. Combined, the radiative feedbacks from LGM to PI act to amplify the climate change by 5.67 W m-2 K-1 and are balanced by an increase in surface evaporation.Item Open Access Seeing the river through the trees: using cottonwood dendrochronology to reconstruct river dynamics in the Upper Missouri River Basin(Colorado State University. Libraries, 2017) Schook, Derek Michael, author; Rathburn, Sara, advisor; Friedman, Jonathan, committee member; Wohl, Ellen, committee member; Covino, Tim, committee member; Denning, Scott, committee memberUnderstanding the past is critical to preparing for the future, especially regarding rivers where extreme events and gradual changes underlie modern forms and processes. Both biological and human communities rely on the abundant resources provided by rivers and floodplains, particularly in dry regions of the western U.S. where water limits growth. To expand temporal perspectives on river processes, I reconstructed flow, channel migration, and riparian forest growth patterns in the Upper Missouri River Basin. Flow reconstructions typically use tree rings from montane conifers. However, I used riparian plains cottonwoods (Populus deltoides ssp. monilifera) directly connected to the alluvial water table to reconstruct flow on the Yellowstone (n = 389 tree cores), Powder (n = 408), and Little Missouri Rivers (n = 643). A two-curve Regional Curve Standardization approach was used to remove age-related growth trends from tree rings at each site. The flow reconstructions explained 57-58% of the variance in historical discharge and extended back to 1742, 1729, and 1643, respectively. Low-frequency flow patterns revealed wet conditions from 1870 to 1980, a period that includes the majority of the historical record. Two 19th century droughts (1816-1823 and 1861-1865) and one pluvial (1826-1829) were more severe than any recorded, revealing that risks are underestimated when using the instrumental period alone. These are the first flow reconstructions for the Lower Yellowstone and Powder Rivers, and they are the farthest downstream among Rocky Mountain rivers east of the Continental Divide. Cottonwood-based flow reconstructions were possible because the trees used river-connected groundwater, and tree ring width strongly correlated with March-June flow magnitude at the Yellowstone River (r = 0.69). Beyond the site-level growth patterns typically used to reconstruct flow, I found that biological and spatial characteristics affected how individual trees responded to flow and climate. Older trees contained stronger signals of non-growing season flow, precipitation, and temperature, which challenges the common dendrochronological assumption of stable tree ring-climate relationships through time. Although trees both near and far from the channel were better correlated to spring flow than precipitation, more distant trees had a stronger relative connection to precipitation, suggesting that greater distance decreases the ability of river water to fulfill transpirative demands. Like annual growth, cottonwood establishment is related to river flows, and tree age indicated fluvial processes including channel migration. I quantified nearly two centuries of channel migration on the Powder River by integrating measured channel cross-sections (1975-2014), air photos (1939-2013), and transects of aged cottonwoods (1830-2014). The combined data revealed that channel migration rates were lower (0.81 m/yr) in the recent and intensively studied cross-section period compared to the longer air photo (1.52 m/yr) and cottonwood (1.62 m/yr) periods. On the Powder River, extreme floods such as those in 1923 and 1978 increase subsequent channel migration rates and initiate decades of channel morphological adjustments. Across the study rivers, data indicate that fundamental fluvial processes have responded to climatic and watershed pressures. By identifying and quantifying past events, diverse research approaches improve understanding of the river, floodplain, and riparian forest processes that are essential to the persistence of these valuable ecosystems.Item Open Access Small government, big problems: climate change adaptation policy in North American Great Lakes localities(Colorado State University. Libraries, 2022) Gelardi, Carrington, author; Schomburg, Madeline, advisor; Scott, Ryan, committee member; Mumme, Stephen, committee member; Denning, Scott, committee memberThe Great Lakes region is home to 30 million people, one of the world's largest economies, and the world's largest freshwater ecosystem. These characteristics make the region uniquely vulnerable to climate change. Local governments in the area are subject to the impacts of climate change whether they are prepared for them or not. To explore this issue, this paper seeks to answer the question, "What is the state of local climate change adaptation policy in the Great Lakes region?" Most literature that exists on local adaptation focuses on larger cities with populations over 50,000 people. This project fills that gap by looking at climate plans from all U.S. local governments that border the Great Lakes regardless of their size. To do this, climate change adaptation plans and policies were gathered from each county and sub-county municipality (such as cities, villages, towns, and townships) in the United States that border the Great Lakes. A text analysis was performed that compared the documents to regional climate science, as well as an inductive content analysis to pull out the major topics in each plan. Local governments in the Great Lakes region are in the beginning stages of adapting to climate change. 6% sent back relevant policies. Many of them were small governments with under 20,000. Findings suggest a lack the capacity to adequately adapt, especially within the smallest governments. The degree of assistance needed from larger institutions to supplement any insufficiencies is still unclear. The results of this project capture a snapshot of how local governments bordering the Great Lakes are (or are not) adapting to climate change. This can be used to foster intergovernmental learning on how sub-state governments in the region can adapt, while also providing insight into the boundaries of local action in the face of a global issue.Item Open Access Snow persistence and hydrologic response across the intermittent-persistent snow transition(Colorado State University. Libraries, 2018) Hammond, John Christopher, author; Kampf, Stephanie, advisor; Covino, Tim, committee member; Denning, Scott, committee member; Fassnacht, Steven, committee memberIn mountainous regions and high latitudes, seasonal snow is a critical component of the surface energy balance and hydrologic cycle. Snowpacks have been declining in many mountain regions, but the hydrologic responses to snow loss have varied due to interactions of climatic, vegetative, topographic and edaphic factors. With continued climatic change, it remains uncertain whether the southwestern U.S. and other subtropical and mid-latitude dry areas may experience significant reductions in water yield. In this dissertation snow persistence and trends are mapped globally; relationships between snow persistence and annual water yield are examined in different climates, and snowmelt and rain partitioning in the critical zone are modelled to examine potential effects of snow loss on hydrologic response. Chapter 2 involves mapping the distribution of snow persistence (SP), the fraction of time that snow is present on the ground for a specific period, using MODIS snow cover data, classifying similar areas into snow zones, assessing how snow persistence relates to climatic variables and elevation, and testing for trends in annual SP. SP is most variable from year to year near the snow line, which has a relatively consistent decrease in elevation with increasing latitude across all continents. At lower elevations, SP is typically best correlated with temperature, whereas precipitation has greater relative importance for SP at high elevations. The largest areas of declining SP are in the seasonal snow zones of the Northern Hemisphere. Trend patterns vary within individual regions, with elevation, and on windward-leeward sides of mountain ranges. This analysis provides a framework for comparing snow between regions, highlights areas with snow changes, and can facilitate analyses of why snow changes vary within and between regions. In Chapter 3, SP is used to evaluate how water yield relates to snow patterns at the annual time scale across the western U.S. in different climates. I first compare snow cover variables derived from MODIS to more commonly utilized metrics (snow fraction and peak snow water equivalent (SWE)). I then evaluate how SP and SWE relate to annual streamflow (Q) for 119 USGS reference watersheds and examine whether these relationships vary for wet/warm (precipitation surplus) and dry/cold (precipitation deficit) watersheds. Results show high correlations between all snow variables, but the slopes of these relationships differ between climates. In dry/cold watersheds, both SP and SNODAS SWE correlate with Q spatially across all watersheds and over time within individual watersheds. I conclude that SP can be used to map spatial patterns of annual streamflow generation in dry/cold parts of the study region. In Chapter 4 of the dissertation, I use a series of one-dimensional simulations to study how snow loss may impact hydrologic response in mountain areas at event to annual time scales. I use Hydrus 1-D simulations with historical inputs from fifteen SNOTEL snow monitoring sites to investigate how inter-annual variability of water input type (snowmelt, rainfall) and timing affect soil saturation and deep drainage in different soil types and depths. Greater input rate and antecedent moisture are observed for snowmelt compared to rain events, resulting in greater runoff efficiencies. At the annual scale runoff efficiencies increase with snowmelt fraction and decrease when all input is rainfall. In contrast, deep drainage has no clear correlation to snowmelt fraction. Input that is concentrated in time leads to greater surface runoff and deep drainage. Soil texture and depth modify partitioning, but these effects are small compared to those caused by variability in climate. This dissertation's findings have direct implications for climate change impacts in cold dry areas globally. Through the synthesis of the chapters described above I highlight areas where hydrologic response to snow loss may be most sensitive, provide methods for comparing regional snow patterns, demonstrate how snow persistence can help estimate annual streamflow generation, and improve process-based knowledge of hydrologic response to rainfall and snowmelt in the western U.S. Collectively these findings indicate that annual water yield is not directly sensitive to whether input is snowmelt vs. rainfall; instead it is more dependent on the effect that snowpack accumulation has on input timing and rate. Loss of concentrated melt from persistent snowpacks may lead to lower streamflow and compromise deep drainage, and thus aquifer recharge, in semi-arid cold regions. The consequences of streamflow and groundwater recharge loss could be severe in regions already water-stressed, and this needs to be addressed in long-term water supply planning.Item Open Access The effects of climate change on high elevation lake ecosystems(Colorado State University. Libraries, 2019) Christianson, Kyle R., author; Johnson, Brett, advisor; Hooten, Mevin, committee member; Denning, Scott, committee member; Myrick, Christopher, committee memberHigh elevation lakes are an important class of the world's fresh water. Nearly 10% of all lakes globally reside above 2,100 m ASL and almost half of the world's population relies on water from high elevation regions. Also, these lakes provide important cool water habitat refugia for aquatic biota. However, high elevation areas are sensitive to changes in climate and are changing faster than other regions. Likewise, secondary effects of a changing climate like drought, forest fire, and eutrophication threaten lake habitats, exacerbating changes from air warming. Despite the importance of high elevation lakes and their increased threat from climate change, little is known about high elevation lakes and their vulnerability to these threats. The goal of my dissertation was first (Chapter 1) to determine historic changes in lake surface temperatures for a set of high elevation lakes in the Southern Rocky Mountains, USA (SRM). Then, I determined potential future changes to thermal stratification (Chapter 2) and the length of the open water season (Chapter 3) for a subset of lakes in the Rawah Wilderness Area (RWA) within the SRM. For these future predictions, I estimated alterations in lake surface and bottom temperatures from multiple stressors, as well as how these changes may affect aquatic habitat for native and nonnative fish species that reside in the region. Although historic lake temperature trend analyses are numerous, remote lakes, including many high elevation lakes, are typically underrepresented due to limited availability of long-term datasets. In Chapter 1, I developed a Bayesian modeling technique to analyze sparse data from high elevation lakes that allowed me to estimate lake surface warming across a large region (SRM). The analysis allowed for inclusion of lakes with few repeated measurements, and observations made prior to 1980 when more intensive lake monitoring began. I accumulated the largest dataset of high elevation lake surface temperatures globally analyzed to date. Data from 590 high elevation lakes in the Southern Rocky Mountains showed a 0.13°C decade-1 increase in surface temperatures and a 14% increase in seasonal degree days since 1955. Like surface temperature trends, many studies have also examined the effects of climate warming on lake thermal stratification, but few have addressed environmental changes concomitant with climate change, such as alterations in water clarity and lake inflow. Although air temperature rise is a predominant factor linked to lake thermal characteristics, climate-driven changes at watershed scales can substantially alter lake clarity and inflow, exacerbating the effects of future air warming on lake thermal conditions. In Chapter 2, I employed the mechanistic General Lake Model (GLM) to simulate future thermal conditions of typical mountain lakes of the western United States. I found that after air temperature, alterations in inflow had the largest effect on lake thermal conditions, changes in wind had the least effect, and large lakes experienced more than double the increase in lake stability than small lakes. Assuming air temperature rise alone, summer stability of mountain lakes of the western United States was predicted to increase by 15-23% at +2°C air temperatures, and by 39-62% at +5°C air temperatures. When accounting for associated changes in clarity and inflow, lake stability was predicted to increase by 208% with +2°C air warming and 318% at +5°C air warming. Finally, the open water duration at high elevations is increasing at a higher rate than at lower elevations. Earlier snowmelt, resulting in decreased ice cover duration, is having a proportionally higher effect on mountain lakes than other regions. But the effect early melt and increased air temperatures have on mountain lake thermal characteristics and implications for fish is unclear. Mountain lakes exhibit a variety of thermal conditions, altering metabolic requirements for ectotherms. In Chapter 3, I coupled GLM with a fish bioenergetics model to assess potential thermal changes and energetic consequences for native Cutthroat Trout (Oncorhynchus clarkii spp.) and nonnative but present Brook Trout (Salvelinus fontinalis) in a continuously mixed polymictic and seasonally stratified dimictic mountain lake during early and nominal snowpack melt in the SRM. I found that early snowmelt alone had a larger consumptive demand for all species than an air temperature increase of 2°C, but combined these environmental changes are most effective. Early melt coupled with 5°C air warming could more than double the food requirements for Cutthroat Trout and Brook Trout. Ultimately, food availability may dictate the future success of fish in mountain regions. My dissertation research expanded the current knowledge of high elevation lake thermal conditions, developed a novel method to utilize sparse datasets, and provided valuable holistic insight to potential future changes in lake thermal structure and habitat suitability for fish while accounting for localized and watershed scale consequences of climate change.Item Open Access Towards understanding the processes that influence global mean temperature(Colorado State University. Libraries, 2011) Mullin, Kathryn A., author; Thompson, David, advisor; Denning, Scott, committee member; Klein, Julia, committee memberGlobal mean surface temperature variability is largely determined by the global mean surface energy budget, which is driven by many natural and anthropogenic forcings. In theory, if all natural sources of global mean temperature variability could be removed from the global mean temperature time series the anthropogenic signal would be clearer. Previous studies have exploited this reasoning to remove the signature of volcanoes, the El-Niño Southern Oscillation (ENSO), and dynamic variability from the global mean temperature time series. This thesis extends previous work by 1) examining the linkages between global mean temperature and natural variability as a function of timescale; and 2) examining the two-way coupling between area-averaged surface temperatures and sea ice concentration. The results reveal a series of unique spatial structures in surface temperatures that drive intraannual, interannual, and decadal variability in global mean temperature. The results confirm the apparent role of hemispheric mean temperatures in driving sea ice variability, and also point to a possible feedback between wintertime sea ice concentration and springtime surface temperatures over the Northern Hemisphere. Linkages between sea ice concentration and surface temperature in the Southern Hemisphere are much weaker, and it can be argued that the hemispheric difference in these linkages may aid in explaining the different trends in sea ice between the two hemispheres.Item Open Access Tracking the terrestrial hydroclimate and paleoclimate response to changes in atmospheric pCO₂ using stable oxygen and carbon isotopes: a proxy-model comparison across Cenozoic Eurasia(Colorado State University. Libraries, 2022) Driscoll, Elizabeth, author; Rugenstein, Jeremy Caves, advisor; Denning, Scott, committee member; Keys, Patrick, committee member; Ronayne, Michael, committee memberDespite well-known constraints on the global hydrologic cycle with future warming, the response of the terrestrial hydrologic cycle – and hence future available freshwater resources – remains uncertain. This is largely due to the difficulty in predicting changes in environmental parameters such as precipitation (P), evapotranspiration (ET), and runoff (q). Shifts in the ratio of P/ET can be quantified using the δ18O of precipitation (δ18Op), given that P decreases δ18Op and ET increases δ18Op. Additionally, the δ13C composition of soil can provide insight into shifts in P and ET by recording the response of vegetation to changing climate. To better understand how the terrestrial water cycle will change with future warming, we utilize the geologic stable oxygen and carbon (δ18O and δ13C) isotope record to reconstruct past spatial distribution and longitudinal gradients of δ18Op and δ13C across Eurasia during periods of high atmospheric pCO2. We compile nearly 15,000 samples of authigenic carbonate and tooth enamel samples that span the Cenozoic Era across Eurasia. Oxygen isotopes in these proxies record the meteoric water δ18O signature during the time of mineral formation, which allows for reconstruction of moisture transport in the past. Soil carbonate δ13C, in turn, captures shifts in primary productivity and aridity, providing a complementary viewpoint to changes in the hydroclimate response to increased atmospheric pCO2. The δ18O record indicates that the westerlies have driven moisture transport across Eurasia since at least the Eocene. Additionally, steeper δ18Op gradients correspond with periods of high pCO2, suggesting a relatively high P/ET ratio and/or a higher evaporative fraction of ET, which preferentially supplies air parcels with 18O and thus steepens the gradient. The δ13C record demonstrates an increase in aridity in Asia during the late Cenozoic concurrent with stable (or slightly increased) primary productivity in Europe. Both isotopic records indicate that hydroclimate is driven predominantly by shifts in global climate due to changes in pCO2 and is only marginally impacted by dramatic changes in Cenozoic paleogeography. The proxy δ18Op gradients were compared to δ18Op gradients produced by both a simple reactive transport model and by an isotope-enabled earth system model. Gradients of δ18Op produced by both models under varying pCO2 demonstrate a mismatch between the proxy and model δ18O gradient response to CO2 – both models generate shallow gradients during periods of high pCO2 (4x Pre-Industrial pCO2) and generate the steepest gradients with the lowest pCO2 (Pre-Industrial pCO2). The proxy-model mismatch implies that these models may be misrepresenting the response of transpiration or the partitioning of P into ET and q at higher atmospheric CO2 concentrations.