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Northern hardwood forests are expected to experience an increase in mean annual air temperatures, and a decrease in winter snowpack and greater frequency of soil freeze/thaw cycles (FTCs) by the end of the century. As a result of these anticipated changes, northern hardwood forests in the northeastern U.S. will also have warmer soil temperatures in the growing season and colder soils in winter. Prior studies show that warmer soils in the growing season increase net primary productivity (NPP) and C storage as a result of increased soil net N mineralization, while increases in soil freezing in winter reduces plant uptake of N and C as a result of root damage. However, the combined effects of warmer soils in the growing season and increased soil freeze/thaw cycles in winter on tree litter mass and chemistry are unknown. We report here results from the Climate Change Across Seasons Experiment (CCASE) at Hubbard Brook Experimental Forest in New Hampshire, USA to characterize the response of leaf litter mass and chemistry to growing season warming combined with soil freeze–thaw cycles in winter. Across the years 2014-2017, litterfall mass and chemistry (%C, %N, C:N) were not significantly affected by changes in soil temperature; however, there was a trend of higher total litterfall mass and litter N mass from plots where soils were warmed in the growing season, but this increase disappeared with the addition of FTCs in winter. These results indicate that while rates of NPP and the total mass of N could be increased with rising soil temperatures over the next century in northern hardwood forests, the combination of warmer soils in the growing season and colder soils in winter may ultimate have little to no impact on litter mass or chemistry. We conclude that considering the combined effects of climate changes in the growing season and in winter is vital for the accurate determination of the response of litterfall mass and chemistry in northern hardwood forests.

The aim of this study was to assess how the plant community and its species diversity are affected by establishment methods and warming. Priority effects, i.e., the order of species arrival in plant establishment, are an important subject when invasive species become more common, and the biodiversity decreases as the global temperature continues to increase. A field experiment was carried out to investigate the effect of establishment methods and temperature on plant communities by using four different establishment methods. The plants were divided into two groups (fast and slow germinating) and sown at two occasions, one earlier than the other. Half of the study plots were then treated with OTC (Open Top Chambers) and the other half were used as control. There was a difference in temperature between the OTC blocks and the control blocks. OTC had no effect on the number of species or the Shannon diversity index but affected the proportion fast germinating, slow germinating, and naturally established species. There was a significant effect of the establishment events on the plant community. There was also a significant interaction between OTC treatment and establishment, only present for the proportion of fast germinating species. The temperature differences can provide information on plant responses to global warming and the results can mostly be explained by priority effects such as numerical superiority and competition in combination with germination rate are. The results of this study can be used to better understand how warming and priority effects may affect plant communities in the future.

Evidence continues to mount regarding the impact of climate change on the ecosystems of the world with increasingly dire predictions about the need for global action to slow warming and its downstream effects. Human beings are not immune to changes in their environment. Growing research demonstrates the impact of climate change on cardiovascular, pulmonary, psychiatric, neurologic and renal diseases, as well as its disruption of overall health through malnutrition, infectious disease, and pregnancy and developmental complications. Stress is known to precipitate, worsen, and maintain chronic disease. Social and community factors are known to impact individual and community mental health. The psychological stress of loss of goods, identity, and social support through weather events brought about by climate change has the potential to worsen the health and wellbeing of populations. Climate change does not impact communities equally. Populations historically and currently disadvantaged by inequitable policies may live in environments more at-risk to natural disaster, and have access to fewer financial, governmental, social, and healthcare resources to respond to climate events. Limitation of individual and community ability to respond to stressors reduces resilience and perpetuates chronic stress. The aim of this thesis is to examine the intersection of mental health and climate change with a particular focus on how social injustice has shaped the capability of populations, particularly those in urban settings, to respond to environmental changes with Philadelphia as a particular example. / Urban Bioethics

Climate warming is increasing fire severity in boreal forests and can alter forest structure and carbon (C) dynamics in Cajander larch (Larix cajanderi) forests of Siberia, which occur over C and ice-rich yedoma permafrost. Altered forest structure may impact understory vegetation through changing canopy cover, permafrost thaw depth, and soil temperatures. The primary objective of this study was to assess the long-term impacts of fire-driven changes in tree density on understory composition, diversity, and C pools and the underlying soil organic layer (SOL). Shrubs dominated low density stands, likely from reduced canopy cover and thaw depth, while mosses dominated high density stands. Consequently, understory C pools decreased from 415.46 to 158.87 g C m-2. Total SOL C pools remained unchanged as tree density increased. These findings suggest that fire-driven changes in tree density may alter understory composition and C pools, which could impact nutrient/water cycling and permafrost stability.

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The framing of news stories is found to be changing throughout time. This thesis advances a quantitative, longitudinal content analysis to examine the news coverage on climate change in five different countries over a period of ten years. Applying Chyi and McCombs two-dimensional measurement scheme, this thesis finds that the international frame was the most deployed spatial frame, while the present frame was the most used temporal frame. The political action, environmental risk and science frames, in their own regard, were the most deployed topical frames. Centrally, the analysis showed that the environmental risked frame is increasingly superseded by the science frame. This suggests that scientific considerations have become increasingly important in climate change journalism. Additionally, a higher climatological vulnerability of a country does not appear to translate to a risk focused framing of news articles. Moreover, the analysis finds that the societal spatial frame is increasingly used, pointing to emphasised national considerations in climate change journalism. Finally, the data of the thesis supports the emergence of a previously unconsidered climate action frame.

Greenhouse gases (GHGs) emissions due to human activities have considerably increased in the past decades which are the main contributors of global warming. In order to limit the consequences of the global climate change happening, all sectors must reduce their carbon emissions and especially the building industry which represents 19% of the carbon footprint of human activities. This paper is giving methods to help reduce the carbon footprint of a building when designing it such as life cycle assessment which allow project teams to compare the global warming potential of all building materials. Those methods are used and challenged in a case study of an architectural competition project named quai d’Issy in Paris, France. Using biobased materials help reduce the carbon footprint of a building, a structure made of timber and concrete elements can emit less than 21% of GHGs than a classic concrete structure. By sourcing reused and recycled building materials, by using geothermal heat pump as heating and cooling systems for example, we have been able to reach for the quai d’Issy project a carbon footprint of 930kgC02eq/m2 of floor area, which is less than level needed for the highest French environmental certification. However, these results can be obtained only if the building materials companies continue their work to develop low-carbon materials and promote recycled and reused materials. This study emphasises the need to spread knowledge of the tools to design low-carbon building to all the actors of the building industry in order to promote behaviours that will limit the consequences of climate change.

A worldwide shift from fossil fuels to zero carbon energy sources is imperative to limit global warming to 1.5°C. While integrating high penetrations of VRE into the grid may introduce the need for upgrading an aging electrical system, renewable energy represents a new opportunity to decarbonize multiple sectors. Otherwise curtailed solar and wind energy can accelerate deep decarbonization in hard-to-reach sectors - transportation, industrial, residential, and commercial buildings, all of which must be decarbonized to limit global warming. With renewable energy as its input, electrolytic H₂ represents a solution to the supply-demand mismatch created by the proliferation of VREs on a grid designed for on-demand power. Electrolytic H₂ can stabilize the grid since the H2 created can be stored and transferred. Thus, Chapter 1 introduces the opportunity of green H2 in the context of low-cost VREs as a means of deep decarbonization through sector coupling, and an overview of the techno-economics, key technologies, and life cycle assessment versus the incumbent steam methane reformation. The growing imbalances between electricity demand and supply from VREs create increasingly large swings in electricity prices. Capable of operating with variable input power and high current densities without prohibitively large ohmic losses, polymer electrolyte membrane (PEM) electrolyzers are well suited to VREs. In Chapter 2, polymer electrolyte membrane (PEM) electrolyzers are shown to help buffer against these supply demand imbalances, while simultaneously minimizing the levelized cost of hydrogen (LCOH) by ramping up production of H2 through high-current-density operation when low-cost electricity is abundant, and ramping down current density to operate efficiently when electricity prices are high. A techno-economic model is introduced that optimizes current density profiles for dynamically operated electrolyzers, while accounting for the potential of increased degradation rates, to minimize LCOH for any given time-of-use (TOU) electricity pricing. This model is used to predict LCOH from different methods of operating a PEM electrolyzer for historical and projected electricity prices in California and Texas, which were chosen due to their high penetration of VREs. Results reveal that dynamic operation could enable reductions in LCOH ranging from 2% to 63% for historical 2020 pricing and 1% to 53% for projected 2030 pricing. Moreover, high-current-density operation above 2.5 A cm−2 is shown to be increasingly justified at electricity prices below $0.03 kWh−1. These findings suggest an actionable means of lowering LCOH and guide PEM electrolyzer development toward devices that can operate efficiently at a range of current densities. Chapter 3 uses techno-economic modeling to analyze the benefits of producing green (zero carbon) hydrogen through dynamically operated PEM electrolyzers connected to off-grid VREs. Dynamic electrolyzer operation is considered for current densities between 0 to 6 A cm-2 and compared to operating a PEM electrolyzer at a constant current density of 2 A cm-2. The analysis was carried out for different combinations of VRE to electrolysis (VRE:E) capacity ratios and compositions of wind and solar electricity in 4 locations – Ludlow, California, Dalhart, Texas, Calvin, North Dakota, and Maple Falls, Washington. For optimal VRE:E and wind:PV capacity ratios, dynamic operation of the PEM electrolyzer was found to reduce the LCOH by 5% to 9%, while increasing H₂ production by 134% to 173%, and decreasing excess (i.e. curtailed) electrical power by 82% to 95% compared to constant current density operation. Under dynamic electrolyzer operation, the minimum LCOH is achieved at higher VRE:E capacity ratios than constant current density operation and a VRE mix that was more skewed to whichever VRE source with the higher capacity factor at a given location. In addition, dynamically operated electrolyzers are found to achieve LCOH values within 10% of the minimum LCOH over a significantly wider range of VRE:E capacity ratios and VRE mixes than constant electrolyzers. As demonstrated, the techno-economic framework described herein may be used to determine the optimal VRE:E capacity and VRE mix for dynamically-operated green hydrogen systems that minimize cost and maximize the amount of H2 produced. Chapter 4 focuses on the production of high-purity water and H₂ from seawater. Current electrolyzers require deionized water so they need to be coupled with desalination units. This study shows that such coupling is cost-effective in H₂ generation, and offers benefits to thermal desalination, which can utilize waste heat from electrolysis. Furthermore, it is shown that such coupling can be optimized when electrolyzers operate at high current density, using low-cost solar and/or wind electricity, as such operation increases both H₂ production and heat generation. Results of techno-economic modeling of PEM electrolyzers define thresholds of electricity pricing, current density, and operating temperature that make clean electrolytic hydrogen cost-competitive with H₂ from steam methane reforming. By using 2020 hourly electricity pricing in California and Texas, H₂ is shown to be produced from seawater in coupled desalination-electrolyzer systems at prices near $2, reaching cost parity with SMR-produced H₂. Chapter 5 concludes the dissertation with an overview of the challenges and research needs for PEM electrolyzers at scale, including projected iridium needs, iridium thrifting, recycling methods, key degradation mechanisms, a failure modes and effects analysis, and LCOH projections.

Long-term warming experiments in arctic tundra have resulted in reduced moss cover and increased vascular plant cover. As mosses have a major impact on microclimatic conditions, changes in community composition can potentially alter direct and indirect drivers of productivity and decomposition, which are low in arctic ecosystems. This can potentially change the carbon balance. Therefore, it is of importance to investigate how the effects of warming on ecosystem processes depend on the identity of the moss species dominating the vegetation. Furthermore, because subarctic climate is expected to get warmer and wetter, it is essential to examine how warming effects depend on variation in precipitation as well. Thus, Gross Primary Production (GPP) and Ecosystem Respiration (Reco) fluxes, moss growth, and decomposition rate, were measured in plot pairs along a natural precipitation gradient in subarctic tundra Sweden. Each pair was dominated by one of three common moss species (Hylocomium splendens, Pleurozium schreberi and Sphagnum spp.), and were subjected to moss removal and Open Top Chamber (OTC) warming treatment combinations. Moss growth measurements were taken as a measure of productivity and Tea Bag Index was used as a measure of decomposition rate. Warming effects on GPP depended on both species and precipitation; Hylocomium and Sphagnum had highest productivity in warmed high-precipitation plots, whereas the effect was negative in all cases for Pleurozium. No significant interactions were found for the other response variables, but there was a positive effect of warming on Reco across all treatments, as well as a negative effect on decomposition. Warming induced increases in GPP and respiration, and decrease in decomposition rate, could imply that the increase in CO2 emissions from arctic ecosystems will be smaller than predicted, as the sequestration of CO2 in that case would exceed what is being emitted. Still, it is a complex matter and therefore crucial to further investigate the role of moss species in relation to warming effects on ecosystem processes.