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Forests perform many important ecological functions and provide numerous environmental services to humanity. Currently forests are under ever increasing pressures from humans through deforestation, changes in land use, and anthropogenic climate change. Managed forests play an important role in supplying forest products to the global population, necessitating the need to predict how forests will respond to climate change and how this will influence future wood product supplies. In this dissertation I used loblolly pine (Pinus taeda), the most extensively cultivated tree species in the United States, as a study system to simulate how climate change and forest management could alter the dynamics of managed forests in the future. Using the land component (the Community Land Model) of the widely used Community Earth System Model (CESM), I developed and validated a set of tools necessary to simulate the loblolly pine plantation system using the vegetation demography model embedded in CESM (FATES). This included developing a representation of loblolly pine using data from the literature, which was better able to capture forest growth and development observed in field studies than FATES's existing conifer tree representation. I added the ability to simulate several aspects of forest management not previously supported in FATES by creating the Vegetation Management Module, which I showed was able to realistically reproduce the common management practice of stand thinning. I used these new tools to perform simulations of how loblolly pine will grow across the Southeastern United States until the end of the 21st century, under the high and low climate change scenarios developed by the scientific community in the Coupled Model Intercomparison Project Phase 6 (CMIP6). Our experiments show that loblolly pine productivity may as much as double by the end of the century, with total wood harvest over that period increasing by almost half. I also showed that different management activities had significant effects on loblolly plantation yields, with mid-rotation stand thinning having an effect under both climate scenarios on par with increases due to the extreme climate change scenario SSP5 RCP8.5. I showed that these changes in wood yields could decrease the forest area in the Southeast required to meet the wood product demands over the rest of the century. These changes in plantation productivity could interact with socioeconomic factors to drive changes in land use and carbon storage in the Southeastern U.S. This work increases our understanding of how managed forests in the U.S.\ will be affected by climate change and how our management choices modulate that response. The techniques and tools developed here open up new areas of research into the role of forest management in the climate system.

Phytoplankton play a key role in regulating the global carbon cycle by exporting carbon to the deep ocean via the biological pump. The ratio of carbon to nutrients in exported organic matter has long been used to simplify biogeochemical cycles, where a fixed Redfield ratio is assumed. However, these ratios are not truly fixed, and many instances of variation have been observed. Here I aim to illustrate the importance of variable stoichiometry in accurately representing ocean biogeochemical cycling the modern steady state ocean. First, I will highlight the role of variable iron to carbon stoichiometry in coupling of the marine carbon and nitrogen cycles and regulating marine ecosystem response to global changes in atmospheric iron deposition. Second, I will show the impacts of fully variable C:N:P:Fe:Si stoichiometry on biogeochemical cycle interactions and how using a fixed implementation may introduce bias in the interactions between the carbon and nitrogen cycles. Lastly, I will show the roles of individual nutrient variability in driving patterns of nutrient stress and limitation in the modern ocean and how variable stoichiometry plays a role in regulating ocean carbon cycling under nutrient stress.

Planetary climates are controlled by the delicate radiative balance between incoming and outgoing energy. Any perturbation to either side of the equation disrupts the surface temperature with implications for the physical environment and (bio)geochemical processes that in their turn impose additional alterations to the radiative energy balance. In this dissertation, I use numerical approaches to investigate the role of changing greenhouse gas concentrations and astronomical forcing (quasi-periodic variations in solar radiation via gradual changes in Earth's orbit and tilt) on the surface climate, carbon cycling, and the environmental consequences of associated feedback processes, on Earth and beyond. First, I address the impact of carbon dioxide release to the atmosphere on geological timescales. Past carbon release events and episodes of global warming that are preserved in paleoclimate records are compared to an ensemble of Earth system model simulations to estimate the carbon input and output fluxes during past events and determine how the Earth responded to such perturbations. Next, I provide the reader with a thorough analysis of the impact of astronomical forcing and its imprint on the marine environment as simulated in an Earth system model. The transient modeling approach allows direct comparison between time-varying model output and multi-million-year-long paleoclimate records and thereby provides an opportunity to determine from model-data (mis)matches what feedback processes are the main drivers for variations in the climate-carbon system on astronomical timescales across tens of thousand to millions of years. Lastly, I examine with the use of orbital and obliquity simulations how the frequency and amplitude of astronomical cycles change as a function of planetary architecture and assess their impact on surface conditions and planetary habitability across million-year timescales. This research contributes to identifying exoplanets in other systems that may have been able to maintain habitable conditions over prolonged periods of time required for the development and evolution of life elsewhere in the universe.

Widespread land surface changes took place in the Northern Hemisphere throughout the Holocene epoch (11,700 years ago to the present). The most notable of these changes were large increases in vegetation cover that occurred throughout the African Sahara. Yet, the varied responses of regional and global climate to this land surface change are not well understood because few modeling studies have directly incorporated it in their experiments. This dissertation presents new Earth system modeling results that include land surface changes in the African Sahara and Northern Hemisphere mid- and high latitudes to directly identify the responses of regional and global climate to these changes. The chapters in this dissertation provide various resolutions to several challenges in reconciling Holocene model and proxy reconstructions of both hydroclimate and temperature. Chapters 2 and 3 investigate challenges related to the impact of African Saharan greening, known as the “Green Sahara”, on the West African monsoon during the mid-Holocene. Conclusions from these chapters better constrain the mid-Holocene response of African hydroclimate to a wide array of land surface processes associated with the Green Sahara. Chapter 2 explores the competing impacts of the direct radiative and indirect aerosol-cloud effects associated with the vegetation-induced reduction in dust aerosols, providing an improved understanding of the West African monsoon response to vegetation and dust forcing in the mid- Holocene. Chapter 3 examines how vegetation-induced changes in the isotopic composition of precipitation and soil water help to constrain estimates of the monsoon’s northernmost limit. By showcasing a previously unknown positive anomaly in the isotopic composition of precipitation, this chapter suggests a mid-Holocene northernmost limit of ~23–28°N for the West African monsoon. Chapters 4 and 5 expand the spatial and temporal scope of this dissertation and investigate external climate responses, both regional and global, to Holocene land surface change. Both of these chapters offer increased Northern Hemisphere vegetation as a resolution to challenges related to reconstructing Holocene climate with model simulations and geochemical proxies. Chapter 4 explores the opposing isotopic signature of the North American monsoon in comparison with other Northern Hemisphere land monsoons and shows that the Green Sahara is likely responsible for this opposing behavior by modulating the local Walker circulation. Seasonal exploration of hydroclimatic and stable water isotopic changes elucidates the factors contributing to the evolution of the North American monsoon. Chapter 5 investigates the controversial model-data disagreement in reconstruction of Holocene global temperatures known as the Holocene Temperature Conundrum. By inclusion of Holocene increases in vegetation in the African Sahara and Northern Hemisphere mid- and high latitudes, the results of this chapter suggest that vegetation change drives a mid-Holocene maximum in annual global mean temperatures and better aligns model simulations with proxy data. Taken together, findings from this dissertation highlight the importance of vegetation in driving past climate change and in reconciling results from model simulations with geochemical proxy data.

Icehouses, such as the current glacial state and Late Paleozoic Ice Age (LPIA; ~340 to 290 million years ago), make up less than 25% of the past billion years of Earth’s history and are associated with major environmental change. Thus, exploring climate dynamics during the LPIA expands our view of the climate system and improves our confidence in future climate projections. Despite major advancement in our understanding of CO2-induced changes in supercontinental glaciation during the LPIA, far less is known about how the superocean and biosphere factored into the climate system. This dissertation presents new understanding of regional and global ocean dynamics and tropical vegetation-climate feedbacks under evolving atmospheric pCO2 during the LPIA by simulating these processes in an Earth system model framework. The chapters in this dissertation integrate novel Earth system model simulations with environmental proxy data to better understand how climate dynamics drove marine and terrestrial change during the LPIA. Chapters 2 and 3 investigate glacial-interglacial changes in ocean circulation and temperature from the global to regional scale to better constrain the nature and role of the superocean in the LPIA. Chapter 2 explores the effects of changing CO2, sea level, and high-latitude ice extent on global ocean circulation, salinity, and temperature patterns. Results from this chapter represent the first global estimates of surface currents and thermohaline circulation for the LPIA and connect insights from coastal proxy records to large-scale ocean dynamics. Chapter 3 refines the spatial scale of interest by investigating the patterns of seawater oxygen isotopic composition, temperature, and circulation in the North American Midcontinent Sea. Results from this chapter demonstrate that local variations in runoff contribute to much of the high spatial variability observed in oxygen isotopic records from the sea. This chapter demonstrates how isotope-enabled Earth system models can be used to constrain local processes in semi-restricted ancient inland seas, and thus improve proxy-based interpretations of seawater temperature and chemistry related to large-scale paleoceanographic events. Chapter 4 shifts from the marine to terrestrial realm and explores vegetation-climate interactions across low-latitude Pangaea to better understand the role of paleo-plant physiology in paleotropical climate change. This chapter presents the first methodology for translating Pennsylvanian fossil leaf characteristics and insights from process-based ecosystem modeling into extinct plant types in an Earth system model. Using novel paleo-plant types that represent key Pennsylvanian tropical plants and their modern plant analogs, this chapter explores tropical vegetation-climate feedbacks under different levels of CO2 as well as wetland and dryland tropical forests. Results from this chapter show that paleo-plants transpire more water overall and respond differently to elevated CO2 compared to modern plants, leading to regional variations in low-latitude precipitation and soil water that better capture the tropical moisture gradient inferred from terrestrial proxy records. This chapter highlights the important role that age-specific paleo-plant physiology plays in vegetation-climate interactions in deep time. In sum, findings from this dissertation demonstrate the importance of atmospheric pCO2 in driving past icehouse climate dynamics and the utility of integrating Earth system models and proxy records in deep time.

One of the largest ecosystem controls in the oceans is the presence of dissolved oxygen. As oxygen levels fall, both micro- and macroorganisms face shrinking habitats and potential mortality. There have been several periods in Earth history where oxygen levels have fallen to anoxic (dissolved O2 concentration < 10 µmol L-1) or hypoxic (< 60 µmol L-1) levels in certain ocean basins or within inland seas and some of these events could potentially be linked to mass extinction events. Several hypotheses exist regarding the depletion of oxygen, the spread of hypoxia-anoxia, and why the low oxygen events occur at certain points in the geologic record, including rapid climate warming, enhanced nutrient inputs, and modifications to the surface biological pump. Unfortunately, there is little agreement on which of these potential hypotheses caused individual events and what might impact the oxygenation of our oceans in the future. This dissertation will test hypotheses related to deep ocean oxygen using the University of Victoria Earth System Climate Model. The first set of experiments feature Late Ordovician winds and paleogeography and test the impacts of atmospheric CO2 and O2, ocean bottom topography, and nutrient loadings on deep ocean oxygen concentrations. The second set of experiments is also within the Late Ordovician, but tests the impacts of remineralization rates, detrital sinking velocities, and ocean surface albedo on ocean oxygenation. The final set of experiments tests the impacts of a warming climate on the oxygenation of near-future oceans, in addition to the impacts of detrital sinking velocities and ocean surface albedo.For the Late Ordovician, the factors most favorable for the spread of anoxia are reduced atmospheric O2, increased loadings of nitrate, and a reduction in ocean surface albedo. Climatic factors (namely, increased CO2) played little role in the spread of anoxia or the depletion of oxygen in these experiments. Similarly, phosphate, enhanced remineralization rates, and slower sinking velocities do little to diminish the dissolved oxygen, but can actually work to increase oxygen below the surface layer, potentially based on where the remineralization is occurring due to these modifications.The future oceans will be impacted by warming; the largest reductions in ocean oxygen can be attributed to decreased dissolution of atmospheric oxygen in warmer surface waters. Shrinking sizes of particles (done here by reducing particle velocities) may actually work to counteract some of the reductions in ocean oxygen due to climate warming, as fewer particles fall into the deep ocean, reducing deep remineralization and uptake of oxygen.

Global continental discharge is an important component of global water cycle, but it is difficult to measure by traditional methods alone. In this dissertation, we provide an overview of current methods of estimating global continental discharge, and provide mass-balance estimates of global discharge using remote sensing and reanalysis products. Evaluating against observation-based estimates, we find that discharge computed from land mass balance using reanalysis-based moisture convergences and GRACE can provide a viable alternative to declining in situ observations’ records for major river basins. Then we conduct a series of sensitivity experiments on an ocean-ice model forced with varying discharge to understand ocean’s response to changes and uncertainties in discharge. We find that sea surface salinity (SSS), mixed layer depth, potential temperature, net primary production, and stratification show notable changes in the coastal and river plume regions, and that for certain river plume regions, interannual variability accounts to up to half of the total variance. Lastly, we introduce another novel method of estimating discharge from space. We isolate discharge and transport signal in SSS after removing atmospheric freshwater flux signal from SSS using joint EOF analysis, and then use discharge-SSS correlation to infer discharge for the Orinoco-Amazon, and Congo-Niger river basins.

Rivers smaller than the Amazon tend to be excluded from earth system modeling efforts. Does it matter? Do sub-grid-scale rivers have significant impacts on offshore primary productivity? Using the Eel River in northern California, the river with the largest sediment yield per drainage area in the continental United States, as a test case, this question is explored using two approaches. First, a data-driven analysis of relevant time series taken on land, by buoy, and from space, demonstrates very little evidence of direct impact of Eel River discharge on contemporaneous coastal ocean primary productivity - but to the extent that that evidence exists, it seems to occur during years of greatest river discharge. To further analyze mechanistic drivers, a coupled mesoscale modeling framework unifying ocean, watershed and atmospheric representations is formulated and run in hindcast over the 2002-2010 period. Monthly average climatologies, interannual variabilities, and event-driven analysis of each year's largest river discharge are all examined for evidence of a river-ocean connection expressed through primary production. Storm event-generated turbulence appears to dominate the primary productivity during the winter months. The impact of the river seems to be largely independent of nutrient load, because its dissolved nitrate is less than that of the coastal ocean. There is no evidence that riverine delivery of gradually bioavailable detritus has a significant effect. Although a sufficiently super-nitrous river shows the ability to sustain a plume-nutrient-driven-bloom even at periods of extremely low flow, this is not currently a realistic scenario for the Eel River. The possibility remains that another micronutrient not studied in the modeling framework, such as iron, could be important to this system.