1,720,991 research outputs found
Radiative Forcings of Well-Mixed Greenhouse Gases
No full textA change in the atmospheric inventory of a greenhouse gas produces a radiative forcing on the atmosphere which results in climatic change. Thus to understand climate change resulting from perturbations to atmospheric greenhouse gas concentrations it is necessary to quantify the radiative forcing. Here, radiative forcings are presented for large changes in atmospheric CO2, CH4, and N2O in the modern atmosphere and large changes in atmospheric CO2, CH4 and 18 other gases for the Archean atmosphere.
For the modern Earth, I present new calculations of radiative forcing at very high concentrations of CO2, CH4, and N2O, relevant to extreme anthropogenic climate change and paleoclimate studies. CO2 forcing is calculated over the range 100 ppmv to 50,000 ppmv. CH4, and N2O forcings are calculated over the range 100 ppbv to 100 ppmv. The sensitivity of these calculations to spatial averaging and tropopause definition are examined. I compare our results with the ``simplified expressions'' reported by IPCC, and find significant differences at high greenhouse gas concentrations. I provide new simplified expressions which agree much better with the calculated forcings, and suggest that these expressions be used in place of the IPCC expressions. Additionally, I provide meridionally resolved forcings which may be used to force simple and intermediate complexity climate models.
For the Archean Earth, I present new calculations of radiative forcing for CO2 (10^-6 - 1 bar), CH4 (500 ppbv - 10,000 ppmv) and 18 other gases (10 ppbv - 10 ppmv). I aim to provide a set of radiative forcing and overlap calculations which can be used as a standard for comparisons. Radiative forcings are calculated for atmospheres with various N2 inventories (0.5, 1, and 2 bar). The effect of overlap and atmospheric pressure on radiative forcing are examined. The CO2 radiative forcings are consistent with previous work, however, I find significantly more shortwave absorption by CH4 than previously reported which may limit warming above 100 ppmv. For the 18 other gases, I find that significant radiative forcings result from low concentrations (<1 ppmv). These forcings are compared to those given in the [email protected]
Nitrogen in the Earth System: planetary budget and cycling during geologic history
Full text linkThe distribution and geologic history of nitrogen on Earth is poorly known. Traditionally thought to be an inert gas, with only a small but important biologic cycle, geochemical investigation highlights that it can also be present in rocks and minerals. Even at low concentrations, the great mass of the solid Earth allows for the possibility of substantial N mass and cycling in the geosphere over Earth history. Thus, the assumption that N on the surface of the Earth has remained in steady state over Earth history can be questioned. The research goals of this thesis are to investigate the Earth System N cycle using both large- and small-scale approaches.
I present a comprehensive literature compilation to ascertain the N budget of Earth. Determining the total abundance of N in all reservoirs of the Earth, including the atmosphere, oceans, crust, mantle, and core is crucial to a discussion of its cycling in the past. This budget study suggests that the majority of planetary N is likely in the core, with the Bulk Silicate Earth a more massive reservoir than the atmosphere. I also present experimental data and data from lunar samples as added context.
As quantification of geologic N is difficult, I present research detailing the adaptation of a fluorometric technique common in aquatic geochemistry for use on geologic samples. I compare fluorometry analysis of geochemical standards to several other techniques: colourimetry, elemental analyzer mass spectrometry, and neutron activation analysis. Fluorometry generally behaves well for crystalline samples, and is a relatively quick and easy alternative to more expensive or intensive techniques. As a preliminary application, I have determined a N budget estimate for the continental crust based on analysis of crystalline crustal rocks and glacial tills from North America. This budget is consistent with published work, suggesting about 2 × 1018 kg N, or half a present atmospheric mass of N, is in the continental crust.
I also present a geochemical study measuring N-isotopes and redox sensitive trace elements from a syn-glacial unit deposited during the the Marinoan Snowball Earth. Snowball Earth events were the most extreme glaciations in Earth history. The measurements presented herein are the first to quantify biologic activity via N-isotopes as well as the redox state of the atmosphere and ocean using trace elements from this intriguing time period in Earth history. The data suggests that there was active N- fixing in the biosphere, persistent but limited O2, nitrification, and nearly quantitative denitrification during the glaciation. After the glacial interval, O2 levels increased and denitrification levels dropped, indicated by near-modern δ15N values. The combined use of N-isotope with redox sensitive trace elements provides a more nuanced and comprehensive view in reconstructing past ocean and biologic conditions.
Lastly, I present an Earth-system N cycle model with nominal results. Previous modelling efforts have agreed with the traditional notion that atmospheric N-levels have remained constant over geologic time. This is in contrast with modern geochemical evidence suggesting net transport of N from the surface into the mantle. The aim, in turn, of this model is to model N cycling over Earth history by explicitly incorporating both biologic and geologic fluxes. The model is driven by a mantle cooling history and calculated plate tectonic speed, as well as a prescribed atmospheric O2 evolution history. This approach is the first of its kind, to my knowledge, and produces stable model runs over Earth history. While tuning and sensitivity studies may be required for publishable results, nominal runs are compelling. In model output, atmospheric N varies by an factor of 2 − 3 over Earth history, and the availability of nutrients (i.e., PO4) exerts a strong control on biologic activity and movement of N throughout the Earth system.
Such a planetary perspective on N serves as an entry point into discussions of planetary evolution as a whole. With the great increase in the number of discovered exoplanets, the scientific community is charged with developing models of planetary evolution and factors that promote habitability. Comparison of Earth to its solar system neighbours and future data on exoplanets will allow a system of evolution pathways to be explored, with the role of N expected to be prominent in discussions of habitability and planetary [email protected]
EONS: A new biogeochemical model of Earth's longterm evolution
Full text linkI present Earth’s Oxygenation and Natural Systematics (EONS): a new, fully coupled biogeochemical model of the atmosphere, ocean, and their interactions with the geosphere, which can reproduce major features of Earth’s evolution fol- lowing the origin of life to the present day. The model includes an interactive biosphere, cycles of carbon, nitrogen, phosphorus, and oxygen, and climate. A nominal model run initialized in the Eoarchean resolves emergent surface oxy- genation, nutrient limitations, and climate feedbacks. The modelled atmosphere oxygenates in stepwise fashion over the course of the Proterozoic; a nearly billion year lag after the evolution of photosynthesis at 3.5 Ga is followed by a great oxi- dation event (GOE) at 2.4 Ga, which appears to be caused by the gradual buildup of organic matter on the continents imposing nutrient limitation on the biosphere by removing key nutrients from the ocean system. The simple climate system shows significant temperature shifts punctuate the oxygenation process, implying that major biological transitions possibly destabilized Earth’s climate. I expand upon this finding by adapting the climate system to include non-linearities such as ice-albedo and supergreenhouse feedbacks in order to investigate potential causes of Paleoproterozoic Snowball Earth events. My preliminary findings suggest that Paleoproterozoic glaciations may have preceded the GOE, and are more likely a result of perturbations to atmospheric CO2 than from declining CH4. This work demonstrates that forward modelling the entirety of Earth’s history with relatively few imposed boundary forcings is feasible, that the Earth system is not at steady state, and that our understanding of coupled C-N-P-O cycling as it functions today can explain much of the Earth’s evolution.Graduat
The State Dependency of Climate Sensitivity and Cloud Responses to CO2 Doubling
No full textCloud feedbacks are a large source of uncertainty in paleoclimate studies and in the constraint of climate sensitivity, and it is thought that climate sensitivity depends on climate state because of these feedbacks. Here we evaluate the state dependence of both climate sensitivity and cloud responses to CO2 doubling for a palate of climates with varying solar luminosity and CO2 concentrations, yet near-equivalent global mean surface temperatures. We find that cloud responses to CO2 doubling and climate sensitivity are largely dependent on the magnitude of radiative forcing from doubling CO2 alone, and are therefore independent of climate state. We also determine that the most important responses in clouds occur in the mid-latitude storm tracks, owing to inhibited baroclinic activity as a consequence of a more equable climate under higher CO2 concentrations.Graduat
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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