Tunneling with negative tension

We describe a new family of thin-wall instantons, with negative tension bubble walls, that mediate tunneling between Minkowski or de Sitter vacua. Some of these instantons can have exponentially enhanced tunneling rates, and would dominate the Euclidean path integral, representing a severe non-perturbative instability in any theory which supports them. We give two constructions of these instantons in theories which are perturbatively stable, but violate the null energy condition. One construction uses a scalar field theory with higher-derivative kinetic term, and is similar to the Coleman-de Luccia positive tension instanton. The other construction employs a negative-tension boundary arising from Z_2 orbifolding: it resembles a "bubble of nothing" which nucleates and grows, consuming the surrounding de Sitter or Minkowski spacetime. We explain how the spectral flow for fluctutations about the thin-wall tunneling solutions automatically protects causality, for both positive and negative tension instantons. We comment briefly on the relation of these solutions to a Kalzua-Klein "bubble of nothing," cosmological models relying on ghost condensates, and string theory orientifolds.Comment: PDFLaTeX, 43pp v2: new appendix, references adde

What can history tell us about the future? Using recent observations and paleoclimate proxies to constrain equilibrium climate sensitivity

Despite improvements in computing power, climate modeling, and basic theoretical understanding, the Earth’s physical response to a doubling of atmospheric carbon dioxide remains uncertain. Can observations be useful in constraining this theoretical quantity? We have high-­‐quality information on recent trends: greenhouse gas concentrations have increased since the industrial revolution, and the planet has warmed in response. But I will argue that this recent history provides only weak constraints on the eventual climate sensitivity: observations of a transient climate are poor predictors of a future equilibrium state. Reconstructions of past equilibria both colder (the Last Glacial Maximum) and warmer (the mid-­‐Pliocene) than the present provide stronger constraints, suggesting that the extremely high climate sensitivities of some state‐of‐the‐art climate models are unrealistic. I’ll present a framework for facilitating apples-to-apples comparisons of past and future climate and discuss how to understand, reduce, and communicate the uncertainties associated with future climate response

African Institutes for Mathematical Sciences: Building a knowledge base for African security

In this paper we explore the role of scientific and technical education in addressing security issues on the African continent. We adopt a definition of “security” that emphasises the well-being of the citizen rather than that of the state, and that encompasses both military and non-military threats. Security and development are interlinked, and this paper will emphasise the need for an effective African technical infrastructure to address these twin issues. To this end, we argue for the establishment of a pan-African network of mathematical institutes, relying on open-source software and educational tools as well as improved communication links throughout the continent. We emphasise the crucial role played by free and open-source tools in improving communication links on the continent. As a case study, we analyse the recently founded African Institute for Mathematical Sciences in South Africa. This unique project is characterised by three main characteristics: a commitment to pan-African cooperation, innovative, project-based teaching methods, and exclusive reliance on open-source software and educational material. We analyse to what extent it can serve as a model for future capacity building initiatives. Finally, we outline the proposed African Mathematical Institute Network, or AMI-Net. This network of interconnected institutes will help to build the scientific and technical knowledge base necessary for sustained African development. We offer suggestions for the implementation of the project, and show how it can contribute to the overall security infrastructure

Sensitivity to Factors Underlying the Hiatus

Recent trends in global mean surface air temperature fall outside the 90 range predicted by models using the CMIP5 forcings and scenarios; this recent period of muted warming is dubbed the hiatus. The hiatus has attracted broad attention in both the popular press and the scientific literature, primarily because of its perceived implications for understanding long-term trends. Many hypotheses have been offered to explain the warming slowdown during the hiatus, and comprehensive studies of this period across multiple variables and spatial scales will likely improve our understanding of the physical mechanisms driving global temperature change and variability.We argue, however, that decadal temperature trends by themselves are unlikely to constrain future trajectories of global mean temperature and that the hiatus does not significantly revise our understanding of overall climate sensitivity. Instead, we demonstrate that, because of the poorly constrained nature of the hiatus, model-observation disagreements over this period may be resolvable via uncertainties in the observations, modeled internal variability, forcing estimates, or (more likely) some combination of all three factors. We define the hiatus interval as 1998-2012, endpoints judiciously chosen to minimize observed warming by including the large 1998 El Nio event and excluding 2014, an exceptionally warm year. Such choices are fundamentally subjective and cannot be considered random, so any probabilistic statements regarding the likelihood of this occurring need to be made carefully. Using this definition, the observed global temperature trend estimates from four datasets fall outside the 5-95 interval predicted by the CMIP5 models. Here we explore some of the plausible explanations for this discrepancy, and show that no unique explanation is likely to fully account for the hiatus

Overestimate of Committed Warming

Palaeoclimate variations are an essential component in constraining future projections of climate change as a function of increasing anthropogenic greenhouse gases. The Earth System Sensitivity (ESS) describes the multi-millennial response of Earth (in terms of global mean temperature) to a doubling of CO2 concentrations. A recent study used a correlation of inferred temperatures and radiative forcing from greenhouse gases over the past 800,000 years to estimate the ESS from present day CO2 is about 9 degrees C, and to imply a long-term commitment of 3-7 degrees C even if greenhouse gas levels remain at present-day concentrations. However, we demonstrate that the methodology of ref. 2 does not reliably estimate the ESS in the presence of orbital forcing of ice age cycles and therefore conclude that the inferred present-day committed warming is considerably overestimated

Challenges in quantifying changes in the global water cycle

Human influences have likely already impacted the large-scale water cycle but natural variability and observational uncertainty are substantial. It is essential to maintain and improve observational capabilities to better characterize changes. Understanding observed changes to the global water cycle is key to predicting future climate changes and their impacts. While many datasets document crucial variables such as precipitation, ocean salinity, runoff, and humidity, most are uncertain for determining long-term changes. In situ networks provide long time-series over land but are sparse in many regions, particularly the tropics. Satellite and reanalysis datasets provide global coverage, but their long-term stability is lacking. However, comparisons of changes among related variables can give insights into the robustness of observed changes. For example, ocean salinity, interpreted with an understanding of ocean processes, can help cross-validate precipitation. Observational evidence for human influences on the water cycle is emerging, but uncertainties resulting from internal variability and observational errors are too large to determine whether the observed and simulated changes are consistent. Improvements to the in situ and satellite observing networks that monitor the changing water cycle are required, yet continued data coverage is threatened by funding reductions. Uncertainty both in the role of anthropogenic aerosols, and due to large climate variability presently limits confidence in attribution of observed changes

Rapidly evolving aerosol emissions are a dangerous omission from near-term climate risk assessments

Anthropogenic aerosol emissions are expected to change rapidly over the coming decades, driving strong, spatially complex trends in temperature, hydroclimate, and extreme events both near and far from emission sources. Under-resourced, highly populated regions often bear the brunt of aerosols’ climate and air quality effects, amplifying risk through heightened exposure and vulnerability. However, many policy-facing evaluations of near-term climate risk, including those in the latest Intergovernmental Panel on Climate Change assessment report, underrepresent aerosols’ complex and regionally diverse climate effects, reducing them to a globally averaged offset to greenhouse gas warming. We argue that this constitutes a major missing element in society’s ability to prepare for future climate change. We outline a pathway towards progress and call for greater interaction between the aerosol research, impact modeling, scenario development, and risk assessment communities

Kate Marvel

Kate Marvelhttps://digitalcommons.montclair.edu/sust-seminar-headshots/1088/thumbnail.jp