26,027 research outputs found
Quantifying statistical uncertainty in the attribution of human influence on severe weather
Event attribution in the context of climate change seeks to understand the
role of anthropogenic greenhouse gas emissions on extreme weather events,
either specific events or classes of events. A common approach to event
attribution uses climate model output under factual (real-world) and
counterfactual (world that might have been without anthropogenic greenhouse gas
emissions) scenarios to estimate the probabilities of the event of interest
under the two scenarios. Event attribution is then quantified by the ratio of
the two probabilities. While this approach has been applied many times in the
last 15 years, the statistical techniques used to estimate the risk ratio based
on climate model ensembles have not drawn on the full set of methods available
in the statistical literature and have in some cases used and interpreted the
bootstrap method in non-standard ways. We present a precise frequentist
statistical framework for quantifying the effect of sampling uncertainty on
estimation of the risk ratio, propose the use of statistical methods that are
new to event attribution, and evaluate a variety of methods using statistical
simulations. We conclude that existing statistical methods not yet in use for
event attribution have several advantages over the widely-used bootstrap,
including better statistical performance in repeated samples and robustness to
small estimated probabilities. Software for using the methods is available
through the climextRemes package available for R or Python. While we focus on
frequentist statistical methods, Bayesian methods are likely to be particularly
useful when considering sources of uncertainty beyond sampling uncertainty.Comment: 41 pages, 11 figures, 1 tabl
The Economic Impacts of Aquatic Invasive Species: A Review of the Literature
Invasive species are a growing threat in the United States, causing losses in biodiversity, changes in ecosystems, and impacts on economic enterprises such as agriculture, fisheries, and international trade. The costs of preventing and controlling invasive species are not well understood or documented, but estimates indicate that the costs are quite high. The costs of aquatic invasive species are even less well understood than those for terrestrial species. A systematic approach is needed to develop a consistent method to estimate the national costs of aquatic invasives. This review of the economic literature on aquatic invasive species is the first stage in the development of that estimate. We reviewed over sixty sources and include both empirical papers that present cost estimates as well as theoretical papers on preventing and mitigating the impacts of aquatic invasive species. Species-specific estimates are included for both animals and plants.aquatic invasive species, costs, literature review, Resource /Energy Economics and Policy,
Interatomic Methods for the Dispersion Energy Derived from the Adiabatic Connection Fluctuation-Dissipation Theorem
Interatomic pairwise methods are currently among the most popular and
accurate ways to include dispersion energy in density functional theory (DFT)
calculations. However, when applied to more than two atoms, these methods are
still frequently perceived to be based on \textit{ad hoc} assumptions, rather
than a rigorous derivation from quantum mechanics. Starting from the adiabatic
connection fluctuation-dissipation (ACFD) theorem, an exact expression for the
electronic exchange-correlation energy, we demonstrate that the pairwise
interatomic dispersion energy for an arbitrary collection of isotropic
polarizable dipoles emerges from the second-order expansion of the ACFD
formula. Moreover, for a system of quantum harmonic oscillators coupled through
a dipole--dipole potential, we prove the equivalence between the full
interaction energy obtained from the Hamiltonian diagonalization and the ACFD
correlation energy in the random-phase approximation. This property makes the
Hamiltonian diagonalization an efficient method for the calculation of the
many-body dispersion energy. In addition, we show that the switching function
used to damp the dispersion interaction at short distances arises from a
short-range screened Coulomb potential, whose role is to account for the
spatial spread of the individual atomic dipole moments. By using the ACFD
formula we gain a deeper understanding of the approximations made in the
interatomic pairwise approaches, providing a powerful formalism for further
development of accurate and efficient methods for the calculation of the
dispersion energy
Efeitos nocivos do manejo inadequado da adubação no crescimento radicular das culturas anuais, com enfase no potássio.
Este documento tem como objetivo discutir os efeitos da localização da adubação, principalmente a potássica, na germinação, crescimento radicular e rendimento de algumas culturas anuais.bitstream/CNPAF/21630/1/doc_158.pd
Quantifying the effect of interannual ocean variability on the attribution of extreme climate events to human influence
In recent years, the climate change research community has become highly
interested in describing the anthropogenic influence on extreme weather events,
commonly termed "event attribution." Limitations in the observational record
and in computational resources motivate the use of uncoupled,
atmosphere/land-only climate models with prescribed ocean conditions run over a
short period, leading up to and including an event of interest. In this
approach, large ensembles of high-resolution simulations can be generated under
factual observed conditions and counterfactual conditions that might have been
observed in the absence of human interference; these can be used to estimate
the change in probability of the given event due to anthropogenic influence.
However, using a prescribed ocean state ignores the possibility that estimates
of attributable risk might be a function of the ocean state. Thus, the
uncertainty in attributable risk is likely underestimated, implying an
over-confidence in anthropogenic influence.
In this work, we estimate the year-to-year variability in calculations of the
anthropogenic contribution to extreme weather based on large ensembles of
atmospheric model simulations. Our results both quantify the magnitude of
year-to-year variability and categorize the degree to which conclusions of
attributable risk are qualitatively affected. The methodology is illustrated by
exploring extreme temperature and precipitation events for the northwest coast
of South America and northern-central Siberia; we also provides results for
regions around the globe. While it remains preferable to perform a full
multi-year analysis, the results presented here can serve as an indication of
where and when attribution researchers should be concerned about the use of
atmosphere-only simulations
Electron Signatures of Satellite Sweeping in the Magnetosphere of Uranus
The Voyager 2 Cosmic Ray System found large-scale macrosignatures of satellite sweeping for MeV electrons near the orbits of the satellites Miranda, Ariel, and Umbriel in the magnetosphere of Uranus. Due to the large magnetic inclinations of satellite orbits at Uranus, sweeping rates vary along the orbits with the McIlwain L parameter. However, no evidence was found, where expected, for fresh sweeping signatures at such positions. Although the maximal electron intensity occurs near Voyager 2's minimum L (4.67) as predicted by the Q_3 field model, the intensity minima in the macrosignatures show large outward displacements (≤0.5 R_U) from minimum-L positions of the associated satellites. These radial displacements increased with measured electron energy and at higher magnetic latitudes. Pitch angle distributions are generally more anisotropic outside the macrosignatures and more isotropic within, as determined from comparison of inbound and outbound intensity profiles at different latitudes. These anisotropy measurements provide the basis for latitudinal flux extrapolation, which when coupled with power law scaling of spectral distributions allow the calculation of phase space density profiles. The latter show local minima in the macrosignatures and are indicative of distributed electron sources in the inner magnetosphere and/or nonadiabatic transport processes such as pitch angle scattering and magnetospheric recirculation. Preliminary diffusion coefficients with values D_(LL) ∼ 10^(−7)–10^(−6) RS² and radial dependence D_(LL) ∼ L^3–L^4 have been estimated for the macrosignatures. The low-order L dependence of D_(LL) is consistent with diffusion driven by ionospheric dynamo. However, quantitative modeling of radial and pitch angle diffusion is required to assess the formative processes for the macrosignatures before more physically meaningful transport parameters can be determined
Zero-energy states in graphene quantum dots and rings
We present exact analytical zero-energy solutions for a class of smooth
decaying potentials, showing that the full confinement of charge carriers in
electrostatic potentials in graphene quantum dots and rings is indeed possible
without recourse to magnetic fields. These exact solutions allow us to draw
conclusions on the general requirements for the potential to support fully
confined states, including a critical value of the potential strength and
spatial extent.Comment: 8 pages, 3 figures, references added, typos corrected, discussion
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