3,074 research outputs found
Changing perspectives on marijuana use during early adolescence and young adulthood: Evidence from a panel of cross-sectional surveys
Introduction. Prior research has often overlooked potential cohort differences in marijuana
views and use across adolescence and young adulthood. To begin to address this gap, we conduct
an exploratory examination of marijuana views and use among American youth using a panel of
cross-sectional surveys. Method. Findings are based on repeated, cross-sectional data collected
annually from adolescents (ages 12-17; n = 230,452) and young adults (ages 18-21; n = 120,588)
surveyed as part of the National Survey on Drug Use and Health between 2002 and 2014. For
each of the birth years between 1986 and 1996, we combined a series of nationally representative
cross-sections to provide multi-year data strings designed to approximate nationally
representative cohorts. Results. Compared to youth born in the mid-to-late 1980s, youth born in
the mid-1990s reported significantly higher levels of marijuana disapproval during the early
adolescent years (Age 14: 1988 = 64.7%, 1994 = 70.4%) but lower levels of disapproval during
the young adult years (Age 19: 1988 = 32.0%, 1994 = 25.0%; Age 20: 1988 = 27.9%, 1994 =
19.7%). Moreover, the prevalence of marijuana use among youth born in 1994 was significantly
lowerâcompared to youth born in 1988âat age 14 (1988: 11.39%, 1994: 8.19%) and
significantly higher at age 18 (1988: 29.67%, 1994: 34.83%). This pattern held even when
adjusting for potential confounding by demographic changes in the population across the study
period. Conclusions. We see evidence of changes in the perceptions of marijuana use among
youth born during the late twentieth century.2018-01-0
Statistically derived contributions of diverse human influences to twentieth-century temperature changes
The warming of the climate system is unequivocal as evidenced by an increase in global temperatures by 0.8â°C over the past century. However, the attribution of the observed warming to human activities remains less clear, particularly because of the apparent slow-down in warming since the late 1990s. Here we analyse radiative forcing and temperature time series with state-of-the-art statistical methods to address this question without climate model simulations. We show that long-term trends in total radiative forcing and temperatures have largely been determined by atmospheric greenhouse gas concentrations, and modulated by other radiative factors. We identify a pronounced increase in the growth rates of both temperatures and radiative forcing around 1960, which marks the onset of sustained global warming. Our analyses also reveal a contribution of human interventions to two periods when global warming slowed down. Our statistical analysis suggests that the reduction in the emissions of ozone-depleting substances under the Montreal Protocol, as well as a reduction in methane emissions, contributed to the lower rate of warming since the 1990s. Furthermore, we identify a contribution from the two world wars and the Great Depression to the documented cooling in the mid-twentieth century, through lower carbon dioxide emissions. We conclude that reductions in greenhouse gas emissions are effective in slowing the rate of warming in the short term.F.E. acknowledges financial support from the Consejo Nacional de Ciencia y Tecnologia (http://www.conacyt.gob.mx) under grant CONACYT-310026, as well as from PASPA DGAPA of the Universidad Nacional Autonoma de Mexico. (CONACYT-310026 - Consejo Nacional de Ciencia y Tecnologia; PASPA DGAPA of the Universidad Nacional Autonoma de Mexico
Velocity shear and current driven instability in a collisional F-region
We have studied how the presence of collisions affects the behavior of instabilities triggered by a combination of shears and parallel currents in the ionosphere under a variety of ion to electron temperature ratios. To this goal we have numerically solved a kinetic dispersion relation, using a relaxation model to describe the effects of ion and electron collisions. We have compared our solutions to expressions derived in a fluid limit which applied only to large electron to ion temperature ratios. We have limited our study to threshold conditions for the current density and the shears. We have studied how the threshold varies as a function of the wave-vector angle direction and as a function of frequency. As expected, we have found that for low frequencies and/or elevated ion to electron temperature ratios, the kinetic dispersion relation has to be used to evaluate the threshold conditions. We have also found that ion velocity shears can significantly lower the field-aligned threshold current needed to trigger the instability, especially for wave-vectors close to the perpendicular to the magnetic field. However the current density and shear requirements remain significantly higher than if collisions are neglected. Therefore, for ionospheric F-region applications, the effect of collisions should be included in the calculation of instabilities associated with horizontal shears in the vertical flow. Furthermore, in many situations of interest the kinetic solutions should be used instead of the fluid limit, in spite of the fact that the latter can be shown to produce qualitatively valid solutions
Widespread phytoplankton blooms triggered by 2019â2020 Australian wildfires
Droughts and climate-change-driven warming are leading to more frequent and intense wildfires1,2,3, arguably contributing to the severe 2019â2020 Australian wildfires4. The environmental and ecological impacts of the fires include loss of habitats and the emission of substantial amounts of atmospheric aerosols5,6,7. Aerosol emissions from wildfires can lead to the atmospheric transport of macronutrients and bio-essential trace metals such as nitrogen and iron, respectively8,9,10. It has been suggested that the oceanic deposition of wildfire aerosols can relieve nutrient limitations and, consequently, enhance marine productivity11,12, but direct observations are lacking. Here we use satellite and autonomous biogeochemical Argo float data to evaluate the effect of 2019â2020 Australian wildfire aerosol deposition on phytoplankton productivity. We find anomalously widespread phytoplankton blooms from December 2019 to March 2020 in the Southern Ocean downwind of Australia. Aerosol samples originating from the Australian wildfires contained a high iron content and atmospheric trajectories show that these aerosols were likely to be transported to the bloom regions, suggesting that the blooms resulted from the fertilization of the iron-limited waters of the Southern Ocean. Climate models project more frequent and severe wildfires in many regions1,2,3. A greater appreciation of the links between wildfires, pyrogenic aerosols13, nutrient cycling and marine photosynthesis could improve our understanding of the contemporary and glacialâinterglacial cycling of atmospheric CO2 and the global climate system.Analyses of satellite aerosol observations used in this study were produced with the Giovanni online data system, developed and maintained by the NASA GES DISC. We thank SeaWiFS and MODIS mission scientists and associated NASA personnel for the production of the data used in this research effort. The BGC-Argo data were collected and made freely available by the International Argo Program and the national programs that contribute to it (http://www.argo.ucsd.edu, http://argo.jcommops.org). The Argo Program is part of the Global Ocean Observing System (https://doi.org/10.17882/42182). W.T. is supported by the Harry H. Hess Postdoctoral Fellowship from Princeton University. N.C. is supported by the âLaboratoire dâExcellenceâ LabexMER (ANRâ10âLABXâ19) and co-funded by a grant from the French government under the program âInvestissements dâAvenirâ. S.B. acknowledges the AXA Research Fund for the support of the long-term research line on Sand and Dust Storms at the Barcelona Supercomputing Center (BSC) and CAMS Global Validation (CAMS-84). P.G.S., J.L., M.M.G.P. and A.R.B. are supported by the Australian Research Council Discovery Projects scheme (DP190103504). P.G.S. and J.W. are supported by the Australian Research Council Centre of Excellence for Climate Extremes (CLEX: CE170100023). J.L. is supported by the European Unionâs Horizon 2020 research and innovation programme under the Marie SkĆodowska-Curie grant agreement no. 754433. A.R.B. is supported by the Australian Research Council Future Fellowship scheme (FT130100037). R.M. is supported by the CSIRO Decadal Climate Forecasting Project. We thank M. Strzelec, M. East, T. Holmes, M. Corkill, S. Meyerink and the Wellington Park Management Trust for help with installation and sampling the Tasmanian aerosol time-series station; A. Townsend for iron aerosol analyses by ICPMS at the University of Tasmania; and A. Benedetti and S. Remy for providing insights on the validation of aerosol reanalysis.Peer Reviewed"Article signat per 15 autors/es: Weiyi Tang, Joan Llort, Jakob Weis, Morgane M. G. Perron, Sara Basart, Zuchuan Li, Shubha Sathyendranath, Thomas Jackson, Estrella Sanz Rodriguez, Bernadette C. Proemse, Andrew R. Bowie, Christina Schallenberg, Peter G. Strutton, Richard Matear & Nicolas Cassar"Postprint (author's final draft
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Biomedical Ph.D. Students Enrolled in Two Elite Universities in the United Kingdom and the United States Report Adopting Multiple Learning Relationships
Objective: The ability to form multiple learning relationships is a key element of the doctoral learning environment in the biomedical sciences. Of these relationships, that between student and supervisor has long been viewed as key. There are, however, limited data to describe the student perspective on what makes this relationship valuable. In the present study, we discuss the findings of semi-structured interviews with biomedical Ph.D. students from the United Kingdom and the United States to: i) determine if the learning relationships identified in an Australian biomedical Ph.D. cohort are also important in a larger international student cohort; and ii) improve our understanding of student perceptions of value in their supervisory relationships. Study Design 32 students from two research intensive universities, one in the United Kingdom (n = 17), and one in the United States (n = 15) were recruited to participate in a semi-structured interview. Verbatim transcripts were transcribed, validated and analysed using a Miles and Huberman method for thematic analysis. Results: Students reported that relationships with other Ph.D. students, post-doctoral scientists and supervisors were all essential to their learning. Effective supervisory relationships were perceived as the primary source of high-level project guidance, intellectual support and confidence. Relationships with fellow students were viewed as essential for the provision of empathetic emotional support. Technical learning was facilitated, almost exclusively, by relationships with postdoctoral staff. Conclusions: These data make two important contributions to the scholarship of doctoral education in the biomedical sciences. Firstly, they provide further evidence for the importance of multiple learning relationships in the biomedical doctorate. Secondly, they clarify the form of a âvaluedâ supervisory relationship from a student perspective. We conclude that biomedical doctoral programs should be designed to contain a minimum level of formalised structure to promote the development of multiple learning relationships that are perceived as key to student learning
Generalized nonuniform dichotomies and local stable manifolds
We establish the existence of local stable manifolds for semiflows generated
by nonlinear perturbations of nonautonomous ordinary linear differential
equations in Banach spaces, assuming the existence of a general type of
nonuniform dichotomy for the evolution operator that contains the nonuniform
exponential and polynomial dichotomies as a very particular case. The family of
dichotomies considered allow situations for which the classical Lyapunov
exponents are zero. Additionally, we give new examples of application of our
stable manifold theorem and study the behavior of the dynamics under
perturbations.Comment: 18 pages. New version with minor corrections and an additional
theorem and an additional exampl
On conformal measures and harmonic functions for group extensions
We prove a Perron-Frobenius-Ruelle theorem for group extensions of
topological Markov chains based on a construction of -finite conformal
measures and give applications to the construction of harmonic functions.Comment: To appear in Proceedings of "New Trends in Onedimensional Dynamics,
celebrating the 70th birthday of Welington de Melo
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