6,401 research outputs found
State-dependence of climate sensitivity: attractor constraints and palaeoclimate regimes
Equilibrium climate sensitivity (ECS) is a key predictor of climate change.
However, it is not very well constrained, either by climate models or by
observational data. The reasons for this include strong internal variability
and forcing on many time scales. In practise this means that the 'equilibrium'
will only be relative to fixing the slow feedback processes before comparing
palaeoclimate sensitivity estimates with estimates from model simulations. In
addition, information from the late Pleistocene ice age cycles indicates that
the climate cycles between cold and warm regimes, and the climate sensitivity
varies considerably between regime because of fast feedback processes changing
relative strength and time scales over one cycle.
In this paper we consider climate sensitivity for quite general climate
dynamics. Using a conceptual Earth system model of Gildor and Tziperman (2001)
(with Milankovich forcing and dynamical ocean biogeochemistry) we explore
various ways of quantifying the state-dependence of climate sensitivity from
unperturbed and perturbed model time series. Even without considering any
perturbations, we suggest that climate sensitivity can be usefully thought of
as a distribution that quantifies variability within the 'climate attractor'
and where there is a strong dependence on climate state and more specificially
on the 'climate regime' where fast processes are approximately in equilibrium.
We also consider perturbations by instantaneous doubling of CO and
similarly find a strong dependence on the climate state using our approach.Comment: 32 pages, 10 figure
Extreme sensitivity and climate tipping points
A climate state close to a tipping point will have a degenerate linear
response to perturbations, which can be associated with extreme values of the
equilibrium climate sensitivity (ECS). In this paper we contrast linearized
(`instantaneous') with fully nonlinear geometric (`two-point') notions of ECS,
in both presence and absence of tipping points. For a stochastic energy balance
model of the global mean surface temperature with two stable regimes, we
confirm that tipping events cause the appearance of extremes in both notions of
ECS. Moreover, multiple regimes with different mean sensitivities are visible
in the two-point ECS. We confirm some of our findings in a physics-based
multi-box model of the climate system.Comment: 11 figure
On the Construction of Common Size, Value and Momentum Factors in International Stock Markets: A Guide with Applications
Demand is growing for a better understanding of how assets are priced in countries outside of the U.S. While financial data are available for many firms world-wide, it is important to have a reliable and replicable method of constructing high-quality systematic risk factors from these data. This paper first documents that appropriately screened data from Thomson Reuters Datastream and Thomson Reuters Worldscope can be used to replicate closely not only U.S. market returns and the corresponding momentum risk factor (as existing work has suggested), but also the widely-used U.S. size and value risk factors. We then build novel pan-European and country-specific momentum, size, and value risk factors. By comparing our pan-European market returns and risk factors with their counterparts in the U.S., we find that they are astonishingly highly correlated. The factors we compute are made available to other researchers.Risk factors; value, size, momentum, international equity markets, asset pricing anomalies
A Unified Picture of the FIP and Inverse FIP Effects
We discuss models for coronal abundance anomalies observed in the coronae of
the sun and other late-type stars following a scenario first introduced by
Schwadron, Fisk & Zurbuchen of the interaction of waves at loop footpoints with
the partially neutral gas. Instead of considering wave heating of ions in this
location, we explore the effects on the upper chromospheric plasma of the wave
ponderomotive forces. These can arise as upward propagating waves from the
chromosphere transmit or reflect upon reaching the chromosphere-corona
boundary, and are in large part determined by the properties of the coronal
loop above. Our scenario has the advantage that for realistic wave energy
densities, both positive and negative changes in the abundance of ionized
species compared to neutrals can result, allowing both FIP and Inverse FIP
effects to come out of the model. We discuss how variations in model parameters
can account for essentially all of the abundance anomalies observed in solar
spectra. Expected variations with stellar spectral type are also qualitatively
consistent with observations of the FIP effect in stellar coronae.Comment: 25 pages, 4 figures, submitted to Ap
Cosmological perturbations of self-accelerating universe in nonlinear massive gravity
We study cosmological perturbations of self-accelerating universe solutions
in the recently proposed nonlinear theory of massive gravity, with general
matter content. While the broken diffeomorphism invariance implies that there
generically are 2 tensor, 2 vector and 2 scalar degrees of freedom in the
gravity sector, we find that the scalar and vector degrees have vanishing
kinetic terms and nonzero mass terms. Depending on their nonlinear behavior,
this indicates either nondynamical nature of these degrees or strong couplings.
Assuming the former, we integrate out the 2 vector and 2 scalar degrees of
freedom. We then find that in the scalar and vector sectors, gauge-invariant
variables constructed from metric and matter perturbations have exactly the
same quadratic action as in general relativity. The difference from general
relativity arises only in the tensor sector, where the graviton mass modifies
the dispersion relation of gravitational waves, with a time-dependent effective
mass. This may lead to modification of stochastic gravitational wave spectrum.Comment: 32 pages, 1 figure; v2: minor update to match the published versio
Test Characteristics of Urinary Lipoarabinomannan and Predictors of Mortality among Hospitalized HIV-Infected Tuberculosis Suspects in Tanzania.
Tuberculosis is the most common cause of death among patients with HIV infection living in tuberculosis endemic countries, but many cases are not diagnosed pre-mortem. We assessed the test characteristics of urinary lipoarabinomannan (LAM) and predictors of mortality among HIV-associated tuberculosis suspects in Tanzania. We prospectively enrolled hospitalized HIV-infected patients in Dar es Salaam, with ≥2 weeks of cough or fever, or weight loss. Subjects gave 2 mLs of urine to test for LAM using a commercially available ELISA, ≥2 sputum specimens for concentrated AFB smear and solid media culture, and 40 mLs of blood for culture. Among 212 evaluable subjects, 143 (68%) were female; mean age was 36 years; and the median CD4 count 86 cells/mm(3). 69 subjects (33%) had culture confirmation of tuberculosis and 65 (31%) were LAM positive. For 69 cases of sputum or blood culture-confirmed tuberculosis, LAM sensitivity was 65% and specificity 86% compared to 36% and 98% for sputum smear. LAM test characteristics were not different in patients with bacteremia but showed higher sensitivity and lower specificity with decreasing CD4 cell count. Two month mortality was 64 (53%) of 121 with outcomes available. In multivariate analysis there was significant association of mortality with absence of anti-retroviral therapy (p = 0.004) and a trend toward association with a positive urine LAM (p = 0.16). Among culture-negative patients mortality was 9 (75%) of 12 in LAM positive patients and 27 (38%) of 71 in LAM negative patients (p = 0.02). Urine LAM is more sensitive than sputum smear and has utility for the rapid diagnosis of culture-confirmed tuberculosis in this high-risk population. Mortality data raise the possibility that urine LAM may also be a marker for culture-negative tuberculosis
Picosecond electric-field-induced threshold switching in phase-change materials
Many chalcogenide glasses undergo a breakdown in electronic resistance above
a critical field strength. Known as threshold switching, this mechanism enables
field-induced crystallization in emerging phase-change memory. Purely
electronic as well as crystal nucleation assisted models have been employed to
explain the electronic breakdown. Here, picosecond electric pulses are used to
excite amorphous AgInSbTe. Field-dependent reversible
changes in conductivity and pulse-driven crystallization are observed. The
present results show that threshold switching can take place within the
electric pulse on sub-picosecond time-scales - faster than crystals can
nucleate. This supports purely electronic models of threshold switching and
reveals potential applications as an ultrafast electronic switch.Comment: 6 pages manuscript with 3 figures and 8 pages supplementary materia
Engineering Hybrid Epitaxial InAsSb/Al Nanowire Materials for Stronger Topological Protection
The combination of strong spin-orbit coupling, large -factors, and the
coupling to a superconductor can be used to create a topologically protected
state in a semiconductor nanowire. Here we report on growth and
characterization of hybrid epitaxial InAsSb/Al nanowires, with varying
composition and crystal structure. We find the strongest spin-orbit interaction
at intermediate compositions in zincblende InAsSb nanowires,
exceeding that of both InAs and InSb materials, confirming recent theoretical
studies \cite{winkler2016topological}. We show that the epitaxial InAsSb/Al
interfaces allows for a hard induced superconducting gap and 2 transport in
Coulomb charging experiments, similar to experiments on InAs/Al and InSb/Al
materials, and find measurements consistent with topological phase transitions
at low magnetic fields due to large effective -factors. Finally we present a
method to grow pure wurtzite InAsSb nanowires which are predicted to exhibit
even stronger spin-orbit coupling than the zincblende structure.Comment: 10 pages and 5 figure
Epicardial adipose tissue predicts incident cardiovascular disease and mortality in patients with type 2 diabetes
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