751 research outputs found
3D climate modeling of close-in land planets: Circulation patterns, climate moist bistability and habitability
The inner edge of the classical habitable zone is often defined by the
critical flux needed to trigger the runaway greenhouse instability. This 1D
notion of a critical flux, however, may not be so relevant for inhomogeneously
irradiated planets, or when the water content is limited (land planets).
Here, based on results from our 3D global climate model, we find that the
circulation pattern can shift from super-rotation to stellar/anti stellar
circulation when the equatorial Rossby deformation radius significantly exceeds
the planetary radius. Using analytical and numerical arguments, we also
demonstrate the presence of systematic biases between mean surface temperatures
or temperature profiles predicted from either 1D or 3D simulations.
Including a complete modeling of the water cycle, we further demonstrate that
for land planets closer than the inner edge of the classical habitable zone,
two stable climate regimes can exist. One is the classical runaway state, and
the other is a collapsed state where water is captured in permanent cold traps.
We identify this "moist" bistability as the result of a competition between the
greenhouse effect of water vapor and its condensation. We also present
synthetic spectra showing the observable signature of these two states.
Taking the example of two prototype planets in this regime, namely Gl581c and
HD85512b, we argue that they could accumulate a significant amount of water ice
at their surface. If such a thick ice cap is present, gravity driven ice flows
and geothermal flux should come into play to produce long-lived liquid water at
the edge and/or bottom of the ice cap. Consequently, the habitability of
planets at smaller orbital distance than the inner edge of the classical
habitable zone cannot be ruled out. Transiting planets in this regime represent
promising targets for upcoming observatories like EChO and JWST.Comment: Accepted for publication in Astronomy and Astrophysics, complete
abstract in the pdf, 18 pages, 18 figure
Estimating the effect of healthcare-associated infections on excess length of hospital stay using inverse probability-weighted survival curves
Background: Studies estimating excess length of stay (LOS) attributable to nosocomial infections have failed to address time-varying confounding, likely leading to overestimation of their impact. We present a methodology based on inverse probability–weighted survival curves to address this limitation.
Methods: A case study focusing on intensive care unit–acquired bacteremia using data from 2 general intensive care units (ICUs) from 2 London teaching hospitals were used to illustrate the methodology. The area under the curve of a conventional Kaplan-Meier curve applied to the observed data was compared with that of an inverse probability–weighted Kaplan-Meier curve applied after treating bacteremia as censoring events. Weights were based on the daily probability of acquiring bacteremia. The difference between the observed average LOS and the average LOS that would be observed if all bacteremia cases could be prevented was multiplied by the number of admitted patients to obtain the total excess LOS.
Results: The estimated total number of extra ICU days caused by 666 bacteremia cases was estimated at 2453 (95% confidence interval [CI], 1803–3103) days. The excess number of days was overestimated when ignoring time-varying confounding (2845 [95% CI, 2276–3415]) or when completely ignoring confounding (2838 [95% CI, 2101–3575]).
Conclusions: ICU-acquired bacteremia was associated with a substantial excess LOS. Wider adoption of inverse probability–weighted survival curves or alternative techniques that address time-varying confounding could lead to better informed decision making around nosocomial infections and other time-dependent exposures
Indication of insensitivity of planetary weathering behavior and habitable zone to surface land fraction
It is likely that unambiguous habitable zone terrestrial planets of unknown
water content will soon be discovered. Water content helps determine surface
land fraction, which influences planetary weathering behavior. This is
important because the silicate weathering feedback determines the width of the
habitable zone in space and time. Here a low-order model of weathering and
climate, useful for gaining qualitative understanding, is developed to examine
climate evolution for planets of various land-ocean fractions. It is pointed
out that, if seafloor weathering does not depend directly on surface
temperature, there can be no weathering-climate feedback on a waterworld. This
would dramatically narrow the habitable zone of a waterworld. Results from our
model indicate that weathering behavior does not depend strongly on land
fraction for partially ocean-covered planets. This is powerful because it
suggests that previous habitable zone theory is robust to changes in land
fraction, as long as there is some land. Finally, a mechanism is proposed for a
waterworld to prevent complete water loss during a moist greenhouse through
rapid weathering of exposed continents. This process is named a "waterworld
self-arrest," and it implies that waterworlds can go through a moist greenhouse
stage and end up as planets like Earth with partial ocean coverage. This work
stresses the importance of surface and geologic effects, in addition to the
usual incident stellar flux, for habitability.Comment: 15 pages, 6 figures, accepted at Ap
Global modelling of the early Martian climate under a denser CO2 atmosphere: Water cycle and ice evolution
We discuss 3D global simulations of the early Martian climate that we have
performed assuming a faint young Sun and denser CO2 atmosphere. We include a
self-consistent representation of the water cycle, with atmosphere-surface
interactions, atmospheric transport, and the radiative effects of CO2 and H2O
gas and clouds taken into account. We find that for atmospheric pressures
greater than a fraction of a bar, the adiabatic cooling effect causes
temperatures in the southern highland valley network regions to fall
significantly below the global average. Long-term climate evolution simulations
indicate that in these circumstances, water ice is transported to the highlands
from low-lying regions for a wide range of orbital obliquities, regardless of
the extent of the Tharsis bulge. In addition, an extended water ice cap forms
on the southern pole, approximately corresponding to the location of the
Noachian/Hesperian era Dorsa Argentea Formation. Even for a multiple-bar CO2
atmosphere, conditions are too cold to allow long-term surface liquid water.
Limited melting occurs on warm summer days in some locations, but only for
surface albedo and thermal inertia conditions that may be unrealistic for water
ice. Nonetheless, meteorite impacts and volcanism could potentially cause
intense episodic melting under such conditions. Because ice migration to higher
altitudes is a robust mechanism for recharging highland water sources after
such events, we suggest that this globally sub-zero, `icy highlands' scenario
for the late Noachian climate may be sufficient to explain most of the fluvial
geology without the need to invoke additional long-term warming mechanisms or
an early warm, wet Mars.Comment: Minor revisions to text, one new table, figs. 1,3 11 and 18 redon
Increased insolation threshold for runaway greenhouse processes on Earth like planets
Because the solar luminosity increases over geological timescales, Earth
climate is expected to warm, increasing water evaporation which, in turn,
enhances the atmospheric greenhouse effect. Above a certain critical
insolation, this destabilizing greenhouse feedback can "runaway" until all the
oceans are evaporated. Through increases in stratospheric humidity, warming may
also cause oceans to escape to space before the runaway greenhouse occurs. The
critical insolation thresholds for these processes, however, remain uncertain
because they have so far been evaluated with unidimensional models that cannot
account for the dynamical and cloud feedback effects that are key stabilizing
features of Earth's climate. Here we use a 3D global climate model to show that
the threshold for the runaway greenhouse is about 375 W/m, significantly
higher than previously thought. Our model is specifically developed to quantify
the climate response of Earth-like planets to increased insolation in hot and
extremely moist atmospheres. In contrast with previous studies, we find that
clouds have a destabilizing feedback on the long term warming. However,
subsident, unsaturated regions created by the Hadley circulation have a
stabilizing effect that is strong enough to defer the runaway greenhouse limit
to higher insolation than inferred from 1D models. Furthermore, because of
wavelength-dependent radiative effects, the stratosphere remains cold and dry
enough to hamper atmospheric water escape, even at large fluxes. This has
strong implications for Venus early water history and extends the size of the
habitable zone around other stars.Comment: Published in Nature. Online publication date: December 12, 2013.
Accepted version before journal editing and with Supplementary Informatio
What Are the Real Procedural Costs of Bariatric Surgery? A Systematic Literature Review of Published Cost Analyses
This is the final version. Available on open access from Springer Verlag via the DOI in this recordThis review aims to evaluate the current literature on the procedural costs of bariatric surgery for the treatment of severe obesity. Using a published framework for the conduct of micro-costing studies for surgical interventions, existing cost estimates from the literature are assessed for their accuracy, reliability and comprehensiveness based on their consideration of seven ‘important’ cost components. MEDLINE, PubMed, key journals and reference lists of included studies were searched up to January 2017. Eligible studies had to report per-case, total procedural costs for any type of bariatric surgery broken down into two or more individual cost components. A total of 998 citations were screened, of which 13 studies were included for analysis. Included studies were mainly conducted from a US hospital perspective, assessed either gastric bypass or adjustable gastric banding procedures and considered a range of different cost components. The mean total procedural costs for all included studies was US7423 to US$33,541). No study considered all of the recommended ‘important’ cost components and estimation methods were poorly reported. The accuracy, reliability and comprehensiveness of the existing cost estimates are, therefore, questionable. There is a need for a comparative cost analysis of the different approaches to bariatric surgery, with the most appropriate costing approach identified to be micro-costing methods. Such an analysis will not only be useful in estimating the relative cost-effectiveness of different surgeries but will also ensure appropriate reimbursement and budgeting by healthcare payers to ensure barriers to access this effective treatment by severely obese patients are minimised.National Institute for Health Research (NIHR
A common functional variant of endoplasmic reticulum aminopeptidase 2 (ERAP2) that reduces major histocompatibility complex class I expression is not associated with ankylosing spondylitis
RUNX3 and T-Bet in Immunopathogenesis of Ankylosing Spondylitis—Novel Targets for Therapy?
Susceptibility to ankylosing spondylitis (AS) is polygenic with more than 100 genes identified to date. These include HLA-B27 and the aminopeptidases (ERAP1, ERAP2, and LNPEPS), which are involved in antigen processing and presentation to T-cells, and several genes (IL23R, IL6R, STAT3, JAK2, IL1R1/2, IL12B, and IL7R) involved in IL23 driven pathways of inflammation. AS is also strongly associated with polymorphisms in two transcription factors, RUNX3 and T-bet (encoded by TBX21), which are important in T-cell development and function. The influence of these genes on the pathogenesis of AS and their potential for identifying drug targets is discussed here
Is Gliese 581d habitable? Some constraints from radiative-convective climate modeling
The recently discovered exoplanet Gl581d is extremely close to the outer edge
of its system's habitable zone, which has led to much speculation on its
possible climate. We have performed a range of simulations to assess whether,
given simple combinations of chemically stable greenhouse gases, the planet
could sustain liquid water on its surface. For best estimates of the surface
gravity, surface albedo and cloud coverage, we find that less than 10 bars of
CO2 is sufficient to maintain a global mean temperature above the melting point
of water. Furthermore, even with the most conservative choices of these
parameters, we calculate temperatures above the water melting point for CO2
partial pressures greater than about 40 bar. However, we note that as Gl581d is
probably in a tidally resonant orbit, further simulations in 3D are required to
test whether such atmospheric conditions are stable against the collapse of CO2
on the surface.Comment: 9 pages, 11 figures. Accepted for publication in Astronomy &
Astrophysic
Myocardial and vascular dysfunction in patients with rheumatoid arthritis: insights from cardiovascular magnetic resonance
- …