532 research outputs found
Соціально-правова та етична природа мусульманської сім‘ї
Relative sea-level variations during the late Pleistocene can only be
reconstructed with the knowledge of ice-sheet history. On the other hand, the
knowledge of regional and global relative sea-level variations is necessary
to learn about the changes in ice volume. Overcoming this problem of
circularity demands a fully coupled system where ice sheets and sea level
vary consistently in space and time and dynamically affect each other. Here
we present results for the past 410 000 years (410 kyr) from the coupling
of a set of 3-D ice-sheet-shelf models to a global sea-level model, which is
based on the solution of the gravitationally self-consistent sea-level
equation. The sea-level model incorporates the glacial isostatic adjustment
feedbacks for a Maxwell viscoelastic and rotating Earth model with coastal
migration. Ice volume is computed with four 3-D ice-sheet-shelf models for
North America, Eurasia, Greenland and Antarctica. Using an inverse approach,
ice volume and temperature are derived from a benthic δ18O stacked
record. The derived surface-air temperature anomaly is added to the
present-day climatology to simulate glacial–interglacial changes in
temperature and hence ice volume. The ice-sheet thickness variations are then
forwarded to the sea-level model to compute the bedrock deformation, the
change in sea-surface height and thus the relative sea-level change. The
latter is then forwarded to the ice-sheet models. To quantify the impact of
relative sea-level variations on ice-volume evolution, we have performed
coupled and uncoupled simulations. The largest differences of ice-sheet
thickness change occur at the edges of the ice sheets, where relative sea-level
change significantly departs from the ocean-averaged sea-level variations
Compensating errors in inversions for subglacial bed roughness: same steady state, different dynamic response
Subglacial bed roughness is one of the main factors
controlling the rate of future Antarctic ice-sheet retreat and also one of
the most uncertain. A common technique to constrain the bed roughness using
ice-sheet models is basal inversion, tuning the roughness to reproduce the
observed present-day ice-sheet geometry and/or surface velocity. However,
many other factors affecting ice-sheet evolution, such as the englacial
temperature and viscosity, the surface and basal mass balance, and the
subglacial topography, also contain substantial uncertainties. Using a basal
inversion technique intrinsically causes any errors in these other
quantities to lead to compensating errors in the inverted bed roughness.
Using a set of idealised-geometry experiments, we quantify these
compensating errors and investigate their effect on the dynamic response of
the ice sheet to a prescribed forcing. We find that relatively small errors
in ice viscosity and subglacial topography require substantial compensating
errors in the bed roughness in order to produce the same steady-state ice
sheet, obscuring the realistic spatial variability in the bed roughness.
When subjected to a retreat-inducing forcing, we find that these different
parameter combinations, which per definition of the inversion procedure
result in the same steady-state geometry, lead to a rate of ice volume loss
that can differ by as much as a factor of 2. This implies that ice-sheet
models that use basal inversion to initialise their model state can still
display a substantial model bias despite having an initial state which is
close to the observations.</p
Climate model boundary conditions for four Cretaceous time slices
International audienceGeneral circulation models (GCMs) are useful tools for investigating the characteristics and dynamics of past climates. Understanding of past climates contributes significantly to our overall understanding of Earth's climate system. One of the most time consuming, and often daunting, tasks facing the paleoclimate modeler, particularly those without a geological background, is the production of surface boundary conditions for past time periods. These boundary conditions consist of, at a minimum, continental configurations derived from plate tectonic modeling, topography, bathymetry, and a vegetation distribution. Typically, each researcher develops a unique set of boundary conditions for use in their simulations. Thus, unlike simulations of modern climate, basic assumptions in paleo surface boundary conditions can vary from researcher to researcher. This makes comparisons between results from multiple researchers difficult and, thus, hinders the integration of studies across the broader community. Unless special changes to surface conditions are warranted, researcher dependent boundary conditions are not the most efficient way to proceed in paleoclimate investigations. Here we present surface boundary conditions (land-sea distribution, paleotopography, paleobathymetry, and paleovegetation distribution) for four Cretaceous time slices (120 Ma, 110 Ma, 90 Ma, and 70 Ma). These boundary conditions are modified from base datasets to be appropriate for incorporation into numerical studies of Earth's climate and are available in NetCDF format upon request from the lead author. The land-sea distribution, bathymetry, and topography are based on the 1°×1° (latitude × longitude) paleo Digital Elevation Models (paleoDEMs) of Christopher Scotese. Those paleoDEMs were adjusted using the paleogeographical reconstructions of Ronald Blakey (Northern Arizona University) and published literature and were then modified for use in GCMs. The paleovegetation distribution is based on published data and reconstructions and consultation with members of the paleobotanical community and is represented as generalized biomes that should be easily translatable to many vegetation-modeling schemes
Application of [email protected] simulations of paleoclimate as forcing for an ice-sheet model, ANICE2.1: set-up and benchmark experiments
Fully coupled ice-sheet–climate
modelling over 10 000–100 000-year timescales at high spatial and temporal
resolution remains beyond the capability of current computational systems.
Forcing an ice-sheet model with precalculated output from a general
circulation model (GCM) offers a middle ground, balancing the need to
accurately capture both long-term processes, in particular circulation-driven
changes in precipitation, and processes requiring a high spatial resolution
like ablation. Here, we present and evaluate a model set-up that forces the
ANICE 3-D thermodynamic ice-sheet–shelf model calculating the four large
continental ice sheets (Antarctica, Greenland, North America, and Eurasia)
with precalculated output from two steady-state simulations with the HadCM3
(GCM) using a so-called matrix method of coupling both components, whereby
simulations with various levels of pCO2 and ice-sheet
configuration are combined to form a time-continuous transient climate
forcing consistent with the modelled ice sheets. We address the difficulties
in downscaling low-resolution GCM output to the higher-resolution grid of an
ice-sheet model and account for differences between GCM and ice-sheet model
surface topography ranging from interglacial to glacial conditions. Although
the approach presented here can be applied to a matrix with any number of GCM
snapshots, we limited our experiments to a matrix of only two snapshots. As a
benchmark experiment to assess the validity of this model set-up, we perform
a simulation of the entire last glacial cycle from 120 kyr ago to present
day. The simulated eustatic sea-level drop at the Last Glacial Maximum (LGM)
for the combined Antarctic, Greenland, Eurasian, and North American ice
sheets amounts to 100 m, in line with many other studies. The simulated ice
sheets at the LGM agree well with the ICE-5G reconstruction and the more
recent DATED-1 reconstruction in terms of total volume and geographical
location of the ice sheets. Moreover, modelled benthic oxygen isotope
abundance and the relative contributions from global ice volume and
deep-water temperature agree well with available data, as do surface
temperature histories for the Greenland and Antarctic ice sheets. This model
strategy can be used to create time-continuous ice-sheet distribution and
sea-level reconstructions for geological periods up to several million years
in duration, capturing climate-model-driven variations in the mass balance of
the ice sheet.</p
Uncertainties in long-term twenty-first century process-based coastal sea-level projections
Many processes affect sea level near the coast. In this paper, we discuss the major uncertainties in coastal sea-level projections from a process-based perspective, at different spatial and temporal scales, and provide an outlook on how these uncertainties may be reduced. Uncertainty in centennial global sea-level rise is dominated by the ice sheet contributions. Geographical variations in projected sea-level change arise mainly from dynamical patterns in the ocean response and other geophysical processes. Finally, the uncertainties in the short-duration extreme sea-level events are controlled by near coastal processes, storms and tides
Modelling Antarctic ice shelf basal melt patterns using the one-layer Antarctic model for dynamical downscaling of ice–ocean exchanges (LADDIE v1.0)
A major source of uncertainty in future sea level projections is the ocean-driven basal melt of Antarctic ice shelves. While ice sheet models require a kilometre-scale resolution to realistically resolve ice shelf stability and grounding line migration, global or regional 3D ocean models are computationally too expensive to produce basal melt forcing fields at this resolution on long timescales. To bridge this resolution gap, we introduce the 2D numerical model LADDIE (one-layer Antarctic model for dynamical downscaling of ice–ocean exchanges), which allows for the computationally efficient modelling of detailed basal melt fields. The model is open source and can be applied easily to different geometries or different ocean forcings. The aim of this study is threefold: to introduce the model to the community, to demonstrate its application and performance in two use cases, and to describe and interpret new basal melt patterns simulated by this model. The two use cases are the small Crosson–Dotson Ice Shelf in the warm Amundsen Sea region and the large Filchner–Ronne Ice Shelf in the cold Weddell Sea. At ice-shelf-wide scales, LADDIE reproduces observed patterns of basal melting and freezing in warm and cold environments without the need to re-tune parameters for individual ice shelves. At scales of 0.5–5 km, which are typically unresolved by 3D ocean models and poorly constrained by observations, LADDIE produces plausible basal melt patterns. Most significantly, the simulated basal melt patterns are physically consistent with the applied ice shelf topography. These patterns are governed by the topographic steering and Coriolis deflection of meltwater flows, two processes that are poorly represented in basal melt parameterisations. The kilometre-scale melt patterns simulated by LADDIE include enhanced melt rates in grounding zones and basal channels and enhanced melt or freezing in shear margins. As these regions are critical for ice shelf stability, we conclude that LADDIE can provide detailed basal melt patterns at the essential resolution that ice sheet models require. The physical consistency between the applied geometry and the simulated basal melt fields indicates that LADDIE can play a valuable role in the development of coupled ice–ocean modelling
Changes in the Isotopic Signature of Atmospheric Nitrous Oxide and Its Global Average Source During the Last Three Millennia
Nitrous oxide (N2O) is a strong greenhouse gas whose mole fraction in the atmosphere has increased over the industrial period. We present a new set of isotope measurements of N2O in air extracted from ice cores covering the last 3,000 years. For the preindustrial (PI) atmosphere, we find an average N2O mole fraction of (267 ± 1) nmol/mol and average tropospheric N2O isotopic values of δ15Nav PI = (9.5 ± 0.1)‰, δ18OPI = (47.1 ± 0.2)‰, δ15Nα PI = (17.8 ± 0.4)‰, and δ15Νβ PI = (1.2 ± 0.4)‰. From PI to modern times all isotope signatures decreased with a total change of δ15Nav = (−2.7 ± 0.2)‰, δ18O = (−2.5 ± 0.4)‰, δ15Nα = (−2.0 ± 0.7)‰, and δ15Νβ (−3.5 ± 0.7)‰. Interestingly, the temporal evolution is not the same for δ15Nav and δ18O. δ18O trends are relatively larger during the early part, and δ15Nav trends are larger during the late part of the industrial period, implying a decoupling of sources over the industrial period. Using a mass balance model, we determined the isotopic composition of the total average N2O source. Assuming that the total present source is the sum of a constant natural source and an increasing anthropogenic source, this anthropogenic source has an isotopic signature of δ15Nav source,anthrop = (−15.0 ± 2.6)‰, δ18Osource,anthrop = (30.0 ± 2.6)‰, δ15Nα source,anthrop = (−4.5 ± 1.7)‰, and δ15Nβ source,anthrop = (−24.0 ± 8.4)‰. The 15N site preference of the source has increased since PI times, which is indicative of a relative shift from denitrification to nitrification sources, consistent with agricultural emissions playing a major role in the N2O increase.SCOPUS: ar.jinfo:eu-repo/semantics/publishe
Highly abundant HCN in the inner hot envelope of GL 2591: probing the birth of a hot core?
We present observations of the v2=0 and vibrationally excited v2=1 J=9-8
rotational lines of HCN at 797 GHz toward the deeply embedded massive young
stellar object GL 2591, which provide the missing link between the extended
envelope traced by lower-J line emission and the small region of hot (T_ex >=
300 K), abundant HCN seen in 14 micron absorption with the Infrared Space
Observatory (ISO). The line ratio yields T_ex=720^+135_-100 K and the line
profiles reveal that the hot gas seen with ISO is at the velocity of the
protostar, arguing against a location in the outflow or in shocks. Radiative
transfer calculations using a depth-dependent density and temperature structure
show that the data rule out a constant abundance throughout the envelope, but
that a model with a jump of the abundance in the inner part by two orders of
magnitude matches the observations. Such a jump is consistent with the sharp
increase in HCN abundance at temperatures >~230 K predicted by recent chemical
models in which atomic oxygen is driven into water at these temperatures.
Together with the evidence for ice evaporation in this source, this result
suggests that we may be witnessing the birth of a hot core. Thus, GL 2591 may
represent a rare class of objects at an evolutionary stage just preceding the
`hot core' stage of massive star formation.Comment: Accepted by ApJ Letters, 11 pages including 3 figures, uses AASTe
Античные и средневековые городища на дне Иссык-Куля
В статье дается обзор результатов многолетних подводных археологических разведок на озере Иссык-Куль. Приводятся данные по местоположению некоторых затопленных городищ античности и средних веков (Тору-Айгыр, Кара-ой, Чигу). Описываются наиболее интересные артефакты, найденные на дне озера.В статті дається огляд результатів багаторічних підводних археологічних розвідок на озері Іссик-Куль. Наводяться дані про місцезнаходження деяких затоплених городищ античності і середньовіччя (Тору-Айгир, Кара-ой, Чігу). Описуються найцікавіші артефакти, знайдені на дні озера.The article is a review of the results of many years’ underwater archaeological researches at lake Issik Kul. Data about the place of location of some Ancient and Medieval towns (Toru-Aygir, Kara-oy, Chigu) are given. Most interesting artefacts found at the bottom of the lake are described
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