6,731 research outputs found
Crop intensification, land use, and on-farm energy-use efficiency during the worldwide spread of the green revolution
We analyzed crop production, physical inputs, and land use at the country level to assess technological changes behind the threefold increase in global crop production from 1961 to 2014. We translated machinery, fuel, and fertilizer to embedded energy units that, when summed up, provided a measure of agricultural intensification (human subsidy per hectare) for crops in the 58 countries responsible for 95% of global production. Worldwide,there was a 137% increase in input use per hectare, reaching 13 EJ, or 2.6% of the world´s primary energy supply, versus only a 10% increase in land use. Intensification was marked in Asia and Latin America, where input-use levels reached those that North America and Europe had in the earlier years of the period; the increase was more accentuated, irrespective of continent, for the 12 countries with mostly irrigated production. Half of the countries (28/58), mainly developed ones, had an average subsidy >5 GJ/ha/y (with fertilizers accounting for 27% in 1961 and 45% in 2014), with most of them (23/28) using about the same area or less than in 1961 (net land sparing of 31 Mha). Most of the remaining countries (24/30 with inputs <5 GJ/ha/y), mainly developing ones, increased their cropped area (net land extensification of 135 Mha). Overall, energy-use efficiency (cropoutput/inputs) followed a U-shaped trajectory starting at about 3 and finishing close to 4. The prospects of a more sustainable intensification are discussed, and the inadequacy of the land-sparing model expectation of protecting wilderness via intensified agriculture is highlighted.Fil: PELLEGRINI, PEDRO. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; ArgentinaFil: FERNANDEZ, Roberto J.. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentin
Complex Obtuse Random Walks and their Continuous-Time Limits
We study a particular class of complex-valued random variables and their
associated random walks: the complex obtuse random variables. They are the
generalization to the complex case of the real-valued obtuse random variables
which were introduced in \cite{A-E} in order to understand the structure of
normal martingales in \RR^n.The extension to the complex case is mainly
motivated by considerations from Quantum Statistical Mechanics, in particular
for the seek of a characterization of those quantum baths acting as classical
noises. The extension of obtuse random variables to the complex case is far
from obvious and hides very interesting algebraical structures. We show that
complex obtuse random variables are characterized by a 3-tensor which admits
certain symmetries which we show to be the exact 3-tensor analogue of the
normal character for 2-tensors (i.e. matrices), that is, a necessary and
sufficient condition for being diagonalizable in some orthonormal basis. We
discuss the passage to the continuous-time limit for these random walks and
show that they converge in distribution to normal martingales in \CC^N. We
show that the 3-tensor associated to these normal martingales encodes their
behavior, in particular the diagonalization directions of the 3-tensor indicate
the directions of the space where the martingale behaves like a diffusion and
those where it behaves like a Poisson process. We finally prove the
convergence, in the continuous-time limit, of the corresponding multiplication
operators on the canonical Fock space, with an explicit expression in terms of
the associated 3-tensor again
Entanglement of Bipartite Quantum Systems driven by Repeated Interactions
We consider a non-interacting bipartite quantum system undergoing repeated quantum interactions with an
environment modeled by a chain of independant quantum systems interacting one
after the other with the bipartite system. The interactions are made so that
the pieces of environment interact first with and then with
. Even though the bipartite systems are not interacting, the
interactions with the environment create an entanglement. We show that, in the
limit of short interaction times, the environment creates an effective
interaction Hamiltonian between the two systems. This interaction Hamiltonian
is explicitly computed and we show that it keeps track of the order of the
successive interactions with and . Particular
physical models are studied, where the evolution of the entanglement can be
explicitly computed. We also show the property of return of equilibrium and
thermalization for a family of examples
The Effect of the AGN Feedback on the Interstellar Medium of Early-Type Galaxies: 2D Hydrodynamical Simulations of the Low-Rotation Case
We present 2D hydrodynamical simulations for the evolution of early-type
galaxies containing central massive black holes (MBHs), starting at age 2 Gyr.
The code contains accurate and physically consistent radiative and mechanical
AGN wind feedback, with parsec-scale central resolution. Mass input comes from
stellar evolution; energy input includes Type Ia and II supernova and stellar
heating; star-formation is included. Realistic, axisymmetric dynamical models
for the galaxies are built solving the Jeans' equations. The lowest mass models
(Mstar = 8 10^{10}Msun) develop global outflows sustained by SNIa's heating,
ending with a significantly lower amount of hot gas and new stars. In more
massive models, nuclear outbursts last to the present epoch, with large and
frequent fluctuations in nuclear emission and from the gas (Lx). Each burst
last ~ 10^{7.5} yr, during which (for r < 2-3 kpc) cold, inflowing, and hot,
outflowing gas phases coexist. The Lx-T relation for the gas matches that of
local galaxies. AGN activity causes positive feedback for star formation.
Roughly half of the total mass loss is recycled into new stars (DeltaMstar),
just ~ 3% of it is accreted on the MBH, the remainder being ejected from the
galaxy. The ratio between the mass of gas expelled to that in to new stars, the
load factor, is ~0.6. Rounder galaxies shapes lead to larger final MBH masses,
DeltaMstar, and Lx. Almost all the time is spent at very low nuclear
luminosities, yet one quarter of the total energy is emitted at an Eddington
ratio > 0.1. The duty-cycle of AGN activity approximates 4% (Abridged).Comment: 26 pages, 15 figure, submitted to ApJ. Comments welcom
Structure and Feedback in 30 Doradus I: Observations
We have completed a a new optical imaging and spectrophotometric survey of a
140 x 80 pc region of 30 Doradus centered on R136, covering key optical
diagnostic emission lines including \Ha, \Hb, \Hg, [O III]
4363, 4959, 5007, [N II] 6548, 6584, [S II]
6717, 6731 [S III] 6312 and in some locations [S III]
9069. We present maps of fluxes and intensity ratios for these lines,
and catalogs of isolated ionizing stars, elephant-trunk pillars, and edge-on
ionization fronts. The final science-quality spectroscopic data products are
available to the public. Our analysis of the new data finds that, while stellar
winds and supernovae undoubtedly produce shocks and are responsible for shaping
the nebula, there are no global spectral signatures to indicate that shocks are
currently an important source of ionization. We conclude that the considerable
region covered by our survey is well described by photoionization from the
central cluster where the ionizing continuum is dominated by the most massive O
stars. We show that if 30 Dor were viewed at a cosmological distance, its
integrated light would be dominated by its extensive regions of lower
surface-brightness rather than by the bright, eye-catching arcs.Comment: 42 pages, 16 figures, Accepted for publication in ApJ
The Kammidɨkadɨ of Little Steamboat Point-1 Rockshelter: Terminal Early Holocene and Early Late Holocene Leporid Processing in Northern Warner Valley, Oregon
The terminal Early Holocene was a time of climatic upheaval in the Great Basin. Archaeofauna from this period are dominated by small game. Many Great Basin sites were abandoned during the Middle Holocene as aridity intensified and human populations focused their settlement systems around remaining sources of water. Following the Middle Holocene, human populations appear to have expanded as conditions improved during the Late Holocene, with many sites being reoccupied after hiatuses. At the Little Steamboat Point-1 Rockshelter, a stratified site containing terminal Early Holocene and Late Holocene deposits in southern Oregon, an extensive archaeofaunal assemblage has been recovered. The assemblage is composed primarily of leporids, the majority of which are jackrabbits (Lepus spp.). The taphonomic analysis presented here strongly suggests that the archaeofauna were deposited by human agents and that the faunal assemblage may represent seasonal usage of the site primarily to hunt leporids
Cooling flows and quasars: II. Detailed models of feedback modulated accretion flows
Most elliptical galaxies contain central black holes (BHs), and most also
contain significant amounts of hot gas capable of accreting on to the central
BH due to cooling times short compared to the Hubble time. Why therefore do we
not see AGNs at the center of most elliptical galaxies rather than in only (at
most) a few per cent of them? We propose here the simple idea that feedback
from accretion events heats the ambient gas retarding subsequent infall. In
this context, we present a class of 1D hydrodynamical evolutionary sequences
for the gas flows in elliptical galaxies with a massive central BH. The
resulting evolution is characterized by strong oscillations, in which very fast
and energetic bursts of the BH are followed by longer periods in which the
X-ray galaxy emission comes from the coronal gas. We also allow for departures
from spherical symmetry by examining scenarios in which the central engine is
either an ADAF or a more conventional accretion disk that is optically thick
except for a polar region. In all cases the duty cycle (fraction of the time
that the system will be seen as an AGN) is quite small and in the range 10^{-4}
- 10^{-3}. Thus, for any reasonable value of the efficiency, the presence of a
massive BH at the center of a galaxy seems to be incompatible with the presence
of a long-lived cooling flow.Comment: 43 pages, 10 figures. Main additions concern observed Compton
temperatures and few extra numerical models. Conclusions unchanged. 1 new
table and 3 new figures. Accepted for publication on ApJ (main journal
Emergent Noncommutative gravity from a consistent deformation of gauge theory
Starting from a standard noncommutative gauge theory and using the
Seiberg-Witten map we propose a new version of a noncommutative gravity. We use
consistent deformation theory starting from a free gauge action and gauging a
killing symmetry of the background metric to construct a deformation of the
gauge theory that we can relate with gravity. The result of this consistent
deformation of the gauge theory is nonpolynomial in A_\mu. From here we can
construct a version of noncommutative gravity that is simpler than previous
attempts. Our proposal is consistent and is not plagued with the problems of
other approaches like twist symmetries or gauging other groups.Comment: 18 pages, references added, typos fixed, some concepts clarified.
Paragraph added below Eq. (77). Match published PRD version
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