1,721,036 research outputs found
An approach to sulfate geoengineering with surface emissions of carbonyl sulfide
No full textSulfate geoengineering (SG) methods based on lower stratospheric tropical injection of sulfur dioxide
(SO2) have been widely discussed in recent years, focusing on the direct and indirect effects they would have on
the climate system. Here a potential alternative method is discussed, where sulfur emissions are located at the
surface or in the troposphere in the form of carbonyl sulfide (COS) gas. There are two time-dependent chemistry–
climate model experiments designed from the years 2021 to 2055, assuming a 40 TgSyr1 artificial global flux
of COS, which is geographically distributed following the present-day anthropogenic COS surface emissions
(SG-COS-SRF) or a 6 TgSyr1 injection of COS in the tropical upper troposphere (SG-COS-TTL). The
budget of COS and sulfur species is discussed, as are the effects of both SG-COS strategies on the stratospheric
sulfate aerosol optical depth ( 1 D 0:080 in the years 2046–2055), aerosol effective radius (0.46 μm), surface
SOx deposition (C8.9% for SG-COS-SRF; C3.3% for SG-COS-TTL), and tropopause radiative forcing (RF;
1:5Wm2 in all-sky conditions in both SG-COS experiments). Indirect effects on ozone, methane and
stratospheric water vapour are also considered, along with the COS direct contribution. According to our model
results, the resulting net RF is 1.3Wm2, for SG-COS-SRF, and 1.5Wm2, for SG-COS-TTL, and it is
comparable to the corresponding RF of 1.7Wm2 obtained with a sustained injection of 4 TgSyr1 in the
tropical lower stratosphere in the form of SO2 (SG-SO2, which is able to produce a comparable increase of the
sulfate aerosol optical depth). Significant changes in the stratospheric ozone response are found in both SG-COS
experiments with respect to SG-SO2 ( 5DU versus C1.4DU globally). According to the model results, the
resulting ultraviolet B (UVB) perturbation at the surface accounts for 4.3% as a global and annual average
(versus 2.4% in the SG-SO2 case), with a springtime Antarctic decrease of 2.7% (versus a C5.8% increase
in the SG-SO2 experiment). Overall, we find that an increase in COS emissions may be feasible and produce a
more latitudinally uniform forcing without the need for the deployment of stratospheric aircraft. However, our
assumption that the rate of COS uptake by soils and plants does not vary with increasing COS concentrations
will need to be investigated in future work, and more studies are needed on the prolonged exposure effects to
higher COS values in humans and ecosystems
Sulfur deposition changes under sulfate geoengineering conditions: quasi-biennial oscillation effects on the transport and lifetime of stratospheric aerosols
No full textAbstract. Sustained injection of sulfur dioxide (SO2) in the tropical lower stratosphere has been proposed as a climate engineering technique for the coming decades. Among several possible environmental side effects, the increase in sulfur deposition deserves additional investigation. In this study we present results from a composition–climate coupled model (University of L'Aquila Composition-Chemistry Model, ULAQ-CCM) and a chemistry-transport model (Goddard Earth Observing System Chemistry-Transport Model, GEOS-Chem), assuming a sustained lower-stratospheric equatorial injection of 8 Tg SO2 yr−1. Total S deposition is found to globally increase by 5.2 % when sulfate geoengineering is deployed, with a clear interhemispheric asymmetry (+3.8 and +10.3 % in the Northern Hemisphere (NH) and the Southern Hemisphere (SH), due to +2.2 and +1.8 Tg S yr−1, respectively). The two models show good consistency, both globally and on a regional scale under background and geoengineering conditions, except for S-deposition changes over Africa and the Arctic. The consistency exists with regard to time-averaged values but also with regard to monthly and interannual deposition changes. The latter is driven essentially by the variability in stratospheric large-scale transport associated with the quasi-biennial oscillation (QBO). Using an externally nudged QBO, it is shown how a zonal wind E shear favors aerosol confinement in the tropical pipe and a significant increase in their effective radius (+13 % with respect to W shear conditions). The net result is an increase in the downward cross-tropopause S flux over the tropics with dominant E shear conditions with respect to W shear periods (+0.61 Tg S yr−1, +42 %, mostly due to enhanced aerosol gravitational settling) and a decrease over the extratropics (−0.86 Tg S yr−1, −35 %, mostly due to decreased large-scale stratosphere–troposphere exchange of geoengineering sulfate). This translates into S-deposition changes that are significantly different under opposite QBO wind shears, with an E–W anomaly of +0.32 in the tropics and −0.67 Tg S yr−1 in the extratropics. Most online QBO schemes predict a significant change in the zonal wind periodicity, up to a blocked E shear condition for large enough injections, so that our results indicate an upper limit for the tropical increase in S deposition of 16.5 % relative to average conditions of unperturbed QBO periodicity and a correspondent extratropical S deposition decrease of 16 %.
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Sulfate geoengineering: a review of the factors controlling the needed injection of sulfur dioxide
Full text linkSulfate geoengineering has been proposed as an affordable and
climate-effective means to temporarily offset the warming produced by the
increase of well-mixed greenhouse gases (WMGHGs). This technique would likely
have to be applied while and after global intergovernmental measures on
emissions of WMGHGs are implemented in order to achieve surface temperature
stabilization. The direct radiative effects of sulfur injection in the
tropical lower stratosphere can be summarized as increasing shortwave
scattering with consequent tropospheric cooling and increasing longwave
absorption with stratospheric warming. Indirect radiative effects are related
to induced changes in the ozone distribution; stratospheric water vapor
abundance,;formation and size of upper-tropospheric cirrus ice particles; and
lifetime of long-lived species, namely CH4 in connection with OH
changes through several photochemical mechanisms. Direct and indirect effects
of sulfate geoengineering both concur to determine the atmospheric response.
A review of previous studies on these effects is presented here, with an
outline of the important factors that control the amount of sulfur dioxide to
be injected in an eventual realization of the experiment. However, we need to
take into account that atmospheric models used for these studies have shown a
wide range of climate sensitivity and differences in the response to
stratospheric volcanic aerosols. In addition, large uncertainties exist in
the estimate of some of these aerosol effects
Upper tropospheric ice sensitivity to sulfate geoengineering
No full textAside from the direct surface cooling sulfate geoengineering (SG) would produce, the investigation on possible sideeffects
of this method is still ongoing, as for instance on upper tropospheric cirrus cloudiness. Goal of the present study is to
better understand the SG thermo-dynamical effects on the homogeneous freezing ice formation process. This is done by comparing
SG model simulations against a RCP4.5 reference case: in one case the aerosol-driven surface cooling is included and
coupled to the 5 stratospheric warming resulting from aerosol absorption of longwave radiation. In a second SG perturbed case,
surface temperatures are kept unchanged with respect to the reference RCP4.5 case. Surface cooling and lower stratospheric
warming, together, tend to stabilize the atmosphere, thus decreasing turbulence and water vapor updraft velocities (-10% in
our modeling study). The net effect is an induced cirrus thinning, which may then produce a significant indirect negative radiative
forcing (RF). This would go in the same direction as the direct effect of solar radiation scattering by the aerosols, thus
10 influencing the amount of sulfur needed to counteract the positive RF due to greenhouse gases. In our study, given a 8 Tg-SO2
equatorial injection in the lower stratosphere, an all-sky net tropopause RF of -2.13 W/m2 is calculated, of which -0.96 W/m2
(45%) from the indirect effect on cirrus thinning (7.5% reduction in ice optical depth). When the surface cooling is ignored, the
ice optical depth reduction is lowered to 5%, with an all-sky net tropopause RF of -1.45 W/m2, of which -0.21 W/m2 (14%)
from cirrus thinning. Relatively to the clear-sky net tropopause RF due to SG aerosols (-2.06 W/m2), the cumulative effect of
15 background clouds and cirrus thinning accounts for -0.07 W/m2, due to close compensation of large positive shortwave (+1.85
W/m2) and negative longwave adjustments (-1.92 W/m2). When the surface cooling is ignored, the net cloud adjustment becomes
+0.71 W/m2, with the shortwave contribution (+1.97 W/m2) significantly larger in magnitude than the longwave one
(-1.26 W/m2). This highlights the importance of including all dynamical feedbacks of SG aerosols
Sulfate aerosols from non-explosive volcanoes: chemical-radiative effects in the troposphere and lower stratosphere
Full text linkImpact of Stratospheric Volcanic Aerosols on Age-of-Air and Transport of Long-Lived Species
Full text linkThe radiative perturbation associated to stratospheric aerosols from major explosive volcanic eruptions may induce significant changes in stratospheric dynamics. The aerosol heating rates warm up the lower stratosphere and cause a westerly wind anomaly, with additional tropical upwelling. Large scale transport of stratospheric trace species may be perturbed as a consequence of this intensified Brewer–Dobson circulation. The radiatively forced changes of the stratospheric circulation during the first two years after the eruption of Mt. Pinatubo (June 1991) may help explain the observed trend decline of long-lived greenhouse gases at surface stations (approximately −8 and −0.4 ppbv/year for CH4 and N2O, respectively). This decline is partly driven by the increased mid- to high-latitude downward flux at the tropopause and also by an increased isolation of the tropical pipe in the vertical layer near the tropopause, with reduced horizontal eddy mixing. Results from a climate-chemistry coupled model are shown for both long-lived trace species and the stratospheric age-of-air. The latter results to be younger by approximately 0.5 year at 30 hPa for 3–4 years after the June 1991 Pinatubo eruption, as a result of the volcanic aerosols radiative perturbation and is consistent with independent estimates based on long time series of in situ profile measurements of SF6 and CO2. Younger age of air is also calculated after Agung, El Chichón and Ruiz eruptions, as well as negative anomalies of the N2O growth rate at the extratropical tropopause layer. This type of analysis is made comparing the results of two ensembles of model simulations (1960–2005), one including stratospheric volcanic aerosols and their radiative interactions and a reference case where the volcanic aerosols do not interact with solar and planetary radiation
Stratospheric Aerosols from Major Volcanic Eruptions: A Composition-Climate Model Study of the Aerosol Cloud Dispersal and e-folding Time
Full text linkLarge explosive volcanic eruptions are capable of injecting considerable amounts of particles and sulfur gases above the tropopause, causing large increases in stratospheric aerosols. Five major volcanic eruptions after 1960 (i.e., Agung, St. Helens, El Chichón, Nevado del Ruiz and Pinatubo) have been considered in a numerical study conducted with a composition-climate coupled model including an aerosol microphysics code for aerosol formation and growth. Model results are compared between an ensemble of numerical simulations including volcanic aerosols and their radiative effects (VE) and a reference simulations ensemble (REF) with no radiative impact of the volcanic aerosols. Differences of VE-REF show enhanced diabatic heating rates; increased stratospheric temperatures and mean zonal westerly winds; increased planetary wave amplitude; and tropical upwelling. The impact on stratospheric upwelling is found to be larger when the volcanically perturbed stratospheric aerosol is confined to the tropics, as tends to be the case for eruptions which were followed by several months with easterly shear of the quasi-biennial oscillation (QBO), e.g., the Pinatubo case. Compared to an eruption followed by a period of westerly QBO, such easterly QBO eruptions are quite different, with meridional transport to mid- and high-latitudes occurring later, and at higher altitude, with a consequent decrease in cross-tropopause removal from the stratosphere, and therefore longer decay timescale. Comparing the model-calculated e-folding time of the volcanic aerosol mass during the first year after the eruptions, an increase is found from 8.1 and 10.3 months for El Chichón and Agung (QBO westerly shear), to 14.6 and 30.7 months for Pinatubo and Ruiz (QBO easterly shear). The corresponding e-folding time of the global-mean radiative flux changes goes from 9.1 and 8.0 months for El Chichón and Agung, to 28.7 and 24.5 months for Pinatubo and Ruiz
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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