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951.
John M. Antle Susan M. Capalbo Edward T. Elliott Keith H. Paustian 《Climatic change》2004,64(3):289-315
In this paper we develop economic measures of vulnerability to climate change with and without adaptation in agricultural production systems. We implement these measures using coupled, site-specific ecosystem and economic simulation models. This modeling approach has two key features needed to study the response of agricultural production systems to climate change: it represents adaptation as an endogenous, non-marginal economic response to climate change; and it provides the capability to represent the spatial variability in bio-physical and economic conditions that interact with adaptive responses. We apply this approach to the dryland grain production systems of the Northern Plains region of the United States. The results support the hypothesis that the most adverse impacts on net returns distributions tend to occur in the areas with the poorest resource endowments and when mitigating effects of CO2 fertilization and adaptation are absent. We find that relative and absolute measures of vulnerability depend on complex interactions between climate change, CO2 level, adaptation, and economic conditions such as relativeoutput prices. The relationship between relative vulnerability and resource endowments varies with assumptions about climate change, adaptation, and economic conditions. Vulnerability measured with respect to an absolute threshold is inversely related to resource endowments in all cases investigated. 相似文献
952.
Johannes Feddema Keith Oleson Gordon Bonan Linda Mearns Warren Washington Gerald Meehl Douglas Nychka 《Climate Dynamics》2005,25(6):581-609
This study assesses the sensitivity of the fully coupled NCAR-DOE PCM to three different representations of present-day land
cover, based on IPCC SRES land cover information. We conclude that there is significant model sensitivity to current land
cover characterization, with an observed average global temperature range of 0.21 K between the simulations. Much larger contrasts
(up to 5 K) are found on the regional scale; however, these changes are largely offsetting on the global scale. These results
show that significant biases can be introduced when outside data sources are used to conduct anthropogenic land cover change
experiments in GCMs that have been calibrated to their own representation of present-day land cover. We conclude that hybrid
systems that combine the natural vegetation from the native GCM datasets combined with human land cover information from other
sources are best for simulating such impacts. We also performed a prehuman simulation, which had a 0.39 K ~higher average
global temperature and, perhaps of greater importance, temperature changes regionally of about 2 K. In this study, the larger
regional changes coincide with large-scale agricultural areas. The initial cooling from energy balance changes appear to create
feedbacks that intensify mid-latitude circulation features and weaken the summer monsoon circulation over Asia, leading to
further cooling. From these results, we conclude that land cover change plays a significant role in anthropogenically forced
climate change. Because these changes coincide with regions of the highest human population this climate impact could have
a disproportionate impact on human systems. Therefore, it is important that land cover change be included in past and future
climate change simulations. 相似文献
953.
On the role of Mn(IV) vacancies in the photoreductive dissolution of hexagonal birnessite 总被引:4,自引:0,他引:4
Photoreductive dissolution of layer type Mn(IV) oxides (birnessite) under sunlight illumination to form soluble Mn(II) has been observed in both field and laboratory settings, leading to a consensus that this process is a key driver of the biogeochemical cycling of Mn in the euphotic zones of marine and freshwater ecosystems. However, the underlying mechanisms for the process remain unknown, although they have been linked to the semiconducting characteristics of hexagonal birnessite, the ubiquitous Mn(IV) oxide produced mainly by bacterial oxidation of soluble Mn(II). One of the universal properties of this biogenic mineral is the presence of Mn(IV) vacancies, long-identified as strong adsorption sites for metal cations. In this paper, the possible role of Mn vacancies in photoreductive dissolution is investigated theoretically using quantum mechanical calculations based on spin-polarized density functional theory (DFT). Our DFT study demonstrates unequivocally that Mn vacancies significantly reduce the band-gap energy for hexagonal birnessite relative to a hypothetical vacancy-free MnO2 and thus would increase the concentration of photo-induced electrons available for Mn(IV) reduction upon illumination of the mineral by sunlight. Calculations of the charge distribution in the presence of vacancies, although not fully conclusive, show a clear separation of photo-induced electrons and holes, implying a slow recombination of these charge-carriers that facilitates the two-electron reduction of Mn(IV) to Mn(II). 相似文献
954.
Yuenyong Panjasawatwong Leonid V. Danyushevsky Anthony J. Crawford Keith L. Harris 《Contributions to Mineralogy and Petrology》1995,118(4):420-432
An experimental investigation of plagioclase crystallization in broadly basaltic/andesitic melts of variable Ca# (Ca/(Ca+Na)*100)
and Al# (Al/(Al+Si)*100) values and H2O contents has been carried out at high pressures (5 and 10 kbar) in a solid media piston-cylinder apparatus. The H2O contents of glasses coexisting with liquidus or near-liquidus plagioclases in each experiment were determined via an FTIR
spectroscopic technique. This study has shown that melt Ca# and Al#, H2O content and crystallization pressure all control the composition of liquidus plagioclase. Increasing melt Ca# and Al# increase
An content of plagioclase, whereas the effect of increasing pressure is the opposite. However, the importance of the role
played by each of these factors during crystallization of natural magmas varies. Melt Ca# has the strongest control on plagioclase
An content, but melt Al# also exerts a significant control. H2O content can notably increase the An content of plagioclase, up to 10 mol % for H2O-undersaturated melts, and 20 mol % for H2O-saturated melts. Exceptionally calcic plagioclases (up to An100) in some primitive subduction-related boninitic and related rocks cannot be attributed to the presence of the demonstrated
amounts of H2O (up to 3 wt %). Rather, they must be due to the involvement of extremely refractory (CaO/Na2O>18) magmas in the petrogenesis of these rocks. Despite the refractory nature of some primitive MORB glasses, none are in
equilibrium with the most calcic plagioclase (An94) found in MORB. These plagioclases were likely produced from more refractory melts with CaO/Na2O = 12–15, or from melts with exceptionally high Al2O3(>18%). Magmas of appropriate compositions to crystallize these most calcic plagioclases are sometimes found as melt inclusions
in near liquidus phenocrysts from these rocks, but are not known among wholerock or glass compositions. The fact that such
melts are not erupted as discrete magma batches indicates that they are effectively mixed and homogenized with volumetrically
dominant, less refractory magmas. The high H2O contents (∼ 6 wt%) in some high-Al basaltic arc magmas may be responsible for the existence of plagioclases up to An95 in arc lavas. However, an alternative possibility is that petrogenesis involving melts with abnormally high CaO/Na2O values (> 8) may account for the presence of highly anorthitic plagioclases in these rocks.
Received: 31 August 1993 / Accepted: 20 May 1994 相似文献
955.
用分子动力学方法,研究了1999 K下,压力由23 MPa上升到15183 MPa的过程中,CaAl_2Si_2O_8成分熔体的微观结构、剪切粘滞度和粒子自扩散系数的压力效应。在此基础上,探讨压力对剪切粘滞度与粒子自扩散系数之间关系的影响,并将它同微观结构的变化联系起来。结果表明,粒子自扩散系数的压力效应与熔体结构有很强的相关性;压力的挤压效应阻碍了粒子的扩散,而Si-O和Al-O 5次配位体的形成又加速了扩散过程,两种相反的作用相互抵消,造成的结果是在0~5 GPa范围内,Si~(4 ),O~(2-)和Al~(3 )等网架形成粒子的自扩散系数随压力变化不明显;当压力继续增大时,挤压效应占了主导,导致自扩散系数值快速减小。Ca~(2 )作为网架修饰粒子,自扩散系数随压力升高单调下降。压力小于5 GPa时,粒子自扩散系数的大小关系是:D_(Ca)>D_(Al)>D_O>D_(Si)。系统粘滞度随压力的变化与熔体中BO的含量密切相关:BO含量小于域值时,一定范围内BO含量的变化不会对粘滞度产生很大的影响,超过域值,BO含量的微小增加会导致粘滞度值迅速增大。有效应用Eyring方程的关键是方程中粒子跳跃距离的确定,本研究发现,Si~(4 )和O~(2-)的跳跃距离可以通过系统中非桥氧的百分含量来获得。这一发现使得我们能够利用系统中NBO的含量,结合Eyring方程有效进行不同压 相似文献
956.
Jenni E. Sherriff Keith N. Wilkinson Poppy Harding Hayley Hawkins Rhys G. O. Timms Daniel S. Adler Emily J. Beverly Simon P. E. Blockley Boris Gasparyan Christina J. Manning Darren Mark Samvel Nahapetyan Katie J. Preece 《第四纪科学杂志》2022,37(1):6-27
The significance of the southern Caucasus in understanding Pleistocene hominin expansions is well established. However, the palaeoenvironments in which Palaeolithic occupation of the region took place are presently poorly defined. The Hrazdan river valley, Armenian Highlands, contains a rich Palaeolithic record alongside Middle Pleistocene volcanic, fluvial and lacustrine strata, and thus offer exciting potential for palaeoenvironmental reconstruction. We present the first results of sedimentological, geochemical, tephrostratigraphical and biological (diatoms) study of the sequence of Bird Farm 1, located in the central part of the valley. These data show six phases of landscape development during the interval 440–200 ka. The sequence represents the first quantitative Pleistocene diatom record from the Armenian Highlands and the southern Caucasus, and indicates the persistence of a deep, stratified lacustrine system, with evidence for changing lake productivity that is tentatively linked to climate. Furthermore, major element chemical characterization of visible and crypto-tephra horizons in the sequence enables the first stages of the development of a regional tephrostratigraphy. Together, the evidence from Bird Farm 1 demonstrates the importance of lacustrine archives in the region for palaeoenvironmental reconstruction and highlights the potential for linkages between archives on both a local and regional scale. 相似文献