The influence of human activity in the Arctic on climate and climate impacts   总被引：1，自引：0，他引：1  

Henry P. Huntington  Michelle Boyle  Gwenn E. Flowers  John W. Weatherly  Lawrence C. Hamilton  Larry Hinzman  Craig Gerlach  Rommel Zulueta  Craig Nicolson  Jonathan Overpeck 《Climatic change》2007,82(1-2):77-92

Human activities in the Arctic are often mentioned as recipients of climate-change impacts. In this paper we consider the more complicated but more likely possibility that human activities themselves can interact with climate or environmental change in ways that either mitigate or exacerbate the human impacts. Although human activities in the Arctic are generally assumed to be modest, our analysis suggests that those activities may have larger influences on the arctic system than previously thought. Moreover, human influences could increase substantially in the near future. First, we illustrate how past human activities in the Arctic have combined with climatic variations to alter biophysical systems upon which fisheries and livestock depend. Second, we describe how current and future human activities could precipitate or affect the timing of major transitions in the arctic system. Past and future analyses both point to ways in which human activities in the Arctic can substantially influence the trajectory of arctic system change.  相似文献   

HST colour–magnitude diagrams of six old globular clusters in the LMC     

K. A. G. Olsen  P. W. Hodge  M. Mateo  E. W. Olszewski  R. A. Schommer  N. B. Suntzeff  & A. R. Walker 《Monthly notices of the Royal Astronomical Society》1998,300(3):665-685

We report on HST observations of six candidate old globular clusters in the Large Magellanic Cloud (LMC): NGC 1754, 1835, 1898, 1916, 2005 and 2019. Deep exposures with the F555W and F814W filters provide us with colour–magnitude diagrams that reach to an apparent magnitude in V of ∼25, well below the main-sequence turn-off. These particular clusters are associated with significantly high LMC field star densities and care was taken to subtract the field stars from the cluster colour–magnitude diagrams accurately. In two cases there is significant variable reddening across at least part of the image, but only for NGC 1916 does the differential reddening preclude accurate measurements of the CMD characteristics. The morphologies of the colour–magnitude diagrams match well those of Galactic globular clusters of similar metallicity. All six have well-developed horizontal branches, while four clearly have stars on both sides of the RR Lyrae gap. The abundances obtained from measurements of the height of the red giant branch above the level of the horizontal branch are 0.3 dex higher, on average, than previously measured spectroscopic abundances. Detailed comparisons with Galactic globular cluster fiducials show that all six clusters are old objects, very similar in age to classical Galactic globulars such as M5, with little age spread among the clusters. This result is consistent with ages derived by measuring the magnitude difference between the horizontal branch and main-sequence turn-off. We also find a similar chronology by comparing the horizontal branch morphologies and abundances with the horizontal branch evolutionary tracks of Lee, Demarque &38; Zinn. Our results imply that the LMC formed at the same time as the Milky Way Galaxy.  相似文献   

Current Convenience,Desperate Deforestation: Ghana's AdjustmentProgram and the Forestry Sector     

J. Henry Owusu 《The Professional geographer》1998,50(4):418-436

Since the early 1980s, most Sub-Saharan African countries have resorted to structural adjustment programs to reform their ailing economies. Adjustment, however, may provide a convenient means for governments of the adjusting economies and international capital to meet their current economic and political interests, usually by sacrificing the physical environment, with huge environmental costs for the adjusting country. This paper uses Ghana's forestry sector, which has been characterized by a dramatic increase in woodexports since adjustment, to demonstrate a direct link between adjustment and environmental destruction. Ghana's dramatic increase in wood exports, involving a rapid and extensive deforestation, results from the government's need to meet its increasing external debt service obligations, and is exacerbated by the series of massive local currency devaluation required under adjustment “to get prices right.” The systematic reduction in government revenue from devaluation to amortize the increasing debts, keeps the government and indebted wood processing firms on a treadmill of export-based extraction/deforestation.  相似文献   

The spatial structure of the dayside ionospheric trough     

S. E. Pryse  L. Kersley  M. J. Williams  I. K. Walker 《Annales Geophysicae》1998,16(10):1169-1179

Tomographic imaging provides a powerful technique for obtaining images of the spatial distribution of ionospheric electron density at polar latitudes. The method, which involves monitoring radio transmissions from the Navy Navigation Satellite System at a meridional chain of ground receivers, has particular potential for complementing temporal measurements by other observing techniques such as the EISCAT incoherent-scatter radar facility. Tomographic reconstructions are presented here from a two-week campaign in November 1995 that show large-scale structuring of the polar ionosphere. Measurements by the EISCAT radar confirm the authenticity of the technique and provide additional information of the plasma electron and ion temperatures. The dayside trough, persistently observed at high latitudes during a geomagnetically quiet period but migrating to lower latitudes with increasing activity, is discussed in relationship to the pattern of the polarcap convection.  相似文献   

Magnesium self-diffusion in orthoenstatite     

Craig S. Schwandt  Randall T. Cygan  Henry R. Westrich 《Contributions to Mineralogy and Petrology》1998,130(3-4):390-396

Magnesium self-diffusion coefficients were determined experimentally for diffusion parallel to each of the three crystallographic directions in natural orthoenstatite (En₈₈Fs₁₂). Experiments were conducted at 1 atm in CO-CO₂ gas mixing furnaces, which provided oxygen fugacities equivalent to the iron-wüstite buffer. Diffusion of ²⁵Mg was induced in polished samples of oriented orthoenstatite using a film of isotopically enriched ²⁵MgO as the source material. Very short (<0.15 μm) diffusional penetration profiles were measured by ion microprobe depth profiling. The diffusion coefficients determined for four temperatures (900, 850, 800, 750 °C) provide the activation energies, E _a , and frequency factors, D _o, where D = D _o exp (−E _a /RT) for Mg self-diffusion parallel to each crystallographic direction: a-axis, E _a  = 360 ± 52 kJ/mole and D _o = 1.10 × 10⁻⁴ m²/s; b-axis, E _a  = 339 ± 77 kJ/mole and D _o = 6.93 × 10⁻⁶ m²/s and c-axis, E _a  = 265 ± 66 kJ/mole and D _o = 4.34 × 10⁻⁹ m²/s. In this temperature range, any possible anisotropy of cation diffusion is very small, however the activation energy for diffusion parallel to the c-axis (001) is the lowest and the activation energies for diffusion parallel to the a-axis (100) and b-axis (010) are higher. Application of these diffusion results to the silicate phases of the Lowicz mesosiderite meteorite provides cooling rates for the silicate portion of the meteorite (4–11 °C/100 years) that are similar, although slower, to previous estimates. These silicate cooling rates are still several orders of magnitude faster than the cooling rates (0.1 °C/10⁶ years) for the metal portions. Received: 22 January 1997 / Accepted: 2 October 1997  相似文献   

The effects of liquid immiscibility and thermal diffusion on oxygen isotopes in silicate liquids   总被引：2，自引：1，他引：1  

T. K. Kyser  C. E. Lesher  D. Walker 《Contributions to Mineralogy and Petrology》1998,133(4):373-381

Differences between the δ¹⁸O values of Si- and Fe-rich immiscible liquids in the system Fe₂SiO₄-KAlSi₂O₆-SiO₂ (Fa-Lc-Q) in isothermal experiments at 0.1 MPa have been determined experimentally to be 0.6 permil. The observed partition of ¹⁸O into the Si-rich liquid is consistent with previous experience with the preferential partition of ¹⁸O into Si-rich minerals in isothermal equilibrium with minerals of less polymerized structure. Crystallochemical principles affect the distribution of oxygen isotopes in coexisting isothermal liquids in the same way as they apply to isothermally coexisting crystals. The effects of Soret (thermal) diffusion on the distribution of oxygen isotopes in silicate liquids above the solvus in the system Fa-Lc-Q under conditions of an imposed temperature gradient of ca. 250 °C over 4 mm and at 2 GPa have also been investigated experimentally. Both the magnitude and the direction of separation of oxygen isotopes as a result of Soret diffusion are unexpected. For each of the silicate liquids, the cold end of the charge is enriched in ¹⁸O by up to 4.7 permil, and the highest δ¹⁸O values are associated with the most silica-poor compositions. The distribution of oxygen isotopes appears to be similar in each liquid, regardless of their chemical compositions, which is in contrast to the behaviour of cations whose distributions are compositionally dependent and characterized by strong crystallochemical effects wherein network-forming species such as Si and Al separate to the hot end and Mg, Fe and Ca are segregated preferentially to the cold end. Structural units in the melts are evidently less selective between oxygen isotopes than between cations, because oxygen redistribution over all possible sites in these units proceeds according to mass. Self-diffusion coefficients of oxygen in basaltic liquids estimated from the Soret experiments are in accord with those from other isotope tracer experiments, and comparable to those of Si. The possible effects of Soret diffusion on the oxygen isotopic composition of metasomatic veins in the mantle are examined in light of these data, and indicate that decay of the thermal gradients in the veins exceeds that of the diffusion of oxygen needed to produce variations in the δ¹⁸O values of mantle minerals. Variations in oxygen isotope ratios in most natural systems as a result of Soret effects are unlikely. Received: 6 January 1997 /  Accepted: 28 June 1998  相似文献   

Seasonal dynamics of aggregates and their typical biocoenosis in the Elbe Estuary     

Heike Zimmermann-Timm  Henry Holst  Stefan Müller 《Estuaries and Coasts》1998,21(4):613-621

In 1995, an extensive investigation was carried out in the Elbe Estuary in Germany. Special attention was paid to organisms, including bacteria, amoebae, ciliates, flagellates, rotifers, larvae ofDreissena polymorpha, and nematodes, dispersed in the water column and associated firmly or loosely with different types of aggregates. The abundance, size, and colonization by microorganisms of the aggregates varied during the seasons. The largest aggregates were found during spring and summer, when diatoms, rotifers, and crustaceans were present. The most aggregates were encountered in spring and from summer to autumn. Most of the species observed during the study were more common in pelagic habitats than in benthic ones. However, the presence of ciliates in the groups Hypotrichida and Sessilida as well as as flagellates in the groups Biosoecida, Cercomonadida, Choanoflagellida, Chrysomonadida, Kinetoplastida, and amoebae and some nematodes in the open water depends, upon the availability of surfaces, because they are sessile or poor swimnters, and some flagellates and amoebae need to attach themselves to an object to feed. Most organisms were much more abundant in or on aggregates than in the surrounding water during spring and summer, which indicates that aggregates enhance the habitat and promote the development of the organisms. From spring through autumn, the structure of the community associated with the aggregates is influenced by the pelagic environment. The presence of the species in the benthic community was detected only during summer.  相似文献   

Infrared mapping of the dust disc around Vega     

Heinrichsen  Walker  & Klaas 《Monthly notices of the Royal Astronomical Society》1998,293(4):L78-L82

ISOPHOT has been used to perform high-resolution 60-μm scans of Vega, and these have been compared with those from γDra, to obtain a Gaussian width of 22±2 arcsec. The dust disc around Vega has been mapped, resolving it at 60 and 90 μm with ISOPHOT. At 90 μm a Gaussian width of 36±3 arsec has been derived. In addition, multi-filter photometry is presented, at 25, 60, 80, 100, 120, 150, 170 and 200 μm. The data are fitted by a modified blackbody with a temperature of 73 K [ Q (λ)∝1/λ^1.1]. The dust disc has a luminosity L _IR/ L _*∼3×10⁻⁵. Using a distance of 7.8 pc, 22 arcsec corresponds to a distance of 86 au, and 36 arcsec to a distance of 140 au.  相似文献   

Optical, infrared and radio observations of the Seyfert galaxy Markarian 315     

M. Nonino  J. P. Henry  C. Fanti  R. Fanti  & J. Davies 《Monthly notices of the Royal Astronomical Society》1998,299(2):332-340

We report on new optical, infrared and radio observations of the Seyfert galaxy Markarian 315. We confirm the detection at all wavelengths of a secondary peak of emission, which lies ≈ 2 arcsec east of the Seyfert nucleus. Moreover, we detect a chain-like structure which surrounds the active nucleus, with peculiar behaviour westward of the nucleus. We consider different interpretations for the origin of the secondary peak emission.  相似文献   

High-redshift galaxies in the Hubble Deep Field— II. Colours and number counts     

Lucia Pozzetti  Piero Madau  Gianni Zamorani  Henry C. Ferguson  & Gustavo Bruzual A. 《Monthly notices of the Royal Astronomical Society》1998,298(4):1133-1144

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