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841.
Temperature variability over the past millennium inferred from Northwestern Alaska tree rings 总被引:3,自引:0,他引:3
Rosanne D’Arrigo Erika Mashig David Frank Rob Wilson Gordon Jacoby 《Climate Dynamics》2005,24(2-3):227-236
We describe a new tree-ring width data set of 14 white spruce chronologies for the Seward Peninsula (SP), Alaska, based on living and subfossil wood dating from 1358 to 2001 AD. A composite chronology derived from these data correlates positively and significantly with summer temperatures at Nome from 1910 to 1970, after which there is some loss of positive temperature response. There is inferred cooling during periods within the Little Ice Age (LIA) from the early to middle 1600s and late 1700s to middle 1800s; and warming from the middle 1600s to early 1700s. We also present a larger composite data set covering 978–2001 AD, utilizing the SP ring-width data in combination with archaeological wood measurements and other recent collections from northwestern Alaska. The Regional Curve Standardization (RCS) method was employed to maximize potential low-frequency information in this data set. The RCS chronology shows intervals of persistent above-average growth around the time of the Medieval Warm Period (MWP) early in the millennium, which are comparable to growth levels in recent centuries. There is a more sustained cold interval during the LIA inferred from the RCS record as compared to the SP ring-width series. The chronologies correlate significantly with Bering and Chukchi Sea sea surface temperatures and with the Pacific Decadal Oscillation index. These atmosphere–ocean linkages probably account for the differences between these records and large-scale reconstructions of Arctic and Northern Hemisphere temperatures based largely on continental interior proxy data. 相似文献
842.
Airborne digital imaging technology: a new overview 总被引:2,自引:1,他引:2
843.
844.
845.
Gordon M. Winder 《New Zealand geographer》2006,62(1):50-64
Abstract: In 1908 Auckland City was the commercial hub for a small and remote provincial economy and cannot have supported more than modest, locally orientated enterprises. This paper argues that such a view is conditioned by a 'landlubber's gaze'. By envisaging a seafarer alighting on the City's wharves from a 1908 steamship to encounter the city's commercial heart, it demonstrates that Queen Street opened to a maritime world where Auckland enterprises enjoyed extensive networks and markets. Indeed, Auckland's 'floating world' rivalled its 'residential world'. Such an approach may force us to recast Auckland Province in archipelago and network terms. 相似文献
846.
847.
Seasonal changes in pasture biomass, production and offtake under the transhumance system in northern Pakistan 总被引:1,自引:0,他引:1
R.M. Omer A.J. Hester I.J. Gordon M.D. Swaine S.M. Raffique 《Journal of Arid Environments》2006,67(4):641-660
An understanding of seasonal changes in pasture biomass, production and offtake of different range types is fundamentally important for the efficient management of livestock grazing. However, few studies have quantified these changes for transhumance systems, despite the fact that transhumance is still the main form of livestock management in several regions of the world and is often critical for the livelihoods of the people. One such area is the Northern Areas of Pakistan, where six villages and their pastures were selected for study. Pastures were categorized within foothill, dry temperate and alpine range types, and seasonal biomass, production and offtake of the vegetation was estimated by clipping paired caged and uncaged quadrats. The alpine range type had by far the highest biomass and offtake; the foothill and dry temperate range types were much more sparsely vegetated. Although alpine pastures were heavily used, particularly in spring, there was no evidence for consistent over-utilization of pasture resources. Within the dry temperate range type, production was highest during spring but significantly under-used. This indicates a potential for increased use of dry temperate pastures during spring, an important period both for early recovery of livestock body condition after winter and to reduce the heavy pressure on the alpine pastures at this time. 相似文献
848.
849.
Pierre Kaufmann Gordon D. Holman Yang Su C. Guillermo Gimenez de?Castro Emilia Correia Luis O. T. Fernandes Rodney V. de Souza Adolfo Marun Pablo Pereyra 《Solar physics》2012,279(2):465-475
The GOES X28 flare of 4 November 2003 was the largest ever recorded in its class. It produced the first evidence for two spectrally separated emission components, one at microwaves and the other in the THz range of frequencies. We analyzed the pre-flare phase of this large flare, twenty?minutes before the onset of the major impulsive burst. This period is characterized by unusual activity in X-rays, sub-THz frequencies, H??, and microwaves. The CME onset occurred before the onset of the large burst by about 6?min. It was preceded by pulsations of 3??C?5?s periods at sub-THz frequencies together with X-ray and microwave enhancements. The sub-THz pulsations faded out as impulsive bursts were detected at 100??C?300?keV and 7?GHz, close to the time of the first H?? brightening and the CME onset. The activities detected prior to and at the CME onset were located nearly 2?arcmin south of the following large flare, suggesting they were separate events. This unusual activity brings new clues to understanding the complex energy buildup mechanisms prior to the CME onset, occurring at a distinct location and well before the major flare that exploded afterwards. 相似文献
850.
Gordon A.H. Walker 《New Astronomy Reviews》2012,56(1):9-15
The reflex motion of a star induced by a planetary companion is too small to detect by photographic astrometry. The apparent discovery in the 1960s of planetary systems around certain nearby stars, in particular Barnard’s star, turned out to be spurious. Conventional stellar radial velocities determined from photographic spectra at that time were also too inaccurate to detect the expected reflex velocity changes. In the late 1970s and early 1980s, the introduction of solid-state, signal-generating detectors and absorption cells to impose wavelength fiducials directly on the starlight, reduced radial velocity errors to the point where such a search became feasible. Beginning in 1980, our team from UBC introduced an absorption cell of hydrogen fluoride gas in front of the CFHT coudé spectrograph and, for 12 years, monitored the radial velocities of some 29 solar-type stars. Since it was assumed that extra-solar planets would most likely resemble Jupiter in mass and orbit, we were awarded only three or four two-night observing runs each year. Our survey highlighted three potential planet hosting stars, γ Cep (K1 IV), β Gem (K0 III), and ? Eri (K2 V). The putative planets all resembled Jovian systems with periods and masses of: 2.5 years and 1.4 MJ, 1.6 years and 2.6 MJ, and 6.9 years and 0.9 MJ, respectively. All three were subsequently confirmed from more extensive data by the Texas group led by Cochran and Hatzes who also derived the currently accepted orbital elements.None of these three systems is simple. All five giant stars and the supergiant in our survey proved to be intrinsic velocity variables. When we first drew attention to a possible planetary companion to γ Cep in 1988 it was classified as a giant, and there was the possibility that its radial velocity variations and those of β Gem (K0 III) were intrinsic to the stars. A further complication for γ Cep was the presence of an unseen secondary star in an orbit with a period initially estimated at some 30 years. The implication was that the planetary orbit might not be stable, and a Jovian planet surviving so close to a giant then seemed improbable. Later observations by others showed the stellar binary period was closer to 67 years, the primary was only a sub-giant and a weak, apparently synchronous chromospheric variation disappeared. Chromospheric activity was considered important because κ1 Cet, one of our program stars, showed a significant correlation of its radial velocity curve with chromospheric activity.? Eri is a young, magnetically active star with spots making it a noisy target for radial velocities. While the signature of a highly elliptical orbit (e = 0.6) has persisted for more than three planetary orbits, some feel that even more extensive coverage is needed to confirm the identification despite an apparent complementary astrometric acceleration detected with the Hubble Space Telescope.We confined our initial analyses of the program stars to looking for circular orbits. In retrospect, it appears that some 10% of our sample did in fact have Jovian planetary companions in orbits with periods of years. 相似文献