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221.
C. de Vegt J. A. Hughes D. Jauncey G. White K. J. Johnston J. Russell G. Nicholson 《Astrophysics and Space Science》1991,177(1-2):199-202
An observational pilot program is described which provides a first step towards the construction of an inertial extragalactic reference frame from optical and radio measurements. Candidate objects will be selected from extragalactic compact radio sources which display optical counterparts. Galactic radio stars will be secondary reference objects. The reference frame will be defined by the radio positions of the extragalactic objects. Based on a coordinated effort in both hemispheres, a global homogeneous net of about 400 quasars and 100 radio stars is expected to result from a five year campaign. 相似文献
222.
Theodore G. Shepherd Emily Boyd Raphael A. Calel Sandra C. Chapman Suraje Dessai Ioana M. Dima-West Hayley J. Fowler Rachel James Douglas Maraun Olivia Martius Catherine A. Senior Adam H. Sobel David A. Stainforth Simon F. B. Tett Kevin E. Trenberth Bart J. J. M. van den Hurk Nicholas W. Watkins Dimitri A. Zenghelis 《Climatic change》2018,147(3-4):555-569
Investigating the relationships between climate extremes and crop yield can help us understand how unfavourable climatic conditions affect crop production. In this study, two statistical models, multiple linear regression and random forest, were used to identify rainfall extremes indices affecting wheat yield in three different regions of the New South Wales wheat belt. The results show that the random forest model explained 41–67% of the year-to-year yield variation, whereas the multiple linear regression model explained 34–58%. In the two models, 3-month timescale standardized precipitation index of Jun.–Aug. (SPIJJA), Sep.–Nov. (SPISON), and consecutive dry days (CDDs) were identified as the three most important indices which can explain yield variability for most of the wheat belt. Our results indicated that the inter-annual variability of rainfall in winter and spring was largely responsible for wheat yield variation, and pre-growing season rainfall played a secondary role. Frequent shortages of rainfall posed a greater threat to crop growth than excessive rainfall in eastern Australia. We concluded that the comparison between multiple linear regression and machine learning algorithm proposed in the present study would be useful to provide robust prediction of yields and new insights of the effects of various rainfall extremes, when suitable climate and yield datasets are available. 相似文献
223.
Stela Cuna Elise Pendall John B. Miller Pieter P. Tans Ed Dlugokencky James W.C. White 《Applied Geochemistry》2008
The Danube Delta-Black Sea region of Romania is an important wetland, and this preliminary study evaluates the significance of this region as a source of atmospheric CH4. Measurements of the mixing ratio and δ13C in CH4 are reported from air and water samples collected at eight sites in the Danube Delta. High mixing ratios of CH4 were found in air (2500–14,000 ppb) and dissolved in water samples (∼1–10 μmol L−1), demonstrating that the Danube Delta is an important natural source of CH4. The intercepts on Keeling plots of about −62‰ show that the main source of CH4 in this region is microbial, probably resulting primarily from acetate fermentation. Atmospheric CH4 and CO data from the NOAA/ESRL (National Oceanic and Atmospheric Administration/Earth System Research Laboratory) were used to make a preliminary estimate of biogenic CH4 at the Black Sea sampling site at Constanta (BSC). These data were used to calculate ratios of CH4/CO in air samples, and using an assumed CH4/CO anthropogenic emissions ratio of 0.6, fossil fuel emissions at BSC were estimated. Biogenic CH4 emissions were then estimated by a simple mass balance approach. Keeling plots of well-mixed air from the BSC site suggested a stronger wetland source in summer and a stronger fossil fuel source in winter. 相似文献
224.
Emily Fung Pablo Imbach Lenin Corrales Sergio Vilchez Nelson Zamora Freddy Argotty Lee Hannah Zayra Ramos 《Climatic change》2017,141(1):77-92
Climate change and land use conversion are global threats to biodiversity. Protected areas and biological corridors have been historically implemented as biodiversity conservation measures and suggested as tools within planning frameworks to respond to climate change. However, few applications to national protected areas systems considering climate change in tropical countries exist. Our goal is to define new priority areas for biodiversity conservation and biological corridors within an existing protected areas network. We aim at preserving samples of all biodiversity under climate change and facilitate species dispersal to reduce the vulnerability of biodiversity. The analysis was based on a three step strategy: i) protect representative samples of various levels of terrestrial biodiversity across protected area systems given future redistributions under climate change, ii) identify and protect areas with reduced climate velocities where populations could persist for relatively longer periods, and iii) ensure species dispersal between conservation areas through climatic connectivity pathways. The study was integrated into a participatory planning approach for biodiversity conservation in Costa Rica. Results showed that there should be an increase of 11 % and 5 % on new conservation areas and biological corridors respectively. Our approach integrates climate change into the design of a network of protected areas for tropical ecosystems and can be applied to other biodiversity rich areas to reduce the vulnerability of biodiversity to global warming. 相似文献
225.
Holocene hydrological cycle changes in the Southern Hemisphere documented in East Antarctic deuterium excess records 总被引:1,自引:0,他引:1
F. Vimeux V. Masson J. Jouzel J. R. Petit E. J. Steig M. Stievenard R. Vaikmae J. W. C. White 《Climate Dynamics》2001,17(7):503-513
Four Holocene-long East Antarctic deuterium excess records are used to study past changes of the hydrological cycle in the
Southern Hemisphere. We combine simple and complex isotopic models to quantify the relationships between Antarctic deuterium
excess fluctuations and the sea surface temperature (SST) integrated over the moisture source areas for Antarctic snow. The
common deuterium excess increasing trend during the first half of the Holocene is therefore interpreted in terms of a warming
of the average ocean moisture source regions over this time. Available Southern Hemisphere SST records exhibit opposite trends
at low latitudes (warming) and at high latitudes (cooling) during the Holocene. The agreement between the Antarctic deuterium
excess and low-latitude SST trends supports the idea that the tropics dominate in providing moisture for Antarctic precipitation.
The opposite trends in SSTs at low and high latitudes can potentially be explained by the decreasing obliquity during the
Holocene inducing opposite trends in the local mean annual insolation between low and high latitudes. It also implies an increased
latitudinal insolation gradient that in turn can maintain a stronger atmospheric circulation transporting more tropical moisture
to Antarctica. This mechanism is supported by results from a mid-Holocene climate simulation performed using a coupled ocean-atmosphere
model.
Received: 7 July 1999 / Accepted: 21 July 2000 相似文献
226.
Absolute Calibration of an Ashtech Z12-T GPS Receiver 总被引:3,自引:0,他引:3
Dual-frequency carrier phase and code measurements from geodetic type receivers are a promising tool for frequency and time
transfer. In order to use them for clock comparisons, all instrumental delays should be calibrated. We have carried out the
calibration of one such receiver, an Ashtech Z12-T type, by two different methods: first, by absolute calibration using a
GPS simulator; second, by differential calibration with respect to a time transfer receiver that had previously been calibrated.
We present the experimental set-ups and the results of the two experiments and estimate the uncertainty budget. An ultimate
uncertainty of order 1 ns in the absolute calibration seems to be attainable. ? 2001 John Wiley & Sons, Inc. 相似文献
227.
228.
White S.H. Zegers T.E. van Haaften W.M. Kloppenburg A. Wijbrans J. 《Geologie en Mijnbouw》1997,76(4):343-347
Geologie en Mijnbouw - 相似文献
229.
Activity–composition relations for the calculation of partial melting equilibria in metabasic rocks 下载免费PDF全文
E. C. R. Green R. W. White J. F. A. Diener R. Powell T. J. B. Holland R. M. Palin 《Journal of Metamorphic Geology》2016,34(9):845-869
A set of thermodynamic models is presented that, for the first time, allows partial melting equilibria to be calculated for metabasic rocks. The models consist of new activity–composition relations combined with end‐member thermodynamic properties from the Holland & Powell dataset, version 6. They allow for forward modelling in the system NaO–CaO–KO–FeO–MgO–AlO–SiO–HO–TiO–FeO. In particular, new activity–composition relations are presented for silicate melt of broadly trondhjemitic–tonalitic composition, and for augitic clinopyroxene with Si–Al mixing on the tetrahedral sites, while existing activity–composition relations for hornblende are extended to include KO and TiO. Calibration of the activity–composition relations was carried out with the aim of reproducing major experimental phase‐in/phase‐out boundaries that define the amphibolite–granulite transition, across a range of bulk compositions, at ≤13 kbar. 相似文献
230.