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421.
422.
Atmospheric nitrogen deposition in the northern Chihuahuan desert: Temporal trends and potential consequences 总被引:9,自引:0,他引:9
S. Bez J. Fargione D.I. Moore S.L. Collins J.R. Gosz 《Journal of Arid Environments》2007,68(4):640-651
Rates and impacts of atmospheric nitrogen (N) deposition are poorly understood in arid land ecosystems where soils are typically low in plant available N. To address this issue, we quantified long-term trends in N deposition and estimated its impact on plant community structure in the northern Chihuahuan desert of Central New Mexico, USA. Annual and seasonal rates of N deposition were strongly positively correlated with precipitation. When precipitation effects were removed statistically, N deposition increased at an annual rate of 0.049 kg ha−1 yr−1 between 1989 and 2004. Based on two independent fertilization studies at our desert grassland field site, continued atmospheric inputs are likely to increase grass cover, decrease legume abundance, and may favor blue grama (Bouteloua gracilis) at the expense of the current dominant species, black grama (Bouteloua eriopoda). We conclude that, although arid lands have low rates of N deposition and are primarily water limited, observed trends in N deposition rates may lead to significant changes in plant community structure. 相似文献
423.
Towards quantifying uncertainty in transient climate change 总被引:2,自引:3,他引:2
Matthew Collins Ben B. B. Booth Glen R. Harris James M. Murphy David M. H. Sexton Mark J. Webb 《Climate Dynamics》2006,27(2-3):127-147
Ensembles of coupled atmosphere–ocean global circulation model simulations are required to make probabilistic predictions of future climate change. “Perturbed physics” ensembles provide a new approach in which modelling uncertainties are sampled systematically by perturbing uncertain parameters. The aim is to provide a basis for probabilistic predictions in which the impact of prior assumptions and observational constraints can be clearly distinguished. Here we report on the first perturbed physics coupled atmosphere–ocean model ensemble in which poorly constrained atmosphere, land and sea-ice component parameters are varied in the third version of the Hadley Centre model (the variation of ocean parameters will be the subject of future study). Flux adjustments are employed, both to reduce regional sea surface temperature (SST) and salinity biases and also to admit the use of combinations of model parameter values which give non-zero values for the global radiation balance. This improves the extent to which the ensemble provides a credible basis for the quantification of uncertainties in climate change, especially at a regional level. However, this particular implementation of flux-adjustments leads to a weakening of the Atlantic overturning circulation, resulting in the development of biases in SST and sea ice in the North Atlantic and Arctic Oceans. Nevertheless, model versions are produced which are of similar quality to the unperturbed and un-flux-adjusted version. The ensemble is used to simulate pre-industrial conditions and a simple scenario of a 1% per year compounded increase in CO2. The range of transient climate response (the 20 year averaged global warming at the time of CO2 doubling) is 1.5–2.6°C, similar to that found in multi-model studies. Measures of global and large scale climate change from the coupled models show simple relationships with associated measures computed from atmosphere-mixed-layer-ocean climate change experiments, suggesting that recent advances in computing the probability density function of climate change under equilibrium conditions using the perturbed physics approach may be extended to the transient case. 相似文献
424.
425.
The walking school bus: extending children's geographies? 总被引:5,自引:0,他引:5
In many Western cities, the journey between home and school has become problematic, due to intensifying traffic and growing fears for children's safety. Accordingly, many parents now chauffeur their children to and from school. This situation has compounded congestion, prompting efforts to identify safe alternatives. One recent innovation is the walking school bus (WSB). In this paper we report on the development of this initiative, and its adoption at a primary school in Auckland, New Zealand. We conclude that although WSBs extend children's geographies they are, at best, an ambivalent response to the hegemony of motorized transport. 相似文献
426.
Paleozoic northward drift of the North Tien Shan (Central Asia) as revealed by Ordovician and Carboniferous paleomagnetism 总被引:7,自引:0,他引:7
Mikhail L. Bazhenov Adam Q. Collins Kirill E. Degtyarev Natalia M. Levashova Alexander V. Mikolaichuk Vladimir E. Pavlov Rob Van der Voo 《Tectonophysics》2003,366(1-2):113-141
Three new Middle–Late Ordovician and two new Early Carboniferous paleomagnetic poles have been obtained from the North Tien Shan Zone (NTZ) of the Ural–Mongol belt in Kyrgyzstan and Kazakhstan. Paleolatitudes for the Carboniferous are unambiguously northerly and average 15.5°N, whereas the Ordovician paleolatitudes (6°, 9°, and 9°) are inferred to be southerly, given that a very large (180°) rotation of the NTZ would be necessary during the middle Paleozoic if the other polarity option was chosen. Thus, the NTZ drifted northward during much of the Paleozoic; east–west drift cannot be determined, as is well known, from paleomagnetic data. In addition, detailed thermal demagnetization analysis reveals two overprints, one of recent age and the other of Permian age, which is a time of strong deformation in the NTZ. The paleolatitude of the combined Permian overprint is 30.5+2°N. The paleolatitudes collectively track those predicted for the area by extrapolation from Baltica very well, but are different from those of Siberia for Ordovician times. This finding is compatible with Sengör and Natal'in's [Sengör, A.M.C., Natal'in, B.A., 1996. Paleotectonics of Asia: fragments of a synthesis. In: Yin A., Harrison, M. (Eds.), The Tectonic Evolution of Asia. Cambridge Univ. Press, Cambridge, pp. 486–640] model of tectonic evolution of the Ural–Mongol belt and disagrees with the models of other researchers. Declinations of the Ordovician and Early Carboniferous results range from northwesterly to northeasterly, and are clearly affected by local relative rotations, which seem characteristic for the entire NTZ, because the Permian overprint declinations also show such a spread. Apparently, the important latest Paleozoic–Triassic deformation involved shear zone-related rotations as well as folding and significant granitic intrusions. 相似文献
427.
Equilibrium coastal profiles: II. Evidence from EOF analysis 总被引:1,自引:0,他引:1
The validity of the concept of coastal equilibrium depends upon a proof of the existence of equilibrium. In addition, the
methods of calculating the actual equilibrium profile and the response time are required. This study evaluates equilibrium
conditions using evidence from EOF (empirical orthogonal function) analysis of the coastal profile data (bed slopes and associated
elevations) obtained from Poole and Christchurch Bays, southern England. The preliminary results show that the largest eigenvalue
is much greater than the other eigenvalues and the temporally-related eigenfunction is stationary; this observation implies
the existence of equilibrium. Further, the spatially-related eigenfunction associated with the largest eigenvalue is used
to calculate the actual beach profile at equilibrium. Although the temporal eigenfunction may fluctuate, it recovers rapidly,
indicating that the response time is short. However, further studies are required to determine the response time accurately.
Contribution No 3335 from the Institute of Oceanology, Chinese Acodemy of Sciences.
Project supported by the SCOPAC (UK) and the CAS (through the “Hundred Talent Programme”, No. A10970203) 相似文献
428.
429.
Lead distribution on a public shotgun range 总被引:1,自引:0,他引:1
James R. Craig David Edwards Donald J. Rimstidt Patrick F. Scanlon Thomas K. Collins Oliver Schabenberger Jeffrey B. Birch 《Environmental Geology》2002,41(8):873-882
430.
Summary An inexpensive and simple form of radiometer capable of attachment to the American Bureau of Standards type radiosonde is described: it measures the upward flux of longwave radiation when used on night ascents. A slight modification to the telemetry circuit of the standard radiosonde enables the radiation signals to be obtained in addition to the usual pressure, temperature and humidity information.Deceased. 相似文献