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21.
Some natural substrates record environmental information and, as such, provide a means to reconstruct the environmental conditions
from the period these substrates were formed. Samples from environmental archives are not always equally spaced in distance.
When a periodic time series model is estimated from these unequally spaced proxy records, the search for reasonable starting
values is the main difficulty. In this work, a non-parametric method based on the regressive Fourier series is first presented,
which reduces averaging errors starting from unequally spaced records. The method is applied to synthetic data and generally
performs well in all circumstances. Secondly, a parametric method for the construction of a time base and the elimination
of averaging errors from unequally spaced records is presented. This parametric method uses the non-parametric method to produce
starting values for the parameters. The method is compared with the time series construction method with the averaging effect
taken into account and it is observed that only the current method produces acceptable results. The statistical performance
of the method is verified with a Monte Carlo simulation and the estimator is proven to be an efficient estimator. The applicability
of the method is demonstrated on the vessel density measurement in a mangrove tree, Rizophora mucronata, which is a proxy for the rainfall in tropical coastal regions. 相似文献
22.
The gas transport through non-volatile random porous media is investigated numerically. We extend our previous research of the transport of molecules inside the uppermost layer of a cometary surface (
[Skorov and Rickman, 1995] and [Skorov et al., 2001]). We assess the validity of the simplified capillary model and its assumptions to simulate the gas flux trough the porous dust mantle as it has been applied in cometary physics. A microphysical computational model for molecular transport in random porous media formed by packed spheres is presented. The main transport characteristics such as the mean free path distribution and the permeability are calculated for a wide range of model parameters and compared with those obtained by more idealized models. The focus in this comparison is on limitations inherent in the capillary model. Finally a practical way is suggested to adjust the algebraic Clausing formula taking into consideration the nonlinear dependence of permeability on layer porosity. The retrieved dependence allows us to accurately calculate the permeability of layers whose thickness and porosity vary in the range of values expected for the near-surface regions of a cometary nucleus. 相似文献
23.
Differential impacts of climate change on communities in the middle hills region of Nepal 总被引：1，自引：1，他引：0
There is a growing understanding that the impacts of climate change affect different communities within a country, in a variety of ways—not always uniformly. This article reports on research conducted in the middle hills region of Nepal that explored climate change vulnerability in terms of exposure, sensitivity and adaptive capacity across different well-being groups, genders of the head of household and household location. In the study region, dry land farming has increasingly experienced climate-induced changes to farm productivity and natural resources. The experience of vulnerability to decreased livelihood options and natural resource hazards due to a changing climate varied according to household wealth and well-being status, with very poor and poor households more vulnerable than medium and well-off households. The research indicates that the climate change adaptation would benefit by considering: (i) differential impacts of vulnerability mainly based on well-being status of households; (ii) understanding of the local socio-political context and underlying causes of vulnerability and its application; and (iii) identifying vulnerable populations for the units of vulnerability analysis and adaptation planning. 相似文献
24.
Nina Keul John W. Morse Rik Wanninkhof Dwight K. Gledhill Thomas S. Bianchi 《Aquatic Geochemistry》2010,16(3):337-351
This paper presents the results of two cruises in the Northern Gulf of Mexico in 2008 that investigated local and short-term
factors influencing the carbonate chemistry dynamics and saturation state with respect to aragonite (Ωaragonite) of surface seawater in this region. One cruise covered much of the northern half of the Gulf, and the other focused on
the coastal zone west of the Atchafalaya Bay outlet of the Mississippi River—the region where the hypoxic “dead zone” occurs
on the Louisiana shelf. Offshore waters (>100 m depth) exhibited only small variations in CO2 fugacity (fCO2), total alkalinity (TA) and Ωaragonite. Values were close to those typically observed in subtropical Atlantic Ocean and Caribbean Sea waters of similar salinity.
However, inner shelf waters (<50 m depth) exhibited large variations in fCO2, TA, and Ωaragonite that were not directly related to salinity or distance from the Mississippi River plume. Changes in TA values were not the
result of simple mixing of end-member freshwater and seawater TA concentrations but exhibited a minimum in values near salinity
of 25. This minimum could be the result of microbial recycling across salinity gradients, biological removal of alkalinity
by formation of calcium carbonate or mixing of a third end-member with a low alkalinity such as Terrebonne Bay. All waters
were supersaturated with respect to aragonite. Offshore waters had an average Ωaragonite of 3.86 with a standard deviation of only ±0.06 and inner shelf waters had a range in Ωaragonite values from 3.9 to 9.7 with a median of 4.3. Shelf water Ωaragonite values were elevated relative to the offshore as a consequence of both high TA input from Mississippi River and biological
drawdown of CO2. A dominant factor controlling Ωaragonite distribution in offshore waters with relatively constant temperatures was fCO2, with higher supersaturation occurring in areas with low fCO2. 相似文献
25.
Veerle Beelaerts Maite Bauwens Emma Versteegh Frank Dehairs Rik Pintelon 《Mathematical Geosciences》2011,43(6):635-662
Identifying a periodic time-series model from environmental records, without imposing the positivity of the growth rate, does
not necessarily respect the time order of the data observations. Consequently, subsequent observations, sampled in the environmental
archive, can be inversed on the time axis, resulting in a non-physical signal model. In this paper an optimization technique
with linear constraints on the signal model parameters is proposed that prevents time inversions. The activation conditions
for this constrained optimization are based upon the physical constraint of the growth rate, namely, that it cannot take values
smaller than zero. The actual constraints are defined for polynomials and first-order splines as basis functions for the nonlinear
contribution in the distance-time relationship. The method is compared with an existing method that eliminates the time inversions,
and its noise sensitivity is tested by means of Monte Carlo simulations. Finally, the usefulness of the method is demonstrated
on the measurements of the vessel density, in a mangrove tree, Rhizophora mucronata, and the measurement of Mg/Ca ratios, in a bivalve, Mytilus trossulus. 相似文献
26.
The land cover and carbon cycle consequences of large-scale utilizations of biomass as an energy source 总被引：1，自引：0，他引：1
Rik Leemans André van Amstel Coos Battjes Eric Kreileman Sander Toet 《Global Environmental Change》1996,6(4):335-357
The use of modern biomass for energy generation has been considered in many studies as a possible measure for reducing or stabilizing global carbon dioxide (CO2) emissions. In this paper we assess the impacts of large-scale global utilization of biomass on regional and grid scale land cover, greenhouse gas emissions, and carbon cycle. We have implemented in the global environmental change model IMAGE the LESS biomass intensive scenario, which was developed for the Second Assessment Report of IPCC. This scenario illustrates the potential for reducing energy related emission by different sets of fuel mixes and a higher energy efficiency. Our analysis especially covers different consequences involved with such modern biomass scenarios. We emphasize influences of CO2 concentrations and climate change on biomass crop yield, land use, competition between food and biomass crops, and the different interregional trade patterns for modern biomass based energy. Our simulations show that the original LESS scenario is rather optimistic on the land requirements for large-scale biomass plantations. Our simulations show that 797 Mha is required while the original LESS scenario is based on 550 Mha. Such expansion of agricultural land will influence deforestation patterns and have significant consequenses for environmental issues, such as biodiversity. Altering modern biomass requirements and the locations where they are grown in the scenario shows that the outcome is sensitive for regional emissions and feedbacks in the C cycle and that competition between food and modern biomass can be significant. We conclude that the cultivation of large quantities of modern biomass is feasible, but that its effectiveness to reduce emissions of greenhouse gases has to be evaluated in combination with many other environmental land use and socio-economic factors. 相似文献
27.
Daniela Müller Rik Tjallingii Mateusz Płóciennik Tomi P. Luoto Bartosz Kotrys Birgit Plessen Arne Ramisch Markus J. Schwab Mirosław Błaszkiewicz Michał Słowiński Achim Brauer 《Boreas: An International Journal of Quaternary Research》2021,50(2):535-555
The sediment profile from Lake Gościąż in central Poland comprises a continuous, seasonally resolved and exceptionally well-preserved archive of the Younger Dryas (YD) climate variation. This provides a unique opportunity for detailed investigation of lake system responses during periods of rapid climate cooling (YD onset) and warming (YD termination). The new varve record of Lake Gościąż presented here spans 1662 years from the late Allerød (AL) to the early Preboreal (PB). Microscopic varve counting provides an independent chronology with a YD duration of 1149+14/–22 years, which confirms previous results of 1140±40 years. We link stable oxygen isotopes and chironomid-based air temperature reconstructions with the response of various geochemical and varve microfacies proxies especially focusing on the onset and termination of the YD. Cooling at the YD onset lasted ~180 years, which is about a century longer than the terminal warming that was completed in ~70 years. During the AL/YD transition, environmental proxy data lagged the onset of cooling by ~90 years and revealed an increase of lake productivity and internal lake re-suspension as well as slightly higher detrital sediment input. In contrast, rapid warming and environmental changes during the YD/PB transition occurred simultaneously. However, initial changes such as declining diatom deposition and detrital input occurred already a few centuries before the rapid warming at the YD/PB transition. These environmental changes likely reflect a gradual increase in summer air temperatures already during the YD. Our data indicate complex and differing environmental responses to the major climate changes related to the YD, which involve different proxy sensitivities and threshold processes. 相似文献
28.
Retrieval of Suspended Sediment from Advanced Hyperspectral Sensor Data in the Scheldt Estuary at Different Stages in the Tidal Cycle 总被引：1，自引：0，他引：1
Hyperspectral airborne remote sensing images and in-situ data are combined to assess the spatial and temporal sediment dynamics in the tidal Scheldt river. A log-linear empirical relationship has been developed between a near-infrared reflectance difference and total suspended matter. The relationship was shown to be relatively insensitive to the varying cirrus cloud cover occurring during data acquisition. The produced sediment maps show good agreement with known variations of turbidity over the tidal cycle: maximum turbidity around high water, gradual settling of the sediment in the succeeding slack water and resuspension at the onset of the ebb flow stage. 相似文献
29.
Time-Series Reconstruction from Natural Archive Data with the Averaging Effect Taken into Account 总被引：2，自引：2，他引：0
Veerle Beelaerts Fjo De Ridder Nele Schmitz Maite Bauwens Rik Pintelon 《Mathematical Geosciences》2010,42(6):705-722
Environmental information of the past can be obtained by processing and analyzing proxies recorded by environmental archives.
Natural archives are sampled at a distance grid along their accretion axis. Starting from these distance series, a time series
needs to be constructed, because comparison of different data records is only meaningful on a time grid. However, distance–time
relationships are nonlinear as the accretion rate of natural archives is dependent on environmental and physiological factors.
Furthermore, in environmental archives, samples are taken over a volume in distance, rather than over a point in distance.
This implies that the sample-values will be averaged over the volume of the sample. In this paper a method is proposed, which
establishes the nonlinear distance–time relationship and corrects for the averaging effects. The method is built upon the
assumption that the proxy record on a time axis is harmonic. If this is not the case, then a harmonic approximation is made.
As a consequence of the nonlinear distance–time relationship, this harmonic proxy signal is nonlinearly distorted on a distance
axis. As such, a harmonic signal model with a nonlinear phase distortion and an averaging effect is fitted on the data. Since
environmental records are short data records, the statistical performance of the estimator on noisy data is verified by means
of Monte Carlo simulations. The applicability of the method is demonstrated on the measurement of the vessel density, in a
mangrove tree, Rhizophora mucronata, which is an indicator of the rainfall in tropical coastal ecosystems. 相似文献
30.
Recent increases in the accuracy of climate models have enhanced the possibilities for analyzing the impacts of climate change on society. This paper explores how the local, economic impacts of climate change can be modeled for a specific eco-region, the Western Sahel. The people in the Sahel are highly dependent on their natural resource base, and these resources are highly vulnerable to climate change, in particular to changes in rainfall. Climate models project substantial changes in rainfall in the Sahel in the coming 50 years, with most models predicting a reduction in rainfall. To connect climate change to changes in ecosystem productivity and local income, we construct an ecological–economic model that incorporates rangeland dynamics, grazing and livestock prices. The model shows that decreased rainfall in the Sahel will considerably reduce local incomes, in particular if combined with increases in rainfall variability. Adaptation to these climate change projections is possible if reductions in rainfall are followed by destocking to reach efficient grazing levels. However, while such a strategy is optimal from the perspective of society, the stocking rate is determined by individual pastoralists that face few incentives to destock. 相似文献