Search Results - (Author, Cooperation:W. Steffen)

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  1. 1
    Staff View
    Publication Date:
    2016-01-09
    Publisher:
    American Association for the Advancement of Science (AAAS)
    Print ISSN:
    0036-8075
    Electronic ISSN:
    1095-9203
    Topics:
    Biology
    Chemistry and Pharmacology
    Computer Science
    Medicine
    Natural Sciences in General
    Physics
    Keywords:
    Aluminum/analysis ; *Biota ; Carbon Cycle ; Climate ; Construction Materials/analysis ; *Earth (Planet) ; Fossil Fuels/adverse effects ; Geologic Sediments/*chemistry ; *Human Activities ; Humans ; Ice/analysis ; Introduced Species ; Plastics/analysis ; Radioactive Fallout/analysis ; Radioisotopes/analysis
    Published by:
    http://www.aaas.org/

    Latest Papers from Table of Contents or Articles in Press
  2. 2
    D. Gerten ; J. Rockstrom ; J. Heinke ; W. Steffen ; K. Richardson ; S. Cornell
    American Association for the Advancement of Science (AAAS)
    Published 2015
    Staff View
    Publication Date:
    2015-06-13
    Publisher:
    American Association for the Advancement of Science (AAAS)
    Print ISSN:
    0036-8075
    Electronic ISSN:
    1095-9203
    Topics:
    Biology
    Chemistry and Pharmacology
    Computer Science
    Medicine
    Natural Sciences in General
    Physics
    Keywords:
    *Biological Evolution ; *Climate Change ; *Earth (Planet) ; Humans ; *Ozone Depletion
    Published by:
    http://www.aaas.org/

    Latest Papers from Table of Contents or Articles in Press
  3. 3
    Staff View
    Publication Date:
    2015-01-17
    Publisher:
    American Association for the Advancement of Science (AAAS)
    Print ISSN:
    0036-8075
    Electronic ISSN:
    1095-9203
    Topics:
    Biology
    Chemistry and Pharmacology
    Computer Science
    Medicine
    Natural Sciences in General
    Physics
    Keywords:
    Atmosphere ; *Biological Evolution ; *Climate Change ; *Earth (Planet) ; Fresh Water ; Humans ; *Ozone Depletion
    Published by:
    http://www.aaas.org/

    Latest Papers from Table of Contents or Articles in Press
  4. 4
    D. Griggs ; M. Stafford-Smith ; O. Gaffney ; J. Rockstrom ; M. C. Ohman ; P. Shyamsundar ; W. Steffen ; G. Glaser ; N. Kanie ; I. Noble
    Nature Publishing Group (NPG)
    Published 2013
    Staff View
    Publication Date:
    2013-03-23
    Publisher:
    Nature Publishing Group (NPG)
    Print ISSN:
    0028-0836
    Electronic ISSN:
    1476-4687
    Topics:
    Biology
    Chemistry and Pharmacology
    Medicine
    Natural Sciences in General
    Physics
    Keywords:
    Animals ; Climate Change ; *Conservation of Natural Resources ; *Economics ; *Environmental Policy ; Humans ; Poverty
    Published by:
    http://www.nature.com/

    Latest Papers from Table of Contents or Articles in Press
  5. 5
    Steffen, W. ; Patkowski, A. ; Meier, G. ; Fischer, E. W.

    College Park, Md. : American Institute of Physics (AIP)
    Published 1992
    Staff View
    ISSN:
    1089-7690
    Source:
    AIP Digital Archive
    Topics:
    Physics
    Chemistry and Pharmacology
    Notes:
    Depolarized Rayleigh spectra of ortho-terphenyl (OTP) were measured in the temperature range from Tg to Tg+190 K. Two samples prepared with and without "clusters'' were used for the measurement. Four different Fabry–Perot interferometers covering the time range from about 0.5 ps to 100 ns were employed. Two relaxation modes were observed: a slow mode with all its characteristics of the α process, and a fast mode with a constant, temperature independent relaxation time of about 3 ps. The fast mode has not been reported heretofore. The intensity of the fast mode vanishes at about the Vogel–Fulcher–Tamman temperature T0. The relaxation times of these two processes are found to be identical for both OTP with and without clusters. The temperature dependence of the relaxation time of the α process is Arrhenius at high temperatures, but shows a Williams–Landel–Ferry (WLF) behavior in the range from Tg to Tg+80 K for both samples with and without the long-range density fluctuations (i.e., cluster).
    Type of Medium:
    Electronic Resource
    URL:
    http://dx.doi.org/10.1063/1.461873

    Articles: DFG German National Licenses
  6. 6
    Floudas, G. ; Steffen, W. ; Fischer, E. W. ; Brown, W.

    College Park, Md. : American Institute of Physics (AIP)
    Published 1993
    Staff View
    ISSN:
    1089-7690
    Source:
    AIP Digital Archive
    Topics:
    Physics
    Chemistry and Pharmacology
    Notes:
    Depolarized Rayleigh scattering (DRS) and dielectric spectroscopy (DS) are employed to study the solvent and polymer dynamics in the polystyrene (PS)/toluene system and for PS concentration up to 0.6 g/ml. The DRS measurements were made in the temperature range from 255 to 373 K with different interferometers making possible the separation of the "fast'' solvent (in the ps time scale) from the "slow'' polymer dynamics (in the ns time scale). The DS measurements were made in the temperature range from 128 to 310 K and two processes could also be observed. With DRS and DS we investigate the very different mobilities in the PS/toluene solutions over a frequency range of ∼10 decades. The prominent features of the experimental data are (i) the existence of two primary (α−) relaxations in the macroscopically homogeneous solutions which have been analyzed using both techniques and (ii) that these relaxations reflect the polymer and solvent dynamics and display the usual Vogel–Fulcher–Tammann behavior, however, with a broad distribution of relaxation times, especially at low temperatures. The existence of two time scales in the solutions is discussed in terms of concentration heterogeneities composed of PS-rich and toluene-rich domains, large enough to exhibit two glass transitions. As a result of the strong temperature dependence for the fast solvent reorientation times the rate of change of solvent friction with polymer concentration becomes temperature dependent at low temperatures.
    Type of Medium:
    Electronic Resource
    URL:
    http://dx.doi.org/10.1063/1.465742

    Articles: DFG German National Licenses
  7. 7
    Köhler, W. ; Fytas, G. ; Steffen, W. ; Reinhardt, L.

    College Park, Md. : American Institute of Physics (AIP)
    Published 1996
    Staff View
    ISSN:
    1089-7690
    Source:
    AIP Digital Archive
    Topics:
    Physics
    Chemistry and Pharmacology
    Notes:
    The decay of a spatial transient temperature grating created within the molecular glass former phenolphthalein-dimethylether (PDE) relates to the hydrodynamic heat diffusion and the dynamics of local density fluctuations. The characteristics of the latter process agree well with the structural α relaxation observed by photon correlation and dielectric spectroscopy near and above the glass transition temperature Tg. When the α-relaxation time approaches the experimental timescale set-in by the thermal diffusivity the specific heat undergoes a dispersion. In either side of this dynamic regime, heat diffusion and α relaxation are independent modes. "Two fluid'' models of the vitreous state conform to the experimental observations. © 1996 American Institute of Physics.
    Type of Medium:
    Electronic Resource
    URL:
    http://dx.doi.org/10.1063/1.470895

    Articles: DFG German National Licenses
  8. 8
    Eckstein, E. ; Qian, J. ; Hentschke, R. ; Thurn-Albrecht, T. ; Steffen, W. ; Fischer, E. W.

    College Park, Md. : American Institute of Physics (AIP)
    Published 2000
    Staff View
    ISSN:
    1089-7690
    Source:
    AIP Digital Archive
    Topics:
    Physics
    Chemistry and Pharmacology
    Notes:
    Wide angle x-ray scattering and classical molecular-dynamics simulations were employed to study structural and dynamic properties of two low-molecular weight glass-forming liquids, propylene carbonate, and salol. We observe pronounced changes in the liquid structure as a function of temperature in a wide temperature range bracketing the glass transition temperature. The experimental and simulation results compare the x-ray scattering from the liquid slightly and far above the experimental glass transition temperature, Tgexp. The simulations largely reproduce the characteristic behavior of the experimental x-ray intensities, which is interpreted in terms of clusterlike heterogeneities forming in the liquid as Tgexp is approached. © 2000 American Institute of Physics.
    Type of Medium:
    Electronic Resource
    URL:
    http://dx.doi.org/10.1063/1.1288907

    Articles: DFG German National Licenses
  9. 9
    Gapinski, J. ; Steffen, W. ; Patkowski, A.

    College Park, Md. : American Institute of Physics (AIP)
    Published 1999
    Staff View
    ISSN:
    1089-7690
    Source:
    AIP Digital Archive
    Topics:
    Physics
    Chemistry and Pharmacology
    Notes:
    A knee-shaped feature observed earlier in light scattering spectra of Ca0.4K0.3(NO3)1.4 (CKN) below Tc is used as a strong argument in favor of mode-coupling theory of the glass transition (MCT). Our careful measurements reveal no "knee" in the spectra of two glass forming liquids, CKN and ortho-terphenyl. Instead of the knee the spectra show nontrivial broadening and an increase of the intensity with a temperature increase. Both variations are confirmed by neutron scattering measurements on CKN and are neither expected in the asymptotic MCT predictions nor in any other model. © 1999 American Institute of Physics.
    Type of Medium:
    Electronic Resource
    URL:
    http://dx.doi.org/10.1063/1.477966

    Articles: DFG German National Licenses
  10. 10
    Patkowski, A. ; Steffen, W. ; Nilgens, H. ; Fischer, E. W.

    College Park, Md. : American Institute of Physics (AIP)
    Published 1997
    Staff View
    ISSN:
    1089-7690
    Source:
    AIP Digital Archive
    Topics:
    Physics
    Chemistry and Pharmacology
    Notes:
    Depolarized dynamic light scattering (DDLS) experiments are performed on the glass forming materials ortho terphenyl (OTP), bis-methyl-methoxy-phenyl-cyclohexane (BMMPC), and bis-methyl-phenyl-cyclohexane (BMPC). Depolarized spectra are obtained for the bulk liquids and for solutions of varying concentration in CCl4. Effective optical anisotropies of the molecules are obtained from the solution spectra. A detailed analysis of the DDLS spectra, as well as a comparison with the results of quasielastic neutron scattering (QENS) experiments, indicates that the DDLS spectra in the bulk liquids result from at least two physical mechanisms: (i) The low frequency part of the spectrum, in the frequency range of the α peak, arises mainly from the molecular reorientation mechanism. The integrated intensity of this contribution is proportional to the square of the effective molecular anisotropy. (ii) At higher frequencies (but still below the Boson peak), the predominant contribution to the DDLS spectrum arises from interaction-induced scattering. The intensity of this contribution increases with increasing temperature (decreasing density) in the way expected for the cancellation effect. The implications of these results for the comparison of DDLS and QENS experiments and for the comparison of the data to the predictions of mode coupling theories of glass forming liquids are discussed. © 1997 American Institute of Physics.
    Type of Medium:
    Electronic Resource
    URL:
    http://dx.doi.org/10.1063/1.474117

    Articles: DFG German National Licenses
  11. 11
    Patkowski, A. ; Steffen, W. ; Nilgens, H. ; Fischer, E. W.

    College Park, Md. : American Institute of Physics (AIP)
    Published 1998
    Staff View
    ISSN:
    1089-7690
    Source:
    AIP Digital Archive
    Topics:
    Physics
    Chemistry and Pharmacology
    Type of Medium:
    Electronic Resource
    URL:
    http://dx.doi.org/10.1063/1.475925

    Articles: DFG German National Licenses
  12. 12
    Kittel, T. G. F. ; Steffen, W. L. ; Chapin, F. S.

    Oxford, UK : Blackwell Publishing Ltd
    Published 2000
    Staff View
    ISSN:
    1365-2486
    Source:
    Blackwell Publishing Journal Backfiles 1879-2005
    Topics:
    Biology
    Energy, Environment Protection, Nuclear Power Engineering
    Geography
    Notes:
    Understanding the distribution and function of Arctic and boreal ecosystems under current conditions and their vulnerability to altered forcing is crucial to our assessment of future global environmental change. Such efforts can be facilitated by the development and application of ecological models that simulate realistic patterns of vegetation change at high latitudes. This paper reviews three classes of ecological models that have been implemented to extrapolate vegetation information in space (e.g. across the Arctic and adjacent domains) and over historical and future periods (e.g. under altered climate and other forcings). These are: (i) equilibrium biogeographical models; (ii) frame-based transient ecosystem models, and (iii) dynamic global vegetation models (DGVMs). The equilibrium response of high-latitude vegetation to scenarios of increased surface air temperatures projected by equilibrium biogeographical models is for tundra to be replaced by a northward shift of boreal woodland and forests. A frame-based model (ALFRESCO) indicates the same directional changes, but illustrates how response time depends on rate of temperature increase and concomitant changes in moisture regime and fire disturbance return period. Key disadvantages of the equilibrium models are that they do not simulate time-dependent responses of vegetation and the role of disturbance is omitted or highly generalized. Disadvantages of the frame-based models are that vegetation type is modelled as a set unit as opposed to an association of individually simulated plant functional types and that the role of ecosystem biogeochemistry in succession is not explicitly considered. DGVMs explicitly model disturbance (e.g. fire), operate on plant functional types, and incorporate constraints of nutrient availability on biomass production in the simulation of vegetation dynamics. Under changing climate, DGVMs detail conversion of tundra to tree-dominated boreal landscapes along with time-dependent responses of biomass, net primary production, and soil organic matter turnover–-which all increase with warming. Key improvements to DGVMs that are needed to portray behaviour of arctic and boreal ecosystems adequately are the inclusion of anaerobic soil processes for inundated landscapes, permafrost dynamics, and moss-lichen layer biogeochemistry, as well as broader explicit accounting of disturbance regimes (including insect outbreaks and land management). Transient simulation of these landscapes can be further tailored to high-latitude processes and issues by spatially interactive, gridded application of arctic/boreal frame-based models and development of dynamic regional vegetation models (DRVMs) utilizing plant functional type schemes that capture the variety of high-latitude environments.
    Type of Medium:
    Electronic Resource
    URL:
    http://dx.doi.org/10.1046/j.1365-2486.2000.06011.x

    Articles: DFG German National Licenses
  13. 13
    Schmidt, Steffen W.

    Oxford, UK : Blackwell Publishing Ltd.
    Published 1988
    Staff View
    ISSN:
    1540-5931
    Source:
    Blackwell Publishing Journal Backfiles 1879-2005
    Topics:
    Ethnic Sciences
    Type of Medium:
    Electronic Resource
    URL:
    http://dx.doi.org/10.1111/j.0022-3840.1988.2201_17.x

    Articles: DFG German National Licenses
  14. 14
    Fuge, H. ; Bastmeyer, M. ; Steffen, W.

    Amsterdam : Elsevier
    Staff View
    ISSN:
    0022-5193
    Source:
    Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics:
    Biology
    Type of Medium:
    Electronic Resource
    URL:
    http://dx.doi.org/10.1016/S0022-5193(85)80199-5

    Articles: DFG German National Licenses
  15. 15
    Staff View
    ISSN:
    0370-2693
    Source:
    Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics:
    Physics
    Type of Medium:
    Electronic Resource
    URL:
    http://linkinghub.elsevier.com/retrieve/pii/0370-2693(82)90987-X

    Articles: DFG German National Licenses
  16. 16
    Staff View
    ISSN:
    0370-2693
    Source:
    Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics:
    Physics
    Type of Medium:
    Electronic Resource
    URL:
    http://linkinghub.elsevier.com/retrieve/pii/0370-2693(84)90447-7

    Articles: DFG German National Licenses
  17. 17
    Staff View
    ISSN:
    0375-9474
    Keywords:
    Nuclear reactions
    Source:
    Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics:
    Physics
    Type of Medium:
    Electronic Resource
    URL:
    http://linkinghub.elsevier.com/retrieve/pii/0375-9474(83)90267-1

    Articles: DFG German National Licenses
  18. 18
    Staff View
    ISSN:
    0375-9474
    Keywords:
    Nuclear reactions
    Source:
    Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics:
    Physics
    Type of Medium:
    Electronic Resource
    URL:
    http://linkinghub.elsevier.com/retrieve/pii/0375-9474(83)90371-8

    Articles: DFG German National Licenses
  19. 19
    Steffen, W. ; Meier, G. ; Patkowski, A. ; Fischer, E.W.

    Amsterdam : Elsevier
    Staff View
    ISSN:
    0378-4371
    Source:
    Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics:
    Physics
    Type of Medium:
    Electronic Resource
    URL:
    http://linkinghub.elsevier.com/retrieve/pii/0378-4371(93)90426-5

    Articles: DFG German National Licenses
  20. 20
    Bohle, D. ; Kuchler, G. ; Richter, A. ; Steffen, W.

    Amsterdam : Elsevier
    Staff View
    ISSN:
    0370-2693
    Source:
    Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics:
    Physics
    Type of Medium:
    Electronic Resource
    URL:
    http://linkinghub.elsevier.com/retrieve/pii/0370-2693(84)90084-4

    Articles: DFG German National Licenses