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Search Results: 1 - 10 of 11 matches for " Birgir Guemundsson "
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Cornelius Castoriadis: A Society Adrift. Interviews & Debates 1974-1997 (translated by Helen Arnold; New York: Fordham University Press, 2010)
Birgir Guemundsson
Nordicum-Mediterraneum , 2012,
Abstract: Cornelius Castoriadis (1922-1997) was not only a powerful political thinker, economist, and psychoanalyst. He was also a militant activist, who in the late 1940s founded, along with Claude Lefort, “Socialisme ou Barbarie”, a French revolutionary group that had split from the Trotskyite Fourth International. Castoriadis and “Socialime ou Barbarie” were influential on the far left for two decades, although splits and disagreements among the group's members and followers characterized it.
"Karlson" - A Stasi "Kontakt Person". An episode of Iceland’s Cold War legacy
Birgir Guemundsson,Markus Meckl
Nordicum-Mediterraneum , 2012,
Abstract: Iceland’s geographical position gave this small nation a special strategic importance in the political and military chess game between East and West during the Cold War era. Iceland constituted an important post for the NATO defence forces and surveillance activities. The military base and the NATO alignment created stark divisions among the population. The political discussion was framed in terms of the Cold War and the press in Iceland continuously suggested that the political motives of their opponents were conspicuously linked to or derived from either the interests of Soviet or Eastern European communism or US capitalist imperialism. It was in this circumstances that in the fifties and sixties young left wing people sought to undertake their university education in the Eastern block. The legacy of heated feelings of the Cold War has in many ways survived the Cold War itself. This article wants to contribute to the objectification of the debate by presenting the files from the archives of the East German secret service on one Icelandic student cooperating with it while studying in East Germany.
How to use the Fast Fourier Transform in Large Finite Fields
Petur Birgir Petersen
Mathematics , 2011,
Abstract: The article contents suggestions on how to perform the Fast Fourier Transform over Large Finite Fields. The technique is to use the fact that the multiplicative groups of specific prime fields are surprisingly composite.
The MCMC split sampler: A block Gibbs sampling scheme for latent Gaussian models
óli Páll Geirsson,Birgir Hrafnkelsson,Daniel Simpson,Helgi Sigurearson
Statistics , 2015,
Abstract: A novel computationally efficient Markov chain Monte Carlo (MCMC) scheme for latent Gaussian models (LGMs) is proposed in this paper. The sampling scheme is a two block Gibbs sampling scheme designed to exploit the model structure of LGMs. We refer to the proposed sampling scheme as the MCMC split sampler. The principle idea behind the MCMC split sampler is to split the latent Gaussian parameters into two vectors. The former vector consists of latent parameters which appear in the data density function, while the latter vector consists of latent parameters which do not appear in it. The former vector is placed in the first block of the proposed sampling scheme and the latter vector is placed in the second block along with any potential hyperparameters. The resulting conditional posterior density functions within the blocks allow the MCMC split sampler to handle, by design, LGMs with latent models imposed on more than just the mean structure of the data density function. The MCMC split sampler is also designed to be applicable for any choice of a parametric data density function. Moreover, it scales well in terms of computational efficiency when the dimension of the latent model increase.
Computationally efficient spatial modeling of annual maximum 24 hour precipitation. An application to data from Iceland
óli Páll Geirsson,Birgir Hrafnkelsson,Daniel Simpson
Statistics , 2014, DOI: 10.1002/env.2343
Abstract: We propose a computationally efficient statistical method to obtain distributional properties of annual maximum 24 hour precipitation on a 1 km by 1 km regular grid over Iceland. A latent Gaussian model is built which takes into account observations, spatial variations and outputs from a local meteorological model. A covariate based on the meteorological model is constructed at each observational site and each grid point in order to assimilate available scientific knowledge about precipitation into the statistical model. The model is applied to two data sets on extreme precipitation, one uncorrected data set and one data set that is corrected for phase and wind. The observations are assumed to follow the generalized extreme value distribution. At the latent level, we implement SPDE spatial models for both the location and scale parameters of the likelihood. An efficient MCMC sampler which exploits the model structure is constructed, which yields fast continuous spatial predictions for spatially varying model parameters and quantiles.
Daptomycin exposure precedes infection and/or colonization with daptomycin non-susceptible enterococcus
Jeremy C Storm, Daniel J Diekema, Jennifer S Kroeger, Sarah J Johnson, Birgir Johannsson
Antimicrobial Resistance and Infection Control , 2012, DOI: 10.1186/2047-2994-1-19
Abstract: The study is a retrospective case-series involving all patients with DNSE infection and/or colonization at UIHC, a 734-bed academic referral center, from June 1, 2005 to June 1, 2011.The majority of patients with DNSE colonization and/or infection had prior daptomycin exposure (15 of 25; 60%), a concomitant gastrointestinal process (19 of 25; 76%), or were immunosuppressed (21 of 25; 84%). DNSE infection was confirmed in 17 of 25 (68%) patients, including 9 patients with bacteremia. Twelve of 17 (71%) patients with DNSE infection had prior daptomycin exposure, including 7 of 9 (78%) patients with bacteremia. Compared to patients without prior daptomycin exposure, patients with prior daptomycin exposure were less likely to harbor E. faecalis (0% vs. 33%; p?=?0.019). A high proportion of patients (10 of 25; 40%) died during their hospitalizations. Most enterococcal isolates were E. faecium (86%), and were vancomycin-resistant (72%). Molecular typing revealed a diverse population of DNSE.Prior daptomycin exposure, immunosuppression, and/or a concomitant gastrointestinal process, may be associated with the development of DNSE. PFGE revealed a diverse population of DNSE, which along with both increasing numbers of DNSE detected yearly and increasing annual rates of daptomycin usage, suggests the emergence of DNSE under antimicrobial pressure.
Greenhouse gas balances of managed peatlands in the Nordic countries – present knowledge and gaps
M. Maljanen, B. D. Sigurdsson, J. Guemundsson, H. óskarsson, J. T. Huttunen ,P. J. Martikainen
Biogeosciences (BG) & Discussions (BGD) , 2010,
Abstract: This article provides an overview of the effects of land-use on the fluxes of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) and from peatlands in the Nordic countries based on the field data from about 100 studies. In addition, this review aims to identify the gaps in the present knowledge on the greenhouse gas (GHG) balances associated with the land-use of these northern ecosystems. Northern peatlands have accumulated, as peat, a vast amount of carbon from the atmosphere since the last glaciation. However, the past land-use and present climate have evidently changed their GHG balance. Unmanaged boreal peatlands may act as net sources or sinks for CO2 and CH4 depending on the weather conditions. Drainage for agriculture has turned peatlands to significant sources of GHGs (mainly N2O and CO2). Annual mean GHG balances including net CH4, N2O and CO2 emissions are 2260, 2280 and 3140 g CO2 eq. m 2 (calculated using 100 year time horizon) for areas drained for grass swards, cereals or those left fallow, respectively. Even after cessetion of the cultivation practices, N2O and CO2 emissions remain high. The mean net GHG emissions in abandoned and afforested agricultural peatlands have been 1580 and 500 g CO2 eq. m 2, respectively. Peat extraction sites are net sources of GHGs with an average emission rate of 770 g CO2 eq. m 2. Cultivation of a perennial grass (e.g., reed canary grass) on an abandoned peat extraction site has been shown to convert such a site into a net sink of GHGs ( 330 g CO2 eq. m 2). In contrast, despite restoration, such sites are known to emit GHGs (mean source of 480 g CO2 eq. m 2, mostly from high CH4 emissions). Peatland forests, originally drained for forestry, may act as net sinks (mean 780 g CO2 eq. m 2). However, the studies where all three GHGs have been measured at an ecosystem level in the forested peatlands are lacking. The data for restored peatland forests (clear cut and rewetted) indicate that such sites are on average a net sink (190 g CO2 eq. m 2). The mean emissions from drained peatlands presented here do not include emissions from ditches which form a part of the drainage network and can contribute significantly to the total GHG budget. Peat soils submerged under water reservoirs have acted as sources of CO2, CH4 and N2O (mean annual emission 240 g CO2 eq. m 2). However, we cannot yet predict accurately the overall greenhouse gas fluxes of organic soils based on the site characteristics and land-use practices alone because the data on many land-use options and our understanding of the biogeochemical cycling associated with the gas fluxes are limited.
Modelling the 20th and 21st century evolution of Hoffellsj kull glacier, SE-Vatnaj kull, Iceland
G. Aealgeirsdóttir,S. Guemundsson,H. Bj?rnsson,F. Pálsson
The Cryosphere , 2011, DOI: 10.5194/tc-5-961-2011
Abstract: The Little Ice Age maximum extent of glaciers in Iceland was reached about 1890 AD and most glaciers in the country have retreated during the 20th century. A model for the surface mass balance and the flow of glaciers is used to reconstruct the 20th century retreat history of Hoffellsj kull, a south-flowing outlet glacier of the ice cap Vatnaj kull, which is located close to the southeastern coast of Iceland. The bedrock topography was surveyed with radio-echo soundings in 2001. A wealth of data are available to force and constrain the model, e.g. surface elevation maps from ~1890, 1936, 1946, 1989, 2001, 2008 and 2010, mass balance observations conducted in 1936–1938 and after 2001, energy balance measurements after 2001, and glacier surface velocity derived by kinematic and differential GPS surveys and correlation of SPOT5 images. The approximately 20% volume loss of this glacier in the period 1895–2010 is realistically simulated with the model. After calibration of the model with past observations, it is used to simulate the future response of the glacier during the 21st century. The mass balance model was forced with an ensemble of temperature and precipitation scenarios derived from 10 global and 3 regional climate model simulations using the A1B emission scenario. If the average climate of 2000–2009 is maintained into the future, the volume of the glacier is projected to be reduced by 30% with respect to the present at the end of this century. If the climate warms, as suggested by most of the climate change scenarios, the model projects this glacier to almost disappear by the end of the 21st century. Runoff from the glacier is predicted to increase for the next 30–40 yr and decrease after that as a consequence of the diminishing ice-covered area.
Modelling the 20th and 21st century evolution of Hoffellsj kull glacier, SE-Vatnaj kull, Iceland
G. Aealgeirsdóttir,S. Guemundsson,H. Bj?rnsson,F. Pálsson
The Cryosphere Discussions , 2011, DOI: 10.5194/tcd-5-1055-2011
Abstract: The Little Ice Age maximum extent of glaciers in Iceland was reached about 1890 AD and most glaciers in the country have retreated during the 20th century. A model for the surface mass balance and the flow of glaciers is used to reconstruct the 20th century retreat history of Hoffellsj kull, a south-flowing outlet glacier of Vatnaj kull, which is located close to the southeast coast of Iceland. The bedrock topography was surveyed with radio-echo soundings in 2001. A wealth of data are available to force and constrain the model, e.g. surface elevation maps from ~1890, 1936, 1946, 1986, 2001, 2008 and 2010, mass balance observations conducted in 1936–1938 and after 2001, energy balance measurements after 2001, and glacier surface velocity derived by DGPS and correlation of SPOT5 images. The 21% volume loss of this glacier in the period 1895–2010 is realistically simulated with the model. After calibration of the model with past observations, it is used to simulate the future response of the glacier during the 21st century. The mass balance model was forced with an ensemble of temperature and precipitation scenarios from a study of the effect of climate change on energy production in the Nordic countries (the CES project). If the average climate of 2000–2009 is maintained into the future, the volume of the glacier is projected to be reduced by 30% with respect to the present at the end of this century, and the glacier will almost disappear if the climate warms as suggested by most of the climate change scenarios. Runoff from the glacier is predicted to increase for the next 30–40 years and decrease after that as a consequence of the diminishing ice-covered area.
Long-period seismic events with strikingly regular temporal patterns on Katla volcano's south flank (Iceland)
Giulia Sgattoni,Zeinab Jeddi,ólafur Guemundsson,Páll Einarsson,Ari Tryggvason,Bj?rn Lund,Federico Lucchi
Physics , 2015,
Abstract: Katla is a threatening volcano in Iceland, partly covered by the M\'yrdalsj\"okull ice cap. The volcano has a large caldera with several active geothermal areas. A peculiar cluster of long-period seismic events started on Katla's south flank in July 2011, during an unrest episode in the caldera that culminated in a glacier outburst. The seismic events were tightly clustered at shallow depth in the Gvendarfell area, 4 km south of the caldera, under a small glacier stream on the southern margin of M\'yrdalsj\"okull. No seismic events were known to have occurred in this area before. The most striking feature of this seismic cluster is its temporal pattern, characterized by regular intervals between repeating seismic events, modulated by a seasonal variation. Remarkable is also the stability of both the time and waveform features over a long time period, around 3.5 years. No comparable examples have been found in the literature. Both volcanic and glacial processes can produce similar waveforms and therefore have to be considered as potential seismic sources. Discerning between these two causes is critical for monitoring glacier-clad volcanoes and has been controversial at Katla. For this new seismic cluster on the south flank we regard volcano-related processes as more likely than glacial ones for the following reasons: 1) the seismic activity started during an unrest episode involving sudden melting of the glacier and a j\"okulhlaup; 2) the glacier stream is small and stagnant; 3) the seismicity remains regular and stable for years; 4) there is no apparent correlation with short-term weather changes, such as rain storms. We suggest that a small, shallow hydrothermal system was activated on Katla's south flank in 2011, either by a minor magmatic injection or by changes of permeability in a local crack system.
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