961 research outputs found
Hydrologic variability and its influence on long‐term peat dynamics
No full textPeatlands are carbon‐rich ecosystems that are extensive in the northern high latitudes where significant 21st century climate changes are expected. In response to climate change, peatlands may become a net source of greenhouse gases, thereby inducing a positive climate feedback effect. In this paper, the impact of precipitation variability and the mean climate state on long‐term peat accumulation is investigated with model simulations. The models couple peat accumulation with the hydrological cycle, which results in peatland bistability, where peatlands may take the physical characteristics from one of two possible alternative stable states. The models consider precipitation as a stochastic forcing variable, temperature‐dependent functions and are parameterized with climatology and peat characteristics to represent the West Siberian Lowlands (WSL) between 55°N and 60°N. Observed WSL peat depths statistically imply bistability. Peatland bistability, however, is eliminated in model simulations with moderate‐to‐large precipitation variability and warmer and wetter climates. This suggests that projected late 21st century climate change would put the thick peatlands in WSL on a transition to thin peatlands. The loss of thick peatlands could significantly increase atmospheric carbon dioxide and provide a positive climate feedback effect. However, the impacts depend on the importance of unaccounted stabilizing factors. The study also shows that precipitation variability induces peatlands to switch between extended periods of accumulation and depletion even if the peatlands are in long‐term equilibrium. Thus, short‐term observations may see only natural fluctuations and new, longer‐term observational strategies are necessary to diagnose if peatlands are undergoing fundamental changes.Rennermalm, Asa K., Nordbotten, Jan M., and Wood, Eric F., "Hydrologic variability and its influence on long‐term peat dynamics." Water Resources Research 46 (Fall 2010), W12546. doi:10.1029/2009WR008242Copyright 2010 by the American Geophysical Union.Peer reviewe
Interannual Variability in Carbon Dioxide Flux from a High Arctic Fen Estimated by Measurements and Modeling
No full textThe response of high arctic ecosystems' carbon dioxide exchange to changing climate is uncertain and may be important from a climate-change perspective. In this study, the net ecosystem carbon dioxide exchange during four growing seasons is examined by combining measurements and modeling from a high arctic fen in northeastern Greenland. The summer-season net ecosystem exchange shows large interannual variations, fluctuating from an uptake of −50 g C m−2 to −123 g C m−2. Through ecosystem modeling, we can observe that leaf area index development and the maximum Rubisco capacity are more important controls on the interannual variability of net ecosystem carbon dioxide exchange than meteorological conditions. Furthermore, we present a hypothesis linking the interannual variability in maximum Rubisco capacity with leaf nitrogen content and leaf area index development. This hypothesis may provide a method to model seasonal net ecosystem carbon dioxide exchange in detail without having to resort to elaborate fitting procedures using measured carbon dioxide flux data.This article was published in Arctic, Antarctic, and Alpine Research (2005), and this Version of Record is archived in RUcore with permission. The published article is available from the Institute of Arctic and Alpine Research (INSTAAR) at: http://instaar.colorado.edu/aaar/journal_issues/abstract.php?id=2353Peer reviewe
Does Sea Ice Influence Greenland Ice Sheet Surface-melt?
No full textRecent decreases in Arctic sea ice and increases in Greenland ice sheet surface-melt may have global impacts, but the interactions between these two processes are unknown. Using microwave satellite data, we explore the spatial and temporal covariance of sea ice extent and ice sheet surface-melt around Greenland from 1979 to 2007. Significant covariance is discovered in several loci in the late summer, with the strongest covariance in western Greenland, particularly in the southwest (Kangerlussuaq). In this region, wind direction patterns and a statistical lag analysis of ice retreat/advance and surface-melt event timings suggest that sea ice extent change is a potential driver of ice sheet melt. Here, late summer wind directions facilitate onshore advection of ocean heat, and enhanced melting on the ice sheet commonly occurs after reductions in offshore sea ice. Hence, this study identifies for the first time the covariability patterns of sea ice and ice sheet melt and suggests that a retreating sea ice margin may enhance melting over the ice sheet.Peer reviewe
Nonbinary Difference:Dionysus, Arianna, and the Fictive Arts of Museum Photography
No full textThis collaborative essay explores a pair of museum photographs depicting the same marble sculpture: a Roman copy of a Greek fourth-century BCE bust of Dionysus, housed at the Capitoline Museums in Rome. The first photograph is an anonymous twentieth-century reproduction – with Arianna printed underneath, crossed out with ballpoint – found by Åsa Johannesson in the Eugenie Strong Collection at the British School at Rome archive. The second is a photograph of the bust made by Johannesson in 2017, taken in response to the archival image. As nonbinary trans people doing artistic research through practice, we explore the assemblage of associations and resonances generated by these images to articulate relations among photography, labeling, gender, museums, and archives. We aim to display complexity and connectivity, rather than to draw firm conclusions. Following Karen Barad, we refer to our approach as a material-discursive method. The interconnections in museum collections – links formed between things by classification and labeling, by grouping and arrangement, by histories and myths – are simultaneously semiotic and material. Mislabeling has material consequences and leaves traces; myths, too, may have tangible effects. These links cannot be reduced to the zero-sum fields of true/false or object/label; instead, they suggest a fictive, contingent mode. With this essay, we are not aiming to discover forgotten truths, nor to place or displace entities within established or novel forms of categorization. Instead, we sketch one method of exploring networks of association in museums to demonstrate the complex and paradoxical operations of what we label nonbinary difference: a generative potential that foregrounds instabilities, rather than a given identity to be represented
Hydrologic controls on coastal suspended sediment plumes around the Greenland Ice Sheet
No full textRising sea levels and increased surface melting of the Greenland ice sheet have heightened the need for direct observations of meltwater release from the ice edge to ocean. Buoyant sediment plumes that develop in fjords downstream of outlet glaciers are controlled by numerous factors, including meltwater runoff. Here, Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery is used to average surface suspended sediment concentration (SSC) in fjords around ∼80% of Greenland from 2000–2009. Spatial and temporal patterns in SSC are compared with positive-degree-days (PDD), a proxy for surface melting, from the Polar MM5 regional climate model. Over this decade significant geographic covariance occurred between ice sheet PDD and fjord SSC, with outlet type (land- vs. marine-terminating glaciers) also important. In general, high SSC is associated with high PDD and/or a high proportion of land-terminating glaciers. Unlike previous site-specific studies of the Watson River plume at Kangerlussuaq, temporal covariance is low, suggesting that plume dimensions best capture interannual runoff dynamics whereas SSC allows assessment of meltwater signals across much broader fjord environments around the ice sheet. Remote sensing of both plume characteristics thus offers a viable approach for observing spatial and temporal patterns of meltwater release from the Greenland ice sheet to the global ocean.Peer reviewe
Proglacial river stage, discharge, and temperature datasets from the Akuliarusiarsuup Kuua River northern tributary, Southwest Greenland, 2008-2011
No full textPressing scientific questions concerning the Greenland ice sheet's climatic sensitivity, hydrology, and contributions to current and future sea level rise require hydrological datasets to resolve. While direct observations of ice sheet meltwater losses can be obtained in terrestrial rivers draining the ice sheet and from lake levels, few such datasets exist. We present a new hydrologic dataset from previously unmonitored sites in the vicinity of Kangerlussuaq, Southwest Greenland. This dataset contains measurements of river stage and discharge for three sites along the Akuliarusiarsuup Kuua (Watson) River's northern tributary, with 30 min temporal resolution between June 2008 and July 2011. Additional data of water temperature, air pressure, and lake stage are also provided. Flow velocity and depth measurements were collected at sites with incised bedrock or structurally reinforced channels to maximize data quality. However, like most proglacial rivers, high turbulence and bedload transport introduce considerable uncertainty to the derived discharge estimates. Eleven propagating error sources were quantified, and reveal that largest uncertainties are associated with flow depth observations. Mean discharge uncertainties (approximately the 68% confidence interval) are two to four times larger (±19% to ±43%) than previously published estimates for Greenland rivers. Despite these uncertainties, this dataset offers a rare collection of direct measurements of ice sheet runoff to the global ocean and is freely available for scientific use at http://dx.doi.org/10.1594/PANGAEA.762818.Peer reviewe
Evidence of Meltwater Retention within the Greenland Ice Sheet
No full textGreenland ice sheet mass losses have increased in recent decades with more than half of these attributed to surface meltwater runoff. However, the magnitudes of englacial storage, firn retention, internal refreezing and other hydrologic processes that delay or reduce true water export to the global ocean remain less understood, partly due to a scarcity of in situ measurements. Here, ice sheet surface meltwater runoff and proglacial river discharge between 2008 and 2010 near Kangerlussuaq, southwestern Greenland were used to establish sub- and englacial meltwater storage for a small ice sheet watershed (36–64 km2). This watershed lacks significant potential meltwater storage in firn, surface lakes on the ice sheet and in the proglacial area, and receives limited proglacial precipitation. Thus, ice sheet surface runoff not accounted for by river discharge can reasonably be attributed to retention in sub- and englacial storage. Evidence for meltwater storage within the ice sheet includes (1) characteristic dampened daily river discharge amplitudes relative to ice sheet runoff; (2) three cold-season river discharge anomalies at times with limited ice sheet surface melt, demonstrating that meltwater may be retained up to 1–6 months; (3) annual ice sheet watershed runoff is not balanced by river discharge, and while near water budget closure is possible as much as 54% of melting season ice sheet runoff may not escape to downstream rivers; (4) even the large meltwater retention estimate (54%) is equivalent to less than 1% of the ice sheet volume, which suggests that storage in en- and subglacial cavities and till is plausible. While this study is the first to provide evidence for meltwater retention and delayed release within the Greenland ice sheet, more information is needed to establish how widespread this is along the Greenland ice sheet perimeter.Peer reviewe
Adaptive Simulated Annealing (ASA): Lessons Learned
No full textAdaptive simulated annealing (ASA) is a global optimization algorithm based on an associated proof that the parameter space can be sampled much more efficiently than by using other previous simulated annealing algorithms. The author's ASA code has been publicly available for over two years. During this time the author has volunteered to help people via e-mail, and the feedback obtained has been used to further develop the code. Some lesson
Some Reflections on Future Organizational Trends of the ASA
No full textThe Collins Report on Future Organizational Trends of the ASA Neglects Three Important Areas: The Distribution of Participation and Membership of Sociologist in the ASA; the Role of Regional Societies; and the Politics of the ASA. This Comment Addresses the Former Two Points, Leaving the Third for the Author to Develop during His Future Tenure as Editor of TAS. © 1991 Springer
Onderzoek naar de morfologie en stabiliteit van ASA bevattende polymeermengsels: de invloed van een component met een zwichtspanning
No full textIn dit verslag is het onderzoek aan mengsels van Polypropeen (PP) met Acrylonitril-Styreen-Acrylester copolymeer (ASA) beschreven. De keuze van de materialen was tweeledig: Ten eerste bevatten beide componenten fysische cross-links, welke de vorming van een zogenaamde bi-continue structuur bevorderen. En ten tweede bezit ASA een zwichtspanning, die in staat moet zijn een gevormde morfologie stabiel te houden. Omdat bleek dat dit systeem zeer incompatibel was, is naast dit mengsel op een aantal punten ook het compatibeler mengsel van ASA met Polycarbonaat (PC) bekeken. De mengsels zijn gemaakt met behulp van een extruder (mengschroef). De mengsels van ASA en PP zijn gekarakteriseerd met behulp van onder andere DSC en DMTA. Er zijn reologische metingen uitgevoerd aan deze materialen met de capillair reometer en het kegel—plaat apparaat. Er is uitvoerig onderzoek verricht naar de morfologie van beide systemen met behulp van de Scanning Elektronen Microscoop en door middel van selectieve extractie. De stabiliteit van deze mengsels is onderzocht door uitvoering van een "anneal"proces. Om de invloed van de zwichtspanning op de stabiliteit nog verder te kunnen vaststellen, zijn opbreekexperimenten uitgevoerd aan ASA draadjes in zowel PP als PCmatrices. Verder zijn de elasticiteitsmoduli bepaald van zowel uitgangsmaterialen (PP en ASA) als mengsels.Applied SciencesTechnologie van Makromolekulaire Stoffe
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