1,630 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
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
ASA 2019 Election: Aldon Morris
No full textOur coverage of the 2019 ASA election continues with an interview of Presidential candidate Aldon Morris
ASA 2019 Election: Shelley Correll
No full textOur coverage of the 2019 ASA election continues with an interview of Presidential candidate Shelley Correl
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
ASA Election 2019: Jennifer Earl
No full textIntroducing ASA Vice Presidential candidate Jennifer Earl from the University of Arizona
Are Macroeconomic Forecasts Informative? Cointegration Evidence from the ASA-NBER Surveys
No full textWe examine the properties of the ASA-NBER forecasts for several US macroeconomic variables, specifically: (i) are the actual and forecast series integrated of the same order; (ii) are they cointegrated, and; (iii) is the cointegrating vector consistent with long run unitary elasticity of expectations with respect to the actual series. We also examine whether forecasts respond to error correction terms. Tests are applied to both final and preliminary versions of the data. We find that the Treasury bill rate, housing starts, industrial production, inflation and their forecasts are trend stationary. The corporate bond rate, GNP, the GNP deflator, unemployment and their forecasts are difference stationary. About half of the these pairs are cointegrated, with the unitary elasticity restriction seldom rejected. Similar results are obtained when using the originally-reported data.
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