Hu F.S.,University of Illinois at Urbana - Champaign |
Higuera P.E.,University of Illinois at Urbana - Champaign |
Higuera P.E.,University of Idaho |
Walsh J.E.,University of Alaska Fairbanks |
And 5 more authors.
Journal of Geophysical Research: Biogeosciences | Year: 2010
Recent climatic warming has resulted in pronounced environmental changes in the Arctic, including shrub cover expansion and sea ice shrinkage. These changes foreshadow more dramatic impacts that will occur if the warming trend continues. Among the major challenges in anticipating these impacts are "surprises" stemming from changes in system components that have remained relatively stable in the historic record. Tundra burning is potentially one such component. Here we report paleoecological evidence showing that recent tundra burning is unprecedented in the central Alaskan Arctic within the last 5000 years. Analysis of lake sediment cores reveals peak values of charcoal accumulation corresponding to the Anaktuvuk River Fire in 2007, with no evidence of other fire events throughout the past five millennia in that area. Atmospheric reanalysis suggests that the fire was favored by exceptionally warm and dry weather conditions in summer and early autumn. Boosted regression tree modeling shows that such conditions also explain 95% of the interannual variability in tundra area burned throughout Alaska over the past 60 years and that the response of tundra burning to climatic warming is nonlinear. These results contribute to an emerging body of evidence suggesting that tundra ecosystems can burn more frequently under suitable climatic and fuel conditions. The Anaktuvuk River Fire coincides with extreme sea ice retreat, and tundra area burned in Alaska is moderately correlated with sea ice extent from 1979 to 2009 (r = -0.43, p = 0.02). Recurrences of large tundra fires as a result of sea ice disappearance may represent a novel manifestation of coupled marine-terrestrial responses to climatic warming. Copyright 2010 by the American Geophysical Union.
Tolaymat T.M.,U.S. Environmental Protection Agency |
Green R.B.,Waste Management Inc. |
Hater G.R.,Waste Management Inc. |
Barlaz M.A.,North Carolina State University |
And 3 more authors.
Journal of the Air and Waste Management Association | Year: 2010
Prediction of the rate of gas production from bioreactor landfills is important for the optimization of energy recovery and for estimating greenhouse gas emissions. To improve the predictability of gas production, landfill gas (LFG) composition and flow rates were monitored for 4 yr from one conventional and two bioreactor landfill cells at the Outer Loop Landfill in Louisville, KY. The ultimate methane yield (Lo) was estimated from the biochemical methane (CH4) potential of freshly buried refuse and the decay rate constant (k) was estimated from measured CH4collection. The site-specific Lo was estimated to be 48.4 m3-CH 4 wet Mg-1. The estimated decay rate in the conventional cell (0.06 yr-1) was comparable to the AP-42 default value of 0.04 yr-1, whereas estimates for the two bioreactor cells were substantially higher (∼0.11 yr-1). The data document the ability of the bioreactor operation to enhance landfill CH4 generation, although the estimated decay rate is sensitive to the selected Lo. The more rapid decomposition in the bioreactor cells reduces the length of time over which gas will be produced and emphasizes the importance of having a LFG collection system operational once the waste receives added moisture. Copyright 2010 Air & Waste Management Association.
Loescher H.,National Ecological Observatory Network NEON |
Loescher H.,University of Colorado at Boulder |
Ayres E.,National Ecological Observatory Network NEON |
Ayres E.,University of Colorado at Boulder |
And 5 more authors.
PLoS ONE | Year: 2014
Soils are highly variable at many spatial scales, which makes designing studies to accurately estimate the mean value of soil properties across space challenging. The spatial correlation structure is critical to develop robust sampling strategies (e.g., sample size and sample spacing). Current guidelines for designing studies recommend conducting preliminary investigation(s) to characterize this structure, but are rarely followed and sampling designs are often defined by logistics rather than quantitative considerations. The spatial variability of soils was assessed across ∼1 ha at 60 sites. Sites were chosen to represent key US ecosystems as part of a scaling strategy deployed by the National Ecological Observatory Network. We measured soil temperature (Ts) and water content (SWC) because these properties mediate biological/ biogeochemical processes below- and above-ground, and quantified spatial variability using semivariograms to estimate spatial correlation. We developed quantitative guidelines to inform sample size and sample spacing for future soil studies, e.g., 20 samples were sufficient to measure Ts to within 10% of the mean with 90% confidence at every temperate and subtropical site during the growing season, whereas an order of magnitude more samples were needed to meet this accuracy at some high-latitude sites. SWC was significantly more variable than Ts at most sites, resulting in at least 10x more SWC samples needed to meet the same accuracy requirement. Previous studies investigated the relationship between the mean and variability (i.e., sill) of SWC across space at individual sites across time and have often (but not always) observed the variance or standard deviation peaking at intermediate values of SWC and decreasing at low and high SWC. Finally, we quantified how far apart samples must be spaced to be statistically independent. Semivariance structures from 10 of the 12- dominant soil orders across the US were estimated, advancing our continental-scale understanding of soil behavior. © 2014 Loescher et al.
Ringham B.M.,University of Colorado at Denver |
Kreidler S.M.,Neptune and Company |
Muller K.E.,University of Florida |
Glueck D.H.,University of Colorado at Denver
Statistics in Medicine | Year: 2016
Multilevel and longitudinal studies are frequently subject to missing data. For example, biomarker studies for oral cancer may involve multiple assays for each participant. Assays may fail, resulting in missing data values that can be assumed to be missing completely at random. Catellier and Muller proposed a data analytic technique to account for data missing at random in multilevel and longitudinal studies. They suggested modifying the degrees of freedom for both the Hotelling–Lawley trace F statistic and its null case reference distribution. We propose parallel adjustments to approximate power for this multivariate test in studies with missing data. The power approximations use a modified non-central F statistic, which is a function of (i) the expected number of complete cases, (ii) the expected number of non-missing pairs of responses, or (iii) the trimmed sample size, which is the planned sample size reduced by the anticipated proportion of missing data. The accuracy of the method is assessed by comparing the theoretical results to the Monte Carlo simulated power for the Catellier and Muller multivariate test. Over all experimental conditions, the closest approximation to the empirical power of the Catellier and Muller multivariate test is obtained by adjusting power calculations with the expected number of complete cases. The utility of the method is demonstrated with a multivariate power analysis for a hypothetical oral cancer biomarkers study. We describe how to implement the method using standard, commercially available software products and give example code. Copyright © 2015 John Wiley & Sons, Ltd. Copyright © 2015 John Wiley & Sons, Ltd.
Johnson J.L.,University of North Carolina at Chapel Hill |
Kreidler S.M.,Neptune and Company |
Catellier D.J.,Rti International |
Murray D.M.,U.S. National Institutes of Health |
And 2 more authors.
Statistics in Medicine | Year: 2015
We used theoretical and simulation-based approaches to study Type I error rates for one-stage and two-stage analytic methods for cluster-randomized designs. The one-stage approach uses the observed data as outcomes and accounts for within-cluster correlation using a general linear mixed model. The two-stage model uses the cluster specific means as the outcomes in a general linear univariate model. We demonstrate analytically that both one-stage and two-stage models achieve exact Type I error rates when cluster sizes are equal. With unbalanced data, an exact size α test does not exist, and Type I error inflation may occur. Via simulation, we compare the Type I error rates for four one-stage and six two-stage hypothesis testing approaches for unbalanced data. With unbalanced data, the two-stage model, weighted by the inverse of the estimated theoretical variance of the cluster means, and with variance constrained to be positive, provided the best Type I error control for studies having at least six clusters per arm. The one-stage model with Kenward-Roger degrees of freedom and unconstrained variance performed well for studies having at least 14 clusters per arm. The popular analytic method of using a one-stage model with denominator degrees of freedom appropriate for balanced data performed poorly for small sample sizes and low intracluster correlation. Because small sample sizes and low intracluster correlation are common features of cluster-randomized trials, the Kenward-Roger method is the preferred one-stage approach. © 2015 John Wiley & Sons, Ltd.
Allan J.D.,University of Michigan |
Yuan L.L.,U.S. Environmental Protection Agency |
Black P.,Neptune and Company |
Stockton T.,Neptune and Company |
And 3 more authors.
Freshwater Biology | Year: 2012
1.Stream reaches found to be impaired by physical, chemical or biological assessment generally are associated with greater extent of urban and agricultural land uses, and lesser amount of undeveloped lands. However, because stream condition commonly is influenced by multiple stressors as well as underlying natural gradients, it can be difficult to establish mechanistic relationships between altered land use and impaired stream condition. 2.This study explores the use of Bayesian belief networks (BBNs) to model presumed causal relationships between stressors and response variables. A BBN depicts the chain of causal relationships resulting in some outcome such as environmental impairment and can make use of evidence from expert judgment as well as observational and experimental data. 3.Three case studies illustrate the flexibility of BBN models. Expert elicitation in a workshop setting was employed to model the effects of sedimentation on benthic invertebrates. A second example used empirical data to explore the influence of natural and anthropogenic gradients on stream habitat in a highly agricultural watershed. The third application drew on several forms of evidence to develop a decision support tool linking grazing and forestry practices to stream reach condition. 4.Although data limitations challenge model development and our ability to narrow the range of possible outcomes, model formulation forces participants to conceptualise causal mechanisms and consider how to resolve data shortfalls. With sufficient effort and resources, models with greater evidentiary strength from observational and experimental data may become practical tools to guide management decisions. 5.Such models may be used to explore possible outcomes associated with a range of scenarios, thus benefiting management decision-making, and to improve insight into likely causal relationships. © 2011 Blackwell Publishing Ltd.
Lloyd A.H.,Middlebury College |
Duffy P.A.,Neptune and Company |
Mann D.H.,University of Alaska Fairbanks
Canadian Journal of Forest Research | Year: 2013
Ongoing warming at high latitudes is expected to lead to large changes in the structure and function of boreal forests. Our objective in this research is to determine the climatic controls over the growth of white spruce (Picea glauca (Moench) Voss) at the warmest driest margins of its range in interior Alaska. We then use those relationships to determine the climate variables most likely to limit future growth. We collected tree cores from white spruce trees growing on steep, south-facing river bluffs at five sites in interior Alaska, and analyzed the relationship between ring widths and climate using boosted regression trees. Precipitation and temperature of the previous growing season are important controls over growth at most sites: trees grow best in the coolest, wettest years. We identify clear thresholds in growth response to a number of variables, including both temperature and precipitation variables. General circulation model (GCM) projections of future climate in this region suggest that optimum climatic conditions for white spruce growth will become increasingly rare in the future. This is likely to cause short-term declines in productivity and, over the longer term, probably lead to a contraction of white spruce to the cooler, moister parts of its range in Alaska.
Brenner D.,Neptune and Company
Journal of the Air and Waste Management Association | Year: 2010
Most of the published empirical data on indoor air concentrations resulting from vapor intrusion of contaminants from underlying groundwater are for residential structures. The National Aeronautics and Space Administration (NASA) Research Park site, located in Moffett Field, CA, and comprised of 213 acres, is being planned for redevelopment as a collaborative research and educational campus with associated facilities. Groundwater contaminated with hydrocarbon and halogenated hydrocarbon volatile organic compounds (VOCs) is the primary environmental medium of concern at the site. Over a 15-month period, approximately 1000 indoor, outdoor ambient, and outdoor ambient background samples were collected from four buildings designated as historical landmarks using Summa canisters and analyzed by the U.S. Environmental Protection Agency TO-15 selective ion mode. Both 24-hr and sequential 8-hr samples were collected. Comparison of daily sampling results relative to daily background results indicates that the measured trichloroethylene (TCE) concentrations were primarily due to the subsurface vapor intrusion pathway, although there is likely some contribution due to infiltration of TCE from the outdoor ambient background concentrations. Analysis of the cis-1,2-dichloroethylene concentrations relative to TCE concentrations with respect to indoor air concentrations and the background air support this hypothesis; however, this indicates that relative contributions of the vapor intrusion and infiltration pathways vary with each building. Indoor TCE concentrations were also compared with indoor benzene and background benzene concentrations. These data indicate significant correlation between background benzene concentrations and the concentration of benzene in the indoor air, indicating benzene was present in the indoor air primarily through infiltration of outdoor air into the indoor space. By comparison, measured TCE indoor air concentrations showed a significantly different relationship to background concentrations. Analysis of the results show that indoor air samples can be used to definitively define the source of the TCE present in the indoor air space of large industrial buildings. Copyright 2010 Air & Waste Management Association.
Tauxe J.,Neptune and Company
International Journal of Technoethics | Year: 2015
Much of humanity's solid waste will outlast the human race, and the waste generated by one generation must be endured and managed by future societies. Radioactive wastes are unique in that their regulation explicitly considers the protection of future generations. But radioactive waste management faces a serious quandary: how to balance the substantial expense of waste isolation against the uncertain mitigation of risks to hypothetical future humans. Most of this uncertainty stems not from natural processes, or from the projected performance of engineered materials, but rather from social actions and human behaviors. Given that these uncertainties become overwhelming when consider the future only a few centuries from now, how far into the future is it useful for us to attempt to assess risks? Government regulators are currently grappling with this question as they rewrite regulations in order to accommodate radioactive wastes that have the potential for unacceptable and perpetual human health risks. This paper discusses the issues surrounding the period of performance expected from radioactive waste management practices, and outlines central conditions for soundly addressing controversial problems. © 2015, IGI Global.
Mann D.,University of Alaska Fairbanks |
Rupp T.,University of Alaska Fairbanks |
Olson M.,University of Alaska Fairbanks |
Duffy P.,Neptune and Company
Arctic, Antarctic, and Alpine Research | Year: 2012
Many boreal forests grow in regions where climate is now warming rapidly. Changes in these vast, cold forests have the potential to affect global climate because they store huge amounts of carbon and because the relative abundances of their different tree species influence how much solar radiation reflects back to space. Both the carbon cycling and albedo of boreal forests are strongly affected by wildland fires, which in turn are closely controlled by summer climate. Here we use a forest disturbance model in both a retrospective and predictive manner to explore how the forests of Interior Alaska respond to changing climate. Results suggest that a widespread shift from coniferous to deciduous vegetation began around A.D. 1990 and will continue over the next several decades. This ecological regime shift is being driven by old, highly flammable spruce stands encountering a warmer climate conducive to larger and more frequent fires. Increased burning promotes the spread of early successional, deciduous species at the expense of spruce. These striking changes in the vegetation composition and fire regime are predicted to alter the biophysics of Alaska's forests. The ground will warm, and a surge of carbon emission is likely. Our modeling results support previous inferences that Alaska's boreal forest is now shifting to a new ecological state and that positive feedbacks to global warming will accompany this change.