24 research outputs found
If all you have is a hammer, everything looks like a nail: spectral trends as a measure of ecological change in the Arctic
No full textThe Arctic is warming four times faster than lower latitudes. Observations from spatially limited tundra field sites show increased vegetation growth, expansion of woody shrub cover, decreased snow cover and extreme permafrost thaw disturbances. Satellite imagery is fundamental to our understanding of these land-surface changes across the Arctic and, thereby, to our ability to predict feedbacks to global climate. Positive trends in the satellite-derived normalised difference vegetation index (NDVI) have been broadly observed and attributed to increased vegetation productivity, instigating a discourse of Arctic greening, while negative trends, or browning, are less common and broader in attribution. However, methodological issues such as spectral mixing within satellite pixels, the saturation of the NDVI, and the period of the analyses have emerged as a source of significant uncertainty in the detection and ecological interpretation of spectral greening and browning trends. In this thesis, I use high-resolution drone and satellite imagery to assess the effect of snow, fractional vegetation cover, and permafrost thaw slumps on our ability to detect and interpret spectral trends.
Snow cover has decreased in extent and duration across much of the Arctic but is poorly accounted for in spectral trend analyses. By using high-resolution drone imagery from one Arctic and one sub-Arctic site, I found that fine-scale snow persistence within satellite pixels is associated with both reduced magnitude and delayed timing of annual peak NDVI, the base metric of spectral trend analyses. These findings indicate that unaccounted changes in fine-scale snow persistence may contribute to Arctic spectral greening and browning trends through either biotic responses of vegetation to snow cover or abiotic integration of snow within the estimated peak NDVI. Across the Arctic, changing snow persistence may drive both underestimation and overestimation of changes in vegetation productivity.
Fractional vegetation cover corresponds with spectral mixing and the saturation of the NDVI. However, vegetation cover is difficult to calculate at the scale of satellite pixels, and the relationship between vegetation cover and spectral trends therefore remains unknown. I found that spectral Sentinel-2 data can predict vegetation cover at a high-latitude and low-latitude tundra site, and subsequently observed that predicted vegetation cover differed significantly between pixels with and without spectral trends. These results suggest that a pixel’s vegetation cover may affect our ability to detect spectral trends, due to spectral mixing within low vegetation cover pixels and saturation of the NDVI within high vegetation cover pixels. Spatial variation in spectral greening and browning across the Arctic may, in places, reflect underlying patterns in fractional vegetation cover more than the presence or absence of vegetation change.
Permafrost disturbance events are an often-cited source of spectral browning, however, the effect of their timing and subsequent recovery on trend detection has received limited attention. I use a pan-Arctic dataset of retrogressive thaw slumps to examine the representation of permafrost disturbance events in spectral trends derived from Landsat imagery. I found that spectral browning occurred over less than half of the analysed thaw slumps (~49%) due to post-disturbance vegetation recovery and the time period of analysis. Ultimately, this may lead to an underestimation of permafrost disturbance-related change across the Arctic.
Together, my thesis findings demonstrate that spectral trend analyses, although familiar, are a somewhat blunt tool for inferring Arctic vegetation change from satellite imagery. Attributing spectral trends to field observations of ecological change is complicated by a lack of methodological nuance, where unaccounted variation in snow, vegetation cover and dynamic permafrost thaw disturbances may obscure the detection or interpretation of trends. Overall, this thesis highlights three confounding effects on spectral trend analyses that should be considered to improve future assessments of Arctic land-surface change
Data accompanying: "Snow persistence lowers and delays peak NDVI, the vegetation index that underpins Arctic greening analyses"
No full textThis dataset contains multispectral orthomosaics generated from repeat drone imagery captured during a single growing season at one Arctic and one sub-Arctic tundra site. The orthomosaics are used to calculate the persistence of fine scale snow cover in: Hoad et al. (in press) - Snow persistence lowers and delays peak NDVI, the vegetation index that underpins Arctic greening analyses All code to analyse and visualise this data can be found in the following GitHub repository: https://github.com/calumhoad/snowpersistence Data summary: This dataset contains an orthomosaic for each date where a drone survey was conducted at three tundra plots: Blæsedalen (BL), Kluane Low (KL) and Kluane High (KH). The Blæsedalen data were captured using the integrated sensor aboard a DJI Mavic 3 Multispectral and the Kluane data were captured using a SAL Engineering MAIA-S2 sensor. For more information on methdology, please refer to our Environmental Research Letters article and supplementary materials. The imagery for each plot is listed below: Blæsedalen (BL): 2023-07-02 2023-07-12 2023-07-18 2023-07-26 Kluane Low (KL): 2022-06-29 2022-07-05 2022-07-18 2022-08-01 2022-08-14 Kluane High (KH): 2022-07-09 2022-07-19 2022-07-29 2022-08-04 2022-08-13 To replicate analyses from the manuscript: Clone the GitHub repository before downloading this data, then place the contents of each subfolder from the dataset into the following folders of the GitHub repo: For Blæsedalen files: data/uav/orthomosaics/m3m/5cm For Kluane Low files: data/uav/orthomosaics/maia/kluane-low/5cm For Kluane High files: data/uav/orthomosaics/maia/kluane-high/5cm Cite as: Hoad, C., Myers-Smith, I.H., Kerby, J.T., Colesie, C. and Assmann, J.J., (in press). Snow persistence lowers and delays peak NDVI, the vegetation index that underpins Arctic greening analyses. Abstract (from manuscript): Satellite imagery is critical for understanding land-surface change in the rapidly warming Arctic. Since the 1980s, studies have found positive trends in the normalised difference vegetation index (NDVI) derived from satellite imagery over the Arctic—commonly referred to as ‘Arctic greening’ and assumed to represent increased vegetation productivity. However, greening analyses use satellite imagery with pixel sizes ranging from tens to hundreds of metres and do not account for the integration of abiotic phenomena such as snow within vegetation indices. Here, we use high-resolution drone data from one Arctic and one sub-Arctic site to show that fine-scale snow persistence within satellite pixels is associated with both reduced magnitude and delayed timing of annual peak NDVI, the base metric of Arctic greening analyses. We found higher snow persistence within Sentinel-2 pixels is associated with a lower magnitude and later peak NDVI, with a mean difference in NDVI of 0.1 and seven days between high and low snow persistence pixels. These effects were stronger in NASA HLSS30 data, representative of Landsat data commonly used in greening analyses. Our findings indicate that unaccounted changes in fine-scale snow persistence may contribute to Arctic spectral greening and browning trends through either biotic responses of vegetation to snow cover or abiotic integration of snow within the estimated peak NDVI. In order to improve our understanding of Arctic land-surface change, studies should integrate very-high-resolution data to estimate the dynamics of late-season snow within coarser satellite pixels. Acknowledgements (from manuscript): We would like to thank everyone who helped with field data collection in the Canadian Yukon during 2022 and in Greenland during 2023, including Joseph Everest, Erica Zaja, Jiri Subrt, Sian Williams and Mariana García Criado. For help with drones and sensors, particular thanks go to Tom Wade at the University of Edinburgh Airborne Research and Innovation facility, and Alex Merrington, Jack Gillespie, Craig Atkins and Robbie Ramsay at the NERC Field Spectroscopy Facility. Additional thanks to Alan Hobbs, Colin Kay and Graham Mitchell from the NERC Geophysical Equipment Facility. We thank Tim Gyger for support and consultation on our statistical methods, Gwenn Flowers for the time taken to provide climate data for Kluane and Kirsten Schmidt-Pedersen for sharing her extensive knowledge of the people, plants and animals of Qeqertarsuaq, Greenland. Funding for this research was provided by NERC through a SENSE CDT studentship (NE/T00939X/1), the NERC Tundra Time project (NE/W006448/1), a 2023 UK-Greenland Arctic Bursary, a NERC Geophysical Equipment Facility loan (1152), and a NERC Field Spectroscopy Facility loan (891.0111). Additional funding was provided by a Scottish Alliance for GeoScience, Environment and Society (SAGES) small grant scheme award to Calum Hoad. We thank Kluane First Nation and Champagne and Aishihik First Nations for their permission to work on their lands. We gratefully acknowledge the people of Kalaallit Nunaat in general, and of Qeqertarsuaq in particular, for being able to conduct this research on their land. We thank Outpost Research Station and Arctic Station for logistical support. The authors acknowledge constructive comments from two anonymous reviewers, which greatly improved the manuscript. Author contributions (from manuscript): Calum G. Hoad: Conceptualisation (lead); Data curation (lead); Formal analysis (equal); Funding acquisition (equal); Investigation (lead); Methodology (lead); Project administration (lead); Resources (lead); Software (equal); Visualisation (lead); Writing – original draft (lead); Writing – review and editing (lead). Isla H. Myers-Smith: Conceptualisation (supporting); Formal analysis (supporting); Funding acquisition (equal); Methodology (supporting); Resources (supporting); Supervision (lead); Visualisation (supporting); Writing – review and editing (equal). Jeff T. Kerby: Conceptualisation (supporting); Methodology (supporting); Supervision (equal); Visualisation (supporting); Writing – review and editing (equal). Claudia Colesie: Conceptualisation (supporting); Funding acquisition (supporting); Supervision (equal); Project administration (supporting); Resources (supporting); Visualisation (supporting); Writing – review and editing (equal). Jakob J. Assmann: Conceptualisation (supporting); Data curation (supporting); Formal analysis (equal); Investigation (supporting); Methodology (supporting); Supervision (lead); Software (equal); Visualisation (supporting); Writing – original draft (supporting); Writing – review and editing (equal).Hoad, C. G., Myers-Smith, I., Kerby, J., Colesie, C., & Assmann, J. J. (2025). Data accompanying: "Snow persistence lowers and delays peak NDVI, the vegetation index that underpins Arctic greening analyses" [Data set]. Zenodo. https://doi.org/10.5281/zenodo.1481020
Multiple sets of solutions for harmonic elimination PWM bipolar waveforms: Analysis and experimental verification
Get PDFMultiple sets of solutions for the selective harmonic elimination pulse-width modulation method for inverter control exist. These sets present an independent solution to the same problem but further investigation reveals that certain sets may offer an improved overall harmonic performance. In this paper, a minimization method is discussed as a way to obtain these multiple sets of switching angles. A simple distortion harmonic factor that takes into account the first two most significant harmonics present in the generated waveform is considered in order to evaluate the performance of each set. The bipolar waveform is thoroughly analyzed and two cases are considered; single-phase patterns which eliminate all odd harmonics and three-phase counterparts which eliminate only the nontriplen odd harmonics from the line-to-neutral pattern but such harmonics are naturally eliminated from the line-to-line waveform. Experimental results support the theoretical considerations reported in the paper
Public perceptions of the risks of deliberate contamination of the food supply in the United States
No full textThe threat of agricultural terrorism has increasing become the focal point of interest for government, industry, and the public. As such, interviews were conducted with 1,001 American adults to examine their knowledge, attitudes, and perceptions of the risks associated with contamination of the food supply and their likely responses to the threat of deliberate food contamination. Respondents were also randomly assigned to be interviewed about attitudes and behaviors related to one of four contaminants (anthrax, botulism, salmonella, and cyanide) that might be used in a deliberate contamination incident. The results suggest that Americans know little about the food supply chain or about likely points of vulnerability within the system. Yet, more than half say they have thought about terrorism and the food supply. The majority also say that future terrorist attacks on the food and water supply within the United States are likely, and are only somewhat confident in the ability of grocery stores and the federal government to ensure food safety. More than one-fifth believe that deliberate contamination already occurs at least ‘somewhat often’. While perceived risk varied based on the type of contaminant used in a deliberate attack on the food supply, there were no differences in respondents’ affective reactions. However, when asked to predict their behaviors in a deliberate food contamination event, there were differences reported in confidence in the food supply, the amount of food respondents would purchase, and in the length of time it would take for them to resume their pre-contamination buying behaviors.Poster presented December 5, 2005 at the 25th Annual Meeting of the Society for Risk Analysis, Orlando, Florida.Peer reviewe
Fault-tolerant operation of single-phase SR generators
Get PDFThis paper studies fault-tolerant operation of multipole single-phase switched reluctance generators (SRGs), in particular, an 8/8-pole switched reluctance machine. The multipole single-phase SRG system is advantageous for reduced cost and higher efficiency compared to polyphase equivalents. However, using the classical phase-leg topology, a phase fault may prevent generating operation completely, since redundancy in the number of phases does not exist like polyphase systems. A new power converter topology which connects two coil banks in parallel is proposed for higher fault tolerance with minimum additional cost. Faulty coils can be disconnected with the proposed converter and the SRG can continue generating operation after coil faults with reduced output power. Output power per coil current under faults is studied. Open- and short-circuit coils are studied through linear analysis, finite-element analysis and static torque measurement. Generated currents under faults with the proposed converter are measured. The capability of the system to disconnect faulty coils dynamically is also shown
Biography: Agricultural Finance Review
No full textPurpose – This paper aims to provide a “biography” of sorts on Agricultural Finance Review. The paper tracks the evolution of Agricultural Finance Review from its introduction in 1938 to its current status. Design/methodology/approach – The paper is based on a complete review of every paper and every issue. Not all papers were read by the author, but key papers of interest that in one way or another made significant contributions to the study of agricultural finance were reviewed. Findings – The paper shows the evolution of agricultural finance from the early days of reporting financial data in the 1930s and 1940s, to its emergence as a major and significant sub discipline of the general field of agricultural economics. Research limitations/implications – As indicated, not all papers were fully reviewed or read. It is possible that papers identified as “firsts” may have been preceded by other papers. Nonetheless the paper identifies the basic evolutionary path of the journal and defines key points in time when a paradigm shift emerged to change the direction of this discipline. Practical implications – As Agricultural Finance Review transitions from the Department of Applied Economics and Management at Cornell University to Emerald Group Publishing Limited, this “biography” provides readers with a general overview of the journal's and the discipline's historical development. Originality/value – This paper is simply a review of the existing literature found in Agricultural Finance Review.Agriculture, Farms, Finance, Risk Management, Serials
Thromboelastometry and Platelet Function during Acclimatization to High Altitude
Get PDFInteraction between hypoxia and coagulation is important given the increased risk of thrombotic diseases in chronically hypoxic patients who reside at sea level and in residents at high altitude. Hypoxia alters the proteome of platelets favouring a prothrombotic phenotype, but studies of activation and consumption of specific coagulation factors in hypoxic humans have yielded conflicting results. We tested blood from 63 healthy lowland volunteers acclimatizing to high altitude (5,200 m) using thromboelastometry and assays of platelet function to examine the effects of hypoxia on haemostasis. Using data from two separate cohorts of patients following identical ascent profiles, we detected a significant delay in clot formation, but increased clot strength by day 7 at 5,200 m. The latter finding may be accounted for by the significant rise in platelet count and fibrinogen concentration that occurred during acclimatization. Platelet function assays revealed evidence of platelet hyper-reactivity, with shortened PFA-100 closure times and increased platelet aggregation in response to adenosine diphosphate. Post-expedition results were consistent with the normalization of coagulation following descent to sea level. These robust findings indicate that hypoxia increases platelet reactivity and, with the exception of the paradoxical delay in thromboelastometry clotting time, suggest a prothrombotic phenotype at altitude. Further work to elucidate the mechanism of platelet activation in hypoxia will be important and could impact upon the management of patients with acute or chronic hypoxic respiratory diseases who are at risk of thrombotic events.
Erratum to: Thromboelastometry and platelet function during acclimatisation to high altitude (doi: 10.1160/TH17-02-0138) http://eprints.whiterose.ac.uk/129510/
In the Original Article by Rocke et al. “Thromboelastometry and platelet function during acclimatization to high altitude” (Thromb Haemost 2018; 118: 063-071) after publication of the article it has come to the corresponding author's attention that an author was inadvertently omitted from the manuscript. The author, Martin MacInnis, made a significant contribution to: 1. initiating the coagulation research that led to the manuscript, 2. designing the research protocol and performing the initial data analysis, 3. recruiting volunteers, writing applications for ethical approval and making other logistical arrangements that were necessary to complete the study. Martin MacInnis has read and approved the published version of the manuscript. Furthermore, a middle initial was added to the updated list (Shona E. Main) and misspelling of Elizabeth Horn's surname was corrected. The amended author list is as above. https://doi.org
Snow persistence lowers and delays peak NDVI, the vegetation index that underpins Arctic greening analyses
Get PDFSatellite imagery is critical for understanding land-surface change in the rapidly warming Arctic. Since the 1980s, studies have found positive trends in the normalised difference vegetation index (NDVI) derived from satellite imagery over the Arctic—commonly referred to as 'Arctic greening' and assumed to represent increased vegetation productivity. However, greening analyses use satellite imagery with pixel sizes ranging from tens to hundreds of metres and do not account for the integration of abiotic phenomena such as snow within vegetation indices. Here, we use high-resolution drone data from one Arctic and one sub-Arctic site to show that fine-scale snow persistence within satellite pixels is associated with both reduced magnitude and delayed timing of annual peak NDVI, the base metric of Arctic greening analyses. We found higher snow persistence within Sentinel-2 pixels is associated with a lower magnitude and later peak NDVI, with a mean difference in NDVI of 0.1 and seven days between high and low snow persistence pixels. These effects were stronger in NASA HLSS30 data, representative of Landsat data commonly used in greening analyses. Our findings indicate that unaccounted changes in fine-scale snow persistence may contribute to Arctic spectral greening and browning trends through either biotic responses of vegetation to snow cover or abiotic integration of snow within the estimated peak NDVI. In order to improve our understanding of Arctic land-surface change, studies should integrate very-high-resolution data to estimate the dynamics of late-season snow within coarser satellite pixels
The association between life course socioeconomic position and life satisfaction in different welfare states:European comparative study of individuals in early old age
Get PDFBACKGROUND: whether socioeconomic position over the life course influences the wellbeing of older people similarly in different societies is not known.OBJECTIVE: to investigate the magnitude of socioeconomic inequalities in life satisfaction among individuals in early old age and the influence of the welfare state regime on the associations. Design: comparative study using data from Wave 2 and SHARELIFE, the retrospective Wave of the Survey of Health, Ageing, and Retirement in Europe (SHARE), collected during 2006-07 and 2008-09, respectively.SETTING: thirteen European countries representing four welfare regimes (Southern, Scandinavian, Post-communist and Bismarckian).SUBJECTS: a total of 17,697 individuals aged 50-75 years.METHODS: slope indices of inequality (SIIs) were calculated for the association between life course socioeconomic position (measured by the number of books in childhood, education level and current wealth) and life satisfaction. Single level linear regression models stratified by welfare regime and multilevel regression models, containing interaction terms between socioeconomic position and welfare regime type, were calculated.RESULTS: socioeconomic inequalities in life satisfaction were present in all welfare regimes. Educational inequalities in life satisfaction were narrowest in Scandinavian and Bismarckian regimes among both genders. Post-communist and Southern countries experienced both lower life satisfaction and larger socioeconomic inequalities in life satisfaction, using most measures of socioeconomic position. Current wealth was associated with large inequalities in life satisfaction across all regimes.CONCLUSIONS: Scandinavian and Bismarckian countries exhibited narrower socioeconomic inequalities in life satisfaction. This suggests that more generous welfare states help to produce a more equitable distribution of wellbeing among older people.</p
Sympathy as cognitive impairment in Robin Jenkins’s The Cone-Gatherers: the limits of homo sacer
No full textThe Scottish author Robin Jenkins's 1955 novel, The Cone Gatherers, is an allegory of the Nazis' persecution of the disabled that lends itself well to reading in terms of Giorgi Agamben's account of homo sacer. The gamekeeper, Duror, who persecutes the mentally and physically disabled cone-gatherer, Calum McPhie, is analogous to Agamben's medicalised sovereign, who issues biopolitical judgements on who has a life worthy of being lived, and who is a "subhuman" homo sacer. Agamben's suspicion of sacrificial narratives of loss and restitution also resonates with The Cone-Gatherers, a text which subverts the idea that death, disability, and stoic endurance are inevitably redeemed. Jenkins's novel, however, diverges from Agamben's theories by offering a detailed representation of sympathy as a faculty that is regarded in contemporary Western culture as a variety of cognitive impairment (and, indeed, as a form of unmanliness). For Jenkins, the social construction of sympathy as a form of disgusting mental disability is the means by which the political and juristic logic outlined by Agamben can overcome our capacity for universalistic moral relations
