Noel Gorelick

Noel Gorelick

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    Exploiting satellite observations for global surface albedo trends monitoring
    Nektarios Chrysoulakis
    Zina Mitraka
    Theoretical and Applied Climatology, 137(2019), pp. 1-2
    Preview abstract Surface albedo is one of the essential climate variables as it influences the radiation budget and the energy balance. Because it is used in a variety of scientific fields, from local to global scale, spatially and temporally disaggregated albedo data are required, which can be derived from satellites. Satellite observations have led to directional-hemispherical (black-sky) and bi-hemispherical (white-sky) albedo products, but time series of high spatial resolution true (blue-sky) albedo estimations at global level are not available. Here, we exploit the capabilities of Google Earth Engine (GEE) for big data analysis to derive global snow-free land surface albedo estimations and trends at a 500-m scale, using satellite observations from 2000 to 2015. Our study reveals negative albedo trends mainly in Mediterranean, India, south-western Africa and Eastern Australia, whereas positive trends mainly in Ukraine, South Russia and Eastern Kazakhstan, Eastern Asia, Brazil, Central and Eastern Africa and Central Australia. The bulk of these trends can be attributed to rainfall, changes in agricultural practices and snow cover duration. Our study also confirms that at local scale, albedo changes are consistent with land cover/use changes that are driven by anthropogenic activities. View details
    Inland surface waters in protected areas globally: Current coverage and 30-year trends
    Lucy Bastin
    Santiago Saura
    Bastian Bertzky
    Grégoire Dubois
    Marie-Josée Fortin
    Jean-Francois Pekel
    PloS one, 14(2019), e0210496
    Preview abstract Inland waters are unique ecosystems offering services and habitat resources upon which many species depend. Despite the importance of, and threats to, inland water, global assessments of protected area (PA) coverage and trends have focused on land habitats or have assessed land and inland waters together. We here provide the first assessment of the level of protection of inland open surface waters and their trends (1984–2015) within PAs for all countries, using a globally consistent, high-resolution (30 m) and validated dataset on permanent and seasonal surface waters based on Landsat images. Globally, 15% of inland surface waters are covered by PAs with mapped boundaries. Estimated inland water protection increases to 16.4% if PAs with reported area but delineated only as points are included as circular buffers. These coverage estimates slightly exceed the comparable figure for land but fall below the 17% goal of the Convention on Biological Diversity’s Aichi Target 11 for 2020. Protection levels are very uneven across countries, half of which do not yet meet the 17% target. The lowest coverage of surface water by PAs (<5%) was found in Africa and in parts of Asia. There was a global trend of permanent water losses and seasonal water gains within PAs, concomitant with an increase of both water types outside PAs. In 38% of countries, PAs lost over 5% of permanent water. Global protection targets for inland waters may well be met by 2020, but much stronger efforts are required to ensure their effective conservation, which will depend not only on sound PA governance and management but also on the sustainable use of water resources outside PAs. Given the pressures on water in a rapidly changing world, integrated management planning of water resources involving multiple sectors and entire basins is therefore necessary. View details
    A LandTrendr multispectral ensemble for forest disturbance detection
    Warren B Cohen
    Zhiqiang Yang
    Sean P Healey
    Robert E Kennedy
    Remote sensing of environment, 205(2018), pp. 131-140
    Preview abstract Monitoring and classifying forest disturbance using Landsat time series has improved greatly over the past decade, with many new algorithms taking advantage of the high-quality, cost free data in the archive. Much of the innovation has been focused on use of sophisticated workflows that consist of a logical sequence of processes and rules, multiple statistical functions, and parameter sets that must be calibrated to accurately classify disturbance. For many algorithms, calibration has been local to areas of interest and the algorithm's classification performance has been good under those circumstances. When applied elsewhere, however, algorithm performance has suffered. An alternative strategy for calibration may be to use the locally tested parameter values in conjunction with a statistical approach (e.g., Random Forests; RF) to align algorithm classification with a reference disturbance dataset, a process we call secondary classification. We tested that strategy here using RF with LandTrendr, an algorithm that runs on one spectral band or index. Disturbance detection using secondary classification was spectral band- or index-dependent, with each spectral dimension providing some unique detections and different error rates. Using secondary classification, we tested whether an integrated multispectral LandTrendr ensemble, with various combinations of the six basic Landsat reflectance bands and seven common spectral indices, improves algorithm performance. Results indicated a substantial reduction in errors relative to secondary classification based on single bands/indices, revealing the importance of a multispectral approach to forest disturbance detection. To explain the importance of specific bands and spectral indices in the multispectral ensemble, we developed a disturbance signal-to-noise metric that clearly highlighted the value of shortwave-infrared reflectance, especially when paired with near-infrared reflectance. View details
    Global Assessment of Supraglacial Debris‐Cover Extents
    Dirk Scherler
    Hendrik Wulf
    Geophysical Research Letters, 45(2018), 11,798-11,805
    Preview abstract Rocky debris on glacier surfaces influences ice melt rates and the response of glaciers to climate change. However, scarce data on the extent and evolution of supraglacial debris cover have so far limited its inclusion in regional to global glacier models. Here we present global data sets of supraglacial debris‐cover extents, based on Landsat 8 and Sentinel‐2 optical satellite imagery. We find that about 4.4% (~26,000 km2) of all glacier areas (excluding the Greenland ice sheet and Antarctica) are covered with debris, but that the distribution is heterogeneous. The largest debris‐covered areas are located in high‐mountain ranges, away from the poles. At a global scale, we find a negative scaling relationship between glacier size and percentage of debris. Therefore, the influence of debris cover on glacier mass balances is expected to increase in the future, as glaciers continue to shrink. View details
    Where we live—A summary of the achievements and planned evolution of the global urban footprint
    Thomas Esch
    Felix Bachofer
    Wieke Heldens
    Andreas Hirner
    Mattia Marconcini
    Daniela Palacios-Lopez
    Achim Roth
    Soner Üreyen
    Julian Zeidler
    Stefan Dech
    Remote Sensing, 10(2018), pp. 895
    Preview abstract The TerraSAR-X (TSX) mission provides a distinguished collection of high resolution satellite images that shows great promise for a global monitoring of human settlements. Hence, the German Aerospace Center (DLR) has developed the Urban Footprint Processor (UFP) that represents an operational framework for the mapping of built-up areas based on a mass processing and analysis of TSX imagery. The UFP includes functionalities for data management, feature extraction, unsupervised classification, mosaicking, and post-editing. Based on> 180.000 TSX StripMap scenes, the UFP was used in 2016 to derive a global map of human presence on Earth in a so far unique spatial resolution of 12 m per grid cell: the Global Urban Footprint (GUF). This work provides a comprehensive summary of the major achievements related to the Global Urban Footprint initiative, with dedicated sections focusing on aspects such as UFP methodology, basic product characteristics (specification, accuracy, global figures on urbanization derived from GUF), the user community, and the already initiated future roadmap of follow-on activities and products. The active community of> 250 institutions already working with the GUF data documents the relevance and suitability of the GUF initiative and the underlying high-resolution SAR imagery with respect to the provision of key information on the human presence on earth and the global human settlements properties and patterns, respectively. View details
    Mapping forest change using stacked generalization: An ensemble approach
    Sean P Healey
    Warren B Cohen
    Zhiqiang Yang
    C Kenneth Brewer
    Evan B Brooks
    Alexander J Hernandez
    Chengquan Huang
    M Joseph Hughes
    Robert E Kennedy
    Thomas R Loveland
    Gretchen G Moisen
    Todd A Schroeder
    Stephen V Stehman
    James E Vogelmann
    Curtis E Woodcock
    Limin Yang
    Zhe Zhu
    Remote Sensing of Environment, 204(2018), pp. 717-728
    Preview abstract The ever-increasing volume and accessibility of remote sensing data has spawned many alternative approaches for mapping important environmental features and processes. For example, there are several viable but highly varied strategies for using time series of Landsat imagery to detect changes in forest cover. Performance among algorithms varies across complex natural systems, and it is reasonable to ask if aggregating the strengths of an ensemble of classifiers might result in increased overall accuracy. Relatively simple rules have been used in the past to aggregate classifications among remotely sensed maps (e.g. using majority predictions), and in other fields, empirical models have been used to create situationally specific algorithm weights. The latter process, called “stacked generalization” (or “stacking”), typically uses a parametric model for the fusion of algorithm outputs. We tested the performance of several leading forest disturbance detection algorithms against ensembles of the outputs of those same algorithms based upon stacking using both parametric and Random Forests-based fusion rules. Stacking using a Random Forests model cut omission and commission error rates in half in many cases in relation to individual change detection algorithms, and cut error rates by one quarter compared to more conventional parametric stacking. Stacking also offers two auxiliary benefits: alignment of outputs to the precise definitions built into a particular set of empirical calibration data; and, outputs which may be adjusted such that map class totals match independent estimates of change in each year. In general, ensemble predictions improve when new inputs are added that are both informative and uncorrelated with existing ensemble components. As increased use of cloud-based computing makes ensemble mapping methods more accessible, the most useful new algorithms may be those that specialize in providing spectral, temporal, or thematic information not already available through members of existing ensembles. View details
    Implementation of the LandTrendr Algorithm on Google Earth Engine
    Robert Kennedy
    Zhiqiang Yang
    Justin Braaten
    Lucas Cavalcante
    Warren Cohen
    Sean Healey
    Remote Sensing, 10(2018), pp. 681
    Preview abstract The LandTrendr (LT) algorithm has been used widely for analysis of change in Landsat spectral time series data, but requires significant pre-processing, data management, and computational resources, and is only accessible to the community in a proprietary programming language (IDL). Here, we introduce LT for the Google Earth Engine (GEE) platform. The GEE platform simplifies pre-processing steps, allowing focus on the translation of the core temporal segmentation algorithm. Temporal segmentation involved a series of repeated random access calls to each pixel’s time series, resulting in a set of breakpoints (“vertices”) that bound straight-line segments. The translation of the algorithm into GEE included both transliteration and code analysis, resulting in improvement and logic error fixes. At six study areas representing diverse land cover types across the US, we conducted a direct comparison of the new LT-GEE code against the heritage code (LT-IDL). The algorithms agreed in most cases, and where disagreements occurred, they were largely attributable to logic error fixes in the code translation process. The practical impact of these changes is minimal, as shown by an example of forest disturbance mapping. We conclude that the LT-GEE algorithm represents a faithful translation of the LT code into a platform easily accessible by the broader user community. View details
    Google Earth Engine: Planetary-Scale Geospatial Analysis for Everyone
    Matt Hancher
    Mike Dixon
    Simon Ilyushchenko
    David Thau
    Remote Sensing of Environment, 202(2017), pp. 18-27
    Preview abstract Google Earth Engine is a cloud-based platform for planetary-scale geospatial analysis that brings Google's massive computational capabilities to bear on a variety of high-impact societal issues including deforestation, drought, disaster, disease, food security, water management, climate monitoring and environmental protection. It is unique in the field as an integrated platform designed to empower not only traditional remote sensing scientists, but also a much wider audience that lacks the technical capacity needed to utilize traditional supercomputers or large-scale commodity cloud computing resources. View details
    High-resolution mapping of global surface water and its long-term changes
    Jean-François Pekel
    Andrew Cottam
    Alan S Belward
    Nature, 540(2016), pp. 418-422
    Preview abstract The location and persistence of surface water (inland and coastal) is both affected by climate and human activity and affects climate, biological diversity4 and human wellbeing. Global data sets documenting surface water location and seasonality have been produced from inventories and national descriptions, statistical extrapolation of regional data and satellite imagery, but measuring long-term changes at high resolution remains a challenge. Here, using three million Landsat satellite images, we quantify changes in global surface water over the past 32 years at 30-metre resolution. We record the months and years when water was present, where occurrence changed and what form changes took in terms of seasonality and persistence. Between 1984 and 2015 permanent surface water has disappeared from an area of almost 90,000 square kilometres, roughly equivalent to that of Lake Superior, though new permanent bodies of surface water covering 184,000 square kilometres have formed elsewhere. All continental regions show a net increase in permanent water, except Oceania, which has a fractional (one per cent) net loss. Much of the increase is from reservoir filling, although climate change is also implicated. Loss is more geographically concentrated than gain. Over 70 per cent of global net permanent water loss occurred in the Middle East and Central Asia, linked to drought and human actions including river diversion or damming and unregulated withdrawal. Losses in Australia and the USA linked to long-term droughts are also evident. This globally consistent, validated data set shows that impacts of climate change and climate oscillations on surface water occurrence can be measured and that evidence can be gathered to show how surface water is altered by human activities. We anticipate that this freely available data will improve the modelling of surface forcing, provide evidence of state and change in wetland ecotones (the transition areas between biomes), and inform water-management decision-making. View details
    Earth's surface water change over the past 30 years.
    Gennadii Donchyts
    Baart Fedor
    Hessel Winsemius,
    Jaap Kwadijk
    Nick van de Giesen
    Nature Climate Change, 6(2016), pp. 810
    Preview abstract Earth's surface gained 115,000 km2 of water and 173,000 km2 of land over the past 30 years, including 20,135 km2 of water and 33,700 km2 of land in coastal areas. Here, we analyse the gains and losses through the Deltares Aqua Monitor — an open tool that detects land and water changes around the globe. View details
    APOSTLE: Longterm Transit Monitoring and Stability Analysis of XO-2b
    Praveen Kundurthy
    Rory Barnes
    Andrew C Becker
    Eric Agol
    Benjamin F Williams
    Amy Rose
    The Astrophysical Journal, 770(2013), pp. 36
    Preview abstract The Apache Point Survey of Transit Lightcurves of Exoplanets (APOSTLE) observed 10 transits of XO-2b over a period of 3 yr. We present measurements that confirm previous estimates of system parameters like the normalized semi-major axis (a/R), stellar density (ρ), impact parameter (b), and orbital inclination (iorb). Our errors on system parameters like a/R and ρ have improved by ∼40% compared to previous best ground-based measurements. Our study of the transit times show no evidence for transit timing variations (TTVs) and we are able to rule out co-planar companions with masses 0.20 M⊕ in low order mean motion resonance with XO-2b. We also explored the stability of the XO-2 system given various orbital configurations of a hypothetical planet near the 2:1 mean motion resonance. We find that a wide range of orbits (including Earth-mass perturbers) are both dynamically stable and produce observable TTVs. We find that up to 51% of our stable simulations show TTVs that are smaller than the typical transit timing errors (∼20 s) measured for XO-2b, and hence remain undetectable. Key View details
    Secular Behavior of Exoplanetary Systems: Self-Consistency and Comparisons With The Planet-Planet Scattering Hypothesis
    Miles Timpe
    Rory Barnes
    Ravikumar Kopparapu
    Sean N. Raymond
    Richard Greenberg
    The Astrophysics Journal, 146(2011), pp. 53
    Preview abstract Planet-planet scattering has been suggested as a mechanism to explain the disproportionate number of planet-planet pairs found to lie on or near an apsidal separatrix, in which one planet's eccentricity periodically drops to near-zero. We present the results of numerical simulations of 2-planet systems having arisen from dynamically unstable 3-planet systems. We show that the distribution of near-separatrix systems arising after an instability is consistent with the observed systems, further strengthening the planet-planet scattering hypothesis. We also note that many observed systems have been found near their extreme eccentricity values. Such a pattern may suggest a bias in exoplanet observations, as planets should have an equal probability of being discovered at any point in their secular cycle. We test this possibility by numerically integrating known multiplanet systems and determining the relative time each planet spends in a given eccentricity range and then comparing this distribution of eccentricity values to the observational uncertainty. We find that planets tend to spend more time near their minimum and maximum values as they represent turning points in the oscillations. Moreover, the uncertainties for many eccentricities are so large that we cannot make strong statements regarding the possibility that planets are being discovered at their extreme eccentricities too often. However, as uncertainties become smaller and more multiplanet systems are discovered, this potential bias should be revisited. View details
    Planet-Planet Scattering Leads to Tightly Packed Planetary Systems.
    Sean N. Raymond
    Rory Barnes
    Dimitri Veras
    Philip J. Armitage
    Richard Greenberg
    The Astrophysical Journal, Letters, 696(2011), pp. 98-101
    Preview abstract The known extrasolar multiple-planet systems share a surprising dynamical attribute: they cluster just beyond the Hill stability boundary. Here we show that the planet-planet scattering model, which naturally explains the observed exoplanet eccentricity distribution, can reproduce the observed distribution of dynamical configurations. We calculated how each of our scattered systems would appear over an appropriate range of viewing geometries; as Hill stability is weakly dependent on the masses, the mass-inclination degeneracy does not significantly affect our results. We consider a wide range of initial planetary mass distributions and find that some are poor fits to the observed systems. In fact, many of our scattering experiments overproduce systems very close to the stability boundary. The distribution of dynamical configurations of two-planet systems actually may provide better discrimination between scattering models than the distribution of eccentricity. Our results imply that, at least in their inner regions which are weakly affected by gas or planetesimal disks, planetary systems should be "packed", with no large gaps between planets. View details
    Planet-Planet Scattering in Planetesimal Disks
    Sean N. Raymond
    Philip J. Armitage
    Astrophysical Journal Letters, 699(2009), L88-L92
    Preview abstract We study the final architecture of planetary systems that evolve under the combined effects of planet-planet and planetesimal scattering. Using N-body simulations we investigate the dynamics of marginally unstable systems of gas and ice giants both in isolation and when the planets form interior to a planetesimal belt. The unstable isolated systems evolve under planet-planet scattering to yield an eccentricity distribution that matches that observed for extrasolar planets. When planetesimals are included the outcome depends upon the total mass of the planets. For M tot gsim 1 MJ the final eccentricity distribution remains broad, whereas for M tot lsim 1 MJ a combination of divergent orbital evolution and recircularization of scattered planets results in a preponderance of nearly circular final orbits. We also study the fate of marginally stable multiple planet systems in the presence of planetesimal disks, and find that for high planet masses the majority of such systems evolve into resonance. A significant fraction leads to resonant chains that are planetary analogs of Jupiter's Galilean satellites. We predict that a transition from eccentric to near-circular orbits will be observed once extrasolar planet surveys detect sub-Jovian mass planets at orbital radii of a sime 5-10 AU. View details
    Mean Motion Resonances from Planet-Planet Scattering
    Raymond, Sean N
    Barnes, Rory
    Armitage, Philip J
    The Astrophysical Journal,, 687, Issue 2(2008), pp. 107-110
    Preview
    A dynamical perspective on additional planets in 55 Cancri
    Sean N. Raymond
    Rory Barnes
    The Astrophysical Journal, 689, Issue 1(2008), pp. 478-491
    Preview
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