Scholarly Works, Geosciences

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  • Disappearing cities on US coasts
    Ohenhen, Leonard O.; Shirzaei, Manoochehr; Ojha, Chandrakanta; Sherpa, Sonam F.; Nicholls, Robert J. (Nature Research, 2024-03-06)
    The sea level along the US coastlines is projected to rise by 0.25–0.3 m by 2050, increasing the probability of more destructive flooding and inundation in major cities. However, these impacts may be exacerbated by coastal subsidence— the sinking of coastal land areas—a factor that is often underrepresented in coastal-management policies and long-term urban planning. In this study, we combine high-resolution vertical land motion (that is, raising or lowering of land) and elevation datasets with projections of sea-level rise to quantify the potential inundated areas in 32 major US coastal cities. Here we show that, even when considering the current coastal-defence structures, further land area of between 1,006 and 1,389 km² is threatened by relative sea-level rise by 2050, posing a threat to a population of 55,000–273,000 people and 31,000–171,000 properties. Our analysis shows that not accounting for spatially variable land subsidence within the cities may lead to inaccurate projections of expected exposure. These potential consequences show the scale of the adaptation challenge, which is not appreciated in most US coastal cities.
  • Controls on the Leeside Angle of Dunes in Shallow Unidirectional Flows
    Cisneros, Julia; Best, Jim (Wiley, 2024)
    Dunes are ubiquitous features in alluvial channels, serve as major agents of sediment transport and contribute significantly to flow resistance. Research in the past decade has illustrated the complexity of dune geometry and widespread occurrence of dunes that have a low leeside angle. However, debate exists concerning the occurrence of such dunes and their formative processes. This paper seeks to further our understanding of low-angle dunes by utilizing data from a robust set of shallow flow laboratory experiments detailing equilibrium bedform morphology across a range of sediment transport conditions. Analysis of bedform morphology demonstrates that dunes with low-angle leesides are generated in shallow laboratory flows, and hence are not restricted to deep rivers. Of the possible processes that have been proposed to explain the formation of low-angle dunes, this finding unequivocally shows that liquefied leeside avalanches, which rely on deep flows for their generation, are not a controlling mechanism. In addition, dunes formed under suspension-dominated conditions possess lower leeside angles compared to those formed in bedload-dominated conditions. However, where bedload transport dominates and sediment suspension is likely of lesser importance, low-angle dunes are still present, and preliminary analysis shows that bedform superimposition can result in lowering of the dune leeside angle. Low and intermediate angle dunes formed in these various conditions also have a lower potential for large-scale, permanent, leeside flow separation compared to angle-of-repose dunes, confirming the need to account for these differences in predictions of flow resistance associated with dune form roughness.
  • Transient ice loss in the Patagonia Icefields during the 2015-2016 El Nino event
    Gomez, Demian D.; Bevis, Michael G.; Smalley, Robert; Durand, Michael; Willis, Michael J.; Caccamise, Dana J.; Kendrick, Eric; Skvarca, Pedro; Sobrero, Franco S.; Parra, Hector; Casassa, Gino (Nature Portfolio, 2022-06-10)
    The Patagonia Icefields (PIF) are the largest non-polar ice mass in the southern hemisphere. The icefields cover an area of approximately 16,500 km2 and are divided into the northern and southern icefields, which are ~ 4000 km2 and ~ 12,500 km2, respectively. While both icefields have been losing mass rapidly, their responsiveness to various climate drivers, such as the El Niño-Southern Oscillation, is not well understood. Using the elastic response of the earth to loading changes and continuous GPS data we separated and estimated ice mass changes observed during the strong El Niño that started in 2015 from the complex hydrological interactions occurring around the PIF. During this single event, our mass balance estimates show that the northern icefield lost ~ 28 Gt of mass while the southern icefield lost ~ 12 Gt. This is the largest ice loss event in the PIF observed to date using geodetic data.
  • A Machine Learning Approach to Flood Depth and Extent Detection Using Sentinel 1A/B Synthetic Aperture Radar
    Tiampo, K.; Woods, C.; Huang, L.; Sharma, P.; Chen, Z.; Kar, B.; Bausch, D.; Simmons, C.; Estrada, R.; Willis, Michael J.; Glasscoe, M. (IEEE, 2021-01-01)
    The rising number of flooding events combined with increased urbanization is contributing to significant economic losses due to damages to structures and infrastructures. Here we present a method for producing all weather maps of flood inundation using a combination of synthetic aperture radar (SAR) remote sensing data and machine learning methods that can be used to provide information on the evolution of flood hazards to DisasterAware©, a global alerting system, that is used to disseminate flood risk information to stakeholders across the globe. While these efforts are still in development, a case study is presented for the major flood event associated with Hurricane Harvey and associated floods that impacted Houston, TX in August of 2017.
  • 3D models of the leader valley using satellite & UAV imagery following the 2016 Kaikoura earthquake
    Zekkos, D.; Clark, M.; Willis, Michael J.; Athanasopoulos-Zekkos, A.; Manousakis, J.; Knoper, L.; Stahl, T.; Massey, C.; Archibald, G.; Greenwood, W.; Medwedeff, W. (2018-01-01)
    The ability to quickly, efficiently and reliably characterize changes in the landscape following an earthquake has remained a challenge for the earthquake engineering profession. The 2016 Mw7.8 Kaikoura earthquake provided a unique opportunity to document changes in topography following an earthquake on a regional scale using satellite derived high-resolution digital models. Along-track stereo satellite imagery had been collected for the pre-event topography. Satellites were tasked and collected stereo-mode post-event imagery. Both sets of images were used to create digital surface models (DSMs) of the affected area before and after the event. The procedure followed and indicative results for the Leader valley are presented with emphasis on the challenges associated with the implementation of the technique for the first time in this environment. The valley is of interest because of the variety of features it includes, i.e., the large Leader landslide, smaller landslides, stable sloping and flat ground as well as fault rupture lineaments. The open-source SETSM software is used to provide multiple DSMs. Our workflow is described and results are compared against the DSM created using Structure-from-Motion with imagery collected by Unmanned Aerial Vehicles (UAV) and aerial LIDAR. Overall, the sub-meter agreement between the DSM created using satellites and the DSM created using UAV and LIDAR datasets demonstrates viability for use in seismic studies, but features smaller than about 0.5 m are more difficult to discern.
  • Understanding of Contemporary Regional Sea-Level Change and the Implications for the Future
    Hamlington, Benjamin D.; Gardner, Alex S.; Ivins, Erik; Lenaerts, Jan T. M.; Reager, J. T.; Trossman, David S.; Zaron, Edward D.; Adhikari, Surendra; Arendt, Anthony; Aschwanden, Andy; Beckley, Brian D.; Bekaert, David PS S.; Blewitt, Geoffrey; Caron, Lambert; Chambers, Don P.; Chandanpurkar, Hrishikesh A.; Christianson, Knut; Csatho, Beata; Cullather, Richard; DeConto, Robert M.; Fasullo, John T.; Frederikse, Thomas; Freymueller, Jeffrey T.; Gilford, Daniel M.; Girotto, Manuela; Hammond, William C.; Hock, Regine; Holschuh, Nicholas; Kopp, Robert E.; Landerer, Felix; Larour, Eric; Menemenlis, Dimitris; Merrifield, Mark; Mitrovica, Jerry X.; Nerem, R. Steven; Nias, Isabel J.; Nieves, Veronica; Nowicki, Sophie; Pangaluru, Kishore; Piecuch, Christopher G.; Ray, Richard D.; Rounce, David R.; Schlegel, Nicole-Jeanne; Seroussi, Helene; Shirzaei, Manoochehr; Sweet, William; Velicogna, Isabella; Vinogradova, Nadya; Wahl, Thomas; Wiese, David N.; Willis, Michael J. (American Geophysical Union, 2020-07-20)
    Global sea level provides an important indicator of the state of the warming climate, but changes in regional sea level are most relevant for coastal communities around the world. With improvements to the sea-level observing system, the knowledge of regional sea-level change has advanced dramatically in recent years. Satellite measurements coupled with in situ observations have allowed for comprehensive study and improved understanding of the diverse set of drivers that lead to variations in sea level in space and time. Despite the advances, gaps in the understanding of contemporary sea-level change remain and inhibit the ability to predict how the relevant processes may lead to future change. These gaps arise in part due to the complexity of the linkages between the drivers of sea-level change. Here we review the individual processes which lead to sea-level change and then describe how they combine and vary regionally. The intent of the paper is to provide an overview of the current state of understanding of the processes that cause regional sea-level change and to identify and discuss limitations and uncertainty in our understanding of these processes. Areas where the lack of understanding or gaps in knowledge inhibit the ability to provide the needed information for comprehensive planning efforts are of particular focus. Finally, a goal of this paper is to highlight the role of the expanded sea-level observation network—particularly as related to satellite observations—in the improved scientific understanding of the contributors to regional sea-level change.
  • Advancing Field-Based GNSS Surveying for Validation of Remotely Sensed Water Surface Elevation Products
    Pitcher, Lincoln H.; Smith, Laurence C.; Cooley, Sarah W.; Zaino, Annie; Carlson, Robert; Pettit, Joseph; Gleason, Colin J.; Minear, J. Toby; Fayne, Jessica V.; Willis, Michael J.; Hansen, Jasmine S.; Easterday, Kelly J.; Harlan, Merritt E.; Langhorst, Theodore; Topp, Simon N.; Dolan, Wayana; Kyzivat, Ethan D.; Pietroniro, Al; Marsh, Philip; Yang, Daqing; Carter, Tom; Onclin, Cuyler; Hosseini, Nasim; Wilcox, Evan; Moreira, Daniel; Berge-Nguyen, Muriel; Cretaux, Jean-Francois; Pavelsky, Tamlin M. (Frontiers, 2020-11-23)
    To advance monitoring of surface water resources, new remote sensing technologies including the forthcoming Surface Water and Ocean Topography (SWOT) satellite (expected launch 2022) and its experimental airborne prototype AirSWOT are being developed to repeatedly map water surface elevation (WSE) and slope (WSS) of the world’s rivers, lakes, and reservoirs. However, the vertical accuracies of these novel technologies are largely unverified; thus, standard and repeatable field procedures to validate remotely sensed WSE and WSS are needed. To that end, we designed, engineered, and operationalized a Water Surface Profiler (WaSP) system that efficiently and accurately surveys WSE and WSS in a variety of surface water environments using Global Navigation Satellite Systems (GNSS) time-averaged measurements with Precise Point Positioning corrections. Here, we present WaSP construction, deployment, and a data processing workflow. We demonstrate WaSP data collections from repeat field deployments in the North Saskatchewan River and three prairie pothole lakes near Saskatoon, Saskatchewan, Canada. We find that WaSP reproducibly measures WSE and WSS with vertical accuracies similar to standard field survey methods [WSE root mean squared difference (RMSD) ∼8 cm, WSS RMSD ∼1.3 cm/km] and that repeat WaSP deployments accurately quantify water level changes (RMSD ∼3 cm). Collectively, these results suggest that WaSP is an easily deployed, self-contained system with sufficient accuracy for validating the decimeter-level expected accuracies of SWOT and AirSWOT. We conclude by discussing the utility of WaSP for validating airborne and spaceborne WSE mappings, present 63 WaSP in situ lake WSE measurements collected in support of NASA’s Arctic-Boreal and Vulnerability Experiment, highlight routine deployment in support of the Lake Observation by Citizen Scientists and Satellites project, and explore WaSP utility for validating a novel GNSS interferometric reflectometry LArge Wave Warning System.
  • 'Boundary': mapping and visualizing climatically changed landscapes at Kaskawulsh Glacier and Kluane Lake, Yukon
    Shugar, D. H.; Colorado, K. A.; Clague, J. J.; Willis, Michael J.; Best, J. L. (Taylor & Francis, 2018-06-04)
    This paper describes a collaboration between a visual artist and geoscientists, who together viewed the same rugged, high mountain landscape through different, yet complementary, lenses. We pair scientific mapping and historic comparative photography with a series of site-specific sculptural installations to interpret the dramatic geological changes that occurred at Kaskawulsh Glacier, Yukon, in the spring of 2016. In the summer of that year, artist K.A. Colorado accompanied geoscientists D.H. Shugar, J.J. Clague, and J.L. Best to the terminus of Kaskawulsh Glacier, as well as Kluane Lake downstream of the glacier, to document the landscape changes that occurred earlier in the year. The Boundary images were created as on-site, three-dimensional, artistic interpretations of the markedly changed boundaries that occurred as a result of climate-induced glacier retreat and the sudden subcontinental-scale reorganization of drainage. Both the scientific study conducted by the geomorphologists and the art installations created by the artist were performed simultaneously. The Boundary installation art project, together with satellite imagery and historical photographs, conveys the death of Slims River as a result of climate change.
  • Controls on Eolian Landscape Evolution in Fractured Bedrock
    Perkins, Jonathan P.; Finnegan, Noah J.; de Silva, Shanaka L.; Willis, Michael J. (American Geophysical Union, 2019-11-08)
    Wind abrasion is important for planetary landscape evolution, and wind-abraded bedrock landscapes contain many landforms that are difficult to interpret. Here we exploit a natural experiment in Chile where topographic shielding by an upwind lava flow yields diverse erosional landforms in a downwind ignimbrite. Using a 3-D topographic wind model, we find that low velocities in the wake of a lava lobe coincide with a transition from landforms reflecting fracture-parallel erosion to flow-parallel erosion. Erosion rates across these landforms vary with shear velocity and abrasion susceptibility of the windward escarpment. We hypothesize that this morphologic threshold is controlled by whether particles can be lofted in suspension and overcome topographic steering imposed by fractured bedrock blocks. Within a phase space set by Rouse and Stokes numbers, our data illustrate that wind-abraded landforms reflect a competition between the material skeleton of the landscape and the strength of the flow that shapes it.
  • Accelerating changes in ice mass within Greenland, and the ice sheet's sensitivity to atmospheric forcing
    Bevis, M.; Harig, C.; Khan, S. A.; Brown, A.; Simons, F. J.; Willis, Michael J.; Fettweis, X.; Van Den Broeke, M. R.; Madsen, F. B.; Kendrick, E.; Caccamise, D. J.; Van Dam, T.; Knudsen, P.; Nylen, T. (Proceedings of the National Academy of Sciences, 2019-01-22)
    From early 2003 to mid-2013, the total mass of ice in Greenland declined at a progressively increasing rate. In mid-2013, an abrupt reversal occurred, and very little net ice loss occurred in the next 12-18 months. Gravity Recovery and Climate Experiment (GRACE) and global positioning system (GPS) observations reveal that the spatial patterns of the sustained acceleration and the abrupt deceleration in mass loss are similar. The strongest accelerations tracked the phase of the North Atlantic Oscillation (NAO). The negative phase of the NAO enhances summertime warming and insolation while reducing snowfall, especially in west Greenland, driving surface mass balance (SMB) more negative, as illustrated using the regional climate model MAR. The spatial pattern of accelerating mass changes reflects the geography of NAO-driven shifts in atmospheric forcing and the ice sheet's sensitivity to that forcing. We infer that southwest Greenland will become a major future contributor to sea level rise.
  • Brief communication: Unabated wastage of the Juneau and Stikine icefields (southeast Alaska) in the early 21st century
    Berthier, Etienne; Larsen, Christopher; Durkin, William J.; Willis, Michael J.; Pritchard, Matthew E. (Copernicus, 2018-04-27)
    The large Juneau and Stikine icefields (Alaska) lost mass rapidly in the second part of the 20th century. Laser altimetry, gravimetry and field measurements suggest continuing mass loss in the early 21st century. However, two recent studies based on time series of Shuttle Radar Topographic Mission (SRTM) and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) digital elevation models (DEMs) indicate a slowdown in mass loss after 2000. Here, the ASTER-based geodetic mass balances are recalculated carefully avoiding the use of the SRTM DEM because of the unknown penetration depth of the C-band radar signal. We find strongly negative mass balances from 2000 to 2016 (-0.68 ± 0.15 m w.e. a-1 for the Juneau Icefield and -0.83 ± 0.12 m w.e. a-1 for the Stikine Icefield), in agreement with laser altimetry, confirming that mass losses are continuing at unabated rates for both icefields. The SRTM DEM should be avoided or used very cautiously to estimate glacier volume change, especially in the North Hemisphere and over timescales of less than ∼ 20 years.
  • Sedimentology and geomorphology of a large tsunamigenic landslide, Taan Fiord, Alaska
    Dufresne, A.; Geertsema, M.; Shugar, D. H.; Koppes, M.; Higman, B.; Haeussler, P. J.; Stark, C.; Venditti, J. G.; Bonno, D.; Larsen, C.; Gulick, S. P. S.; McCall, N.; Walton, M.; Loso, M. G.; Willis, Michael J. (Elsevier, 2018-02)
    On 17 October 2015, a landslide of roughly 60 × 106 m3 occurred at the terminus of Tyndall Glacier in Taan Fiord, southeastern Alaska. It caused a tsunami that inundated an area over 20 km2, whereas the landslide debris itself deposited within a much smaller area of approximately 2 km2. It is a unique event in that the landslide debris was deposited into three very different environments: on the glacier surface, on land, and in the marine waters of the fjord. Part of the debris traversed the width of the fjord and re-emerged onto land, depositing coherent hummocks with preserved source stratigraphy on an alluvial fan and adjacent moraines on the far side of the fjord. Imagery from before the landslide shows that the catastrophic slope failure was preceded by deformation and sliding for at least the two decades since the glacier retreated to its current terminus location, exposing steep and extensively faulted slopes. A small volume of the total slide mass remains within the source area and is topped by striated blocks (> 10 m across) and standing trees that were transported down the slope in intact positions during the landslide. Field work was carried out in the summer of 2016, and by the time this paper was written, almost all of the supraglacial debris was advected into the fjord and half the subaerial hummocks were buried by glacial advance; this rapid change illustrates how highly active sedimentary processes in high-altitude glacial settings can skew any landslide-frequency analyses, and emphasizes the need for timely field investigations of these natural hazards.
  • Direct measurements of meltwater runoff on the Greenland ice sheet surface
    Smith, Laurence C.; Yang, Kang; Pitcher, Lincoln H.; Overstreet, Brandon T.; Chu, Vena W.; Rennermalm, Asa K.; Ryan, Jonathan C.; Cooper, Matthew G.; Gleason, Colin J.; Tedesco, Marco; Jeyaratnam, Jeyavinoth; van As, Dirk; van den Broeke, Michiel R.; van de Berg, Willem Jan; Noel, Brice; Langen, Peter L.; Cullather, Richard I.; Zhao, Bin; Willis, Michael J.; Hubbard, Alun; Box, Jason E.; Jenner, Brittany A.; Behar, Alberto E. (National Academy of Sciences, 2017-12-05)
    Meltwater runoff from the Greenland ice sheet surface influences surface mass balance (SMB), ice dynamics, and global sea level rise, but is estimated with climate models and thus difficult to validate. We present a way to measure ice surface runoff directly, from hourly in situ supraglacial river discharge measurements and simultaneous high-resolution satellite/drone remote sensing of upstream fluvial catchment area. A first 72-h trial for a 63.1-km2 moulin-terminating internally drained catchment (IDC) on Greenland’s midelevation (1,207–1,381 m above sea level) ablation zone is compared with melt and runoff simulations from HIRHAM5, MAR3.6, RACMO2.3, MERRA-2, and SEB climate/SMB models. Current models cannot reproduce peak discharges or timing of runoff entering moulins but are improved using synthetic unit hydrograph (SUH) theory. Retroactive SUH applications to two older field studies reproduce their findings, signifying that remotely sensed IDC area, shape, and supraglacial river length are useful for predicting delays in peak runoff delivery to moulins. Applying SUH to HIRHAM5, MAR3.6, and RACMO2.3 gridded melt products for 799 surrounding IDCs suggests their terminal moulins receive lower peak discharges, less diurnal variability, and asynchronous runoff timing relative to climate/SMB model output alone. Conversely, large IDCs produce high moulin discharges, even at high elevations where melt rates are low. During this particular field experiment, models overestimated runoff by +21 to +58%, linked to overestimated surface ablation and possible meltwater retention in bare, porous, low-density ice. Direct measurements of ice surface runoff will improve climate/SMB models, and incorporating remotely sensed IDCs will aid coupling of SMB with ice dynamics and subglacial systems.
  • Estimating supraglacial lake depth in western Greenland using Landsat 8 and comparison with other multispectral methods
    Pope, A.; Scambos, T. A.; Moussavi, M.; Tedesco, M.; Willis, Michael J.; Shean, D.; Grigsby, S. (Copernicus, 2015-06-22)
    Liquid water stored on the surface of ice sheets and glaciers impacts surface mass balance, ice dynamics, and heat transport. Supraglacial lakes, therefore, play a significant role in ice sheet behavior as a surface expression of the glacial hydrological system. Multispectral remote sensing can be used to detect supraglacial lakes and estimate their depth and area. In this study, we use in situ spectral and bathymetric data to assess lake depth retrieval using the recently launched Landsat 8 Operational Land Imager (OLI). We also extend our analysis to other multispectral sensors to evaluate their performance with similar methods. Digital elevation models derived from WorldView stereo imagery (pre-lake filling and post-drainage) are used to validate spectrally derived depths, combined with a lake edge determination from imagery. The optimal supraglacial lake depth retrieval is a physically based single-band model applied to two OLI bands independently (red and panchromatic) that are then averaged together. When OLI- and WorldView-derived depths are differenced, they yield a mean and standard deviation of 0.0 ± 1.6 m. This method is then applied to OLI data for the Sermeq Kujalleq (Jakobshavn Isbrae) region of Greenland to study the spatial and intra-seasonal variability of supraglacial lakes during summer 2014. We also give coefficients for estimating supraglacial lake depth using a similar method as OLI with other multispectral sensors.
  • Satellite-derived volume loss rates and glacier speeds for the Cordillera Darwin Icefield, Chile
    Melkonian, A. K.; Willis, Michael J.; Pritchard, M. E.; Rivera, A.; Bown, F.; Bernstein, S. A. (Copernicus, 2013-05-14)
    We produce the first icefield-wide volume change rate and glacier velocity estimates for the Cordillera Darwin Icefield (CDI), a 2605 km2 temperate icefield in southern Chile (69.6°W, 54.6° S). Velocities are measured from optical and radar imagery between 2001-2011. Thirty-six digital elevation models (DEMs) from ASTER and the SRTM DEM are stacked and a weighted linear regression is applied to elevations on a pixel-by-pixel basis to estimate volume change rates. The CDI lost mass at an average rate of -3.9±1.5 Gt yr-1 between 2000 and 2011, equivalent to a sea level rise (SLR) of 0.01±0.004mmyr-1 and an area-averaged thinning rate of -1.5±0.6mw.e.(water equivalent) yr-1. Thinning is widespread, with concentrations near the front of two northern glaciers (Marinelli, Darwin) and one western (CDI-08) glacier. Thickening is apparent in the south, most notably over the advancing Garibaldi Glacier. The northeastern part of the CDI has an average thinning rate of -1.9±0.7mw.e. yr-1, while the southwestern part has an average thinning rate of -1.0±0.4mw.e. yr -1. Velocities are obtained over many of the CDI glaciers for the first time. We provide a repeat speed time series at the Marinelli Glacier. There we measure maximum front speeds of 7.5±0.2mday-1 in 2001, 9.5±0.6mday-1 in 2003 and 10±0.3mday-1 in 2011. The maintenance of high front speeds from 2001 to 2011 supports the hypothesis that Marinelli is in the retreat phase of the tidewater cycle, withdynamic thinning governed by the fjord bathymetry. © Author(s) 2013.
  • Assessment of Sea Level Rise and Associated Impacts for Tuvalu
    Adams, Kyra; Blackwood, Carmen; Cullather, Richard; Hamlington, Benjamin; Heijkoop, Eduard; Karnauskas, Kristopher; Kopp, Robert; Larour, Eric; Lee, Tong; Nerem, R. Steven; Nowicki, Sophie; Piecuch, Christopher G.; Ray, Richard; Rounce, David; Thompson, Philip; Vinogradova, Nadya; Wang, Ou; Willis, Michael J. (2023)
    For low-lying island nations in the Pacific Ocean, increasing sea levels pose an existential threat. One of these nations, Tuvalu, has already begun experiencing impacts driven by the combined effects of the rising ocean, storms, naturally-occurring ocean variability, and changes in other physical processes. These impacts are expected to worsen in the future, and planning and adaptation is underway in Tuvalu. In this technical report, the NASA Sea Level Change Team (NSLCT) assesses the available observations and latest scientific understanding to provide information on future sea level rise and associated impacts for Tuvalu. This report is written in support of the objectives of the Rising Nations Initiative (RNI), enabled by the UN Global Center for Climate Mobility, and serves as a scientific foundation upon which activities and plans can be based.
  • Morphodynamic and modeling insights from global sensitivity analysis of a barrier island evolution model
    Hoagland, Steven; Irish, Jennifer L.; Weiss, Robert (Elsevier, 2024-02)
    Recently developed models of coastal barrier morphodynamics include marsh and lagoon processes that have been shown to impact barrier island evolution. To gain additional insights into the simulated barrier-backbarrier system dynamics, this study explores the parameter space of a barrier evolution model using global sensitivity analysis. Influential parameters, their interactions with one another, and regions of sensitivity within the parameter space were identified using Sobol indices and factor mapping techniques for model results through the end of the century. The results of this study highlight an important relationship between initial and critical barrier island geometries and suggest that narrow and low-relief barriers are most vulnerable to be eroded away (width drowning) or overtaken by sea level rise (height drowning), respectively. Width drowning was also strongly associated with other model input parameters such as toe depth, sea level rise rate, and backbarrier critical bed shear stress, which suggests that sub-centennial drowning is dependent on a unique combination of input parameter values and may be averted (or delayed) with a single input parameter change. Barrier dynamics were significantly influenced by the backbarrier marsh platform, which was more impacted by sediment transport parameters such as critical bed shear stress and ocean sediment concentration than maximum annual overwash flux. This suggests that inorganic sediment deposition through tidal inlet dispersion is much more significant to the backbarrier marsh and lagoon system than overwash over sub-centennial timescales and can help to reduce the risk of width drowning.
  • A modern pulse of ultrafast exhumation and diachronous crustal melting in the Nanga Parbat Massif
    Guevara, Victor E.; Smye, Andrew J.; Caddick, Mark J.; Searle, Michael P.; Olsen, Telemak; Whalen, Lisa; Kylander-Clark, Andrew RC C.; Jercinovic, Michael J.; Waters, David J. (AAAS, 2022-08-05)
    We combine monazite petrochronology with thermal modeling to evaluate the relative roles of crustal melting, surface denudation, and tectonics in facilitating ultrafast exhumation of the Nanga Parbat Massif in the western Himalayan syntaxis. Our results reveal diachronous melting histories between samples and a pulse of ultrafast exhumation (9 to 13 mm/year) that began ∼1 Ma and was preceded by several million years of slower, but still rapid, exhumation (2 to 5 mm/year). Recent studies show that an exhumation pulse of similar timing and magnitude occurred in the eastern Himalayan syntaxis. A synchronous exhumation pulse in both Himalayan syntaxes suggests that neither erosion by rivers and/or glaciers nor a pulse of crustal melting was a primary trigger for accelerated exhumation. Rather, our results, combined with those of recent studies in the eastern syntaxis, imply that larger-scale tectonic processes impose the dominant control on the current tempo of rapid exhumation in the Himalayan syntaxes.
  • Spatiotemporal Groundwater Storage Dynamics and Aquifer Mechanical Properties in the Santa Clara Valley Inferred From InSAR Deformation Over 2017-2022
    Ghobadi-Far, Khosro; Werth, Susanna; Shirzaei, Manoochehr; Burgmann, Roland (American Geophysical Union, 2023-11-22)
    We used Interferometric Synthetic Aperture Radar (InSAR)-derived vertical land motion (VLM) timeseries during 2017–2022 to examine the compounding impacts of natural and anthropogenic processes on groundwater dynamics in the Santa Clara Valley (SCV). VLM strongly correlates (>0.75) with groundwater level in both unconfined and confined aquifers. We show that VLM in SCV is mainly driven by groundwater dynamics in deep aquifer layers below 120 m. Our results show that during the most recent drought from March 2019 to November 2021, Santa Clara County subsided up to 30 mm due to groundwater depletion, three times as large as average seasonal amplitude of VLM. Owing to the managed aquifer recharge, the region has been able to avoid unrecoverable land subsidence. We utilize InSAR data to calibrate storage coefficient and lag time related to delayed response of clay interbeds to groundwater level changes, which further serves to estimate groundwater volume loss in confined aquifer units during drought.
  • Ordovician origin and subsequent diversification of the brown algae
    Choi, Seok-Wan; Graf, Louis; Choi, Ji Won; Jo, Jihoon; Boo, Ga Hun; Kawai, Hiroshi; Choi, Chang Geun; Xiao, Shuhai; Knoll, Andrew H.; Andersen, Robert A.; Yoon, Hwan Su (Elsevier, 2024-01-19)
    Brown algae are the only group of heterokont protists exhibiting complex multicellularity. Since their origin, brown algae have adapted to various marine habitats, evolving diverse thallus morphologies and gamete types. However, the evolutionary processes behind these transitions remain unclear due to a lack of a robust phylogenetic framework and problems with time estimation. To address these issues, we employed plastid genome data from 138 species, including heterokont algae, red algae, and other red-derived algae. Based on a robust phylogeny and new interpretations of algal fossils, we estimated the geological times for brown algal origin and diversification. The results reveal that brown algae first evolved true multicellularity, with plasmodesmata and reproductive cell differentiation, during the late Ordovician Period (ca. 450 Ma), coinciding with a major diversification of marine fauna (the Great Ordovician Biodiversification Event) and a proliferation of multicellular green algae. Despite its early Paleozoic origin, the diversification of major orders within this brown algal clade accelerated only during the Mesozoic Era, coincident with both Pangea rifting and the diversification of other heterokont algae (e.g., diatoms), coccolithophores, and dinoflagellates, with their red algal-derived plastids. The transition from ancestral isogamy to oogamy was followed by three simultaneous reappearances of isogamy during the Cretaceous Period. These are concordant with a positive character correlation between parthenogenesis and isogamy. Our new brown algal timeline, combined with a knowledge of past environmental conditions, shed new light on brown algal diversification and the intertwined evolution of multicellularity and sexual reproduction.