Building the terrestrial planets: Constrained accretion in the inner Solar System

To date, no accretion model has succeeded in reproducing all observed constraints in the inner Solar System. These constraints include: (1) the orbits, in particular the small eccentricities, and (2) the masses of the terrestrial planets - Mars’ relatively small mass in particular has not been adequately reproduced in previous simulations; (3) the formation timescales of Earth and Mars, as interpreted from Hf/W isotopes; (4) the bulk structure of the asteroid belt, in particular the lack of an imprint of planetary embryo-sized objects; and (5) Earth’s relatively large water content, assuming that it was delivered in the form of water-rich primitive asteroidal material. Here we present results of 40 high-resolution (N = 1000-2000) dynamical simulations of late-stage planetary accretion with the goal of reproducing these constraints, although neglecting the planet Mercury. We assume that Jupiter and Saturn are fully-formed at the start of each simulation, and test orbital configurations that are both consistent with and contrary to the “Nice model”. We find that a configuration with Jupiter and Saturn on circular orbits forms low-eccentricity terrestrial planets and a water-rich Earth on the correct timescale, but Mars’ mass is too large by a factor of 5-10 and embryos are often stranded in the asteroid belt. A configuration with Jupiter and Saturn in their current locations but with slightly higher initial eccentricities (e = 0.07-0.1) produces a small Mars, an embryo-free asteroid belt, and a reasonable Earth analog but rarely allows water delivery to Earth. None of the configurations we tested reproduced all the observed constraints. Our simulations leave us with a problem: we can reasonably satisfy the observed constraints (except for Earth’s water) with a configuration of Jupiter and Saturn that is at best marginally consistent with models of the outer Solar System, as it does not allow for any outer planet migration after a few Myr. Alternately, giant planet configurations which are consistent with the Nice model fail to reproduce Mars’ small size.     

East African food security as influenced by future climate change and land use change at local to regional scales

Climate change impacts food production systems, particularly in locations with large, vulnerable populations. Elevated greenhouse gases (GHG), as well as land cover/land use change (LCLUC), can influence regional climate dynamics. Biophysical factors such as topography, soil type, and seasonal rainfall can strongly affect crop yields. We used a regional climate model derived from the Regional Atmospheric Modeling System (RAMS) to compare the effects of projected future GHG and future LCLUC on spatial variability of crop yields in East Africa. Crop yields were estimated with a process-based simulation model. The results suggest that: (1) GHG-influenced and LCLUC-influenced yield changes are highly heterogeneous across this region; (2) LCLUC effects are significant drivers of yield change; and (3) high spatial variability in yield is indicated for several key agricultural sub-regions of East Africa. Food production risk when considered at the household scale is largely dependent on the occurrence of extremes, so mean yield in some cases may be an incomplete predictor of risk. The broad range of projected crop yields reflects enormous variability in key parameters that underlie regional food security; hence, donor institutions’ strategies and investments might benefit from considering the spatial distribution around mean impacts for a given region. Ultimately, global assessments of food security risk would benefit from including regional and local assessments of climate impacts on food production. This may be less of a consideration in other regions. This study supports the concept that LCLUC is a first-order factor in assessing food production risk.     

Quality of geological CO2 storage to avoid jeopardizing climate targets

We explore allowable leakage for carbon capture and geological storage to be consistent with maximum global warming targets of 2.5 and 3 °C by 2100. Given plausible fossil fuel use and carbon capture and storage scenarios, and based on modeling of time-dependent leakage of CO₂, we employ a climate model to calculate the long-term temperature response of CO₂ emissions. We assume that half of the stored CO₂ is permanently trapped by fast mechanisms. If 40?% of global CO₂ emissions are stored in the second half of this century, the temperature effect of escaped CO₂ is too small to compromise a 2.5 °C target. If 80?% of CO₂ is captured, escaped CO₂ must peak 300?years or later for consistency with this climate target. Due to much more CO₂ stored for the 3 than the 2.5 °C target, quality of storage becomes more important. Thus for the 3 °C target escaped CO₂ must peak 400?years or later in the 40?% scenario, and 3000?years or later in the 80?% scenario. Consequently CO₂ escaped from geological storage can compromise the less stringent 3 °C target in the long-run if most of global CO₂ emissions have been stored. If less CO₂ is stored only a very high escape scenario can compromise the more stringent 2.5 °C target. For the two remaining combinations of storage scenarios and climate targets, leakage must be high to compromise these climate targets.     

Impact of ocean model resolution on CCSM climate simulations

The current literature provides compelling evidence suggesting that an eddy-resolving (as opposed to eddy-permitting or eddy-parameterized) ocean component model will significantly impact the simulation of the large-scale climate, although this has not been fully tested to date in multi-decadal global coupled climate simulations. The purpose of this paper is to examine how resolved ocean fronts and eddies impact the simulation of large-scale climate. The model used for this study is the NCAR Community Climate System Model version 3.5 (CCSM3.5)—the forerunner to CCSM4. Two experiments are reported here. The control experiment is a 155-year present-day climate simulation using a 0.5° atmosphere component (zonal resolution 0.625 meridional resolution 0.5°; land surface component at the same resolution) coupled to ocean and sea-ice components with zonal resolution of 1.2° and meridional resolution varying from 0.27° at the equator to 0.54° in the mid-latitudes. The second simulation uses the same atmospheric and land-surface models coupled to eddy-resolving 0.1° ocean and sea-ice component models. The simulations are compared in terms of how the representation of smaller scale features in the time mean ocean circulation and ocean eddies impact the mean and variable climate. In terms of the global mean surface temperature, the enhanced ocean resolution leads to a ubiquitous surface warming with a global mean surface temperature increase of about 0.2?°C relative to the control. The warming is largest in the Arctic and regions of strong ocean fronts and ocean eddy activity (i.e., Southern Ocean, western boundary currents). The Arctic warming is associated with significant losses of sea-ice in the high-resolution simulation. The sea surface temperature gradients in the North Atlantic, in particular, are better resolved in the high-resolution model leading to significantly sharper temperature gradients and associated large-scale shifts in the rainfall. In the extra-tropics, the interannual temperature variability is increased with the resolved eddies, and a notable increases in the amplitude of the El Ni?o and the Southern Oscillation is also detected. Changes in global temperature anomaly teleconnections and local air-sea feedbacks are also documented and show large changes in ocean–atmosphere coupling. In particular, local air-sea feedbacks are significantly modified by the increased ocean resolution. In the high-resolution simulation in the extra-tropics there is compelling evidence of stronger forcing of the atmosphere by SST variability arising from ocean dynamics. This coupling is very weak or absent in the low-resolution model.     

Effect of changes in water level on sediment pore water redox geochemistry at a reservoir shoreline

Pore water samplers with high vertical resolution were used to evaluate the response of sediment redox geochemistry during transient hydrologic conditions at Lake Powell, a large reservoir in Utah and Arizona, USA. Samplers were deployed at two different yet proximal shoreline locations, White and Farley Canyons, before and after exposure of sediment to air and subsequent resubmersion, which resulted from fluctuations in the water level of the reservoir. Before exposure to air, an observed increase in dissolved Mn concentrations and, at Farley Canyon, an observed decrease in dissolved U concentrations across and immediately below the sediment–water interface indicated reducing conditions in the sub-surface. After exposure and resubmersion of the sediment, pore water profiles at each site differed distinctly from those observed before the fluctuation in water level. At White Canyon, an increase in U concentrations and a decrease in Mn concentrations in pore water after exposure and subsequent resubmersion are suggestive of oxidative processes occurring during the period of sediment exposure. Data from Farley Canyon suggest that the same processes may be occurring, but to a lesser extent. Depth profiles of As and Pb were also examined, but were relatively featureless compared to those of Mn and U. At both sites, sediment evaluated for pore water chemistry in the second sampling was only fully resubmerged for 2–5 days prior to the second sampling event, yet reducing conditions were clearly evident in the Mn pore water profiles. This suggests that the dynamics of the biogeochemical processes occurring in surface sediment at Lake Powell are responsive on the timescale defined by the fluctuating water levels in the reservoir.     

An improved crustal magnetic field map of Mars from electron reflectometry: Highland volcano magmatic history and the end of the martian dynamo

We apply improved kinetic modeling of electron transport in the martian thermosphere to fit pitch angle distributions measured by the Mars Global Surveyor (MGS) Magnetometer/Electron Reflectometer (MAG/ER), together with appropriate filtering, binning, averaging and error correction techniques, to create the most reliable ER global map to date of crustal magnetic field magnitude at 185 km altitude, with twice the spatial resolution and considerably higher sensitivity to crustal fields than global maps of magnetic field components produced with MAG data alone. This map compares favorably to sparsely sampled dayside MAG data taken at similar altitudes, insofar as a direct comparison is meaningful. Using this map, we present two case studies. The first compares the magnetic signatures of two highland volcanoes, concluding that the comparatively greater thermal demagnetization at Syrtis Major compared with Tyrrhena Patera is likely due to a higher ratio of intruded to extruded magmas. The second uses the map along with topographic data to compare the magnetic signatures and crater retention ages of the demagnetized Hellas impact basin and magnetized Ladon impact basin. From this comparison, we determine that the martian global dynamo magnetic field went from substantial to very weak or nonexistent in the absolute model age time interval 4.15±0.05 to 4.07±0.05 Ga ago.     

An anisotropy of magnetic susceptibility (AMS) investigation of the till fabric of drumlins: support for an accretionary origin

This paper describes the results of a spatially dense anisotropy of magnetic susceptibility (AMS) till fabric study of a single drumlin in the Weedsport Drumlin Field, New York State, USA. AMS till fabrics provide a robust, quantitative and unbiased approach to assess subglacial till kinematics and infer ice‐flow dynamics. The drumlin selected for this detailed investigation was systematically sampled at 18 locations to evaluate the patterns of ice flow and associated till kinematics within a drumlin and to test erosional vs. depositional models for its formation. AMS till fabric analysis yielded strong fabrics that increase in strength towards the drumlin crest, indicating that bed deformation occurred during till deposition and that deformation within the drumlin was greater than that in the interdrumlin low. Fabric orientations reveal drumlin convergent, divergent and parallel ice‐flow paths that illustrate a complex interaction between ice flow and the drumlin form; fabric strength and shape reveal systematic differences in bed deformation between the interdrumlin and drumlin regions. These observations are inconsistent with purely erosional models of drumlin genesis; instead, these observations are more consistent with syndepositional streamlining of till transported, probably locally as a deforming bed, from the interdrumlin low towards the drumlin locality.     

Evaluation of a surface energy balance method based on optical and thermal satellite imagery to estimate root‐zone soil moisture

Various remote‐sensing methods are available to estimate soil moisture, but few address the fine spatial resolutions (e.g. 30‐m grid cells) and root‐zone depth requirements of agricultural and other similar applications. One approach that has been previously proposed to estimate fine‐resolution soil moisture is to first estimate the evaporative fraction from an energy balance that is inferred from optical and thermal remote‐sensing images [e.g. using the Remote Sensing of Evapotranspiration (ReSET) algorithm] and then estimate soil moisture through an empirical relationship to evaporative fraction. A similar approach has also been proposed to estimate the degree of saturation. The primary objective of this study is to evaluate these methods for estimating soil moisture and degree of saturation, particularly for a semi‐arid grassland with relatively dry conditions. Soil moisture was monitored at 28 field locations in south‐eastern Colorado with herbaceous vegetation during the summer months of 3 years. In situ soil moisture and degree of saturation observations are compared with estimates calculated from Landsat imagery using the ReSET algorithm. The in situ observations suggest that the empirical relationships with evaporative fraction that have been proposed in previous studies typically provide overestimates of soil moisture and degree of saturation in this region. However, calibrated functions produce estimates with an accuracy that may be adequate for various applications. The estimates produced by this approach are more reliable for degree of saturation than for soil moisture, and the method is more successful at identifying temporal variability than spatial variability in degree of saturation for this region. Copyright © 2015 John Wiley & Sons, Ltd.     

Climate teleconnections influence on West Africa's terrestrial water storage

There is some evidence of rapid changes in the global atmosphere and hydrological cycle caused by the influence of climate variability. In West Africa, such changes impact directly on water resources leading to incessant extreme hydro‐meteorological conditions. This study examines the association of three global climate teleconnections—El‐Niño Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), and Atlantic Multi‐decadal Oscillation (AMO) with changes in terrestrial water storage (TWS) derived from both Modern‐Era Retrospective Analysis for Research and Applications (MERRA, 1980–2015) and Gravity Recovery and Climate Experiment (GRACE, 2002–2014). In the Sahel region, positive phase of AMO coincided with above‐normal rainfall (wet conditions) and the negative phase with drought conditions and confirms the observed statistically significant association (r = 0.62) between AMO and the temporal evolutions of standardised precipitation index. This relationship corroborates the observed presence of AMO‐driven TWS in much of the Sahel region (though considerably weak in some areas). Although ENSO appears to be more associated with GRACE‐derived TWS over the Volta basin (r =?0.40), this study also shows a strong presence of AMO‐ and ENSO‐induced TWS derived from MERRA reanalysis data in the coastal West African countries and most of the regions below latitude 10°N. The observed presence of ENSO‐ and AMO‐driven TWS is noticeable in tropical areas with relatively high annual/bimodal rainfall and strong inter‐annual variations in surface water. The AMO has a wider footprint and sphere of influence on the region's TWS and suggests the important role of North Atlantic Ocean. IOD‐related TWS also exists in West Africa and its influence on the region's hydrology maybe secondary and somewhat complementary. Nonetheless, presumptive evidence from the study indicates that ENSO and AMO are the two major climatic indices more likely to impact on West Africa's TWS.