ALBERT

Description: Recently constructed tomographic models of the lateral heterogeneity of elastic properties in the Earth's mantle are contrasted in terms of their implications concerning the extent to which the endothermic phase transformation at 660 km depth is influencing the radial style of mixing. Previously published whole mantle and split mantle tomographic reconstructions, SH8/WMI3 and SH8/U4L8 respectively, fit the seismic observations equally well but disagree on the extent to which radial mixing may be impeded across this depth horizon. We show that inferences from seismic tomographic images based on the application of diagnostic functions (global and regional variance spectra and the radial correlation function) lead to the conclusion that the mantle circulation is whole mantle in style if model SH8/WM13 is employed. The split mantle tomographic inversion SHS/U4L8 leads to the contradictory conclusion that the mantle circulation is significantly impeded across the 660 km depth horizon. This latter interpretation is reinforced when we employ the new higher resolution split mantle model SH12/U7L5 in our calculations. We demonstrate that the depth-dependent radial heat flow delivered by both of the split models implies the existence of a thermal boundary layer at 660 km depth, and therefore significant layering, whereas that delivered by the whole mantle model does not. By insisting that the depth-dependent viscosity profile of the mantle be compatible with the thermal structure if the flow were layered, we argue that the split mantle tomographic inversions lead to a self-consistent description of geodynamic constraints (geoid and postglacial rebound data).

Description: We identify the "hard snowball" bifurcation point at which total sea-ice cover of the oceans is expected by employing the comprehensive coupled climate model CCSM3 (Community Climate System Model version 3) for two realistic Neoproterozoic continental configurations, namely a low-latitude configuration appropriate for the 720 Ma Sturtian glaciation and a higher southern latitude configuration reconstructed for 570 Ma but which has often been employed in the past to study the later 635 Ma Marinoan glaciation. Contrary to previous suggestions, we find that for the same total solar insolation (TSI) and atmospheric CO2 concentration (pCO2), the 570 Ma continental configuration is characterized by colder climate than the 720 Ma continental configuration and enters the hard snowball state more easily on account of the following three factors: the higher effective albedo of the snow-covered land compared to that of sea ice, the more negative net cloud forcing near the ice front in the Northern Hemisphere (NH), and, more importantly, the more efficient sea-ice transport towards the Equator in the NH due to the absence of blockage by continents. Beside the paleogeography, we also find the optical depth of aerosol to have a significant influence on this important bifurcation point. When the high value (recommended by CCSM3 but demonstrated to be a significant overestimate) is employed, the critical values of pCO2, beyond which a hard snowball will be realized, are between 80 and 90 ppmv (sea-ice fraction 55%) and between 140 and 150 ppmv (sea-ice fraction 50%) for the Sturtian and Marinoan continental configurations, respectively. However, if a lower value is employed that enables the model to approximately reproduce the present-day climate, then the critical values of pCO2 become 50–60 ppmv (sea-ice fraction 57%) and 100–110 ppmv (sea-ice fraction 48%) for the two continental configurations, respectively. All of these values are higher than previously obtained for the present-day geography (17–35 ppmv) using the same model, primarily due to the absence of vegetation, which increases the surface albedo, but are much lower than that obtained previously for the Marinoan continental configuration using the ECHAM5/MPI-OM model in its standard configuration (~500 ppmv). However, when the sea-ice albedo in that model was reduced from 0.75 to a more appropriate value of 0.45, the critical pCO2 becomes ~204 ppmv, closer to the values obtained here. Our results are similar to those obtained with the present-day geography (70–100 ppmv) when the most recent version of the NCAR model, CCSM4, was employed.

Description: Geochemical and geological evidence suggested that several global-scale glaciation events occurred during the Neoproterozoic era at 750–580 million years ago. The initiation of these glaciations is thought to have been a consequence of the combined influence of a result of low-level carbon dioxide and an approximately 6% weakening of solar luminosity. The latest version of the Community Climate System Model (CCSM4) is employed herein to explore the detailed combination of forcings required to trigger such extreme glaciation under present-day geography and topography conditions. It is found that runaway glaciation occurs in the model under the following conditions: (1) a 8–9% reduction in solar radiation with 286 ppmv CO2 or (2) a 6% reduction in solar radiation with 70–100 ppmv CO2. These thresholds are only moderately different from those found to be characteristic of the previous CCSM3 model reported recently in Yang et al. (2011a,b) for which the respective critical points corresponded to a 10–10.5% reduction in solar radiation with 286 ppmv CO2 or a 6% reduction in solar radiation with 17.5–20 ppmv CO2. The most important reason for these differences is that the sea-ice/snow albedo in CCSM4 is somewhat higher than in CCSM3. Differences in cloud radiative forcings and oceanic and atmospheric heat transports between CCSM3 and CCSM4 also influence the bifurcation points. The forcings required to trigger a "hard Snowball" Earth in either CCSM3 or CCSM4 may be not met by the conditions expected to be characteristic of the Neoproterozoic. Furthermore, there exist "soft Snowball" Earth states, in which the sea-ice coverage reaches approximately 60–65%, land masses in low latitudes are covered by perennial snow, and runaway glaciation does not develop. This is also qualitatively consistent with our previous results of the CCSM3 model. These results suggest that a "soft Snowball" solution for the Neoproterozoic is entirely plausible and may in fact be preferred.

Description: Geochemical and geological evidence has suggested that several global-scale glaciation events occurred during the Neoproterozoic Era in the interval from 750–580 million years ago. The initiation of these glaciations is thought to have been a consequence of the combined influence of a low level of atmospheric carbon dioxide concentration and an approximately 6% weakening of solar luminosity. The latest version of the Community Climate System Model (CCSM4) is employed herein to explore the detailed combination of forcings required to trigger such extreme glaciation conditions under present-day circumstances of geography and topography. It is found that runaway glaciation occurs in the model under the following conditions: (1) an 8–9% reduction in solar radiation with 286 ppmv CO2 or (2) a 6% reduction in solar radiation with 70–100 ppmv CO2. These thresholds are moderately different from those found to be characteristic of the previously employd CCSM3 model reported recently in Yang et al. (2012a,b), for which the respective critical points corresponded to a 10–10.5% reduction in solar radiation with 286 ppmv CO2 or a 6% reduction in solar radiation with 17.5–20 ppmv CO2. The most important reason for these differences is that the sea ice/snow albedo parameterization employed in CCSM4 is believed to be more realistic than that in CCSM3. Differences in cloud radiative forcings and ocean and atmosphere heat transports also influence the bifurcation points. These results are potentially very important, as they are to serve as control on further calculations which will be devoted to an investigation of the impact of continental configuration. We demonstrate that there exist ''soft Snowball'' Earth states, in which the fractional sea ice coverage reaches approximately 60–65%, land masses in low latitudes are covered by perennial snow, and runaway glaciation does not develop. This is consistent with our previous results based upon CCSM3. Although our results cannot exclude the possibility of a ''hard Snowball'' solution, it is suggested that a ''soft Snowball'' solution for the Neoproterozoic remains entirely plausible.

Description: This study analyses the response of the Atlantic meridional overturning circulation (AMOC) to LGM forcings and boundary conditions in nine PMIP coupled model simulations, including both GCMs and Earth system Models of Intermediate Complexity. Model results differ widely. The AMOC slows down considerably (by 20–40%) during the LGM as compared to the modern climate in four models, there is a slight reduction in one model and four models show a substantial increase in AMOC strength (by 10–40%). It is found that a major controlling factor for the AMOC response is the density contrast between Antarctic Bottom Water (AABW) and North Atlantic Deep Water (NADW) at their source regions. Changes in the density contrast are determined by the opposing effects of changes in temperature and salinity, with more saline AABW as compared to NADW consistently found in all models and less cooling of AABW in all models but one. In only two models is the AMOC response during the LGM directly related to the response in net evaporation over the Atlantic basin. Most models show large changes in the ocean freshwater transports into the basin, but this does not seem to affect the AMOC response. Finally, there is some dependence on the accuracy of the control state.

Description: One of the critical issues of the Snowball Earth hypothesis is the CO2 threshold for triggering the deglaciation. Using Community Atmospheric Model version 3.0 (CAM3), we study the problem for the CO2 threshold. Our simulations show large differences from previous results (e.g. Pierrehumbert, 2004, 2005; Le Hir et al., 2007). At 0.2 bars of CO2, the January maximum near-surface temperature is about 268 K, about 13 K higher than that in Pierrehumbert (2004, 2005), but lower than the value of 270 K for 0.1 bar of CO2 in Le Hir et al. (2007). It is found that the difference of simulation results is mainly due to model sensitivity of greenhouse effect and longwave cloud forcing to increasing CO2. At 0.2 bars of CO2, CAM3 yields 117 Wm−2 of clear-sky greenhouse effect and 32 Wm−2 of longwave cloud forcing, versus only about 77 Wm−2 and 10.5 Wm−2 in Pierrehumbert (2004, 2005), respectively. CAM3 has comparable clear-sky greenhouse effect to that in Le Hir et al. (2007), but lower longwave cloud forcing. CAM3 also produces much stronger Hadley cells than that in Pierrehumbert (2005). Effects of pressure broadening and collision-induced absorption are also studied using a radiative-convective model and CAM3. Both effects substantially increase surface temperature and thus lower the CO2 threshold. The radiative-convective model yields a CO2 threshold of about 0.21 bars with surface albedo of 0.663. Without considering the effects of pressure broadening and collision-induced absorption, CAM3 yields an approximate CO2 threshold of about 1.0 bar for surface albedo of about 0.6. However, the threshold is lowered to 0.38 bars as both effects are considered.

Description: Some geochemical and geological evidence suggests that the concentration of atmospheric oxygen was only 1–10% of the present level in the time interval from 750 to 580 million years ago when several nearly global glaciations or Snowball Earth events occurred. This low concentration of oxygen would have been accompanied by lower ozone concentration than present. Since ozone is a greenhouse gas, this change in ozone concentration would alter surface temperature, and thereby could have an important influence on the climate of the Snowball Earth. Previous works for either initiation or deglaciation of the proposed Snowball Earth have not taken the radiative effects of ozone changes into account. We address this issue herein by performing a series of simulations using an atmospheric general circulation model with various ozone concentrations. Our simulation results demonstrate that as ozone concentration is uniformly reduced from 100% to 50%, surface temperature decreases by approximately 0.8 K at the equator, with the largest decreases located in the middle latitudes reaching as high as 2.5 K, primarily due to a strengthened snow-albedo feedback. When ozone concentration is reduced and its vertical and horizontal distribution is simultaneously modulated, surface temperature decreases by 0.4–1.0 K at the equator and by 4–7 K in polar regions. These results suggest that ozone has significant effects on the climate during the Neoproterozoic glaciations.

Description: Some geochemical and geological evidence has been interpreted to suggest that the concentration of atmospheric oxygen was only 1–10 % of the present level in the time interval from 750 to 580 million years ago when several nearly global glaciations or Snowball Earth events occurred. This low concentration of oxygen would have been accompanied by a lower ozone concentration than exists at present. Since ozone is a greenhouse gas, this change in ozone concentration would alter surface temperature, and thereby could have an important influence on the climate of the Snowball Earth. Previous works that have focused either on initiation or deglaciation of the proposed Snowball Earth has not taken the radiative effects of ozone changes into account. We address this issue herein by performing a series of simulations using an atmospheric general circulation model with various ozone concentrations. Our simulation results demonstrate that, as ozone concentration is uniformly reduced from 100 % to 50 %, surface temperature decreases by approximately 0.8 K at the Equator, with the largest decreases located in the middle latitudes reaching as high as 2.5 K. When ozone concentration is reduced and its vertical and horizontal distribution is simultaneously modulated, surface temperature decreases by 0.4–1.0 K at the Equator and by 4–7 K in polar regions. These results here have uncertainties, depending on model parameterizations of cloud, surface snow albedo, and relevant feedback processes, while they are qualitatively consistent with radiative-convective model results that do not involve such parameterizations and feedbacks. These results suggest that ozone variations could have had a moderate impact on the climate during the Neoproterozoic glaciations.

Description: The simulation of the Atlantic thermohaline circulation (THC) during the Last Glacial Maximum (LGM) provides an important benchmark for models used to predict future climatic changes. This study analyses the THC response to LGM forcings and boundary conditions in nine PMIP simulations, including both GCMs and Earth system Models of Intermediate Complexity. It is examined whether the mechanism put forward in the literature for a glacial THC reduction in one model also plays a dominant role in other models. In five models the THC reduces during the LGM (by 5–40%), whereas four models show an increase (by 10–40%). In all models but one a reduced (enhanced) THC goes with a stronger (weaker) reversed deep overturning cell associated with the formation of Antarctic Bottom Water (AABW). It is found that a major controlling factor for the THC response is the density contrast between AABW and North Atlantic Deep Water (NADW) during the LGM as compared to the modern climate. More saline AABW is consistently found in all simulations, while all models but one show less cooling of AABW as compared to NADW. In five out of nine models a reduced (enhanced) THC during the LGM is associated with more (less) dense AABW at its source region, which in turn is determined by the balance between the opposing effects of salinity and temperature on the density of AABW versus that of NADW. The response in net evaporation over the Atlantic basin is relatively small in most models, so that changes in the freshwater budget are dominated by ocean transports. In only two models is the THC response during the LGM directly related to the response in net evaporation.