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| Titel |
The initiation of Neoproterozoic "snowball" climates in CCSM3: the influence of paleocontinental configuration |
| VerfasserIn |
Y. Liu, W. R. Peltier, J. Yang, G. Vettoretti |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1814-9324
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| Digitales Dokument |
URL |
| Erschienen |
In: Climate of the Past ; 9, no. 6 ; Nr. 9, no. 6 (2013-11-14), S.2555-2577 |
| Datensatznummer |
250085260
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| Publikation (Nr.) |
 copernicus.org/cp-9-2555-2013.pdf |
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| Zusammenfassung |
| 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. |
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