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| Titel |
Sea-ice dynamics strongly promote Snowball Earth initiation and destabilize tropical sea-ice margins |
| VerfasserIn |
A. Voigt, D. S. Abbot |
| 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 ; 8, no. 6 ; Nr. 8, no. 6 (2012-12-21), S.2079-2092 |
| Datensatznummer |
250005989
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| Publikation (Nr.) |
 copernicus.org/cp-8-2079-2012.pdf |
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| Zusammenfassung |
| The Snowball Earth bifurcation, or runaway ice-albedo feedback, is
defined for particular boundary conditions by a critical CO2 and
a critical sea-ice cover (SI), both of which are essential for
evaluating hypotheses related to Neoproterozoic
glaciations. Previous work has shown that the Snowball Earth
bifurcation, denoted as (CO2, SI)*, differs greatly among
climate models. Here, we study the effect of bare sea-ice albedo,
sea-ice dynamics and ocean heat transport on (CO2, SI)*
in the atmosphere–ocean general circulation model ECHAM5/MPI-OM with
Marinoan (~ 635 Ma) continents and solar insolation (94%
of modern). In its standard setup, ECHAM5/MPI-OM initiates
a~Snowball Earth much more easily than other climate models at
(CO2, SI)* ≈ (500 ppm, 55%). Replacing the
model's standard bare sea-ice albedo of 0.75 by a much lower value
of 0.45, we find (CO2, SI)* ≈ (204 ppm,
70%). This is consistent with previous work and results from net
evaporation and local melting near the sea-ice margin. When we
additionally disable sea-ice dynamics, we find that the Snowball
Earth bifurcation can be pushed even closer to the equator and
occurs at a hundred times lower CO2:
(CO2, SI)* ≈ (2 ppm, 85%). Therefore, the
simulation of sea-ice dynamics in ECHAM5/MPI-OM is a dominant
determinant of its high critical CO2 for Snowball initiation
relative to other models. Ocean heat transport has no effect on the
critical sea-ice cover and only slightly decreases the critical
CO2. For disabled sea-ice dynamics, the state with 85%
sea-ice cover is stabilized by the Jormungand mechanism and shares
characteristics with the Jormungand climate states. However, there
is no indication of the Jormungand bifurcation and hysteresis in
ECHAM5/MPI-OM. The state with 85% sea-ice cover therefore is
a soft Snowball state rather
than a true Jormungand state. Overall, our results demonstrate that differences
in sea-ice dynamics schemes can be at least as important as
differences in sea-ice albedo for causing the spread in climate
models' estimates of the Snowball Earth bifurcation. A detailed
understanding of Snowball Earth initiation therefore requires future
research on sea-ice dynamics to determine which model's simulation
is most realistic. |
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