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| Titel |
Clouds and the Faint Young Sun Paradox |
| VerfasserIn |
C. Goldblatt, K. J. Zahnle |
| 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 ; 7, no. 1 ; Nr. 7, no. 1 (2011-03-04), S.203-220 |
| Datensatznummer |
250004406
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| Publikation (Nr.) |
 copernicus.org/cp-7-203-2011.pdf |
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| Zusammenfassung |
| We investigate the role which clouds could play in resolving the Faint
Young Sun Paradox (FYSP). Lower solar luminosity in the past means
that less energy was absorbed on Earth (a forcing of
−50 W m−2 during the late Archean), but geological
evidence points to the Earth having been at least as warm as it is today,
with only very occasional glaciations. We perform radiative
calculations on a single global mean atmospheric column. We select
a nominal set of three layered, randomly overlapping clouds, which are
both consistent with observed cloud climatologies and reproduced the
observed global mean energy budget of Earth. By varying the fraction,
thickness, height and particle size of these clouds we conduct a wide
exploration of how changed clouds could affect climate, thus
constraining how clouds could contribute to resolving the FYSP. Low
clouds reflect sunlight but have little greenhouse effect. Removing
them entirely gives a forcing of +25 W m−2 whilst more
modest reduction in their efficacy gives a forcing of +10 to
+15 W m−2. For high clouds, the greenhouse effect
dominates. It is possible to generate +50 W m−2 forcing
from enhancing these, but this requires making them 3.5 times thicker
and 14 K colder than the standard high cloud in our nominal set and
expanding their coverage to 100% of the sky. Such changes are not
credible. More plausible changes would generate no more
than +15 W m−2 forcing. Thus neither fewer low clouds nor more
high clouds can provide enough forcing to resolve the FYSP. Decreased
surface albedo can contribute no more than +5 W m−2
forcing. Some models which have been applied to the FYSP do not
include clouds at all. These overestimate the forcing due to increased
CO2 by 20 to 25% when pCO2 is 0.01 to 0.1 bar. |
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