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| Titel |
A 500-year record of summer cloud cover from stable carbon isotopes in Norwegian tree-rings |
| VerfasserIn |
G. H. F. Young, D. McCarroll, N. J. Loader, A. J. Kirchhefer |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250026912
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| Zusammenfassung |
| Tree-ring stable carbon isotope ratios (δ13C) in moist environments are likely to be driven by
photosynthetic rate, and thus sunshine, rather than temperature, should represent the most
direct controlling factor. Due, often, to the lack of availability of long instrumental records of
sunshine δ13C have, however, typically been calibrated with and, therefore, used to
reconstruct growing season temperatures. This, often necessary, but imperfect, interpretation
relies on the assumption that temperature and sunshine co-vary. This is tested using a
δ13C series from Scots pine trees (Pinus sylvestris L.) in NW Norway, where long
(>100 yr) records of both summer temperature and cloud cover are available. It is
demonstrated that when summer temperature and δ13C diverge, summer temperature
and cloud cover also diverge, suggesting that cloud cover, and not temperature,
provides the stronger and more consistent parameter with which to calibrate tree-ring
δ13C series in this area. When a 500-year reconstruction of summer cloudiness
is compared with a published reconstruction of summer temperatures, based on
tree-ring maximum densities, in northern Sweden, the two series largely agree with
high levels of annual-decadal coherence. There are, however, three distinct periods
of lower frequency divergence; two (AD 1600-1650 and AD 1900-1927) when
it appears likely that summers were cool but sunny, and one during the first half
of the 16th century when summers were warm but cloudy. These periods where
the covariance between temperature and sunshine are uncoupled may represent
large scale changes in circulation as recorded in the Arctic Oscillation (AO) index.
Strongly negative values of the summer AO index, as in the early 20th century, are
associated with persistent high pressure over northern Norway and Fennoscandia,
bringing cool summers with clear skies. Long reconstructions of cloudiness from
tree-ring δ13C series at suitable locations, may prove extremely valuable for testing
General Circulation Models (GCMs), as the generation of cloud is a strong control
on temperature evolution, but remains one of the greatest sources of uncertainty. |
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