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| Titel |
Evaluating 20th century warming trends with modern Porites corals from the western Indian Ocean |
| VerfasserIn |
M. Pfeiffer, J. Zinke, W.-C. Dullo  , S. Y. Cahyarini |
| 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 |
250024501
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| Zusammenfassung |
| Recent studies suggest that instrumental records of sea surface temperature (SST) are
unreliable prior to 1965 due to changes in the measurement procedures. Thompson et al.
(2008) identified an artificial cool bias of up to 0.3Ë C in global mean SSTs between 1945
and the mid-1960s.
Geochemical parameters in skeletons of massive corals can be used to infer past
changes in climate on seasonal to centennial time scales. The Sr/Ca ratio of coral
aragonite is a widely used tool for deriving high-resolution proxy records of past
sea surface temperatures. Application of the Sr/Ca paleothermometer relies on the
assumption that coral Sr/Ca varies predictably with temperature and that seawater
Sr/Ca is invariant on millennial timescales due to the long residence time of Sr and
Ca in the ocean. In contrast, the oxygen isotope ratios (δ18O) of coral aragonite
vary in response to temperature and changes in the δ18O of seawater, the latter
depending on the freshwater balance. Thus, coral δ18O may be used as a record
of past sea surface temperatures only at sites were δ18O seawater variations are
negligible.
SST in the western tropical Indian Ocean closely follows global mean temperature trends
(Funk et al., 2008). Here we present a set of Porites coral Sr/Ca and/or oxygen isotope
records from the tropical Indian Ocean covering the past 120-336 years (Seychelles, Chagos
Archipelago). We computed a composite sea surface temperature record for the Western
Indian Ocean using δ18O (Seychelles) and Sr/Ca (Chagos). This record clearly follows
instrumental SST trends in the Western tropical Indian Ocean, except in the 1945-1965
interval, were instrumental SST data show a pronounced cooling not evident in the coral
proxy index (the linear correlation coefficient between the coral index and instrumental SST
is r=0.86 if we omit the most problematic time period from 1945 to 1960). However, the coral
index follows the global land surface air temperature trend, which is free of systematic biases
due to changes in the measurement technique. We therefore conclude that the cooling of the
tropical Indian Ocean indicated by instrumental SST is an artifact that may result
from changes in the SST measurement techniques, as suggested by Thompson et
al., 2008). Our results will have important implications for the investigation of
large-scale teleconnections in the oceans (for example, the apparent shift in the Indian
Ocean SSTs around 1945 has been related to a regime change in the North Pacific
Ocean).
Thompson et al. (2008): A large discontinuity in the mid-twentieth century in observed
global-mean surface temperature, Nature, 453: 646-649, DOI:10.1038/nature06982.
Funk, C. et al. (2008): Warming of the Indian Ocean threatens eastern and southern
African food security but could be mitigated by agricultural development, PNAS, 105:
11081–11086, DOI: 10.1073_pnas.0708196105. |
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