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Indian Ocean warming from the peak of the Little Ice Age to the 21st century: Zanzibar Island, Western Indian Ocean (IndOC-W)

Left: Coral core drilling in Zanzibar (by Prof. Dr. Jens Zinke). Right: Spatial correlation map of Zanzibar SST with global SST during the 20th century (Plotted with the KNMI Climate Explorer).

The Indian Ocean, which gathers one-third of the world’s population around its rim, has been warming faster than any other ocean basin during the 20th century and is strongly coupled to global mean temperatures. The mechanisms underlying this steady warming are not fully understood, in part due to uncertain/sparse historical temperature data, but the current rise in atmospheric CO2 clearly plays an important role. Indian Ocean warming affects the hydrology in the countries around its rim and threatens coral reef ecosystems, which experienced repeated thermal stress events that caused coral bleaching and mass mortality. These thermal stress events are caused by year-to-year modes of climate variability inherent in the Indo-Pacific. It is therefore vital to reconstruct Indian Ocean sea surface temperatures beyond the period influenced by anthropogenic forcing, in order to determine the onset of Indian Ocean warming and its relationship with global surface temperatures in the pre-industrial. Massive-growing tropical corals such as Porites form annual growth bands and preserve geochemical proxies of sea surface temperature (SST) and hydrology in their aragonite skeletons. Replicated, multi-core coral chronologies from the tropical Indian Ocean can provide SST reconstructions (including uncertainties) that extend over several centuries and can be used to evaluate historical temperature trends, the onset of anthropogenic warming and its impact on hydrology and coral growth. IndOC-W aims to analyse a large set of long-lived massive Porites corals from Zanzibar Island to establish the first multi-core Sr/Ca-SST reconstruction from the tropical Indian Ocean that covers more than 300 years and extends into the coldest interval of the Little Ice Age. In addition, we will measure coral δ18O in selected time windows to investigate the impact of extreme temperatures in the Indian Ocean on hydrology in equatorial East Africa. In addition, this study will be the first to assess the impact of past thermal stress events on trace element signatures in Porites skeletons by using a new LA-ICP-OES/MS technique that will be developed at Kiel University. In a companion project (IndOC-E), T. Felis will reconstruct eastern Indian Ocean temperature and hydrology using corals from the Andaman Sea. Results of IndOC-W and IndOC-E allow us to estimate (I) the basin-scale warming of the Indian Ocean from the Little Ice Age to the present and (II) spatial variations in warming rates relative to the basin mean. This is crucial to assess the impact of Indian Ocean warming on hydrology in the countries bordering the Indian Ocean and to validate climate model results. Our project significantly contributes to the overall scientific mission of the SPP 2299, namely to “understand the interaction between global climate change and modes of tropical climate variability, and their combined impact on coral reef ecosystems and tropical societies in a warming world.”

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