Indian Ocean corals reveal crucial role of World War II bias for twentieth century warming estimates

M Pfeiffer,W.-C Dullo,D Garbe-Schönberg,M E Weber,J Zinke,M Latif

doi:10.1038/s41598-017-14352-6

Abstract

The western Indian Ocean has been warming faster than any other tropical ocean during the 20th century, and is the largest contributor to the global mean sea surface temperature (SST) rise. However, the temporal pattern of Indian Ocean warming is poorly constrained and depends on the historical SST product. As all SST products are derived from the International Comprehensive Ocean-Atmosphere dataset (ICOADS), it is challenging to evaluate which product is superior. Here, we present a new, independent SST reconstruction from a set of Porites coral geochemical records from the western Indian Ocean. Our coral reconstruction shows that the World War II bias in the historical sea surface temperature record is the main reason for the differences between the SST products, and affects western Indian Ocean and global mean temperature trends. The 20th century Indian Ocean warming pattern portrayed by the corals is consistent with the SST product from the Hadley Centre (HadSST3), and suggests that the latter should be used in climate studies that include Indian Ocean SSTs. Our data shows that multi-core coral temperature reconstructions help to evaluate the SST products. Proxy records can provide estimates of 20th century SST that are truly independent from the ICOADS data base.

Highlights

Surface warming of the Indian Ocean is larger than the global average sea surface temperature (SST) rise[1,2]
Differences between SST products are largest during and after World War II (WW II) and appear as a warm anomaly followed by an abrupt cooling after 1945 in some SST products (Fig. 1b; Supplementary Fig. S5), while others show a steady warming during the first half of the 20th century followed by a slight cooling trend that ends with a shift towards warmer temperatures in the mid-1970s (Fig. 1c; Supplementary Fig. S5)
Our multi-core coral reconstruction comprises a set of Porites coral oxygen isotope (δ18O; The Seychelles, 50°E, 5°S)[10,11] and Sr/Ca records (Chagos Archipelago, 70°E, 5°S)[12,13] covering the past 50 to 155 years (Fig. 2a,b, Supplementary Figs S1–S4)

Summary

Introduction

Maximum warming occurs in the central equatorial region, along the so-called Seychelles-Chagos thermocline dome[2,3], and appears to be caused by a decrease of upwelling-related oceanic cooling in addition to anthropogenic forcing[2,4]. In the western Indian Ocean, the World War II anomalies dominate the 20th century SST trend. They appear prominently in the global mean SST record (Fig. 1b), and this should reflect an impact of the Indian Ocean on global mean SST. With high confidence 20th century western Indian Ocean warming in order to evaluate its impact on regional and global temperature variability. We can evaluate and compare 20th century warming patterns in the coral and historical temperature datasets from various sources

Methods

Results

Conclusion