Studying the history of the Indian Ocean can shape an understanding of its future in a warming world

Almost 150 years ago, Christmas, aboard the SMS Gazelle, a mid-sized German warship that circumnavigated the globe in the late 19th century, was a day of activity. The vessel’s naval officers and surveyors were recording temperature data in the southern Indian Ocean.

Combined with cruise reports of two other German expeditions, Valdivia (1898-1899) and SMS Planet (1906-1907), the relatively lesser-known vessels yielded over 500 temperature observations in the Indian Ocean at depths spanning from the surface to the seabed.

Physical oceanographer Jacob Wenegrat who analysed the subsurface temperature data of the three expeditions, says cruise reports are underutilised data sources. “These techniques have been proven with the 1870s Challenger ocean expedition’s data (that were used to infer 20th century warming and cooling in the Atlantic and Pacific Oceans) and now with these German deep-sea expeditions,” he says.

Wenegrat’s team digitised the data from these three expeditions, making this historical information available to other researchers trying to unravel the complexities of ocean warming. These observations provide a first look at subsurface temperature change in the Indian Ocean over the 20th century.

The Indian Ocean plays a key role in influencing the Indian monsoon. The monsoon season forms the lifeline for millions of people’s water availability and agricultural activities. However, attention on the Indian Ocean has only...

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