Analysing The History Of The Indian Ocean With Cruise Records, Sediment Cores
Dec 5, 2023 | Pratirodh Bureau
Whaling logbook at the Providence Public Library (Photo by Jayne Doucette/Woods Hole Oceanographic Institution)
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 sub-surface 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 been recent, while its Atlantic and Pacific counterparts are far better understood.
Historical records such as cruise records extend the available observational record back more than a century, providing important baseline data to study human-caused changes in the Indian Ocean as it warms rapidly, with far-reaching effects on weather and climate.
Researchers from several fields use the term ‘Anthropocene’ informally to refer to the current geological time interval, in which human activity is influencing Earth’s conditions and processes. This proposed new geological epoch, according to geologists, began when humans started altering the planet with various forms of industrial and radioactive materials in the 1950s.
“The Indian Ocean basin could be considered as a canary in a coal mine because changes that are now being observed in this ocean basin could also happen in other oceans,” states oceanographer Caroline Ummenhofer at the Woods Hole Oceanographic Institution in the United States.
Ummenhofer mines climate clues tucked away in weather data logged by 19th century American whalers who also voyaged to the Indian Ocean. The New England whaling ship logbooks extend the knowledge of weather conditions over the oceans back to the early 1800s. “This long-term context is essential to understand what aspects of shifting wind and weather patterns observed today might be due to natural variability and which ones due to human-induced climate change,” she adds.
This distinction has important implications for climate risk assessments for vulnerable Indian Ocean rim communities because how one prepares for changes in the monsoons or storm impacts along the coast, for example, will look different when one anticipates conditions to be adverse only for a few years or a decade before reverting again due to natural variability.
“Instead, when we know to expect a trend of progressively worsening conditions and increased climate risks for decades to come, communities need to prepare quite differently,” she says.