Coral and stalagmite records trace one 40-year Indian-Pacific climate break to volcanoes

The link under test runs from the Pacific into the Indian Ocean. Conditions across the Indian Ocean often respond to changes in the Pacific, and that response helps set rainfall, temperature and atmospheric circulation patterns across the tropics ^[2]. Since the 1980s the Indian Ocean has increasingly departed from what Pacific conditions would predict, a weakening researchers have linked to warming ^[3]. Judging whether that departure is unusual is the harder problem, because reliable instrumental records cover less than a century, too short a sample to separate an unusual event from a long natural cycle ^[4].

So the team reconstructed both basins from corals, tree rings and stalagmites, back to the early 1600s, and paired the reconstruction with model simulations ^[1]. That extension adds exactly one earlier case. Through most of the past 400 years the basins stayed closely connected, and only between 1810 and 1850 did the relationship change significantly, which the researchers link to a series of major tropical eruptions that weakened the Pacific's usual influence ^[5]. The window is 40 years ^[1], about a tenth of the reconstructed period ^[2]. It also falls outside instrumental coverage: a record reaching back less than a century begins no earlier than about 1926, some 76 years after that episode ended ^[3].

Proxies establish coincidence in time. The causal case comes from the model runs, and simulations covering the past thousand years supported the volcanic interpretation ^[6]. Those runs also found that the strength of the disruption depended on both eruption size and the background climate state at the time ^[7], so a future eruption of similar magnitude need not produce a similar weakening.

"The modern data we have is limited and doesn't go back far enough. With climate models and paleo-records, we are now able to say with more confidence that the recent changes we are seeing are really quite exceptional," said lead author Shawn Wang ^[8], a former WHOI graduate student now a postdoc at the University of Colorado Boulder ^[9]. The release describes the modern shift as exceptional ^[15]. It gives no correlation coefficient, and does not say what window the comparison covers or where the modern value falls in the reconstructed distribution ^[4]. The paper's title keeps the peer-reviewed claim on the older event: "Coupling of Pacific and Indian Ocean variability disrupted by 19th century volcanism" ^[12].

For a seasonal forecast, the operational question is whether a Pacific-to-Indian Ocean relationship fitted on twentieth century data still holds. This record contains one earlier interval in which it did not ^[5], and the modern observations show the same relationship loosening again ^[3]. Measuring what that costs in forecast skill is beyond a 400-year proxy reconstruction.

Caroline Ummenhofer, a senior scientist at WHOI and a co-author ^[11], said the work is "one of the first studies to examine the breakdown in the connection between the Pacific and Indian oceans using evidence from past climates, modern observations, and climate models" ^[13]. She also said that "global warming and human emissions are now overwhelming the Pacific's natural influence on the Indian Ocean" ^[10].