A colony of Porites lobata coral was quietly building itself, ring by ring, year by year, and absorbing the chemistry of the surrounding water off the northeastern tip of the Philippines, close to the point where the Kuroshio Current bends westward and presses into the South China Sea. No one was observing. No one was taking measurements. However, the record was being maintained. Because of this, the results that were published in Science Advances last February seem to be more than just a significant scientific achievement. The ocean seems to have finally given up its own journal.
Drilling a core through one of these coral colonies off Palaui Island, researchers from the Institute of Oceanology of the Chinese Academy of Sciences extracted oxygen isotope data from 1894 to 2022. The data created a continuous monthly reconstruction of what oceanographers refer to as the Luzon Strait Transport—the amount of water that pushes through the main gateway between the Pacific Ocean and the South China Sea—when combined with satellite altimeter records from the previous thirty years. Seldom has this number been directly observed over extended periods of time. Scientists now have 129 years of it for the first time.
The discovery in the headline is not subtle. Since the late nineteenth century, the South China Sea Throughflow—that enormous, imperceptible flow of freshwater and heat that connects the Pacific and Indian Oceans—has slowed by more than a third. Over the course of the entire period, the total decline is 1.81 Sverdrups, falling at a rate of roughly 0.14 Sv every ten years. Physically speaking, one Sverdrup is equivalent to about a million cubic meters of water per second. The startling figures point to a system that is under constant, ongoing strain.
The intensification of trade winds across the tropical western Pacific is the driver, according to the team’s analysis using a theory known as the Time-Dependent Island Rule. It seems that global warming has forced the tropical Pacific into what scientists refer to as “Cold Tongue Mode,” a pattern in which the eastern equatorial Pacific cools while the western Pacific warms. The trade winds have been tightened by this temperature differential, creating anomalies in wind stress that cause downwelling Rossby waves to roll eastward and increase sea-level pressure east of the Philippines. An anticyclonic circulation that cascades through the system prevents the Kuroshio Current from penetrating the South China Sea as deeply as it formerly did. Weaker throughflow results from less intrusion. Before anyone had instruments in the water to detect it, a series of events started.
This seems to be precisely the kind of discovery that is initially undervalued. It sounds abstract when a current slows down. The South China Sea, however, is not an abstract area. Its throughflow controls heat budgets, salinity structures, eddy behavior, and the cycling of nutrients that marine life depends on. It is one of the world’s most biologically productive and geopolitically active ocean basins. Reduced phytoplankton growth and decreasing chlorophyll-a concentrations in the basin—the base of the food chain, subtly thinning—have already been linked to the weakening Kuroshio intrusion. Fish habitats are changing. Although the extent of those ecological changes is still unknown, the trend is not promising.
The water renewal time in the South China Sea has also changed, albeit in ways that are more difficult to observe directly. The water in the sea used to cycle about every 2.6 decades. Due to a decrease in inflow, that figure has now increased to 3.5 decades. Slower renewal results in different thermocline structures, slower heat and chemical flushing, and possibly rearranged air-sea interactions that contribute to the East Asian monsoon. Naturally, the coral was unaware of all of this. It was simply expanding and recording, just like reefs do. It’s difficult to ignore the subtle irony that, despite a century of industrial change, a non-human organism anchored to the Philippine seafloor was the best witness to it all.
The reconstruction, according to NAN Feng, the lead author of the study, establishes the first centennial-scale baseline for the throughflow—a method of knowing where the system has been before attempting to predict its future. With this longer view, future models might be able to make more accurate predictions about how the Indo-Pacific circulation will change as global warming persists. A strengthening Indonesian throughflow may be counterbalanced by the slowing South China Sea throughflow, indicating that the two systems are changing simultaneously and compensating, adjusting, and balancing in ways that oceanographers are just now starting to map.
Drilled from a reef off a small island in the Philippines, a single coral core provided science with a window into over a century of ocean history. Perhaps the most interesting part is that the ocean was changing long before there were satellites to observe it.
