Enormous swaths of ocean are nutrient-poor and harbor little life. Seen from space, these areas are as blue and empty as the sky. But green, in the ocean as on land, is one of life’s signature colors.
Most of the Pacific Ocean is unremittingly blue. But Woods Hole Oceanographic Institute (WHOI) climate scientist Kris Karnauskas was readjusting the viewing frame of his satellite maps one day in October 2010 when he spotted a tiny telltale smudge of green slicing through the azure sea. The green plume trailed from one of the Gilbert Islands, a group of coral atolls straddling the equator. Karnauskas had nearly missed it because his map was usually centered on the continents, so its edge nearly bisected the Gilberts; Karnauskas only saw the green blip when he shifted the Pacific Ocean to the center of his view. The lucky move sparked an idea connecting islands, ocean currents, and climate change.
“I’d seen the same color during my PhD research on the Galapagos,” explains Karnauskas, “so I knew exactly what it meant.”
The green plume trailed from one of the Gilbert Islands, a group of coral atolls straddling the equator.
Much like the Galapagos, the Gilbert Islands are situated directly in the Pacific’s easterly equatorial undercurrent, which essentially traverses the entire ocean, says Karnauskas. Because nutrients are concentrated near the ocean bottom, the presence of the algal plume meant that deep, cold water was being pushed to the surface as the currents encountered the islands. The water hits the islands “like air rushing over a mountain,” but because the islands are too tall to go over, it goes up and around, bringing nourishment to photosynthetic phytoplankton near the water’s surface that lend a chlorophyll-tinged hue to the currents sweeping past.
The upwelling cold water could be important for the Gilberts, reasoned Karnauskas and his collaborator, Anne Cohen, a coral researcher at WHOI. Ocean temperatures are rising, making life more difficult for delicate organisms like the corals that built the atoll. Corals exist in a symbiotic relationship with certain species of algae: the unicellular plants contribute nutrients to the coral while absorbing the colonial animals’ nutrient-rich waste. But when water temperatures are high for periods as long as several weeks or more, corals jettison the algae in a process known as “coral bleaching,” explains Simon Donner, a University of British Columbia climate scientist who studies the Gilberts’ corals. If they can’t replace their algal symbionts, corals die.
Karnauskas and Cohen wanted to determine how much colder the upwelling could make the ocean temperatures surrounding the Gilberts. But putting the tiny Gilberts into the context of global climate models took some doing, explains Karnauskas. Climate models that describe worldwide climate change aren’t detailed enough to take into account sea temperature changes happening around a few islands, most only a couple of miles across. So Karnauskas added in an ocean current model that could track nuances in sea temperature. He found that ocean temperatures around the Gilberts will lag behind the rest of the Pacific by about 25 percent, or more than half a degree Celsius, over the next century.
Cohen and Karnauskas think that the delayed warming around the Gilberts may be good news for coral. They hypothesize that the Gilberts could serve as “coral refuges,” where corals and their algae may receive a respite from the bleaching that is already impacting other reefs around the Pacific.
“It’s cool oceanography,” Donner says, “and the results make sense”—as far as cooler temperatures go. But the relationship between corals and high temperatures may be more complicated than warmer water causing coral die-off, Donner cautions. The Gilberts, for one thing, are subject to regularly vacillating temperatures as the result of El Niño, which is why Donner uses them as a model for coral response to warming. Donner’s work has actually shown a beneficial effect of higher temperatures—those corals that have been exposed to higher temperatures in the past, and survived, are more likely to survive future heat waves.
Donner also points out that 0.7° C over 100 years may not be enough to buffer the corals against larger-scale temperature changes. He notes that, as fine-grained as Karnauskas’s and Cohen’s models are, they are still lacking in crucial details, such as how close, exactly, the cold water is to shore. The Gilberts are the tips of volcanoes, just breaking the water surface, says Donner, and the coral growing on them is very close to the shore. If the cold water effect “is even a kilometer or two offshore, that may not be [close] enough,” he explains.
Karnauskas acknowledges the need for more detail about where the cool water is flowing, the precise temperature changes of the sea surrounding the Gilberts, and how this relates to specific coral ecosystems at the islands. This summer, the collaborators are planning a cruise to take more accurate measurements, comparing the east and west sides of the islands. They also hope to look back in time, Karnauskas says, by comparing coral growth rings to historical data on ocean temperatures.
While the climate projections generally look bad for corals, Donner is confident that coral reefs will survive—if not always in the form we’d like best. “They may not be as diverse or pretty,” he predicts, “but they will persist.”