Kim Cobb and two team members, clad in black scuba gear, have been scouring the coral-studded seabed near the equatorial Pacific's Christmas Island here for nearly an hour. Then Cobb emerges with a victorious "Yes!"
A few minutes later, Cobb, a paleoclimatologist at the Georgia Institute of Technology in Atlanta, dives back to the bottom and, from under a coral head, extracts the prize: two small containers encrusted with coralline algae. Inside are recorders of salinity and temperature that captured in excruciating detail the 2015–16 El Niño event, which brought a pulse of abnormally warm water to the tropical Pacific. The recorders showed that during the disturbance, which wreaked climatic havoc around the globe, the warming here set a record: 3°C above normal. The extreme warmth, Cobb says, reflected not just the natural El Niño cycle, but a new factor: global warming caused by human activity.
As she will report next week at the American Geophysical Union meeting in San Francisco, California, a detailed, long-term temperature record derived from corals on Christmas and other Pacific islands shows that over the last 7000 years, El Niños waxed and waned. Then, during the 20th century, with global warming taking hold, their intensity began to climb. The trend is likely to continue, boding ever more destructive El Niños, she says. "It's yet another impact of global warming that we'd like to avoid."
Gavin Schmidt, director of NASA's Goddard Institute for Space Studies in New York City, says Cobb's temperature archive offers the first historical picture of El Niño and its changes. "It's unique and gives a fascinating window into an otherwise totally obscure, but vitally important, part of climate history."
El Niños arrive every 3 to 7 years when winds fail in the tropical Pacific, allowing warm water to pool in the eastern part of the ocean. The oceanic warmth disrupts fisheries and can bleach and kill corals—85% of Christmas Island's corals may have died in the latest El Niño, which ended in May. El Niños also trigger widespread floods and droughts that, during the extreme El Niño of 1997–98, caused $35 billion in damage and claimed an estimated 23,000 lives.
Yet no one knew whether the gradual warming of the globe is intensifying these events, in part because records are short and spotty in the remote parts of the Pacific where El Niño hits hardest. To get the long view, Cobb and her collaborators gathered hundreds of lumps of old coral washed up on beaches on Christmas and Fanning islands (both part of the Republic of Kiribati) and the U.S. island of Palmyra. By applying uranium-thorium dating to the corals and measuring ratios of oxygen isotopes in their skeletons, her lab reconstructed ocean temperatures for much of the last 7000 years.
During that time, Cobb says, "all kinds of stuff was going on in the climate, but it had no discernible effect on El Niño events." But the corals, supplemented by sensors like the one Cobb recovered off Christmas, show that over the past century El Niño intensity has increased by 25%. "There is no century even remotely resembling the 20th in the record for at least 5000 years," she says.
Cobb's finding is consistent with a 2013 study of tree rings suggesting that El Niño–related weather havoc has intensified across much of the globe in recent decades, notes Wenju Cai, a climate modeler at the Commonwealth Scientific and Industrial Research Organisation in Melbourne, Australia. But the tree ring record is too short, he says, to show whether global warming is to blame. Cobb's 7000-year archive, in contrast, "clearly shows that 20th century El Niños are more extreme and intense than they were before the industrial era, and that points to global warming as a cause."
Eric Guilyardi, a climate scientist at the Pierre Simon Laplace Institute in Paris, hopes Cobb's results will inspire others to develop similar temperature records elsewhere in the equatorial Pacific. "This will give us the spatial view needed to be sure El Niño is indeed changing."
Now, Cobb plans to push her temperature record into the future. "We're now tracking 30 corals that have survived this latest El Niño," she says. They're waiting to record the next one—and even hotter seas to come.