A recent study reveals a potentially important climate tipping point: The occurrence of more than five major El Niño events per century causes dramatic changes in marine and terrestrial animal life. As climate change spurs stronger, more frequent El Niño events, understanding such tipping points could help predict when fisheries and coastal habitats will change. The research was published in Science.
Today, towering kelp forests tickle the coast from Vancouver down to San Diego, as well as the shorelines of Peru, Ecuador, and Chilean Patagonia. Stellar sea lions chase colorful rockfish and calico bass through the kelp branches. Abalone and blue mussels cling to the rocks and tide pools. This diverse food web hinges on a wind system that pushes warm surface waters away from the coast, in turn causing the upwelling of colder, deeper water that carries nutrients to the surface.
In years when El Niño occurs—those infamous climate phenomena that bring warm, wet conditions to the west coasts of North and South America—these winds stall, and the layer of warm surface water thickens, exposing many coldwater fish and mollusks to high temperatures. Warm temperatures also bring heavy rains, which nourish plant, insect, and inland bird diversity in turn.
Thousands of years of this coastal history is recorded in layers of fish and bird bones, scattered under a rock shelter called Abrigo de los Escorpiones, or “shelter of the scorpions,” in Baja California, about 95 miles south of Tijuana. The site is a rocky overhang; raptors living up above have dropped numerous animal carcasses over the centuries, according to lead author Jack Broughton, a zooarchaeologist at the University of Utah in Salt Lake City. People have also lived there intermittently. Broughton’s research group has worked at Escorpiones since the early-2000s, where they’ve derived 97 radiocarbon dates from bone and charcoal fragments, allowing them to date the entire bone collection back 12,000 years.
In their recent study, Broughton and collaborators compared this archaeological record with ancient climate records, also dating back 12,000 years, from Laguna Pallcacocha, a glacier-carved basin sitting high in the Ecuadorian Andes. The warm conditions of El Niño bring heavy rains to Pallcacocha, eroding the mountainsides into the lake and leaving a record of much thicker sediment layers in El Niño years than in normal years.
Some 12,000 years of habitat change are recorded in layers of bone fragments of the Escorpiones archaeological site in Baja, CA. Image credit: Isaac Hart.
One “potential weakness” of the study, Broughton says, is that there is only one 12,000-year continuous record of El Niño, the one found in Pallcacocha—which is located thousands of miles from Baja.
But the scale of El Niño events should make it feasible to glean effects across great distances. El Niño’s impacts occur up and down the coast, meaning a big event impacts both South America and California—so it’s “not so crazy,” Broughton says, to compare a faunal record from Baja with an El Niño record from Ecuador. He and collaborators scrutinized the evidence century-by-century, to count how many El Niño events occurred and to assess the relative abundance of marine birds versus terrestrial birds, and coldwater fish versus warmer water generalist fish, over time. Based on these comparisons, Broughton uncovered an unsettling pattern.
The relative abundance of marine birds and coldwater fish plunged dramatically in centuries with five or more strong El Niño events, in favor of a few dominant groups: generalist fishes such as surfperch, as well as land-associated birds such as quail. Broughton also found that once low diversity conditions occur, they tend to remain relatively stable.
This latest work isn’t the first to suggest a climatic and faunal tipping point driven by El Niño. Previous studies in South America hinted at the idea in 2001 and in 2020. A group led by archaeologist Dan Sandweiss at the University of Maine, in Orono, for instance, found mollusk declines in Peru coinciding with a period of frequent El Niño events about 2,900 years ago in the 2001 study. But before this recent article, archaeologists hadn’t derived the precise number of events, Sandweiss says. Now that researchers have confirmed that there is a tipping point, the next step is to refine it with more analyses of animal remains from more sites, Sandweiss writes in an accompanying perspective.
Keeping an eye on that tipping point is especially important now, Broughton says, because climate change is expected to make El Niño events stronger and more frequent. Researchers, he adds, would like to discern when we might cross a threshold of five or more strong events per century and what coastal resource managers can do to prepare. His research group is now analyzing more bone fragments from marine mammals and rodents collected at the Escorpiones site, to test whether those fauna point to the same threshold of five events per century.
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