In 1983, a heat wave struck the waters of the eastern Pacific Ocean, resulting in a die-off that killed 95% of the corals in the Galápagos Islands. Fifteen years later, intense rains drenched Kenya, washing away roads and railways and causing billions of dollars in crop damage.
Linking these disasters is El Niño, said paleoclimatologist Julia Cole of the University of Michigan. This warm phase in a multiyear cycle of heating and cooling takes place in the tropical waters of the eastern Pacific, but it triggers droughts, floods, heat waves, and other climate extremes across the globe.
A new analysis of Galápagos corals by Cole and her colleagues revealed that sea surface temperatures in the eastern Pacific have been swinging between more intense highs and lows in the past few decades than at any time in the past thousand years. This strong tie between the intensity of El Niño and climate change may help refine models that predict future El Niño patterns. The researchers will present their work today (15 December) at AGU’s Fall Meeting 2022.
Records Locked in Corals
Every year, waters in the eastern Pacific heat or cool in a cycle known as the El Niño–Southern Oscillation (ENSO). Meteorologists eagerly monitor the magnitude of these shifts in ocean temperature; the cooler pattern, called La Niña, can also set off far-flung weather events.
However, little was known about the history of these cycles in the Galápagos Islands, which lie at ground zero for ENSO fluctuations. To fill these gaps, Cole and her colleagues turned to corals. As corals grow, they lay down thin layers of the mineral calcium carbonate. These layers, preserved in the corals’ skeletons, retain a chemical memory of the surrounding seawater conditions, including temperature.
Using large gas-powered hand drills, the researchers extracted samples from living and fossil corals in the Galápagos. Their analysis relied on the levels of different isotopes of oxygen and the ratios of calcium to strontium within the layers, which are both known tracers of marine temperature conditions. The technique exposed patterns of ocean temperatures with seasonal precision over several decades for every coral sample.
“I’m not going to shy away from saying that this is a result of human-caused climate change.”
The set of samples recorded temperature shifts dating back to 1050 CE. For almost 900 years, ENSO cycles varied by more or less the same amount in the Galápagos—an average of 0.9°C of warming or cooling with each cycle. But starting in 1900, the magnitudes of those temperature variations crept up. Since the 1990s, corals have recorded temperature swings that are 30%–50% more intense than those in the past thousand years.
The jump in variability and the clarity of the trend surprised the team, Cole said. The results confirmed that recent increases in variability were not just the result of a longer natural cycle or fluctuation. “I’m not going to shy away from saying that this is a result of human-caused climate change,” she said.
“I definitely think [the coral study] is a significant advance,” said Kim Cobb, director of the Institute at Brown for Environment and Society at Brown University in Providence, R.I., who was not involved with the study. “It’s still exceedingly rare to be able to look back so far into the history of the El Niño–Southern Oscillation.”
Extreme by Association
Though the study focused on the Galápagos, the implications are far-reaching. “ENSO got more extreme,” said climate scientist Kris Karnauskas of the University of Colorado Boulder, who was not involved with the research. “That means anything that’s connected to it got more extreme by association.”
Current climate forecasts are divided about whether ENSO cycles will become more or less intense in the future. Accurate models could be essential to understanding and predicting destructive weather events caused by ENSO. Whether the cycle will grow stronger in the future “is still a very thorny issue with extremely high stakes,” said Cobb. Data sets like those from the Galápagos corals “are really going to help modelers understand the key drivers in terms of El Niño intensities.”
—Luis Melecio-Zambrano (@ZambranoMelecio), Science Writer