Coral Skeletons Reveal a Stormier Climate
Uranium-thorium dating, a technique geologists developed to determine the age of ancient rocks and mineral deposits, relies on the radioactive decay of uranium into thorium at an incredibly steady rate. By measuring the ratio of these two elements, scientists can calculate how long ago a material formed. The method proved invaluable for dating geological formations, volcanic ash layers, and even early human artifacts. But when environmental scientists needed to know exactly when ancient corals lived, this geological dating tool became their key. The corals themselves were the unexpected bridge between geology and climate research.
The Galápagos Islands sit in the eastern Pacific Ocean, directly in the path of the strongest El Niño events. There, coral colonies grow their calcium carbonate skeletons in annual bands, much like tree rings. The chemistry of each band records the water temperature at the time it formed. Warmer water means less strontium gets incorporated into the skeleton. Oxygen isotopes tell a similar story, shifting with temperature changes. By extracting cores from both living and fossilized corals, researchers can read these chemical records. Living colonies provide data for recent decades, which scientists can verify against modern instruments. Dead corals, some washed up on beaches, extend the record far into the past. Some of these fossilized corals are hundreds of years old. Others reach back nearly a millennium.
Julie Cole, an environmental scientist at the University of Michigan, led the research team. Her group analyzed corals from five different islands in the Galápagos archipelago. The location matters enormously. “The Galápagos are located at 90 degrees west, and you have to go all the way to 160 degrees west to find another island in that near-equatorial zone,” Cole explained. This isolation makes the islands uniquely positioned to capture the full strength of eastern Pacific El Niño events. The corals there grow one to two centimeters per year. Sampling the extracted cores millimeter by millimeter yields more than a dozen measurements annually. The result is a monthly temperature log spanning the life of each coral colony. The team assembled 28 separate time series from these samples. Together, they cover scattered years stretching back to around 1100 CE.
A Thousand Years of Ocean Temperatures
The uranium-thorium dating method allowed the team to place each coral sample in time with remarkable precision. Even corals that are 500 years old can be dated to within a few years of accuracy. “We were also able to find corals that are dead and washed up on the beach,” Cole said. “They may be 100 years old, 200 years old, or 900 years old, or even older.” These beach specimens proved crucial for filling gaps in the record. The dating technique, originally developed for geology, became the backbone of a climate reconstruction. Without it, the team could not have distinguished between corals from different centuries. The method works because uranium dissolves readily in seawater while thorium does not. When corals build their skeletons, they incorporate uranium but almost no thorium. Over time, the uranium decays into thorium at a known rate. Measuring the thorium-to-uranium ratio reveals the coral’s age with remarkable accuracy.
The resulting temperature record is patchy but extraordinarily long. No continuous record of eastern Pacific temperatures exists for the past millennium. This reconstruction is the closest scientists have come to filling that gap. The data reveal something striking about the modern era. Temperature variability since 1984 runs 36.5 percent higher than it was during the preindustrial stretch from 1000 to 1850 CE. Even compared to the period from 1851 to 1982, the recent variability is 16.2 percent higher. All three intervals are statistically distinct from one another. The trend only goes upward. These numbers translate into dramatic changes in actual events. Events that once ran a degree or two above normal in the Galápagos now run three or four degrees above normal. The intensification is not subtle. It represents a fundamental shift in how the climate system behaves.
The corals also recorded something called skewness, which describes whether swings are lopsided in one direction. “The skewness turned positive,” Cole said. “And the positive skewness indicates that it’s the warmer events that got stronger.” This finding matters because it identifies which type of event is intensifying. It is not that both warm and cold swings are becoming more extreme. The warming events are the ones growing stronger. El Niño, the warm phase of the ENSO cycle, is the culprit. The cold counterpart, La Niña, does not show the same intensification. This asymmetry provides a fingerprint for what is happening in the climate system. It also helps scientists understand the mechanisms driving the change.
Models Confirm What Corals Suggest
The coral record alone could not prove that human activity caused the intensification. Natural climate variability might explain the pattern. ENSO fluctuates on its own, with quiet decades followed by wild ones. No external push is required for these natural swings. The question was whether recent decades represent something truly unusual, or whether the cycle simply happened to be caught in a more energetic period. To answer this, the team turned to climate models. They ran simulations covering the last millennium using only natural factors. Volcanoes and solar variability were included. Changes in greenhouse gas concentrations were left out entirely. This allowed the team to ask a specific question: how often does a climate untouched by human impact produce swings this large on its own?
The answer was clear. “We tried to see if there was some evidence for this in climate models, and we found none,” Cole said. The actual increase recorded in the corals sits outside the models’ internal variability. The confidence level approaches 99 percent. This means the current intensification would not happen without human influence. The models provided the control experiment that observational data could not. “We don’t have a direct source of observational data to compare our records to in the eastern Pacific,” Cole said. “If we had another long record, we wouldn’t be publishing this in Science.” The models filled that gap. They showed that natural variability alone cannot produce what the corals reveal.
The picture in the central Pacific is murkier. Corals from the Line Islands show a similar mean increase. But the data there have far wider scatter. The researchers attribute this partly to the recording method. Those records rely on oxygen isotopes alone. Heavy El Niño rainfall in the region can account for half or more of the signal. The rain changes the oxygen levels in the water. This complicates the temperature reading. But an ambiguous central Pacific record is exactly what climate models predict. A forced, human-caused increase should appear first in the eastern Pacific. “The faster emergence in the eastern Pacific is exactly consistent with the models,” Cole said. The pattern matches expectations.
What This Means for the Future
Global warming is already expected to intensify ENSO’s hydrological consequences. Wetter wet phases and drier dry ones are projected. This happens regardless of whether temperature swings themselves change. Even if El Niño’s temperature swings stayed exactly the same magnitude, its floods and droughts would still get worse. The reason is simple physics. A hotter atmosphere holds more water vapor. More water vapor means more intense rainfall when conditions are wet. It also means more evaporation and drying when conditions are dry. The hydrological cycle amplifies in both directions. This finding adds another layer of concern. Not only are temperature swings intensifying, but the consequences of those swings are also growing more severe.
The coral record shows that modern El Niño events are unprecedented in a millennium. “The events don’t look anything like what we see in the pre-industrial era,” Cole said. “That was exciting. And a little surprising, actually.” Neither the Little Ice Age nor the Medieval warm period left such a distinct fingerprint on ENSO variability. The current intensification stands alone in the record. This raises a profound question. How much further will the intensification go? The coral record ends with the present day. It cannot show what comes next. The models suggest continued change as greenhouse gas concentrations rise. But the exact trajectory remains uncertain. The eastern Pacific has already crossed a threshold that took a thousand years to establish. The path forward depends on decisions made now.