To many, the Antarctic Ice Sheet is a blank, frozen wasteland. But to those who study Earth’s past climate, it contains a wealth of information. Gas bubbles trapped in ancient ice preserve long-ago atmospheres, and chemical changes in the ice trace fluctuations in Earth’s temperature.
Climate scientists have used these “ice core diaries” to reconstruct long-term trends in Earth’s temperature, such as the comings and goings of ice ages. But so far, it has been difficult to obtain long-term records of a hot-and-cold pattern that is more familiar to us: the seasonal cycle.
In January, a team of scientists presented a seasonal temperature record dating back 11,000 years. The ice revealed a connection between intense solar radiation and hot summers in Antarctica.
“[This] is the first record of its kind,” said Tyler Jones, a polar climatologist at the University of Colorado Boulder’s Institute of Arctic and Alpine Research (INSTAAR) and lead author of the study. Seasonal temperature data help researchers understand Antarctica’s natural rhythm, which is critical for anticipating the polar regions’ responses to warming.
Temperatures in the Time of Mammoths
Researchers can infer past temperatures by measuring the ratios between isotopes—atoms with the same number of protons but different numbers of neutrons. Because hydrogen is a main constituent of water, paleoclimatologists often focus on the ratio between common hydrogen and its heavier sibling, deuterium: The warmer the regional average temperature was when the ice formed, the higher the deuterium concentration is.
By measuring the relative abundances of common hydrogen and deuterium along 3,405 meters (11,171 feet) of the West Antarctic Ice Sheet Divide core (WDC), Jones and his coauthors reconstructed temperatures into the early Holocene, a time when humans were just starting to develop agriculture and mammoths still roamed Siberia and North America.
The data showed that summer temperatures in West Antarctica were higher when the region received a more intense dose of sunlight. This deceptively simple observation is connected to Milankovitch cycles, a major tenet of climate science. According to Milankovitch theory, the amount of sunlight reaching Earth’s surface—which depends on Earth’s rotation and orbit around the Sun—drives long-term climate change. The study validated the link between sunlight and climate on a seasonal scale: Intensely sunny summers lead to warm temperatures that can potentially trigger large-scale melting of ice.
Deciphering Ice Core Diaries
Ice doesn’t reveal its secrets easily. To construct a continuous temperature record, the scientists needed to be reasonably sure the ice core didn’t contain any significant gaps: A long period without snowfall can create gaps in the information stored inside.
Zeroing in on seasonal cycles requires ice deposited in different seasons to be distinguishable within the ice core. Imagine trying to identify the layers of a tiered cake 11,000 feet tall—a challenging task in any case, but impossible if the layers are too thin to be differentiated. The WDC was ideal for seasonal measurements because its ice accumulated quickly, producing annual layers 10–50 centimeters (4–20 inches) thick.
The researchers measured water isotopes at 5-millimeter (0.2-inch) intervals along the ice core. Because each of those intervals represented several weeks’ worth of ice, the resulting temperature record resolved seasonal changes. The researchers compared the data with the amount of sunlight that reached the pole, determined using numerical models.
Even an ideal ice core can be difficult to decipher, though. The researchers had to account for uneven snowfall between seasons, sporadic storms, and natural diffusion of water particles in ice. Correcting for diffusion and natural noise was a long-haul effort that required “working through a mountain of modeling and statistical considerations,” said Jones, who began contributing to the project as a doctoral student in 2013. “It is both a celebration, and certainly some relief, seeing our results published.”
The results have garnered interest from ice core researchers, some of whom questioned whether the group’s statistical methods are sufficient to tease out seasonal patterns from the ice. Nevertheless, Christo Buizert, a paleoclimatologist at Oregon State University who was not involved with the research, called it “one of the most convincing studies to date to resolve both the summertime and wintertime temperatures from a single location.”
The Antarctic record of small-scale temperature fluctuations feeds into what we know—and don’t know—about Earth’s climate. Seasonal differences are especially relevant in polar regions, where warm summers melt the ice, Buizert explained. “Snow and ice care about the summer temperature, not the annual mean temperature, so having seasonally resolved temperatures is really exciting.”
By enabling researchers to connect cause and effect—as with sunlight and summer temperatures—seasonal records can offer deep insights into climate patterns. Jones said he hopes the research will inspire similar studies on other ice cores: “We could learn a lot from focusing on seasonal climate in the future,” he said, “provided such records can be obtained.”
—Caroline Hasler (@carbonbasedcary), Science Writer