Examining Earth’s climatic past is getting weirder

Erin McClymont’s lab has six kitchen freezers, each with an alarm. This is to protect itself: in the event of a power outage or appliance breakdown, it must act quickly to prevent the nauseating odor of old fish from escaping. The source of this stench: solidified blocks of 50,000-year-old regurgitated stomach oil from Antarctic Snow Petrelspieces of which line the shelves of every freezer.

“One of my colleagues who did some of the initial sampling…did some field work where they had to throw away their coats at the end because they couldn’t get the smell out,” says McClymont, a paleoclimatologist at the University of Durham in England. The blockages “are revolting”.

Solidified oil is an indirect record, or proxy, of the past that scientists rely on “because we don’t have a time machine,” says Tyler Karp, a paleoecologist at the University of Chicago. Researchers trying to understand Earth’s climate and ecosystems must trace precipitation, ice cover, fires and other factors over thousands or millions of years, far longer than human records. But the most common proxies, including tree rings, pollen, and ice cores containing pockets of ancient air, have already been well studied. To learn something new, researchers must be creative.

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That’s why scientists like McClymont are looking for clever ways to study obscure aspects of the past, like his research into how Antarctica’s environment has changed over tens of thousands of years. Sea ice makes drilling samples from the Antarctic seafloor particularly difficult. So his collaborators found a more obscure polar record to explore: seabird stomach oil. Snow petrels spit the oil in front of their nests, mainly to ward off predators with its smell and stickiness. This vomit builds up in layers across avian generations, trapping 50,000 years of data on bird diet and sea ice environment.

Seabird oil is a good indicator because it is composed of waxes and fats, which break down more slowly than proteins and carbohydrates. The pieces can be radiocarbon dated and biopsied to determine the source of their carbon and nitrogen, a process that first requires using a large saw to cut what looks like “a mild cheddar: slightly soft and spongy,” McClymont says. As sea ice shrinks and expands, the ocean surface warms or cools. This in turn affects nutrient cycling and where different species can live, which is reflected in birds’ regurgitations.

Such methods revealed that as Antarctica’s ice sheets expanded during the Last Glacial Maximum, sea ice was pushed further offshore, forcing krill away from the petrels’ feeding range. These results suggest that snow petrels are able to temporarily adapt to different food sources and may do so again during future climate changes.

Tripti Bhattacharya, a paleoclimatologist at Syracuse University, also benefits from waxy substances, although much less odorous. She subjects fossilized and freeze-dried sediments to a “glorified espresso machine” to extract the hydrophobic outer layer of ancient leaves, called leaf wax. As plants use rainwater to grow, their leaves pick up the rain’s characteristic ratio of hydrogen isotopes, that is, atoms with the same number of protons and a different number of neutrons. Hydrogen isotope ratios, in particular, can be traced to reveal how much and when water fell on a plant.

Bhattacharya found that the last time carbon dioxide levels in the atmosphere exceeded 400 parts per million, as they do today, Southern California experienced rainy summers; Today, winters are rainy. The discovery helps explain the warm, humid environment that made California suitable for tropical animals such as crocodiles three million years ago. Climate change probably won’t bring crocodiles back to the West Coast, but ecologists can use old weather data to predict the types of species that might thrive in a similar future. “Proxy data may sound like an obscure scientific thing, but it actually contributes directly to our efforts to manage climate risks,” says Bhattacharya.

Precipitation inspires even stranger indicators than leaf sediment: When ostriches eat plants that grew in rainy conditions, a characteristic isotopic ratio of nitrogen present in the soil is transferred to their bodies and, ultimately, to their eggs.

PhD in geochemistry from Princeton University. Student Mingzhe (Damon) Dai is collaborating with archaeologists to obtain samples of ostrich eggs buried among early human settlements whose residents ate the birds and used the egg shells as water containers. Nitrogen isotope ratios from eggs can thus help reconstruct the precipitation experienced by early humans in Africa and Asia.

Dai’s initial results revealed that precipitation was low in South Africa during the last glacial maximum and increased as the planet warmed. Dai says changes in human culture and behavior occurred at the same time as these climate changes, suggesting that they could have been an important driver of these changes in the way people live.

Although scientists can use conventional records such as sediment and seafloor cores to reconstruct the global climate of the earliest days of civilization, “the resolution is too [coarse]so it’s not much fun for us to discuss something that happened on a local scale, that is relevant to human history,” says Dai. Coincidentally, ostriches and early humans moved around roughly the same area during their lifetimes: 85 square kilometers. If an ostrich egg reveals rainy conditions, that means that an individual human settlement probably also felt that rainfall – and perhaps the inhabitants changed their behavior accordingly.

Karp likes the way these unusual recordings allow him to “use the past as an experience.” He mainly studies the chemical remains of burned plants, called polycyclic aromatic hydrocarbons (PAHs), to trace ancient fires. He also examines chemicals called stanols, which are found in the droppings of herbivores and which help reveal the times when animals such as elephants, zebras, hippos and impalas roamed the land. As ecologists consider reintroducing fire or animals to landscapes where human activity has suppressed them, Karp says it’s useful to check such strange records to see how environments have responded in the past, such as in a natural field test.

Ultimately, every proxy has its limitations; some, like PAHs, can come from multiple sources, so it’s difficult to guarantee you’re tracking the right phenomenon. Others, like seabird oil, may be older than radiocarbon dating can reveal. And usually, you need several detailed experimental steps to connect such an unusual recording to the model you’re trying to study, Bhattacharya says, which adds uncertainty and errors to each step.

But for many researchers, these ancient remains are often the only clues available. “I don’t think that when I started studying science [that] I expected to study poop,” Karp says. But “the more different tools you can use to look at the same question from multiple angles, the more we come to a good consensus about what really happened.”