Understanding the climate record through objects

When Melissa Charenko was working on her Ph.D. in the history of science at the University of Wisconsin-Madison, she started attending meetings of a paleoecology lab. On one excursion, she rode behind a snowmobile across a frozen Wisconsin lake as climate scientists drilled through the ice to collect sediment cores containing fossil pollen grains.

Charenko, now an assistant professor in Penn’s Department of History & Sociology of Science, says, “I was literally along for the ride as a historian.” She notes that the cores collected that day allowed scientists to infer climate conditions for a few thousand years of history. The experience got her thinking about how “past climates become explanations for the present,” she says.

Sediment cores containing pollen are one example of an indirect measure of past climates—known as proxies—whereas direct measures include thermometers and rain gauges. But instrumental records only go back about 150 years, Charenko says, meaning other methods like proxies are needed to get climate insights for most of Earth’s history.

Her new book, “Climate by Proxy: A History of Scientific Reconstructions of the Past and Future,” traces the history of climate proxies, showing that they are critical to getting a fuller picture of anthropogenic, or human-caused, climate change and are important for reliable climate models. Knowledge provided by proxies matters, Charenko says, because this history shows that current rates of warming are much faster than past shifts. She argues that proxies are crucial for addressing today’s climate challenges.

The proxies discussed in her book take scientists back to the last ice age, which ended about 13,000 years ago. Along with pollen, other examples of climate proxies include tree rings and ice cores. “Different trees produce different types of pollen, and the quantities of these types of pollen will tell us something about former climates because trees have certain climatic preferences,” Charenko explains.

Proxies therefore help scientists understand the climate record from different parts of the globe, but Charenko also highlights where these sources are incomplete. For example, trees in the tropics are less well sampled for colonial and historical reasons, and they don’t produce rings in the same way as trees in temperate zones. Still, she says that scientists are developing new methods to extract information from tropical tree rings.

Charenko says she was drawn to studying biological objects because they “seemed important to disciplines that had been left out of the story.” While much of climate science history comes from earth and atmospheric sciences, she wanted to spotlight the work of biologists and ecologists, highlighting the impact former climates had not only on the atmosphere but also on people’s lives on Earth.

But, she adds, proxies are not limited to such biological records—they can also include things like ships’ logs that show when ships might have passed through an ice-free port and paintings depicting what people ate.

Charenko traces the history of scientists using climate proxies to the late 19th century, explaining that as Scandinavia industrialized and more land was needed for agriculture, people drained many bogs. What they found in the peat—five distinct layers of vegetation of tree species that no longer existed—surprised them, since they had assumed that the climate since the last ice age was stable. This discovery eventually helped establish the prevailing notion that climate had indeed changed.

Overall, Charenko says her work points to the need for a plurality of different responses to climate change. “Climate is a complex amalgam of interacting processes taking place in the atmosphere, biosphere, hydrosphere, lithosphere, and cryosphere that occur at different scales,” she says. The atmospheric vision of science came to be the dominant one, but her book shows “potential alternatives that may be more helpful for solving the climate crisis.”

“We need to understand the objects by which we know climate so that we understand what climate is and what we should do about it,” she says.