Wildfires have been part of Earth’s landscape since prehistoric times. At first they were random natural events that occurred when dry grass and wood were hit by lightning. Then at some point our ancestors also began using fire deliberately to shape the landscape, Stefanie Kaboth-Bahr, professor of paleoclimatology at Freie Universität, explains. She wants to know when exactly this shift in our relationship to fire took place.

Together with an international research team, Kaboth-Bahr recently analyzed a 300,000-year-old sediment core from the South China Sea. It shows a sharp rise in fire activity in East Asia about 50,000 years ago – roughly the time that modern humans (Homo sapiens) were moving out of Africa into other parts of the world. Comparative samples indicate that similar changes happened at the same time in Europe, Southeast Asia, and the Papua New Guinea-Australia region. 

But why would these people deliberately alter landscapes with fire? Kaboth-Bahr suggests several possible reasons: for example, fire was used to turn dense forests into open terrain that was easier to travel through. It might also have promoted the growth of certain edible plants that were important to people’s diets. Did this development take place at the same time worldwide, or were people in Africa perhaps using fire deliberately even earlier than other groups?

Investigating the Fire Record

To find out more specifics about the history of fire and its use by humans, Kaboth‑Bahr is studying a sediment core from an Ethiopian lake together with colleagues from Ethiopia, Freie Universität Berlin, the University of Potsdam, and the University of Cologne. The core was taken from a depth of 350 meters and gives an insight into more than 600,000 years of climate history.

By studying this core, Kaboth‑Bahr and her team hope to understand how fire dynamics in Africa evolved over hundreds of thousands of years and whether these patterns were altered by human activity. “Our species has a long history with fire,” says Kaboth-Bahr. 

At first, our ancestors only used fire passively, taking advantage of it when it occurred naturally, for example, after lightning strikes. They could use it to keep warm, cook food, and ward off predators. It was only later that they learned how to make fire themselves – a discovery that laid the foundation for more control over where and when it could be used. “In recent summers we have been reminded of how even vast areas can be destroyed by just a few individuals with a careless attitude to fire. And the question is: When did we begin altering ecosystems so profoundly that evidence of this can be seen in the sediments, i.e., in the natural archives of our climate?” 

This evidence can be found in the form of charred plant remains, or charcoal, which are often well preserved in lake sediments. Researchers can tell whether these remains belong to grasses or trees based on the size and shape of the particles. The quantity of charcoal in a certain deposit lets researchers estimate how much plant matter burned during a specific fire, while the frequency of charcoal layers shows how often fires occurred. The geochemical signature of the charcoal particles can even be used to estimate the temperatures at which these fires burned.

Lake sediments trap wind‑blown particles, making them a particularly good archive of charcoal. When charcoal particles reach a lake, they quickly sink into the sediment at the bottom. Once buried, they can be preserved for a long period of time.

Whether fires were natural or human‑caused can often be inferred from patterns in the charcoal deposits, explains Kaboth‑Bahr. “Natural fires, for example, those caused by lightning, tend to occur more often in certain seasons and can spread rapidly across the landscape with the help of wind,” she says.

Such fires would usually break out in between the rainy season and the dry season, and could be expected to reach average temperatures. On the other hand, deliberate fires caused by humans can occur at any time of year, and are started with nonseasonal materials – often during dry spells when, for example, habitats such as savannas are particularly susceptible to fire. These fires burn longer and hotter.

Using the Past to Predict the Future

Fire analysis is a relatively new tool for learning more about humanity’s early history. Charcoal can provide insights into periods for which we have no direct information about humans in Africa. By examining charcoal remains, researchers can gauge whether people were already present and whether they were influencing their environment, for example, by lighting fires that altered the vegetation around them. In addition to lake sediments, which mainly capture local and regional indicators of fire dynamics, the oceans surrounding Africa also serve as important archives. Wind transports charcoal particles far offshore, where they eventually settle. These deposits combine information from large portions of the continent, offering additional ways to reconstruct fire activity across Africa. “If humans truly affected the fire regime, that influence should also show up in deep‑sea core samples,” explains Kaboth‑Bahr.

While fire is certainly one major focus of Kaboth-Bahr’s research, water also plays a large role in her work. With climate change, both droughts and heavy rain events are expected to increase. “We are already able to use climate models to generate fairly robust simulations of temperature trajectories driven by rising levels of CO₂. However, precipitation forecasts are much more unreliable because cloud formation and rainfall are very region‑specific and highly variable at small scales,” says Kaboth‑Bahr. “Our goal is to provide data on changes in humidity and precipitation during warm periods of Earth’s history so as to better assess how rainfall patterns might evolve in the future.

Ecosystems that Endure 

To do this, the researchers are looking back to periods of time that were markedly warmer than today, for example, the Eocene, a geological epoch between 56 million and 34 million years ago. Core samples from the Messel Pit, a former oil‑shale quarry near Darmstadt with a uniquely rich fossil deposit, capture the climate and environment from around 47 million years ago. 

During extreme warm intervals – or “hyperthermal events” – recorded in the Messel sediment sequence, CO₂ concentrations spiked dramatically, and global temperatures rose noticeably again. “The lake’s ecosystem changed dramatically,” says Kaboth‑Bahr.

Early analyses indicate that algal blooms increased and the composition of fish species shifted fundamentally during this time. This development can be traced in detail thanks to the exceptionally well‑preserved fish fossils found at the site. However, the changes were never dramatic enough so as to push the ecosystem over a tipping point. Once temperatures fell again, the system reverted to its original state. Unlike many modern lake ecosystems that can suddenly and irreversibly collapse due to climate change and human pressures, the prehistoric Messel ecosystem demonstrated remarkable resilience.

How can we improve the capacity of today’s natural systems to stabilize themselves? Kaboth‑Bahr is asking herself the same question. Even if we succeeded in actively pulling carbon dioxide out of the atmosphere, it would take a very long time for concentrations to fall again. That’s why it is all the more essential that we immediately and drastically begin to reduce our CO₂ emissions – so that ecosystems have a chance of healing themselves.

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This article originally appeared in German in the Tagesspiegel newspaper supplement published by Freie Universität Berlin.