From Ancient Charcoal, Hints of Wildfires to Come

The oldest evidence of wildfire in the world can be found in a laboratory on the fourth floor of a brick building in Waterville, Maine. To the untrained eye, it looks like a speck of black lint, not much larger than the tip of a pin. To Ian J. Glasspool, a paleobotanist at Colby College, it is a 430-million-year-old piece of charcoal.

The specimen, which Dr. Glasspool discovered in a mudstone from southern Wales, is one of many pieces of ancient charcoal that have been studied in recent years to explore how fires burned in the past. Together, these remnants are helping scientists understand how fires have shaped and been shaped by environmental change through geologic time.

“They are tedious-looking things,” Dr. Glasspool said, lifting a sample embedded in a small resin disc. “But there’s a whole heap you can get out of them.”

These ancient insights may not help us manage individual wildfires today, Dr. Glasspool said. But they can provide a clearer sense of the global phenomenon of fire and how it shapes Earth’s climate. This, in turn, can help modelers make more accurate projections of the future climate.

“The geologic record shows that it is a lot more complicated than ‘it gets hot, there will be more fires,’” said Jennifer M. Galloway, a paleoecologist with the Geological Survey of Canada. Dr. Galloway recently published a paper in the journal Evolving Earth on the merits of studying ancient wildfires as a way to understand climate dynamics today.

Fire is a fairly recent phenomenon in Earth’s 4.54-billion-year history. For more than 90 percent of that timeline, the planet’s atmosphere and continents lacked the oxygen and kindling required to sustain a flame. Lightning strikes might have charred bits of microbial mat here and there, but combustion would have been short-lived; smoke and embers were all but absent. Only after plants appeared on land some 458 million years ago did sustained burns — and, eventually, a geologic record of fire — become possible.

The earliest fires burned not forests, which were still millions of years from evolving, but simpler growths like mosses and liverworts. “We are talking about stuff that by and large you could walk through and they wouldn’t even get the tops of your boots wet,” Dr. Glasspool said. An enigmatic group of larger growths called nematophytes also dotted landscapes at this time, and these might have helped fuel the earliest flames as well, he added.

To study remnants of these ancient fires, Dr. Glasspool first dissolves his rock samples in acid and then sieves out the tiny black specks left behind. To manipulate and orient each fleck for analysis, he uses a wooden skewer that has a single whisker from his cat, Bingo, duct-taped to the end.

“Low-budget, do-it-yourself,” he said in February in his laboratory. If he used a store-bought paintbrush, his tiny samples might get caught up in the hairs; Bingo’s whisker lends him more control.

Viewed with a simple light microscope, these charcoals reveal the marbleized cellular walls that have been pristinely preserved through the act of charring. That process burns away all volatile organic material and leaves behind only inert carbon, which can remain unchanged for hundreds of millions of years.

Charcoal has a distinct silky luster that helps distinguish it from coal, another form of carbon, which looks more matte under a microscope.

By tracking charcoal abundance at different intervals in the rock record, Dr. Glasspool and his colleagues have identified fire patterns that emerged during past periods of global warming. He andhis team discovered a fivefold increase in charcoal in 200-million-year-old sedimentary rocks collected in East Greenland. This period marked the end of the Triassic, when intense volcanism raised global temperatures by some 6 degrees Celsius and led to one of the worst mass extinctions in Earth’s history.

In 2010, Dr. Glasspool’s team reported that rising atmospheric heat could have increased wildfire activity in a number of ways. For instance, the warmth could have generated thunderstorms with more frequent lightning strikes, the leading natural cause of wildfires both in deep time and today. Just 1 degree Celsius of warming can increase rates of lightning by some 40 percent, according to a study out of Imperial College London. This may partly explain why wildfires were so widespread at the end of the Triassic, Dr. Glasspool said.

The fossil record also indicates that plants with small, narrow leaves became more common as temperatures rose, while species with broader leaves largely disappeared from the landscape. This, his team reported, was most likely a response to the warmth, since smaller leaves can rid themselves of heat more easily than larger leaves can.

The small-leaved species would have fueled more intense fires, much as ripped-up shreds of paper burn faster than intact ones. “They dried more rapidly and were more combustible,” Dr. Glasspool said.

More combustible plants, more smoke and more carbon dioxide in the atmosphere would have further warmed Earth, perhaps fueling more flames, more changes in vegetation and more intense thunderstorms — a positive feedback loop not unlike what seems to be playing out today.

The rock record provides a sense of how long ecosystems might take to recover after such perturbations. Deposits from the end-Permian mass extinction — a period of warming some 252 million years ago that marked the greatest loss of life in all of Earth’s history — suggest that charred wetlands took millions of years to recover after drying out and burning.

“Let’s hope we don’t re-enact that,” said Chris Mays, a paleontologist at University College Cork in Ireland who published studies on these deposits in 2022.

Modern global temperatures have increased far less than they did back then — just 1.1 degree Celsius since 1880, compared with some 10 degrees Celsius during the tens of thousands of years of the end-Permian extinction. But the rates of change today far surpass those of the past. This fast-paced warming has already made wetlands more prone to fire: The Pantanal region of South America, 42 million acres of tropical wetland, has begun seasonally burning at alarming rates. Deposits from the end-Permian offer a sobering view of what might happen should climate change continue unabated.

“There are a bunch of levers we can pull to prevent it from getting that bad,” Dr. Mays said. “But we use it as an absolute worst-case scenario.”

Sean Parks, a research ecologist with the U.S. Forest Service at the Rocky Mountain Research Station in Missoula, Mont., noted that the scope and severity of such fires are also the result of human behavior and land use practices, not just climate change.

Still, Dr. Parks said, studies of the geologic record and ancient climate patterns can help improve global climate models that inform land management decisions: “It is interesting and excellent background information.”

Fernanda Santos, a staff scientist at the Oak Ridge National Laboratory in Tennessee who studies modern fires in Alaska and works closely with climate modelers, agreed.

“I really value ancient data because they can give us this new perspective and new base line,” Dr. Santos said.