Evidence of the world's oldest forest fire can be found in a laboratory on the fourth floor of a brick building in Waterville, Maine. To the untrained eye, it looks like black lint not much larger than the tip of a pin. To Ian J. Glasspool, a paleontologist at Colby College, it's a 430-million-year-old piece of charcoal.
The specimen that Dr Glassfull found in mudstone in south Wales is one of several pieces of ancient charcoal that has been studied in recent years to explore how fire burned in the past. These remains help scientists understand how fires formed and were shaped by environmental changes over geological time.
“They were boring-looking things,” said Dr. Glasspool, holding up a sample embedded in a small resin disk. “But there is so much to be gained from it.”
These ancient insights may not help us manage individual wildfires today, Dr. Glasspool said. But they could provide a clearer sense of the global phenomenon of fire and how it shapes Earth's climate. This, in turn, could help modelers predict future climate more accurately.
“The geological record shows it’s much more complex than ‘the hotter it gets, the more fires there will be,’” said Jennifer M. Galloway, a paleoecologist with the Geological Survey of Canada. Dr. Galloway recently published her paper in the journal Evolving Earth. Discusses the advantages of studying ancient wildfires as a way to understand present-day climate dynamics.
Fires are a fairly recent phenomenon in Earth's 4.54 billion year history. For more than 90% of that time, the planet's atmosphere and continents lacked the oxygen and ignition needed to sustain the flame. Lightning may have carbonized microbial mats here and there, but the combustion is short-lived. There was very little smoke and embers. Only after plants appeared on land about 458 million years ago did sustained burns become possible, and eventually geological fire records.
The earliest fires burned simple growth such as moss and liverwort, not forests that were millions of years removed from evolution. “We’re mostly talking about things that you can walk over and not even get the tops of your boots wet,” Dr. Glasspool said. An enigmatic group of larger growths called nematophytes also dotted the landscape during this time, and this may have helped fuel the early flames, he added.
To study the remains of these ancient fires, Dr. Glasspool first dissolves rock samples in acid and then sieves out the tiny black specks that are left behind. To manipulate and orient each spot for analysis, he uses a wooden skewer with one of Bingo the Cat's whiskers taped to the end.
“It’s low budget, you can do it yourself,” he said from his lab in February. If he uses a store-bought brush, his little sample might get caught in his hair. Bingo's beard gives him more control.
When observed under a simple light microscope, this charcoal reveals marble cell walls that have been perfectly preserved through the carbonization process. This process burns off all volatile organic substances, leaving only the inert carbon, which can remain unchanged for hundreds of millions of years.
Charcoal has a unique soft luster that distinguishes it from coal, another form of carbon that appears more matte when viewed under a microscope.
Dr. Glasspool and his colleagues identified fire patterns that emerged during past periods of global warming by tracking the abundance of charcoal at various intervals in the rock record. He and his team found a five-fold increase in charcoal in 200 million-year-old sedimentary rocks collected in eastern Greenland. This period marks the end of the Triassic period, when intense volcanic activity caused global temperatures to rise by around 6 degrees Celsius and led to one of the worst mass extinctions in Earth's history.
In 2010, Glasspool's team reported that rising atmospheric heat may have increased wildfire activity in several ways. For example, the warmth may have led to more frequent lightning and thunderstorms, which are a major natural cause of wildfires both in ancient times and today. A rise of just 1 degree Celsius can increase lightning incidence by around 40%, according to a study by Imperial College London. This may partly explain why wildfires were so widespread in the late Triassic, Dr Glasspool said.
The fossil record also indicates that as temperatures increased, plants with small, narrow leaves became more common, while species with wider leaves largely disappeared from the landscape. His team reported that this was most likely a response to warmth. This is because small leaves can remove heat more easily than larger leaves.
Just as torn pieces of paper burn faster than intact pieces of paper, species with smaller leaves would have created more intense fires. “They dried faster and were more flammable,” Dr. Glasspool said.
More flammable plants, more smoke, and more carbon dioxide in the atmosphere would have warmed the Earth further, possibly fueling more flames, causing more changes in vegetation and more intense thunderstorms. This is a positive feedback loop, not unlike what appears to be happening today.
The rock record tells us how long it can take for an ecosystem to recover after such a disturbance. Sediments from the end-Permian mass extinction (about 252 million years ago, when warming caused the greatest loss of life in Earth's history) suggest that scorched wetlands took millions of years to recover after drying out and burning.
“I hope we don’t replicate that,” said Chris Mays, a paleontologist at University College Cork in Ireland who published a study of these deposits in 2022.
Modern global temperatures have risen much less than they did back then. That compares with a mere 1.1 degrees Celsius since 1880, compared with an increase of about 10 degrees Celsius over tens of thousands of years for the end-Permian extinction. But today's rates of change far exceed those of the past. This rapid warming is already making wetlands more prone to fire. South America's Pantanal region, a 42 million-acre tropical wetland, has begun burning at an alarming, seasonal rate. Late Permian sediments provide a sobering look at what could happen if climate change continues unabated.
“There are a lot of levers we can pull to prevent things from getting that bad,” Dr. Mays said. “But we use that as the absolute worst-case scenario.”
Sean Parks, a U.S. Forest Service research ecologist at the Rocky Mountain Research Station in Missoula, Montana, noted that the extent and severity of such fires are a result of not only climate change, but also human behavior and land-use practices.
Nonetheless, Dr Parks said the study of the geological record and ancient climate patterns could help improve global climate models that inform land management decisions. “It’s interesting and great background information.”
Fernanda Santos, a staff scientist at Oak Ridge National Laboratory in Tennessee who studies modern fires in Alaska and works closely with climate modelers, agreed.
“I find ancient data very important because it can give us new perspectives and new baselines,” Dr. Santos said.