Are Humans Heating Up Earth? If So, It May Not Be the First Time 

DMU Associate Professor Co-Authors Science Journal Article

This summer’s record-high global temperatures are increasing concerns about the impact of human-caused climate change, but this may not the first time people have helped raise the heat on Earth: A group of scientists, including DMU Anatomy Associate Professor Julie Meachen, Ph.D., found evidence that may show humans literally fired up the planet as long as 15,000 years ago, contributing to the extinction of two-thirds of Earth’s large animals outside of Africa at the end of the Ice Age. 

The group’s findings, published this month in the journal Science, were based on analysis of radiocarbon dates from specimens of eight “megafauna” species trapped in the Rancho La Brea Tar Pits in Southern California during the Pleistocene, commonly known as the Ice Age. At the end of that age, most of Earth’s large mammals went extinct. The causes and consequences of their disappearances are poorly understood, the researchers state, “despite the obvious relevance for modern global change research.”  

The scientists’ latest data sheds light on those disappearances and represent the first statistical modeling of extinction causality in Southern California.  

In their research, Meachen and her colleagues analyzed reliably dated large-animal fossils compared to documented environmental change and human activity during the Pleistocene. They argue the extinctions “may have been triggered by human-ignited fires in an ecosystem made vulnerable by rapid warming and drying.” 

“If our hypothesis is true, it’s pretty damning evidence about human impact on the planet,” Meachen says. “It may show that throughout our history, we modify our habitat to the detriment of every other species.” 

Connecting Causes to Consequences 

In addition to Meachen, the authors of the Science article represent universities and research museums from across the U.S., in Italy and France. They obtained radiocarbon dates on 172 specimens from seven extinct and one existing megafauna species, including saber-toothed cats, dire wolves, ancient bison and wild horses, that spanned 15,600 to 10,000 years ago. They compared the estimated time of extinction of the species to changes in the environment and human activity during the period.  

The La Brea Tar Pits specimens provide a “unique opportunity” to investigate animal populations and changes they experienced during the late Pleistocene. 

“At La Brea, naturally occurring asphalt seeps entrapped and preserved the bones of thousands of individual animals representing numerous megafaunal species across the last 50,000 years,” the researchers explain in the article. “Nearly all of these bones preserve original collagen that enables precise radiocarbon dating analysis.” 

The researchers compared that analysis with known environmental changes and human activity and population growth at the end of the Ice Age to establish causal relationships to the disappearance of the large animal species. They analyzed charcoal accumulation rates in the region and continent-wide, which were low prior to the appearance of humans and increased modestly as humans arrived and the climate warmed and dried beginning around 13,500 years ago. However, around 13,200 years ago, “charcoal accumulation rates suddenly increased thirty-fold,” the researchers found. 

This “unprecedented fire activity” as well as the Earth’s rapidly warming and drying environment coincided with the complete destruction, or “extirpation,” of the large animals they studied. This combination of events “parallels processes occurring in Mediterranean-type ecosystems today,” the researchers conclude in the article. 

The source of the increased charcoal input “is open to question,” they state, “but increased human impacts on the ecosystem should be considered as potential causal factors in the intense burning event.” 

“The ferocity of the fires at the time suggests they did not occur naturally,” Meachen says. “While they may not have killed the animals directly, they destroyed the animals’ habitat.” 

Are We Repeating History? 

Meachen and her research colleagues note that the use of fire by small populations of humans can have disproportionate impacts on landscapes by directly killing wildlife and altering the structure and function of vegetation, which in turn affects resources for animals, alters migration patterns and “can put animals at higher risk of predation.”  

“Today, changing fire regimes resulting from climate change and human activities are again driving some ecosystems towards tipping points,” they state in the Science article.  

Data from the National Oceanic and Atmospheric Administration show that Southern California has warmed substantially over the last century, faster than warming during the late Pleistocene. This human-caused climate warming “is increasing the risk of prolonged droughts and wildfire activity in highly biodiverse Mediterranean regions worldwide,” they say, and these events are predicted to worsen in coming decades. 

“As critical thresholds are reached in Mediterranean ecosystems, state shifts are likely to occur, as they did at the end of the Pleistocene. Some such transitions have already begun: in the western USA, wildfire-burned area has increased four-fold in two decades. Moreover, post-fire ecosystems are not recovering to pre-burned states, suggesting critical thresholds for re-establishment have already been crossed. The conditions that led to terminal-Pleistocene state-shift in Southern California are recurring today across the western USA and in numerous other ecosystems worldwide.” 

That grim prognosis underscores the importance of the research by Meachen and others in this area. She has worked with several of her co-authors on related research funded by a three-year National Science Foundation grant she received in 2017 that’s been extended until 2024 due in part to the COVID-19 pandemic.  

“Understanding the interplay of climatic and anthropogenic changes in driving this past extinction event may be helpful in mitigating future biodiversity loss in the face of similar pressures,” the researchers state in the Science article. 

“Our work has been a huge team effort, and we have lots more questions to ask,” Meachen says. “I anticipate we will have many more papers published based on this work.” 

Science Journal citation: O’Keefe, F. R., R. E. Dunn, E. M. Weitzel, M. R. Waters, Martinez, L., W. J. Binder, J. R. Southon, J. E. Cohen, J. A. Meachen, L. R. G. DeSantis, M. Kirby, E. Ghezzo, J. B. Coltrain, B. T. Fuller, A. B. Farrell, G. T. Takeuchi, G. MacDonald, E. B. Davis, E. L. Lindsey. Accepted. Pre-Younger Dryas megafaunal extirpation at Rancho La Brea linked to fire-driven state shift. Science.