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Earth's fiery past revealed in ice core record

Smoke particles preserved in the ice core record show that more fires were burning in the Southern Hemisphere during pre-industrial times than previously thought. The discovery modifies our understanding of the heat-trapping effects of greenhouse gas emissions and of future global warming. 

Study first author Pengfei Liu is a former graduate student and postdoctoral fellow at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS).

“Up until now, the magnitude of past fire activity, and thus the amount of smoke in the preindustrial atmosphere, has not been well characterized,” said Liu. 

“These results have importance for understanding the evolution of climate change from the 1750s until today, and for predicting future climate.”

When it comes to predicting future climate change, one of the biggest uncertainties is how fast surface temperatures will rise in response to greenhouse gases. Forecasting these temperatures involves the calculation of competing warming and cooling effects in the atmosphere. 

Heat-trapping greenhouse gases trigger global warming, while aerosols from volcanoes and fires block the sunlight and cool the planet. To validate today’s climate models, scientists rely on estimates of these heating and cooling effects. However, the amount of smoke aerosols in the pre-industrial atmosphere is not well documented. 

To model smoke in the pre-industrial Southern Hemisphere, the researchers focused on Antarctica, where smoke particles emitted from fires in Australia, Africa, and South America remain trapped in the ice.  

Study co-authors Joseph McConnell and Nathan Chellman are ice core scientists at the Desert Research Institute in Nevada. They measured soot, a key component of smoke, deposited in 14 ice cores from across the continent.

“Soot deposited in glacier ice directly reflects past atmospheric concentrations so well-dated ice cores provide the most reliable long-term records,” said McConnell.

According to study senior author Loretta Mickley, what the experts found in the ice core record was unexpected. “While most studies have assumed less fire took place in the pre-industrial era, the ice cores suggested a much fierier past, at least in the Southern Hemisphere.”

To account for the levels of smoke estimated from the ice cores, the researchers ran computer simulations including wildfires and the burning practices of Indigenous people.

“The computer simulations of fire show that the atmosphere of the Southern Hemisphere could have been very smoky in the century before the Industrial Revolution,” explained study co-author Professor Jed Kaplan.

“Soot concentrations in the atmosphere were up to four times greater than previous studies suggested. Most of this was caused by widespread and regular burning practiced by Indigenous peoples in the pre-colonial period.”

This idea is supported by the ice core records, which show that soot was abundant before the start of the industrial era and has remained relatively constant through the 20th century. The results of computer simulations suggest that emissions from industry increased as land use changes decreased fire activity.

The findings indicate that climate models have overestimated the warming effect of greenhouse gas emissions by underestimating the cooling effect of smoke particles in pre-industrial times. 

“Climate scientists have known that the most recent generation of climate models have been overestimating surface temperature sensitivity to greenhouse gasses, but we haven’t known why or by how much,” said Liu. “This research offers a possible explanation.”

“Clearly the world is warming but the key question is how fast will it warm as greenhouse gas emissions continue to rise. This research allows us to refine our predictions moving forward,” said Mickley.

The study is published in the journal Science Advances.

By Chrissy Sexton, Staff Writer

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