Fiery Past Sheds Light on Future of Climate Change

Atmosphere-cooling aerosols were much more prevalent years ago than previously thought, protecting the planet’s surface from harmful greenhouse gas effects.

A column of volcanic smoke rises from the crater on the Shinmoedake volcano after its eruption in Kirishima, southern Japan in 2018. (Kyodo News via AP)

(CN) — A recent discovery of soot and smoke particles within the ice cores from Antarctica has yielded new clues about what the atmosphere of the preindustrial Southern Hemisphere was like, which could have important implications for our future.

In a new study published Friday in the journal Science Advances, first author Pengfei Liu, a former graduate student and postdoctoral fellow at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS), and her team of researchers and co-authors suggest the climate of the Southern Hemisphere in the 1700s was probably much more fiery than previously thought.

“Up till 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.”

As climate change continues to progress, fueled by the burning of fossil fuels and other human activity, the scientific community has been frantically looking for clues about what comes next based on what has already happened. Looking into past climate events and changes in atmospheric conditions can provide insight into how the world will respond to further stress and global warming. Already, global temperatures have been pushed past what’s been deemed safe, carbon dioxide in the atmosphere is at an unprecedented level and all over the world, human-caused natural disasters have become more frequent. 

The latest climate question scientists sought to answer is how fast surface temperatures going to climb once aggravated by greenhouse gases. When greenhouse gases like carbon dioxide, methane and nitrous oxide enter the atmosphere, they can stay there for hundreds of years — trapping heat and warming Earth’s surface. Assessing how much this phenomenon will alter surface temperature is a task easier said than done, as experts will need to factor in both the heating and cooling effects.

While greenhouse gases warm Earth, superfine aerosol particles help cool the planet’s surface by creating cloud cover and blocking sunlight. For example, sea spray from the ocean’s waves makes the air at beaches significantly cooler. Other aerosols produced by volcanoes, fires and other combustion methods help to absorb sunlight and reflect solar radiation, effectively cooling the surface temperature.

In order to predict how climate change will continue to alter surface temperature, scientists need to understand how both heating and cooling processes will play into this.

The study authors sought out to fill a blank that exists within current climate models that rely on past levels of greenhouse gases and atmospheric aerosols. They’ve found that while preindustrial greenhouse gas levels have been accurately recorded, very little is known about aerosols in the preindustrial atmosphere. Scientists have believed aerosols were much lower than today’s levels and high in regions with natural aerosol-producing qualities, but so far climate models have been unable to recreate such conditions.

The authors’ research led them to Antarctica, where hundreds of years of smoke particles from Australia, Africa and South America remain trapped in the ice. The goal was to collect enough evidence to create an accurate model of smoke aerosols in the Southern Hemisphere from the desired time frame. 

The team was accompanied by ice core scientists and study co-authors Joseph McConnell and Nathan Chellman from the Desert Research Institute in Nevada. They were looking to measure soot left behind by the smoke, ultimately finding deposits of it in 14 ice cores from around the continent with assistance from other international collaborators.

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

Their findings yielded surprising results, showing that smoke was much more prevalent in the atmosphere than what previous studies had hypothesized.

“While most studies have assumed less fire took place in the preindustrial era, the ice cores suggested a much fierier past, at least in the Southern Hemisphere,” said senior author Loretta Mickley, senior research fellow in chemistry-climate interactions at SEAS.

The team conducted computer simulations based on their discoveries to model a climate with this new unexpected amount of smoke, now accounting for wildfires and routine 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. 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,” said Jed Kaplan, Associate Professor at the University of Hong Kong and co-author of the study.

The results of the simulation matched up with the levels of soot collected from the ice cores, proving that smoke aerosols were very present in the preindustrial atmosphere. The findings also suggested that the levels of aerosols have not changed much since then, showing that as the number of fires dampened, production was replaced by manmade combustion.

The implication means previous climate models that accounted for much fewer aerosols in the atmosphere most likely overestimated how much greenhouse gases would warm surface temperature.

“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.

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