A fiery past sheds new light on the future of global climate change. Ice core samples reveal significant smoke aerosols in the pre-industrial Southern Hemisphere
From: Harvard (the John A. Paulson
School of Engineering and Applied Sciences)
May 28, 2021 -- Centuries-old smoke
particles preserved in the ice reveal a fiery past in the Southern Hemisphere
and shed new light on the future impacts of global climate change, according to
new research published in Science Advances.
"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 Pengfei Liu, a former graduate
student and postdoctoral fellow at the Harvard John A. Paulson School of
Engineering and Applied Sciences (SEAS) and first author of the paper.
"These results have importance for understanding the evolution of climate
change from the 1750s until today, and for predicting future climate."
One of the biggest uncertainties when it
comes to predicting the future impacts of climate change is how fast surface
temperatures will rise in response to increases in greenhouse gases. Predicting
these temperatures is complicated since it involves the calculation of
competing warming and cooling effects in the atmosphere. Greenhouse gases trap
heat and warm the planet's surface while aerosol particles in the atmosphere
from volcanoes, fires and other combustion cool the planet by blocking sunlight
or seeding cloud cover. Understanding how sensitive surface temperature is to
each of these effects and how they interact is critical to predicting the
future impact of climate change.
Many of today's climate models rely on
past levels of greenhouse gasses and aerosols to validate their predictions for
the future. But there's a problem: While pre-industrial levels of greenhouse
gasses are well documented, the amount of smoke aerosols in the preindustrial
atmosphere is not.
To model smoke in the pre-industrial
Southern Hemisphere, the research team looked to Antarctica, where the ice
trapped smoke particles emitted from fires in Australia, Africa and South
America. Ice core scientists and co-authors of the study, Joseph McConnell and
Nathan Chellman from the Desert Research Institute in Nevada, measured soot, a
key component of smoke, deposited in an array of 14 ice cores from across the
continent, many provided by 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.
What they found was unexpected.
"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 Loretta Mickley,
Senior Research Fellow in Chemistry-Climate Interactions at SEAS and senior
author of the paper.
To account for these levels of smoke,
the researchers ran computer simulations that account for both 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. 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.
This result agrees with the ice core
records that also show that soot was abundant before the start of the
industrial era and has remained relatively constant through the 20th century.
The modelling suggests that as land use changes decreased fire activity,
emissions from industry increased.
What does this finding mean for future
surface temperatures?
By underestimating the cooling effect of
smoke particles in the pre-industrial world, climate models might have
over-estimated the warming effect of carbon dioxide and other greenhouse gasses
in order to account for the observed increases in surface temperatures.
"Climate scientists have known that
the most recent generation of climate models have been over-estimating 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.
https://www.sciencedaily.com/releases/2021/05/210528152517.htm
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