There’s new evidence, this time from the Southern Hemisphere, that human activities altered Earth’s atmosphere long before the start of the fossil-fueled industrial age that kicked global warming into high gear.
Research published Oct. 6 in Nature suggests that soot from the land-burning practices of the seafaring Polynesians that settled New Zealand spread widely around the Southern Hemisphere. The detailed analysis of six ice cores from Antarctica found a sharp spike in depositions of climate-altering black carbon starting in about 1300.
Using models of winds in the Southern Hemisphere, along with other records showing possible sources of black carbon, the scientists found that the most likely source was fires started by the Māori to clear forests for agriculture and to ease their hunts as they settled the islands.
Other studies analyzing lake-bottom charcoal sediments have told a similar story, but the new findings spurred some critical feedback from Māori scientists in New Zealand, who said the study should have included researchers who could have helped present the findings in the context of Māori history and culture.
Understanding how early human activities like deforestation, fires and the spread of agriculture affected the climate is critical to precisely measuring how much warming has been caused by modern industrial pollution from burning fossil fuels and how the climate will respond to interventions like planting trees and reducing aerosol emissions. It could also provide clues about the climate effects of the surge in large wildfires in recent years.
According to the study, wildfires in New Zealand emitted soot for centuries at different levels. Black carbon aerosol particles don’t persist for a very long time, but their impacts on the climate can be significant and complex. While they are suspended in air, the particles absorb heat and warm the atmosphere. But they also can brighten clouds, which reflect the sun’s heat away from the planet to cool its surface.
When the soot falls into the ocean, the researchers said, it can also affect the climate because it feeds phytoplankton, tiny organisms that use carbon to grow. When they die, they sink to the bottom of the ocean and store the carbon in sediment, one of the key ways carbon dioxide is removed from the atmosphere, along with forests.
That could be a critical factor, because much of the fallout from the fires likely dropped on a part of the South Pacific that is “one of the most nutrient-limited oceans on Earth,” said Lead author Joe McConnell, with the Desert Research Institute in Reno, Nevada. “Our speculation in the paper is, the fires would have fertilized the Southern Ocean,” he said.
McConnell pointed to recent research that showed exactly that effect from the massive 2019-2020 wildfires in Australia. That study tracked plumes of smoke from the fires and measured phytoplankton growth where the soot fell.
“It showed exactly what we were postulating that happened 700 years ago,” he said. “Measurements of the plumes from the Australia wildfires in Hobart, Tasmania, showed that the nutrients in the smoke were highly available as nutrients—much more so than the nutrients from mineral dust.”
Several recent studies reinforce the importance of understanding pre-industrial aerosols in the context of modeling modern climate change, he added, singling out research by aerosol expert Ken Carslaw, with University of Leeds.
Until recently, such emissions were believed to be so low that researchers weren’t interested in studying their impacts on the climate, he said, so the idea that pre-industrial human activity caused such a big change in atmospheric black carbon was surprising. The findings show that, even in remote and sparsely populated regions, the environment wasn’t pristine, said McConnell, who has also traced lead pollution in ice cores from Greenland to the Roman empire.
“It’s clear from this study that humans have been impacting the environment over the Southern Ocean and the Antarctica Peninsula for at least the last 700 years,” he said.
Co-author Andreas Stohl, with the University of Vienna’s Institute for Meteorology and Geophysics, said the study complements similar recent research in the Northern Hemisphere that also suggests human climate impacts started earlier, and were perhaps larger than previously estimated, though still small compared to the effects of the carbon dioxide emissions from burning coal and oil.
“In the Northern hemisphere, we know humans have had a strong influence on burning. The Southern Hemisphere was thought to be more pristine, but now we see a huge impact on areas thousands of kilometers away, and with a huge impact on aerosol loading,” he said.
The new study focuses on a limited slice of time in a remote region, but there is a growing body of science suggesting that pre-industrial climate impacts started earlier and were more significant than previously thought, said William Ruddiman, a professor emeritus at the University of Virginia.
And it’s not just fires. Deforestation before 1850—a date often used to demarcate the start of the industrial era—resulted in 300 million tons of carbon going into the atmosphere, Ruddiman said, but only about half as much since then.
While there is no controversy about the dramatic impact of human emissions of carbon dioxide and methane on the climate during the last 100 to 200 years, he said there is debate about the scope of human impacts before that time, although that doesn’t reduce the urgency of addressing today’s emissions of climate-harming greenhouse gases, he emphasized.
“You could easily make a claim that the anthropocene began thousands of years ago, causing a slow warming that stopped a natural cycle of cooling,” he said. “It kept the climate warmer than it would have been.” The emerging science suggests that warming driven by human activity started earlier, and makes up more of the measured heating of the atmosphere than acknowledged up to now, he added.
For instance, the once widely held assumption that most deforestation happened in the last 150 years or so is starting to crumble under the weight of new data, he said. The evidence includes “hundreds of pollen records from lakes showing forests at the maximum extent about 6,000 to 7,000 years ago,” when mass forest clearing and slow warming started, he added.
That climate effect isn’t fully considered in key climate reports like the assessments by the Intergovernmental Panel on Climate Change, which “mainly documents changes during the industrial era caused by greenhouse gas emissions,” he said, “but that story ignores the earlier clearance of forests, or early agricultural practices that emitted methane.”
“If you start in 1850 what do you do with all the greenhouse gas emissions caused by deforestation before that?” he asked. “So much happened before 1850, should we just ignore it?”
Understanding those past changes won’t lead to a “magic solution” to climate change, he added, but “knowing can help us, because it shows that, if we can regrow a large percentage of the forests that have been cleared, we could do something to reduce our CO2 impacts on the atmosphere.”
McConnell said part of the reason his study didn’t include Māori researchers is because the study wasn’t intended to look specifically at fires in New Zealand. The research by his lab is focused on “measuring things in ice cores that people didn’t traditionally measure,” he said.
Most of those measurements are associated with aerosols, and for a long time, he focused on analyzing traces of rare earth elements to track aerosols related to mineral dust, blowing off deserts, fields and mountains.
“About 2007 we started thinking about fires in a serious way, and specifically aerosols associated with black carbon,” he said. Along with Ross Edwards, a colleague at the Desert Research Institute, he developed a new method for measuring black carbon that could detect much smaller amounts, which “allowed us to measure black carbon in ice cores in a detailed way that wasn’t possible before,” he said.
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“A lot of what I do is trying to improve the prehistory record of aerosols for climate models,” he said. “Black carbon, or soot, is a very important one because it has a strong impact.”
Going into the research, he expected to see variations driven by natural climate cycles.
“I thought we were going to see black carbon variations…going up and down in response to changes in the temperature and moisture levels in Australia, New Zealand, Patagonia and Southern Africa,” he said. The team was surprised to see a big deviation around 1300.
“There was relatively little burning in this area, so there was a signal when humans came to New Zealand,” he said. “The idea that a preindustrial society could have had a hemispheric-scale impact is surprising. I think this is the earliest evidence of large-scale impact of humans.”
The way the findings were presented elicited a critical response from some scientists in New Zealand, who said the research team “hasn’t caught up with the positive changes in research and science in this country, where policy demands Māori involvement, Māori participation and Māori leadership,” said Sandy Morrison, acting dean of the Faculty of Māori and Indigenous Studies at the University of Waikato, and co-lead for Vision Mātauranga, New Zealand’s Antarctic Science Platform. “This involvement starts from the basic premise that we as Māori will tell our own stories and control our own knowledge.”
The research is fascinating, but could have been much better if it had been more inclusive, said Priscilla Wehi, director of Te Pūnaha Matatini Centre of Research Excellence in Complex Systems.
“It is scientifically spectacular to see an analysis of Antarctic ice cores show fire patterns in Aotearoa over the last millennia so clearly,” she said. But the work also made her reflect on diversity and inclusion in science, with research from Australasia and other parts of the global south often missing from the work of European and North American scientists.
In this case, the authors “apparently lack New Zealand collaboration despite the central topic of Māori burning and fire use,” she said. Research by people who live and work far from the subject of their studies is under growing scrutiny because it can miss important insights. “The ethics of such ‘helicopter science’ have been debated widely over the last year or so, as concerns over the exclusion of different groups from research, including Indigenous peoples, have escalated,” she added.
McConnell interpreted the criticism as a potentially dangerous attack on science. He said that in a New Zealand radio interview, he was asked if he thought he should do more politically correct research in the future.
“I said, ‘to me, that’s an oxymoron, those are two separate things.’ Science is supposed to be about facts, observation and peer review,” he said. “But apparently, there is a different current going on in New Zealand.”
He said he was confused about the backlash, because Māori are very proud of fire, which they see as one of three of Earth’s main elements, given to them by a goddess with fingernails of flames. “Apparently, they are offended by the idea that I’m saying we can see it in Antarctic ice cores,” he said, speculating that it somehow conflicts with the Māori self-image of being in harmony with nature.
“I don’t think we said anything culturally insensitive. It’s science. It’s really about atmospheric chemistry and atmospheric transport,” he said. “All I know about is what I see in my ice cores and what the modeling tells me. The idea that you’re going to pick and choose what science is telling us depending how it fits your preconceived ideas, your traditions or your ethnic identification is what leads us to climate denial,” he said.
“Science isn’t about choosing to believe something,” he said. “Science is about ‘what does the evidence show us?’”
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