The Ruddiman Hypothesis: A Debated Theory Progresses Along Interdisciplinary Lines
In the 1990s marine geologist and current University of Virginia emeritus professor William Ruddiman participated in the summer gatherings of the Cooperative Holocene Mapping Project (COHMAP), which focused on Earth’s conditions beginning around 20,000 years ago during the last glacial period and continuing on into the present interglacial Holocene that began around 12,000 years ago. In COHMAP, Ruddiman interfaced with pollen experts, climate scientists, and others who specialized on the Holocene.
Around the time COHMAP wrapped up, a substantial Antarctic ice record detailing past atmosphere composition came out, showing high methane concentrations ten thousand years ago and decreasing steadily for a few thousand years. The record indicated that around 6000 years ago, atmospheric methane and carbon dioxide rebounded and rose in concentration, instead of expectedly declining, given the Earth’s orientation in the solar system – its “orbital” configuration – that shapes the Earth’s temperature and carbon and methane geochemistry.
Shortly thereafter, Antarctic ice core data from earlier interglacials started coming in, and Ruddiman noted that in previous interglacials methane and carbon dioxide trends kept going down. “It seemed obvious to me that this interglacial was anomalous. And the only thing I could think of was that it might be humans.”
Expanding his focus, Ruddiman began developing his hypothesis that it was primarily humans’ forest clearing for agriculture that released carbon dioxide and methane-emitting rice cultivation that accounted for the upward spike in greenhouse gases emissions before modern times.
Some climate scientists, especially geochemists like the well-known Wally Broecker -- objected to Ruddiman’s explanation and claimed that geochemical ocean dynamics caused the unexplained rise in the carbon release. Importantly, Broecker and others claimed that the Stage 11 Interglacial was the best analogue to the Holocene and its comparison did not support Ruddiman’s claim. The objectors also held that before the 19th century, the less numerous humans could not have massively cleared forests with the carbon impacts Ruddiman suggested (see here; and for background on debate in media and science, see Richard Blaustein’s 2015 article).
But in recent years that is what ecologists, botanists, and archaeologists have been establishing – massive and early preindustrial deforestation. Moreover, archaeobotanists, prominently Dorian Fuller of University College London, have documented the large expansion of rice patty agriculture thousands of years ago, explicating the methane rise. And offering strong support for the Ruddiman hypothesis, a consortium of over 250 archaeologists published a well-noted August 2019 Science article, “Archaeology assessment reveals Earth’s early transformation through land use,” that posited “a planet largely transformed by hunter-gatherers, farmers and pastoralists by 3000 years ago.” (Stephens et al., 2019)
...
Scientists earlier held that the Marine Isotope Stage 11 (MIS11) interglacial that occurred from 424,000 and 374,000 years ago as the best Holocene analogue. Ruddiman disagreed as early as 2005, saying it mismatched with the Holocene’s complete orbital configuration – which is made up of “eccentricity” (the Earth’s rotation around the sun), its “obliquity” (the Earth’s tilt axis); and “precession” (the Earth’s wobble on the axis). In particular, the Stage 11 obliquity does not match up to the Holocene pattern by thousands of years.
Over time, more ice data and glacial-interglacial time refinements came out, and making use of this more precise data, Steve Vavrus and colleagues (2018) underscore a current appreciation of that Stage 19 is a better analogue than Stage 11. In addition to Vavrus, Ruddiman points to the work of University College London palaeogeographer Polychronis Tzedakis, who, along with Ruddiman was a co-author on Vavrus’s Scientific Reports paper, as particularly important for clarifying the interglacial past and MIS19.
Comparing the MIS19 conditions that would match with 1850 in the Holocene, Vavrus’s 2018 Scientific Reports s paper states: “The mean-annual global temperature falls by 1.27 K while the 5-6 K cooling in the high Arctic is the most pronounced anywhere.” While the Earth would not have had another ice age, in the Holocene absent an early greenhouse gas upturn, a year-round “glacial inception” would have set in parts of Canada and Russia that today are snowed over seasonally.
Ruddiman and Vavrus highlight different but complementary aspects of the Vavrus-led study. “We now have six or seven previous interglaciations to look at the carbon dioxide trends and all of those must have been natural – humans were not an active force on the land,” Ruddiman says. “And none of those previous interglaciations show any kind of rising trend like what is happening in past 7000 years. In my mind, the record from the natural previous interglaciations rules out any natural explanation for the rise in carbon dioxide in this interglaciation.”
Vavrus highlights that the simulation’s outcomes have the same ultimate portent to which his earlier research points. “The argument that a few numbers of humans had a huge impact on the environment – if that is true, and our evidence suggests that it is – is all the more reason to be concerned about the much bigger impact we are having on the present-day environment with so many more people in the world and the amount of carbon emissions much higher in the present,” Vavrus says.
...
The study found that Europe forest cover peaked 8,000-6,000 years ago, and that forests then covered around three quarters of Europe. Shortly after 6,000 years ago, significant deforestation began. The northern Europe needle-leaf forests persisted further in time, while deciduous forests in mid-latitude western Europe were felled in earnest for agriculture early, beginning around 6000 years ago. By 3,000 years ago, quite extensive European deforestation had taken place. Today, fragmented forests cover less than half of Europe. Woodbridge and colleagues offer that most forest losses occurred before the industrial revolution.
Ruddiman adds that while “Europe’s Lost Forests” covers Europe and is not a global estimate, it is complemented by much research in China with 50,000 archaeological communal sites that point to enormous population increases 8,000 to 4,000 years ago, indicating deforestation. “Since these people are farmers, and we are talking about areas of natural forests, they had to clear those forests to get the sunlight to the land so they could grow crops.” Ruddiman gives a rough estimate that Europe and China combined account for roughly a preindustrial removal of 45% of natural forest.
https://www.globalpolicyjournal.com/blog/24/02/2021/ruddiman-hypothesis-debated-theory-progresses-along-interdisciplinary-linesGreat overview of science supporting Ruddimans Early Anthropocene theory.
