Thursday, July 15, 2021

Fire Initiated The Anthropocene

James C. Scott in his book Against The Grain: A Deep History of the Earliest States makes the case for the transformational impact of fire on our environment.

" Hominids' use of fire is historically deep and pervasive. Evidence for human fires is at least 400,000 years old, long before our species appeared on the scene. Thanks to hominids, much of the world's flora and fauna consist of fire adapted species (pyrophytes) that have been encourage by burning. The effects of anthropogenic fire are so massive that they might be judged, in an evenhanded account of the human impact on the natural world, to overwhelm crop and livestock domestication. Why human fire as landscape architect doesn't register as it ought to in our historical accounts is perhaps that its effects were spread over hundreds of millennia and were accomplished by "precivilized" peoples also known as "savages". In our age of dynamite and bulldozers, it was a very slow-motion sort of environmental landscaping. But is aggregate effects were momentous."

The impact of human activity on the earth's outer skin has been so considerable that atmospheric chemist Paul Crutzen proposed that we are now living in a new geologic epoch, which he called the Anthropocene. This has sparked a vigorous debate on whether a new division of our time scale is justified, and if it is, on where to place its beginning. James C. Scott make a distinction between what he terms the "thick" Anthropocene, contrasting with the idea of a  "thin" Anthropocene. 

A "thick" Anthropocene is manifest by a sudden appearance of a worldwide signal of human activity. Examples of this could be the advent of the Industrial Revolution, or even more catastrophically, the nuclear age in the 1940's which left global radioactive markers.  The "thick" Anthropocene appears to fit more closely geologic convention which demands that the beginning of a new geologic time unit need be recorded by a widespread and more or less synchronous preservation of biological and chemical changes. 

Scott calls fire, agriculture, and domestication as part of the complex that comprises a "thin" Anthropocene. These inventions changed the world patchily and slowly. Its signals appear here and there, not encoded in one geologic layer, but in many, smeared over the past few hundred thousand years. 

The term "more or less synchronous" I used to describe a new geologic boundary is relative to where in the geologic past the observer is. More recent changes are resolvable to a finer degree either as a matter of historical record or by methods like counting tree rings and annual/decadal growth layers in stalactites, cross calibrated by either carbon dating or some other radiometric dating method. The error bar increases as one goes further back in time. Take the great mass extinctions of the past. There will be a sediment layer to which a geologist can point to and state that this marks the boundary between say the Ordovician and the Silurian or the Permian and the Triassic. But that sediment layer certainly wasn't deposited instantaneously. It likely represents a few thousand years of elapsed time. If you were an observer who spent a few years in the Late Ordovician 443.8 million years ago, the situation would have been more akin to Scott's "thin" Anthropocene, with small changes occurring at different times in different places. You would have been unlikely to have anticipated the profound cumulative shift that would eventually accumulate.

Most mass extinctions which form the basis of the big divisions of geologic time unfolded over thousands of years, but their material record is collapsed into a few feet of sediment. We perceive these geologic turnovers as 'sudden' because the preceding and succeeding periods of relative stability lasted tens of  millions of years and the time the 'boundary layer' spans is unresolvable using our current dating methods.

The exception to this is that fateful day 66.03 million years ago, when a large meteorite struck what is now the Yucatan Peninsula. By the time the dust had settled (literally) in a few weeks, the world had irrevocably changed. The Cretaceous-Paleogene boundary layer is in an absolute time sense a truly instantaneous deposit. We interpret it as instantaneous, not by radiometric dating, but by using our understanding of the physical sedimentation processes that would have been triggered by the impact.

The lesson we can draw from the transformational events from deep geological time is not about the debates over the timing of Anthropocene but on its effect. Like those distant ecologic disruptions, we too have set off biological, chemical and physical processes on a different trajectory than they were several thousands of years ago. The Anthropocene will leave a permanent mark on the many future worlds to come.

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