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The Anthropocene

January 18, 2016

The Greek word ʿολος (holos) can be translated as entirety in English. Whenever I see holos, I'm reminded of the phrase, "Life, the universe and everything," from The Hitchhiker's Guide to the Galaxy. As everyone knows, the answer to this ultimate question is 42. This answer was an Easter egg incorporated into the source code of older spreadsheet versions of the once popular open source program, Open Office. Open Office has been succeeded by LibreOffice, whose word processor I used to write my books.

The geological epoch in which we live is called the Holocene, a name constructed from holo combined with a suffix, -cene, indicating something recent. So, we live in a period that's "entirely recent," which makes a lot of sense. The Holocene is defined as starting 11,700 years before the year 2000, and it includes all the written history of mankind and a lot of prehistory. The Holocene itself is divided into smaller climatic ages, as shown below.

Climate ages of the Holocene

The climatic ages of the Holocene are the Preboreal (10.3-9 ka), Boreal (9-7.5 ka), Atlantic (7.5-5 ka), Subboreal (5-2.5 ka), and the Subatlantic (2.5 ka-present). (Created using Inkscape.)


The start of the Holocene also marked the start of the Neolithic Era of human development, the "New Stone Age," when technology started to develop and humans began to reshape their world. Estimating the human population at the start of the Neolithic is difficult, but it's likely only about five million people. From that time, humanity has grown more than a thousand fold to our present population of 7.3 billion. A few million people are hardly enough to make dramatic changes in their environment, but billions of people certainly will.

The most important technology was agriculture. The ability to create their own food supply released humans from the need to forage, and families were finally able to stay in one place. Animal husbandry of sheep, goats and cattle started at about the same time, and such managed resources allowed population expansion. By the first century, there were more than a hundred million people, and this increased to a billion by 1800.

Early agriculture in Egypt

Agriculture in Egypt, circa 1200 BC - A farmer plowing.

(Mural from the burial chamber of Sennedjem, Thebes, via Wikimedia Commons.)


Increased population and industrialization now have a significant impact on the environment. Animal habitat has been lost from logging, farming and urbanization. The burning of fossil fuel has caused not only global warming, but also ocean acidification as atmospheric carbon dioxide dissolves in water. Such acidic water weakens the calcite shells of organisms such as corals and mollusks.

The need to feed so many people has led to overfishing and the extinction of many species. Unfortunately, as one species of fish becomes extinct, fishermen increase their take of other species, and it becomes a race to the bottom of the trophic pyramid. Agriculture adds another burden to the survival of marine species, since eutrophication from fertilizer runoff has created dead zones near shorelines.

Since humans are now modifying the surface of the planet on a grand scale, and geologic epochs are marked by sudden discontinuities in strata, geologists are now proposing that we've left the Holocene and entered the Anthropocene. The etymology of this word is quite apparent. It derives from the Greek word for human, ανθρωπος. Recent papers in the journals, Science and Nature,[1-2] co-authored by a plethora of international geologists, make the case for the end of the Holocene and the start of the Anthropocene,[1-6]

The United States at night (NASA)

A signal from the Anthropocene.

This is a composite of night images of the United States acquired by the Suomi NPP satellite in 2012.

(NASA image.)


The Nature paper examined the pairing of species in ecological communities; that is, how often a particular pair of plant or animal species was found. The analysis showed that about 6,000 years ago the frequency of such pairing changed, apparently in response to human agriculture.[1,3] The connection is unproven, but one could place the start of the Anthropocene at 6,000 years ago. Says Anna K. Behrensmeyer, a study co-author with the Smithsonian Museum of Natural History,
“The pattern of co-occurring species remained stable through the evolution of land organisms from the earliest tetrapods through dinosaurs, flowering plants and mammals... This pattern didn't change because of previous mass extinctions or ancient climate variability, but instead, early human activities 6,000 years ago suddenly began resetting a basic property of natural communities.”[3]

The science paper, written by the Anthropocene Working Group, looked at more recent evidence of human activity in the geologic record. Several candidates for defining the Anthropocene were identified, as follow:[2]

Material Waste
As a materials scientist, I might be forgiven for placing this item first on the list. As the Science authors note, industry produces about 300 million tons of plastic annually, which has about the same mass of the 7.3 billion humans in the present population. Geologic sediments also contain remnants of aluminum and concrete.[2] The world's concrete amounts to about a kilogram for every square meter of Earth's surface.[5] Fossil fuel combustion has left remnants of inorganic ash and black carbon in recent sediments.[2]

Surface Restructuring
We've transformed the mostly forested surface of the planet with the establishment of farms, cities, strip mines, toxic waste dumps, interstate highways, and much more. Mining, especially, involves modification of the composition of typical surface sediments on a grand scale, and trawlers do the same for the ocean floor.

Above-Ground Nuclear Testing
Considerable quantities of radioactive debris were lofted into the atmosphere before the end of atmospheric testing of nuclear weapons. Such testing between 1952 and 1980 produced a major spike in geologic concentration of carbon-14 and plutonium-239.[2] Plutonium-239 has a half-life of 24,100 years.

Chemical Equilibrium
Combustion of fossil fuels has increased the concentration of carbon dioxide in the atmosphere by 35%, leading to changes in the environment as noted earlier. The use of synthetic fertilizer has changed the Earth's natural nitrogen cycle. Global warming has increased sea level, and human intervention has caused the rapid introduction of invasive species.[2]

A Sixth Mass Extinction
There have been five major extinction events for life on Earth, the most recent being the Cretaceous–Paleogene extinction event (K–Pg, formerly known as the Cretaceous–Tertiary, K–T, extinction) about 66 million years ago. About three quarters of species became extinct in this event, thought to have been triggered by the impact of a large asteroid. All non-avian species of dinosaurs were extinguished in this event. Human activity is on track to match a species extinction of this magnitude within the next few centuries.

A 1626 drawing of a dodo by Adriaen van de Venne

Poster species for extinction caused by human activity.

A 1626 drawing of a dodo (Raphus cucullatus) by Adriaen van de Venne (circa 1589–1662).

(Via Wikimedia Commons.)


So, when did the Anthropocene start? This translates to the question of what's the most definitive marker of human activity found in the geologic record, and a marker that will remain for millions of years. As shown above, quite a few candidate markers are available, but the decision is up to the International Commission on Stratigraphy.[4]

References:

  1. S. Kathleen Lyons, Kathryn L. Amatangelo, Anna K. Behrensmeyer, Antoine Bercovici, Jessica L. Blois, Matt Davis, William A. DiMichele, Andrew Du, Jussi T. Eronen, J. Tyler Faith, Gary R. Graves, Nathan Jud, Conrad Labandeira, Cindy V. Looy, Brian McGill, Joshua H. Miller, David Patterson, Silvia Pineda-Munoz, Richard Potts, Brett Riddle, Rebecca Terry, Anikó Tóth, Werner Ulrich, Amelia Villaseñor, Scott Wing, et al., "Holocene shifts in the assembly of plant and animal communities implicate human impacts," Nature, vol. 529, no. 7584 (January 7, 2016), pp. 80-83, doi:10.1038/nature16447.
  2. Colin N. Waters, Jan Zalasiewicz, Colin Summerhayes, Anthony D. Barnosky, Clément Poirier, Agnieszka Gałuszka, Alejandro Cearreta, Matt Edgeworth, Erle C. Ellis, Michael Ellis, Catherine Jeandel, Reinhold Leinfelder, J. R. McNeill, Daniel deB. Richter, Will Steffen, James Syvitski, Davor Vidas, Michael Wagreich, Mark Williams, An Zhisheng, Jacques Grinevald, Eric Odada, Naomi Oreskes, Alexander P. Wolfe, "The Anthropocene is functionally and stratigraphically distinct from the Holocene," Science, vol. 351, no. 6269 (January 8, 2016), DOI: 10.1126/science.aad2622.
  3. Joshua E. Brown, "Scientists Peg Anthropocene to First Farmers," University of Vermont Press Release, December 17, 2015.
  4. Jonathan Amos, "'Case is made' for Anthropocene Epoch," BBC, January 8, 2016.
  5. Alister Doyle, "The Anthropocene: Are we in a new, human-induced epoch?" Reuters, January 8, 2016.
  6. Eric Roston, "Welcome to the Anthropocene: Five Signs Earth Is in a Man-Made Epoch," Bloomberg News, January 7, 2016.

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