Tipping Points Revisited - Have We Initiated the Anthropocene?
How much of a geologic force have we become?
Two long-time colleagues, Bill Peterjohn and John Pastor, provided responses to the most recent essay on tipping points and that has me looking at the topic again from a different perspective. It is possible to become so used to the fact that we are driving rapid change in the global environment that the continuation of existing trends is seen as nothing extraordinary. I was guilty of that lapse in dismissing the importance of tipping points.
Tipping Points
John provided a mathematical approach to defining tipping points that I will save for another time, but also a geological perspective to be found at the end of this essay. Bill suggested a different look at the numbers on arctic sea ice coverage. I used the graph on the left below to suggest that there was a long-term trend but no tipping points, no sudden changes. Bill pointed out that a longer timeframe, as presented in the figure on the right below, captures a very different reality.
If tipping points are marked by events that produce rapid and irreversible change, then something happened around 1800 that created a tipping point for Arctic sea ice.
It will not surprise readers of these essays that several other crucial measures of the global environment show the same pattern reflected in Arctic sea ice. The figure above captures the "Keeling Curve" of increases in carbon dioxide concentrations in the atmosphere (top), as well as temperature (middle) and sea ice extent (lower), all tipping at about the same time.
Different precise start dates can be and have been identified for each of the increases in this figure, but there is no doubt that this tipping point is driven by population growth, the burning of fossil fuels, and agricultural expansion.
It is also clear that increasing temperatures resulting from greenhouse gas warming are driving the melting of ice caps and a rise in sea level that, when the inertia inherent in the slow rate of melting is overcome, will result in several meters of sea level rise. The Zombie Ice in Greenland, ice that will melt even with no further increase in temperatures, is enough to raise sea levels 10-32 inches. The only question is how fast this will occur.
The original topic for this essay was defining the concept of the Anthropocene and putting that concept into the context of how different geological timelines are defined. This may seem a bit academic compared with the drastic nature of the changes themselves, but the exercise has also provided an additional context and perspective.
So with the understanding that population, industry, and agriculture have driven an integrated tipping point in the global environment, how does the current rate of change compare with previous geologic timelines?
The question has been put this way: Have humans initiated a new geologic time? Just how big a perturbation to the global environment have we become, and how does this compare to previous disruptions?
The Anthropocene
This is the name given, by popular usage if not yet academic concurrence, to the current age, where humans are the major force driving environmental change and when measurements of the global environment have been driven into ranges not seen during the current geologic Epoch or Period. Those capitalized terms have specific meanings for geologists and exploring their meaning provides the perspective I am seeking in this essay.
Geologic Time
It has always interested me that geologists have used changes in species composition to identify boundaries in Earth's history - geology entwined with, even defined by, biology.
The Encyclopedia Britannica summarizes the relationship between biology and geology this way:
The fossil forms that occur in the rocks...provide the chief means of establishing a geologic time scale, with the timing of the emergence and disappearance of widespread species from the fossil record being used to delineate the beginnings and endings of ages, epochs, periods, and other intervals.
According to James Lovelock of Gaia fame, the biggest disruption to biology in the life of the planet began about 600 million years ago when photosynthesis finally got ahead of decomposition and chemical reactions, leading to a dramatic increase in oxygen in the atmosphere. Life shifted from dominance by forms requiring the absence of oxygen to those requiring its presence.
That abrupt transition led to what is termed the Cambrian Explosion, a riotous diversification of life forms on land and in the seas beginning about 540 million years ago. One source has it that nearly all animal phyla first evolved at this time in a period of under 25 million years - a geological heartbeat.
It is oxygen that powers the high energy forms of life that now dominate the biosphere, and the Cambrian Explosion marks the boundary between the Precambrian and Phanerozoic Eons.
Within the Phanerozoic Eon, three Eras have been defined by two major extinction events - times when a significant fraction of species on Earth were eliminated by major environmental changes. What caused these changes is still debated, but I favor an integrated theory that links meteor strikes to massive volcanic activity that together resulted in extreme shifts in global chemistry and climate.
Extinction events also play a role in defining breaks between Periods within the Eras, but in our current Era, the Cenozoic, extinctions have (until recently) been at a lower level and shifts in environmental conditions and species distributions have played a larger role.
We are currently in the Quaternary Period that covers the most recent 2.6 million years within the Cenozoic Era and is characterized by the growth and loss of massive ice sheets (the recurring ice ages). Our current Epoch within the Quaternary is the Holocene, covering the last ~12,000 years since the end of the last glaciation, and encompassing all of the development of modern human culture.
So if we now have a sense of the kinds of disruptions that lead to the initiation of new geologic categories, what of the Anthropocene? Have we initiated a new mass extinction and new Era to replace the Cenozoic? A new Period of rapid environmental change to replace the Quaternary? Are we displacing the Holocene Epoch by forcing rapid changes in climate that move us outside the repeating cycles of the ice ages?
Where do the impacts of modern human life rank on this geologic scale?
Species Extinctions
It is certain that humans are the major cause of extinctions in the modern world, and this is not a recent phenomenon, especially for what we call "megafauna."
If you are ever in Los Angeles, think about visiting the La Brea Tar Pits Museum. Displayed there is the huge cache of bones of extinct animals preserved in the anaerobic tar that bubbles to the surface. At the museum, you are likely to hear school children ask, "where are the dinosaurs?" But the animals here are much more recent - exotic mammals that roamed the plains of what is now Southern California until early in the Holocene.
The list includes mastodons and wooly mammoths, giant ground sloths, horses, and sabretooth tigers. Why did they go extinct?
For some time, hunting by newly arrived humans crossing the land bridge from Asia at the end of the last glaciation about 13,000 years ago (different dates can be found) was projected as the primary cause. This theory appears to have run afoul of modern thinking, such that climate change is offered as another reason, even though these same species survived previous glacial cycles, and then apparently disappeared over just a 300 year period coinciding with the arrival of humans.
The rather graphic images of early hunters with spears attacking mammoths and mastodons have been removed from the Museum's website.
One very recent paper offers a more integrated and nuanced explanation - that both the newly arrived hunters and a drier climate caused an increase in fire frequency and intensity, leading to changes in plant communities. Not being pre-adapted to hunting pressure, increased fire or the new vegetation types, the existing megafauna disappeared, again over a very short span of time.
The role of human hunting or landscape manipulation in the extinction of megafauna species in other regions is hotly debated, but lists of recently extinct and currently endangered species from every continent show that land conversion, hunting, chemical pollution, and now climate change are driving the vast majority of these cases.
The current rate of extinction is itself a hot topic, and one that cannot be resolved here. Estimates of extinction rates for plants and animals vary from 1.5 to 40,000 species per year, and 10 to 10,000 times "normal" or background rates. Differences have much to do with how finely split subspecies and subpopulations are delineated, and estimates of the number of still undiscovered species.
Still, no one disputes that humans have accelerated extinction rates, and perhaps the last word here can go to Stephen J. Gould. During his last major university appearance, at the University of New Hampshire on Earth Day, 2002, Gould noted that any increase in extinction rate that we measure in a human time frame will become a mass extinction event if extended to any length of geologic time.
Environmental Change
In contrast to the apparent uncertainties about extinction rates, changes in the global environment can be measured with some precision. Essays here have dealt with many of the key indicators of change, and 12 are summarized as trends to follow in a dashboard essay.
Let's put some of those indicators into the context of geologic time by summarizing how much they have changed or when the last time numbers were as they are now. Ice cores give us a directly measurable timeline of changes in the atmosphere going back 800,000 years, while environmental proxies of various kinds can give estimates going back millions of years.
Here are four strongly inter-related trends. For population, there has never been a time when a single species dominated the Earth to the extent we do now. For the three major greenhouse gases, current concentrations are outside values over the last 800,000 years, which encompasses all of the Holocene and almost one-third of the Quaternary.
A number of proxy measurements allow a reconstruction of global average temperature going back 60 million years. This figure, with projected ranges for future temperatures due to global warming, suggests that a 2 degree C increase will represent temperatures not seen for 3 million years. A possible increase of 5 degrees C would push us back about 10 million years.
Sea levels have risen and fallen by as much as 100 meters during successive ice ages as huge amounts of frozen water accumulated in the massive ice sheets. Modern human cultural history began in an interglacial period when sea levels were already near a maximum. Projected increases would take sea levels back to values not seen for the last 3-5 million years.
Summing Up
These 6 indicators only scratch the surface, but they form a coherent picture of integrated changes in the Earth system driven by human activity. Geologically, we seem to be a force strong enough to create at least a new Epoch (replacing the Holocene) and probably a new Period (replacing the Quaternary). Defining a new Era to replace the Cenozoic would, by historical geologic standards, require a truly major extinction event.
But John Pastor also raised another issue in reviewing an earlier version of this essay. Geologists need to find an identifiable layer in the rocks to mark a named time. Except in cases of meteor strikes, it takes a long time for the accumulation of sediments to create such a layer.
Just how long will the Anthropocene last?
Sources
The figure on changes in atmospheric carbon dioxide, temperature and Arctic sea ice coverage is from:
The Britannica quote is from:
https://www.britannica.com/science/geologic-time
The original Gaia book - and the one to read, is:
Lovelock, J. 1979. Gaia: A New Look at Life on Earth. Oxford University Press
Wikipedia pages on oxygen in the atmosphere and the Cambrian Explosion are:
https://en.wikipedia.org/wiki/Great_Oxidation_Event
https://en.wikipedia.org/wiki/Cambrian_explosion
The table of geologic time periods is extracted from:
https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/geological-timechart/
The timeline of major extinction events is from:
https://en.wikipedia.org/wiki/Extinction_event
A wonderful book about the K/T extinction, the best known and best documented of the major extinction events is:
Alverez, W. 1997. T. Rex and the Crater of Doom by Walter Alvarez. Random House
The diagram of the Quaternary Period is from:
http://quaternary.stratigraphy.org/major-divisions/
Articles on megafauna extinctions include:
https://www.nature.com/articles/s41467-018-07897-1
https://www.pnas.org/doi/10.1073/pnas.1302698110
https://www.pnas.org/doi/10.1073/pnas.2015032117
The most recent related to the La Brea Tar Pits, and identifying the 300 year period over which many extinctions occurred is here:
https://www.science.org/doi/10.1126/science.abo3594
and a media version is here:
The website for the La Brea Tarpits Museum is:
https://tarpits.org/
The figure of megafauna extinctions on different continents at the time of human in-migration is modified from:
https://ourworldindata.org/quaternary-megafauna-extinction
Use of the original figure is authorized through the Creative Common BY license
The diagram of human population increase is modified from:
https://en.wikipedia.org/wiki/Estimates_of_historical_world_population
Changes in the concentration of carbon dioxide, methane and nitrous oxide over the last 800,000 years are from
And can also be found here:
https://gml.noaa.gov/ccgg/trends/history.html
The long-term temperature reconstruction is from:
https://www.pnas.org/doi/10.1073/pnas.1809600115
The 5 million year+ reconstruction of sea level can be found here:
https://serc.carleton.edu/integrate/teaching_materials/coastlines/student_materials/901