Humans, and scientists in particular, love classifying things. Biologists devised the Linnean system for stuffing species into neat boxes, and a Red List to classify species according to whether we’re killing them off (and to tabulate how we’re doing that). Geologists worked out the passage of time on Earth into distinct ages: Cambrian, Cretaceous, Jurassic, etc. So it’s probably not a surprise that during the last few years, the idea has been catching on that humans are now having such a big impact on the Earth that we should classify modern times as a new epoch: the Anthropocene. But when did the Anthropocene begin? A new paper in Nature looks at some of the evidence, and concludes that we should officially make 1610 the date.

When geologists classify Earth’s periods into ages, they also have to decide where the boundaries lie. Basically, consecutive time periods should be different enough to be distinguishable, perhaps because an entire group of species -- such as the dinosaurs -- were present in the earlier period but either partly or totally disappeared after the boundary.

Ideally the boundary itself should be easy to spot. The preferred method for defining a boundary is to find a “Global Stratotype Section and Point” (GSSP), better known as a “golden spike”. This is a section of rock (or similar) where there is a clear boundary that can be used to delimit two time periods. For example, the boundary between the Cretaceous and Paleogene is easy to see because there is a layer of iridium there in the rocks that dates back to 66 million years ago.

So if we are to have an Anthropocene then geologists must find a clear place in time where they can draw a line between the Anthropocene and those times when the Earth was relatively free of human influence. But where should it be? This is the question that Simon Lewis, of University College, London and the University of Leeds, and Mark Maslin of University College, London, seek to answer. In their paper, they consider several alternatives, and ask whether there is a signature of global change in each period and if a golden spike can be seen there. Their candidates are:

Pleistocene. Either fire caused by humans, or the extinctions of big mammals between 50,000 and 10,000 years ago, could be used as signals of the start of the Anthropocene. The authors discard both of these because they are a series of local events rather than a global event, so they are not a good golden spike.

Development of Farming. This has had a big impact in the long run, but the effects are diffuse since farming started at different times (11,000 years ago in southwest Asia, south America and northern China, to roughly 4-5 thousand years ago in parts of Africa, Asia, and North America). But by approximately 8000 years ago, most humans depended on agriculture for food, so perhaps a date near then could be chosen. But what might serve as the golden spike? There was a rise in methane concentrations about 5020 years ago (see figure 1b), which may be associated with both increased rice cultivation and more farting cows. Unfortunately, there are alternative explanations for rising methane levels so Lewis and Maslin reject this date, too.

Collision of the Old and New Worlds. The authors decide this started in 1492, when rich globe-trotting Europeans discovered the Caribbean. The result of this was the “Colombian Exchange”, with crops being swapped between the Old and New Worlds; maize and potatoes went to Europe, and sugarcane and wheat went to the Americas. The authors note there were other effects, too:

Besides permanently and dramatically altering the diet of almost all of humanity, the arrival of Europeans in the Americas also led to a large decline in human numbers. Regional population estimates sum to a total of 54 million people in the Americas in 1492, with recent population modelling estimates of 61 million people. Numbers rapidly declined to a minimum of about 6 million people by 1650 via exposure to diseases carried by Europeans, plus war, enslavement and famine. The accompanying near-cessation of farming and reduction in fire use resulted in the regeneration of over 50 million hectares of forest, woody savanna and grassland with a carbon uptake by vegetation and soils estimated at 5–40 Pg within around 100 years. The approximate magnitude and timing of carbon sequestration suggest that this event significantly contributed to the observed decline in atmospheric CO 2 of 7–10 p.p.m. (1 p.p.m. CO 2 = 2.1 Pg of carbon) between 1570 and 1620 documented in two high-resolution Antarctic ice core records. [...] This dip in atmospheric CO 2 is the most prominent feature, in terms of both rate of change and magnitude, in pre-industrial atmospheric CO 2 records over the past 2,000 years.

So, Lewis and Maslin suggest that 1610 is a good candidate and refer to it as the “Orbis Spike”. This is when CO 2 was at its minimum (figure 1c), although there are a few similar markers around the same time. However a lot of the climate signature seems to be a blip, rather than a pattern of long-term change, which contrasts with their next suggestion...

Industrialization. The industrial revolution (between about 1760 and 1880) is when most of us think that the Anthropocene really got going. But the Industrial Revolution’s impact is cumulative; it lacks a specific point in time where there is an identifiable change, so the authors can’t point to a single golden spike.

The Great Acceleration. This is the period after World War 2, when human populations and their impacts exploded -- sometimes literally. So a possible golden spike could be nuclear fallout from the detonation of bombs, with an identifiable spike of 14C in the 1960s (figure 1d). But this spike did not itself do much harm. Instead, it is an identifiable symbol of wider changes that humans have caused.

Figure 1: (a) Current boundary between the Pleistocene and Holocene (dashed line), with global temperature (blue), and atmospheric CO2 (red). (b) Early Anthropogenic methane GSSP suggested boundary (dashed line), atmospheric methane (in parts per billion, p.p.b.) (green), global temperature (blue), and atmospheric CO2 (red). (c) Orbis GSSP suggested boundary (dashed line), atmospheric CO2 (blue), and global temperature (red). (d) Bomb GSSP suggested boundary (dashed line), atmospheric radiocarbon from annual tree-rings (black), atmospheric CO2 (red), and global temperature (blue). Illustration: Simon L. Lews & Mark A. Maslin (doi:10.1038/nature14258).

Lewis and Maslin like either 1610 or 1964 as the candidate date for their golden spike. They prefer 1610 since the effect after that date seems larger and, they argue, 1610 is also an important precursor to the industrial revolution. On the other hand, the industrial revolution is widely viewed as the clear start of global changes in both climate and environment, but there is no clear golden spike to mark this geologic boundary.

The authors’ choice of 1610 as the golden spike that signals the beginning of the Anthropocene looks like a rather Euro-centric view. After all, there were people in the Americas already, and people had been carrying food crops and other species all around the world long before then. So declaring the European invasion of the Americas as the beginning of the Anthropocene is a very political act. But any date chosen as the beginning of the Anthropocene will be inherently political. We certainly don’t learn anything new about the past by deciding if 1610 or 1963 is the start of the Anthropocene, but classifying our times in this way draws a line in the sand. We can point to a time when we think the planet started changing significantly due to human influences. If we are to save the world and try to reconstruct it as it was, when should we set the clock back to; 1963 or 1610? When will we date our self-expulsion from the Garden of Eden?

Source:

Simon L. Lews & Mark A. Maslin. (2015) Defining the Anthropocene. Nature, 519: 171–180 doi:10.1038/nature14258 [₤]

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Bob O’Hara is a biostatisician who conducts research at the Biodiversity and Climate Research Centre (BiK-F) in Germany. In his free time, he writes the blog, Deep Thoughts and Silliness, and tweets from @BobOHara.

GrrlScientist is an evolutionary biologist and science writer. She’s very active on twitter @GrrlScientist and sometimes lurks on social media: facebook, G+, LinkedIn, and Pinterest.