Bill & Fred's Excellent Adventure!

It is commonly thought that pre-industrial human societies lived in harmony with the environment (Steffen et al 2007), but evidence showing early modification of landscapes (Neolithic forest clearance, extensive use of fire, and agricultural expansion) proves this myth is just that: a myth. By now, I am sure you realise the biggest debate within the Anthropocene literature (besides whether it should even exist) is when it should begin. Defining the start date is not an easy task as it comes paired with huge implications for how we define human impact (Mackay 2015*), and there are many contrasting opinions. Cast your minds back to one of my first ever blog posts, where I summarised some key proposals for the Anthropocene start date. This post looks in detail at one of the main suggestions, the Early Anthropogenic Hypothesis (EAH) championed by William (aka Bill) Ruddiman (Ruddiman and Thompson 2001; Ruddiman 2003; Ruddiman 2007; Ruddiman 2013).

All the new agricultural expansion and livestock farming at the
dawn of the Anthropocene (according to Ruddiman),
meant our good friend Fred Flintstone could finally enjoy
some bread on his burger! (Source)

Ruddiman's EAH suggests that early agriculture in the mid-Holocene, from 8000-5000 years ago, affected the functioning of Earth Systems to such an extent the onset of a new glaciation was significantly delayed (Ruddiman 2003; see an earlier post of mine about this ice age prevention). In Ruddiman's view, the concept of the Anthropocene focuses on when humans first had a detectable global impact, but not necessarily the largest impact. There are two key facets to Ruddiman's hypothesis:

1. Increased atmospheric CO2 (carbon dioxide) concentration from forest clearance (fewer carbon sinks) particularly in Eurasia, permitting rapid expansion of agriculture 8000 years ago (Ruddiman 2003). 

2. Increased atmospheric CH4 (methane) concentration from rice cultivation in irrigated, water intensive paddy fields starting in South-East Asia 5000 years ago (Ruddiman and Thompson 2001).

Cyclic variations in methane and carbon dioxide driven by Milankovitch cycles (Earth's orbital changes of obliquity, eccentricity, and precession) over the last ~400,000 years imply a declining trend in CO2 and CH4 should have been seen throughout the Holocene. Ruddiman (2003) argues that the anomalous increases of CO2 found in Greenland ice cores 8000 years ago, and CH4 5000 years ago, show anthropogenic influence on the Earth System, whereby GHGs rose enough to prevent a new glacial inception. Ruddiman (2003) insists forest clearance and irrigation contributed to a small, yet sustained rise in global atmospheric temperatures (Mackay 2015*). 

In 2007, Ruddiman responded to a series of challenges against the original EAH (2003). Some of the updates to the hypothesis include CO2 and CH4 anomalies of 35ppm and 230ppb, respectively, which are ~90% of those proposed originally (Ruddiman 2003; 2007). Another revision to the original hypothesis was accepting that deforestation and burning of coal could only account for ~25% of the 35ppm anomaly in CO2 concentration, though Ruddiman (2007) still claims the CO2 rise is anomalous and likely to stem from human origin. Ruddiman (2007) suggests the only other likely source of the remaining 75% of the CO2 anomaly is carbon from oceans in the Holocene that did not cool like they had in earlier interglacials, thus becoming less efficient carbon sinks. Though the CH4 anomaly is largely explained by agricultural activities and climate feedbacks, the CO2 anomaly is the biggest uncertainty in his hypothesis and the reason why many academics have dismissed the credibility of his proposal.

Ruddiman's EAH: a) Anthropogenic increases in methane.
b) Anthropogenic increases in carbon dioxide.
c) Anthropogenic warming caused by GHGs.
Note the similar timing/shape of graph with warming and GHGs.
(Source: Ruddiman 2007).

Supporters of Ruddiman's EAH:

Ruddiman has received cross-disciplinary support from Dorian Fuller, an archeologist at UCL. A paper by Fuller et al (2011) concluded that wet rice paddy cultivation in Asia produces a logarithmic growth in methane emissions on a similar timeline to those posited by Ruddiman's anthropogenic methane increase. Fermentation of organic matter in these wet paddy rice fields produces significant amounts of methane (Smith and Zeder 2013). The tending of methane-emitting livestock are postulated to be another key contributor to the anomalous rise in methane, though the impacts are yet to be quantified. 

A further study by Jed Kaplan et al (2011) looked at Holocene CO2 emissions as a result of human-induced changes in land cover. The authors, which include Ruddiman, show that human residents and farmers from the late Palaeolithic to the start of industrialisation used much more land per person than any humans after them. The land-use model used by Kaplan et al (2011) has added significant weight to Ruddiman's hypothesis, as the issue with the CO2 anomaly is somewhat solved by the model, as it accounts for 24ppm rise in CO2.

Critics of Ruddiman's EAH:

Broecker and Stocker (2006) believe Ruddiman's (2003) claim that anomalous atmospheric CO2 rise 8000 years ago is anthropogenic and not natural should be examined more carefully. Ruddiman (2003) argues that without human activity, CO2 content of the atmosphere would've fallen to 240ppm instead of rising to 280ppm. Broecker and Stocker (2006) are critical of this, as huge amounts of deforestation would have needed to take place to account for a 40ppm increase in CO2. Essentially, by comparing the Holocene's orbital parameters to the early part of Marine Istope Stage 11 (MIS11), Broecker and Stocker (2006) argue that the CO2 rise over the last 8000 years stems from natural causes and not from human influence. 

Steffen et al (2007) are proponents of the Anthropocene beginning in ~1800 AD with the central feature being the exponentially growing use of fossil fuels. Steffen et al (2007) only recognise Ruddiman's EAH as a 'Pre-Anthropocene' event and not the main event, as rates of forest clearance 8000 years ago were minimal compared to deforestation in recent centuries and decades. Steffen et al (2007) argue that pre-industrial societies, though having some clear impact on the environment, did not cause changes ranging outside the normal bounds of natural variability in the Holocene, and societies could not compete with the magnitude or rate of the forces of Nature, and thus dismiss this idea as a starting date for the Anthropocene.

Thoughts and comments:

The hypothesis has sparked a large literary debate from all sorts of disciplines, having had over 50 papers published for and against Ruddiman's ideas in the last 10 years. Ultimately, this suggestion for the start of the Anthropocene between 5000-8000 years ago also has wider implications for the Holocene as we know it and creates new problems, such as whether we even need the Holocene. If the Anthropocene did begin 8000 years ago, what would be the point in the Holocene epoch existing for a mere 3,700 years? Furthermore, in 2013, Ruddiman concluded his paper stating he thinks the Anthropocene should remain an informal term, rather than applying the "simplifying" term to the "rich complexity of human history". I think this shows Ruddiman now sees the Anthropocene more as a unit of human history, than of Earth history (see my earlier post for debates on this). Ruddiman's hypothesis, though unlikely to win over the AWG for the onset of the Anthropocene, carries important messages about life on Earth today. If we were able to alter the climate and atmosphere with such little intended impact in the past, then the rate of change today (not even remotely comparable to the past) has huge implications for the future of our planet!

*Mackay, A.W. (2015) Anthropocene Epoch. International Encyclopaedia for Social and Behavioural Sciences, 2^ndEdition.

"The scheduled service to the 'Next Ice Age' is delayed..."

There is change afoot at Earth station central, where humankind appear to be rewriting the timetables. Trouble is, this could be inviting unforeseen circumstances further down the line...

Imagine one morning, you wake up (or rather, you don't), buried under 10 feet of snow. You desperately fight your way to a window, a gap in the snow, anything which allows you to look outside. Searching for a familiar landscape of rolling green hills and townscape, instead you find a barren, desolate, ice-covered horizon. Am I dreaming, you think? What has happened to planet Earth as we know it?

The Ice Age which never arrived - artificial scenes from The Day After Tomorrow (Source)

Well, this (slightly dramatic) scenario could have been a cold, hard truth for humanity over the next millennium, though not taking place on the overnight time-frame as suggested in films like The Day After Tomorrow. Ever since humans started inadvertently tampering with the CO2 dial in the atmosphere, Earth's future path has veered somewhat off-piste. Before copious layers were sewn to a seemingly ever-growing blanket of greenhouse gases in the atmosphere, it is believed the planet was en route to a new ice age. Now, however, this glaciation isn't likely to happen any time into the foreseeable future, and it's because of humans.

Research led by Chronis Tzedakis, a professor at UCL, has suggested that the onset of a new ice age would begin in 1,500 years time, IF (and this is a very important 'if') CO2 concentration were still at pre-industrial level (~280 parts per million (ppm)). Since the dawn of human civilisation, increased agriculture, the industrial revolution, and the most recent Great Acceleration period (see my last post), CO2 has risen to an average today of almost 400ppm, nearly double the threshold of ~240ppm required for glaciation! Even if anthropogenic emissions were shut down tomorrow, it would take a considerable amount of time to bring atmospheric CO2 concentrations down enough to support a big freeze.

Interglacial-glacial cycles are largely controlled by the slow, cyclical changes in the nature of Earth's orbit around the Sun. These are called Milankovitch cycles, and take place on timescales of tens of thousands of years. A key trigger for ice age inception is weak insolation (low heat from the Sun) during the summer months. Tzedakis et al (2012) compared present day to an interglacial 780,000 years ago, Marine Isotope Stage (MIS) 19c. Both time periods have similarities in orbital configuration regarding weak summer insolation. The difference is that CO2 is much higher today, and this is overpowering the orbital forces which should be driving present-day glaciation.

Milankovitch Cycles of eccentricity, obliquity, and precession (Source)

Though ice ages are ultimately forced by orbital changes, internal climate feedbacks are also vital. When summer solar radiation is low, snow and ice that fell in the previous winter are preserved at high latitudes year-round, and thus are added to the following winter. With this pattern repeating annually for hundreds and thousands of years, Earth starts embarking on a slippery slope to a new ice age. An important impact of increased snow and ice is that an albedo positive feedback loop is created, as light-coloured snow/ice reflects more heat than darker-coloured land, cooling things down just that bit more (see diagram below).

Albedo is greater with snow/ice covered land surface, which means more heat is reflected, thus more snow/ice stays, which reflects more heat etc. A positive feedback loop is formed which has a cooling effect (Source)

Postponing a new ice age might seem like cause for celebration (I'm looking at you, you sun-worshipping, beach-loving, tanned folk). But consider this. Humanity's modification of the atmosphere through anthropogenic carbon dioxide emissions is enough of a planetary-scale forcing to seriously delay the onset of what would've been a natural ice age in Earth's life cycle. The increased albedo needed to shift Earth into a new glacial state cannot and will not happen whilst humans are a dominant geological force. We're tampering with a very sensitive climate system, approaching terra incognita, accompanied by some serious unprecedented consequences. 

Regardless of when human influence of this type upon the planet begun (8000, 200, or 65 years ago), it can be considered unnatural and unhealthy, as it is preventing Earth from undergoing normal cyclical changes. If there's any evidence needed to show that humanity's impact is planetary-scale and significant enough to warrant recognition of the Anthropocene, this is it. This time, it won't be leaves on the line that delays our arrival at the next station 'Ice Age', but something much bigger...

Tzedakis et al's paper is really worth a read, but is perhaps a little heavy reading for those of you unfamiliar with paleoclimatology. Instead, check out Andrew Revkin's 'dotearth' blog post, which nicely summarises the paper and contains some interesting responses from other scientists regarding the findings!