Are clathrates a credible alternative to farming as a methane source?

One particular sentence caught my eye in the Schmidt 2004 paper that I mentioned in the previous post:

"Most importantly, clathrates can be explosively unstable if the temperature increases or the pressure decreases - which can happen as a function of climate change, tectonic uplift or undersea landslides."

Early Holocene archaeologists get excited by the idea of undersea landslides because a major event dated around 8100 +/-100 BP, the Storegga Slide, is thought to be the cause of the inundation of Doggerland, the area between the British Isles, Denmark and northern Germany now known as the North Sea (Weninger et al, 2008, pp.16-17). The date of the that slide is exciting because it is around the time Ruddiman's carbon dioxide anomaly starts to develop and when there is a methane bounce-back from the 8200 BP Holocene cold event (Ruddiman, 2003, pp.263-4; p.266). Methane has a relatively short atmospheric life before it decays to carbon dioxide and other gases (Archer, 2007, p.523); this last point helped resolve my need to find mechanisms that act both on methane and carbon dioxide.



Location of the Storegga Slide (Nature, 2013)

Beget and Addison (2007) hypothesised that the Storegga Slide released "huge amounts" of methane which caused the sudden and significant increase in atmospheric methane, measured in GRIP ice cores, starting around 8100 BP. This is an example of the "clathrate gun hypothesis" which suggests that undersea landslides periodically release climate-affecting quantities of methane (Kennett et al., 2003).

Sea-bed sulfates are a proxy for the concentration of methane; the depth of the sulfate-methane interface (SMI) is a function of the period during which sulfates have been deposited. Paull et al. (2007) measured SMIs along the Storegga Slide and at its margins. They assumed that the sea-bed exposed by the slide would have contained no sulfate. The depths of the measured SMIs were inconsistent with estimates of large quantities of methane being in the material which was displaced in the slide and so they argue that there was not an associated major release of methane.

Dawson et al. (2011, p.1170), who actually set out to date the slide more accurately, compared Beget and Anderson with Paull et al. and concluded:

"the Storegga Slide represents a case where one of the world’s largest slides does not appear to have released significant quantities of methane gas-hydrates into the atmosphere".

which begs two questions:

1. Why did this huge slide not release large quantities of methane?
2. Where did the atmospheric methane come from?

Question 1 is outside the scope of this blog but it is illustrative of the uncertainties that pervade the research of the complex climate-affecting events of the past.

Question 2, frustratingly, takes us back to the beginning.

Public criticism of the Early Anthropogenic hypothesis

Inevitably, while trawling the internet for useful information sources, one encounters discussion and criticism of scientific papers, especially those that relate to contemporary controversies. These may come from scientists specialising in the same field, from academics in other disciplines and from people outside the academic world. Such discussions are not peer-reviewed, in the academic sense, but can be an important and value-added part of the dialectic as they provide open access for constructive criticisms and novel ideas.

Doug L. Hoffman's 2009 blog post on his website www.thereslientearth.com concerns the Early Anthropogenic Hypothesis (EAH). Ignoring the pejorative language, the first six paragraphs provide a reasonable if simplified summary of EAH. He then proceeds to seek papers that refute it; I thought it would be informative follow his trail...

The first (uncited) reference is to Schmidt's 2004 commentary which does not address EAH directly but does, usefully, highlight the possible effect of methane clathrates without directly relating them to anomalous Holocene warming. Next, we are directed to (Zimov et al, 2006) who explore how permafrost stores and releases carbon dioxide and methane and explain how radiocarbon (14C) can be used to differentiate between carbon released into the atmosphere at different times; they conclude by stating that a warming climate will melt permafrost and so cause more carbon to be released. The implication is, I think, that the EAH has not accounted for this effect but (i) such a release would happen during the warming period of every interglacial and (ii) if anthropogenic activity is causing a warmer climate then the efffect would be to increase that release, adding to and compounding the effect of anthropogenic warming caused by early farming. The referenced paper concerning the seasonal fluxing of methane releases (Mastepanov et al, 2008) is very interesting but does not suggest any specific connection with anthropogenic warming and neither do the quotes attributed to Christensen, which are from a separate 2008 Reuters interview.

A supplementary 2009 comment refers to a paper (Elsig et al, 2009) which considers the changes in δ13C in Holocene Antarctic ice cores to determine the origins of atmospheric carbon dioxide. This effectively rejects the EAH and attributes the rise to an "increase in land biosphere and to changes in the marine carbonate cycle" (p.509). This paper does not address methane changes and does not extend the methodology to pre-Holocene time periods to verify the model.

Finally, in a supplementary 2011 comment, Hoffman invokes a paper (Singarayer et al, 2011) which, using climate models, projects and compares the Holocene and Eemian methane releases based on vegetation cover and insolation. This found that the predicted methane concentrations were consistent with the ice core records, suggesting that the EAH is wrong; it is however noticeable that the effect of CO2 on CH4 concentrations was modelled but the resulting CO2 concentrations were not, and that the response of boreal warming (as discussed by Zimov et al above) was specifically excluded from the model.

What did I learn from being sceptical about Hoffman? His sources and arguments are selective and incomplete, and his personalised criticism, language and writing are unacceptable for someone who claims an academic background. But he has pointed me in the direction of some interesting alternatives to EAH. And I am questioning my earlier assertion that refuting EAH must deal with both CO2 and CH4 in the same counter-hypothesis.

24.12.2014: As an addendum to this post I was amused to find this criticism of Hoffman and Simmons' book "The Resilient Earth" at Amazon:

"However, the reader should be strongly cautioned that the source document is likely to be saying something completely opposite to the point being made in this book. Probably it would be best to just check out the source documents, and then optionally read this book afterward. As an example, very few of the N.A.S.A., NOAA or NAS references support the conclusions about climate science presented in the book when taken in their entirety."

The Anthropocene is not just about gases.

I started this blog with a restatement of the Ruddiman hypothesis that forest clearances during the Neolithic to create room for farming started the anomalous upward trend in greenhouse gases. I set that in contrast with the accepted hypothesis that industrialisation during the last 200 years was the cause. The research and reading since that post has made me reappraise my thinking. Now I realise that this is not about accepting or rejecting the Ruddiman position, it is about understanding the complete picture, including what happened to other environmental components between 8,000 BP and 200 BP.

Early agriculture spread from the Fertile Crescent across Europe between 9000 BP and 5000 BP (Isern et al, 2012). The coastal expansion is known as the Impressed Ware trajectory, from the delightful habit of the culture of decorating ceramics with patterns made with cockle (Cardium sp.)shells (Cunliffe, 2011, 115-6), which makes it readily identifiable. The following case study briefly explores an example, starting around 7,400 BP.


Computer-generated model of the Neolithic expansion into Europe based on radiocarbon dates (Isern et al, 2012, e51106)

Anthracological analysis of charcoal fragments from forest clearance in Alicante, south-eastern Spain (Badal et al, 1994) showed that there was a sequence of changes in the prevailing vegetation that paralleled the changes in the Neolithic and Bronze Age cultural horizons. The archaeological evidence showed that agriculture intensified with the introduction of mixed crop rotation and animal traction for ploughing. At the same time black pines (Pinus nigra) and evergreen oak (Quercus ilex) are replaced by olives (Olea europaea) mastic (Pistacia lentiscus) and heather (Erica multiflora) along with the appearance of domesticates such as cereals and beans and the Aleppo pine (Pinus halepensis). Similar changes are observed in many sites around the Mediterranean leading to the prevalence of the now familiar scrubby landscapes. This occurred during a period of generally stable rainfall and temperature.

A significant secondary product of livestock husbandry is dung. The use of dung and manure for agricultural soil enhancement spread across Europe following the Neolithic expansion (Baakels, 1997). Plaggens are slabs of grassy or heather turf used as bedding material in ruminant byres and so soaked in animal waste; across northern Europe, evidence has shown that farmers have been creating deep anthrosols, by the addition of composted plaggens to sandy soils, since the Bronze Age (Blume and Leinweber, 2004).

Soil enhancement is not solely a European phenomenon, the pre-Columbian Amazonian Formative period has also left a legacy of anthropogenic soils, known as terra platas and terras mulatas, that have their origins in Early and Mid Holocene anthropogenic activity (Arroyo-Kalin, 2010, 483-4; 490).

Continuous tillage and soil enhancement has a long term environmental effect. The addition of organic material to soil, by manuring or from forest clearance and burning, significantly elevates carbon levels. This effect can last significantly longer than the duration of the agriculture, depending on the climate and farming systems (Johnston, 1986, 98-9; McLauchlan, 2006, 1369-70), suggesting that ecosystems may be able to retain carbon after agriculture stops.

The ability to detect a global stratigraphic marker in soils and sediments is a geological pre-requisite for an epoch, as we saw in this post. The studies discussed here are examples of markers left in the soil as a result of agricultural intensification. They support the pre-industrial start date for the Anthropocene,for which Certini and Scalenghe (2011) argue, though they precede their preferred date of 2000 BP.

The empirical geoarchaeological evidence discussed here provides a convincing argument for farming to be an initiating factor of the Anthropocene.


References cited that are not available online:

Cunliffe, B. W., 2011. Europe between the oceans: themes and variations: 9000 BC to AD 1000. Yale University Press.

How fast did agriculture spread?

The title question is not about how quickly agricultural skills and knowledge pass between peoples, though that is an intensely interesting subject for archaeologists. It is about the rate of spread from the originating centres which I discussed in my post of 7th November 2014. Modelling the rate of spread depends on understanding the different nature and effects of intensification and extensification.

Intensification is about productivity. It is about getting more yield from a field by tilling, fertilising, irrigating and weeding it. Intensification replaces the nutrients and minerals taken up by the crops harvested in previous years so that fallow periods, during which those nutrients and minerals would be naturally replenished, can be eliminated so that more crops can be planted. The seminal work on these processes is The Conditions of Agricultural Growth: The Economics of Agrarian Change under Population Pressure by Ester Boserup, a Danish economist, though that work has since been challenged and reappraised by many academics. For example, Kathleen Morrison (1994) shows that intensification is not only a response to population increase but may be  driven by trade or food preferences. Agricultural intensification is the reason proposed by Ruddiman (2003, p.264-5) for the anomalous rise in atmospheric methane.

Extensification is about production. It is about getting more yield by expanding the area which may be farmed by clearing forests, creating space for new fields. When those fields lose their fertility, further clearances make space for more fields. Agricultural extensification is the reason proposed by Ruddiman (2003, p.272-3) for the anomalous rise in atmospheric carbon dioxide.

The problem with modelling the different rates of spread of these two processes is that they are not mutually exclusive. Extensification is constrained by the practical limitations of travel to and from fields while some forms of intensification require less labour than does forest clearance. Prehistoric people would have made the choice between intensification, extensification or a combination of the two based on local environmental conditions and local competition for resources.

In their paper Used planet: a global history, Ellis et al. (2013) considered methods for quantifying land use for the period before the availability of historical and instrumental records. Existing attempts typically created models of land use for the historic period and extrapolated them backwards using algorithms based on contemporary land management practices. The two alternatives reviewed used population as a proxy measure for land use.

The HYDE 3.1 model (Klein Goldewijk et al, 2010)  assigned areas of cropland and pasture to a grid of population densities, estimated from a range of sources, for the period 1700-2000. The model runs from 10,000 BCE to date. The initial human population used was 2 million and assumptions are made about regional variations in population growth related to the waxing and waning of empires. A number of uncertainties in the assumptions and results were acknowledged. The prehistoric climate and biome models were derived from those of today, the population start value is subject to great uncertainty and the per capita land-use (assumed to vary little over 12,000 years) may vary greatly depending on the degree of intensification.

The KK10 model (Kaplan et al, 2011) assumed that populations would expand to use all available land before turning to intensification and, importantly, compared a Boserupian scenario, in which per capita land-use reduces over time, with a fixed per capita land-use scenario. The model runs from 6,000 BCE to date. Population estimates were from the same sources used in the HYDE 3.1 study. The model also incorporated climate, soil and CO2 data into a Dynamic Global Vegetation Model.

Results from the HYDE 3.1 model compared with those from the KK10 model (Ellis et al., 2013, p.7980)

The two different models offer contrasting pictures of the potential impact of human agricultural expansion on the environment. The KK10 model clearly impacts earlier and more deeply on both forested areas and savannahs across most regions of the world, consistent with the Early Anthropogenic Hypothesis for the origin of increased greenhouse gases. Neither the researchers who constructed each of the two models, nor the authors of the comparative review, claim that one or other model is correct, indeed they both highlight the uncertainties and weaknesses in the models. What is important is that, with increased empirical evidence and refined models, scientists are improving the understanding of how pre-industrial anthropogenic activity may have contributed to climate change during the Holocene.

Putting gas-masks on cows - how to measure the methane output of ruminants

One day last year I was in an archaeology lecture discussing whether the methane output of domesticated cattle was greater than that of the American Plains bison because if one species just replaced another then the methane contribution of cattle had to be offset by that of the bison. Then some-one asked the inevitable question: "How do you measure the methane?". Following a witty exchange of banter between students and tutor, I set out to research the question. Here is what I found out.

Cattle, which are ruminants, generate methane through the process of biological fermentation that takes place in the rumen and reticulum, the front two parts of the stomach which are involved in the familiar ruminant behaviour called "chewing the cud". This grinding and chemical processing is needed to extract the nutrients from the cellulose in grass. The good news is that over 95% of methane is expelled through the mouth in a process called 'eructation' - belching. I'll pass over the other 5%.

A jersey cow undergoing methane measurement (www.afbini.gov.uk)

One method researchers used for measuring methane output involved placing the animal in a respiration chamber and measuring the build up of the gas. Another involved placing a hood over the head of the test beast to capture the exhaled gases. These methods have now been replaced with a system which uses a chemical marker, sulphur hexaflouride (SF6), which is released at a constant rate from a bolus placed in the rumen and which is captured using a pipe system which connects the beast's nostrils to a collector. This system has been shown to provide an accurate measurement of the methane output of cattle and may be capable of measuring carbon dioxide emissions if correctly calibrated. The method has been successfully applied to cattle and sheep, though there are variations relating to breed, diet and lactation which may cause issues if we try and apply these measures retrospectively to Neolithic pastoralist practices.

The annual contribution of methane to the atmosphere from all this belching by the main domesticated species was estimated in 1983 as 74 Tg/ year of which 54 Tg/year came from cattle and compared with ~6 Tg/year emitted by wild ruminants. The contribution from 36 million cattle across 10 US states in 2008 was estimated at 2.5 Tg compared with a notional displaced population of 30 million bison which would have produced 2.2 Tg/year. This suggests that the cattle and buffalo methane contributions balance out. However, in 1983 there were 115 million cattle in the whole USA while the maximum population estimate for North American bison before 1492 is 60 million. The net difference means that American cattle account for around 8% of the total anthropogenic methane contribution from domestic cattle with about another 4% from Australia and 3% from Canada.

Of course, we can't just project these figures back 8,000 years; the number of cattle appears to be linked to the exponential rise in the human population and the ratio of people to cattle has undoubtedly varied over time. However, there is a promising area of research to understand to what extent early domestication of animals has contributed to the anomalous atmospheric concentrations of carbon dioxide and methane.

What do the archaeologists have to say - where did farming start?

For the Ruddiman hypothesis to work, it is necessary to show that there was sufficient anthropogenic activity starting around 8,000 BP to initiate the kind of environmental changes suggested. Evidence is needed to show that agricultural behaviour had spread over a wide enough area of the planet, early enough for it to manifest the suggested impacts on the atmospheric concentrations of CO2 and CH4. We need to look to archaeological evidence to understand where agriculture originated, what form it took and how far it had spread by when. Then we need to use that evidence to calculate how much of a contribution agriculture made to CO2 and CH4 concentrations.

To model the spread of agriculture we need to establish a time and place for the point of origin and then estimate the rate of diffusion. The problem is that there is significant debate and disagreement between researchers over when and where farming started and how far and fast it spread. What are the alternatives?

The classic "fertile crescent" origin proposed that farming emerged between 12,000 and 8,000 years BP, after the Younger Dryas cool period, in the valleys of the Taurus mountains of southern Anatolia and of the Levant. While that is the likely earliest origin, archaeological evidence now supports the theory that farming originated at possibly more than twenty locations across most continents over time and involved a number of species of animal and plant.


From: Current perspectives and the future of domestication studies (Larson et al, 2014)

All these animals and plants originated in wild species. Zooarchaeologists and palaeobotanists have developed ways of detecting whether a plant or animal has been domesticated, that is the species has been intentionally modified by human selective actions to change it genetically, morphologically or behaviourally. For example, archaeological wheat seeds, often found in dumps and pits created by Neolithic peoples, are examined microscopically to see if they are shattering or non-shattering; the non-shattering form is indicative of domestication as it is the form which is harvested most efficiently with scythes.

The rate of change during domestication is another area of research which has shown that plant domestication can be an extended process taking thousands of years. The early rice project not only seeks out the origins and nature of rice domestications but also links the effects of rice with increased atmospheric CH4. We are now starting to understand and quantify the environmental effects of irrigation, terracing, slash-and-burn forest clearance and other anthropogenic behaviours that come with farming.

There are many reasons why people may have started to farm. The warm period after the Younger Dryas favoured plants. The human population was increasing and with it came increased competition for land and resources. Humans were developing a shared knowledgebase of experience which was passed between generations and enriched through cultural transmission. Once the commitment to farming had been made by investing energy in modifying landscapes, was it too hard to revert to the hunting and gathering strategies for food procurement that humans had used for 200,000 years? A comfortable niche had been constructed, how far and fast did it spread out across the planet?

Inaugural UCL Lecture on Climate and Human History by Bill Ruddiman - a critical review

Bill Ruddiman delivered this lecture, entitled "Top-down and Bottom-up Evidence of Large Early Anthropogenic Climate Change", on 21st October, 2014.

Ruddiman focussed on those areas where the early anthropogenic warming hypothesis have been challenged. These areas have also been addressed in his new afterword, published in the 2010 edition of his book "Plows, Plagues and Petroleum".

The original hypothesis compared the variations in atmospheric CO2 and CH4 during the Holocene with those of the preceding three interglacials. That comparison now stretches back 800,000 years over seven interglacials to MIS19, made possible by the Antarctic ice-coring carried out by EPICA. This has two advantages; firstly there are more interglacials with which to compare the Holocene and secondly the orbital forcing that occurred during MIS 19 is similar to that of the Holocene, enabling a like-for-like comparison.

I feel there is a degree of sleight of hand when comparing the trends from the various interglacials.  Ruddiman uses the first insolation minimum in each interglacial as a common point for aligning the trends; is that a valid point of alignment? Each interglacial lasts for a different length of time; over what time-period should those trends be aligned? Should the interglacials be matched by stretching and compressing them to fit a common scale? He omits MIS3, MIS13 and MIS15 from the comparisons, reasonably, because of the significantly different orbital forcing characteristics of those periods.

Ruddiman's hypothesis concentrates on what he characterises as anomalous increases in CO2 and CH4 towards the first insolation minimum of the current interglacial. Is it possible that the extended periods of low temperature during the preceding MIS3 and MIS5 (Jouzel et al, 2007, p.794) created a build-up of CO2 and CH4 which is now being released? This possibility is something I would like to see addressed.


Antarctic temperature variations from MIS5 to MIS1 (Jouzel et al, 2007, p.794)

Ruddiman's comparison with MIS19 was elegant, especially when the insolation similarities were highlighted. He also noted a feedback from his proposed anthropogenic effects, that a warm ocean cannot hold as much CO2 as a cold one, which usefully corrects for shortfalls seen in the results of his macro-scale analysis.

The bottom-up evidence was drawn mainly from archaeological sources and, as promised, I shall discuss those in another post.

This was an enjoyable lecture. Ruddiman has taken on board the criticisms and answered them coherently while expanding the evidence on which he draws to support his hypothesis.

Later, Ruddiman also commented on the efforts to define the Anthropocene (which I explored in the previous post). He felt that the term "Anthropocene", with a capital A, was fine if the geologists felt the need for it but the term "anthropocene", with a lower-case a, was an important mechanism for bringing together and co-ordinating the archaeological and palaeoclimatological research into how humans are affecting our environment.

References cited not available online:
Ruddiman, W., 2010. Plows, Plagues and Petroleum: How Humans Took Control of Climate. Oxford: Princeton University Press.

The Guardian: Anthropocene: is this the new epoch of humans?

This article in today's Guardian newspaper caught my eye.

The story concerns the conference of the Working Group on the Anthropocene which is meeting in Berlin this week to consider the definition and the time-frame of the Anthropocene geological unit. Geological units have a boundary which is visible in the geological record called a Global Stratigraphic Section and Point (GSSP) or "Golden Spike". For the Anthropocene to be recognised, it too should manifest a global stratigraphic marker. Commentators from within the group argue for various start dates for the Anthropocene, the start of industrialisation still being favourite but the start of atomic era is also suggested. These dates are usefully supported by stratigraphic markers.

Have humans truly displaced volcanoes and plate tectonics as the key agent of geological change on our world? Probably not; all signs of our civilisation will one day be erased as they slide under an adjacent plate in the earth's crust. But this conference, it seems, is not considering that argument; it appears to be considering when the impact of human behaviour started to affect the planet as a whole and how the start of that impact may be detected in the stratigraphic record. It is debating which start date to adopt.

This comment from Mike Ellis of the British Geological Survey is unequivocal: "The principal process of change on the planet is us, so the name of our epoch should reflect that. It’s as simple as that."

Might the Working Group consider the emergence of farming as another option for the start of the Anthropocene? What is the geological marker from farming that would provide the Golden Spike?

I await the outcome with interest.

What does Ruddiman’s hypothesis state?

Introduction

This post summarises the evidence and the hypothesis set out in Ruddiman's 2003 paper which considers two atmospheric gasses, methane (CH₄) and carbon dioxide (CO₂), and compares the currently observed concentrations with the concentrations that would be expected under natural climate change driven by orbital forces.

The Methane anomaly

For most of the last 350,000 years, levels of atmospheric methane have tracked the axial precession of the orbit of the earth through the rising and falling of summer insolation. An increase in the quantity of solar energy falling on land masses causes air to rise which sucks in air from over the oceans. As this moist air rises over land it heats and deposits the moisture as monsoon rains, causing methane to be released from soils. The Greenland Ice Core Project (Blunier et al, 1994) evidence showed an anomalous increase in methane levels since 5000 BP when a decrease was predicted. The actual increase in methane was ~100 parts per billion. Given the predicted reduction, the likely excess methane is ~250 pbb.

The anomalous increase in atmospheric methane since 5000 BP (modified from Ruddiman, 2003)

The carbon dioxide anomaly

Atmospheric carbon dioxide concentrations also track orbital forces with varying degrees of latency. The forcing is more complex as it is affected by the axial precession cycle of 23,000 years, the axial tilt variation cycle of 41,000 years and the orbital eccentricity cycle of 100,000 years. Hays et al (1976) showed that these cycles are responsible for a substantial amount of climate variation including glaciation events and atmospheric carbon dioxide concentrations. The carbon dioxide levels predicted by those cycles and matched to the three of the most recent interglacials are not matched by those observed for the current interglacial. The predicted contemporary concentration is 240-245 parts per million but the observed concentration is 280-285 ppm, an anomalous increase of 40 ppm; this divergence commenced ~8000 BP.

Explaining the anomalies

Loss of terrestrial vegetation would reduce the uptake of carbon dioxide by plants. Various scenarios were modelled (Foley, 1995) to assess whether natural forces, such as desert expansion caused by reduced monsoon precipitation, could explain the anomaly. An alternative theory (Broeker et al, 1999) proposed that the expansion of forests after the Last Glacial Maximum had reduced atmospheric carbon dioxide; when forest expansion ceased the ocean's acidity increased dissolving calcium carbonate and releasing carbon dioxide into the atmosphere; Ruddiman chooses to dismiss these explanations because the underlying theories should have manifested the same outcomes in previous interglacials, those outcomes were not observed.

The alternative explanation offered is that biomass was reduced by anthropogenic action, the clearance of Eurasian forests for agriculture. There is considerable and persuasive archaeological evidence that such deforestation has taken place since 8000 BP, complemented by historical evidence from Greek and Roman writers. Carbon dioxide concentrations also track human population variations caused by plagues.

Commentary

This is a long post but I wanted to establish the arguments for the hypothesis and the complexity of the natural forces that affect climate. What I learned by writing it is that:

• the effects of astronomical forcing on climate vary as the different cycles harmonise and compete with one another
• those astronomical forces continue to affect climate as anthropogenic effects also manifest themselves
• both the methane and carbon dioxide anomalies must be explained in a complementary manner

Next, I will explore the archaeological evidence.

Why does the Anthropocene matter?

Hello and welcome to my blog: "What started the Anthropocene, Farming or Factories?"

I am writing this blog as part of my Masters in Environmental Archaeology at University College London. Environmental Archaeology studies past human behaviours and societies through analysis of prehistoric plant and animal artefacts and analysis of pre-historic soils and landscapes. Analysis of these things allows us to describe models of past climates; why would we want to do this?

We live in an age where the impact of human behaviours on the planet has become a matter of global political and fiscal concern. It is no longer the domain of scientists or environmentalists who, in the past, have been sidelined as eccentric or misguided. Fundamental decisions about how we feed and power all our societies are now being taken with consideration about sustainability and environmental impact. These decisions must be informed by well-grounded and robust evidence for the likely consequences.

We need to look to the past to understand the history of environmental change and to determine how and why those environmental changes happened; we need to understand what we can control, what we cannot control and what we should control. Analysing past climatic and environmental change in the context of the emergence of modern civilisation informs us about the positive and negative impacts, and the risks and rewards of our behaviours. Environmental Archaeology is one tool we can deploy to achieve that understanding.

It is recognised that the development of industrialisation powered by fossil fuels has had a measurable impact on our environment. In the last thirty years some scientists have started to link those impacts with climate change. This has led to the development of the concept of the "Anthropocene" which is a 200-year geological period during which human behaviours have had a greater long-term impact on the environment than the natural forces acting on our planet.

The recent and short-term nature of contribution of humans to climate change is now being challenged by archaeologists, climatologists and geographers, amongst other disciplines, who have posited the theory that our pre-historic behaviours such as the adoption of sedentism, the development of agriculture, the clearance of great forests, the enrichment of soils, etc. is when the human impact started to outweigh the effects of natural forces and that the Anthropocene may have started as long as 8000 years ago. William Ruddiman's 2003 paper "The Anthropogenic Greenhouse Era Began Thousands of Years Ago" lays out the arguments for this hypothesis.

This blog will explore critically the evidence for that earlier emergence of the Anthropocene and consider the question: What started the Anthropocene, Farming or Factories?

I look forward to your comments, criticisms and contributions to this debate over the next few months.