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.