when did domestication begin?

In this post I am going to discuss a more historical aspect to domestication after reading a couple of papers debating when domestication first occurred.  The oldest evidence for agriculture, a few rye grains, has been found in Syria, which is in the 'Fertile Crescent' where agriculture is thought to have begun (as shown on map below).

During the last ice age humans existed as part of sparse populations, belonging to hunter gatherer societies.  As the climate became milder towards the end of the ice age, they built  permanent houses and made tools, this was an important step towards more modern settlements.  However, an abrupt cooling event lasting 1300 years occurred, called the Younger Dryas happened between 12,900 and 11,600 years ago.  Pollen records from within the Fertile Crescent show that a cooling of the climate was felt in this area.    

Both Balter 2010 and Pringle 1998 discuss the arguments surrounding the beginnings of domestication.  One hypothesis, as believed by Bar-Yosef is that the cold period brought on by the Younger Dryas caused domestication, to provide a more stable food source so humans had a sufficient amount to eat.  There is some evidence supporting this, rye grains have been found in Abu Hureyra settlement in Syria dating back to 13000 years ago around the beginning of the Younger Dryas.     

The other hypothesis, opposing this is that there was actually a return to a more mobile lifestyle during the Younger Dryas, meaning a return to hunter gatherer styles of society.  They argue that it was not until warming began after the Younger Dryas that domestication occurred, and the evidence found is not strong enough to definitely suggest crop cultivation.  Wilcox and Rosen are supporters of this hypothesis, Wilcox dismisses the evidence found in Abu Hureyra as no other evidence has been found in other locations.  Rosen on the other hand believes that it is more likely that humans domesticated once the Holocene had begun and warmer temperatures had returned as populations will have grown putting pressure on resources leading to domestication.  

Personally, after reading the two articles and looking at the varying viewpoints I agree with the latter hypothesis.  This is because I believe a return to colder climates would have caused human populations to return to hunting as they did throughout the last ice age rather than domesticate.  Also, populations are likely to have been smaller during the cold period, meaning there would be less population pressure to domesticate.  

Nitrogen fertilisers and climate change

Over recent years, primarily since the ‘Green Revolution’ in the 1960s, nitrogen has been increasingly used in agriculture, now more nitrogen is produced artificially for fertilisers than is produced naturally by the earth.  

Nitrogen naturally forms in soils in tropical and temperate regions of the earth and in the oceans.  However recently it has been used as a fertiliser, is spread over fields (see picture below) as it increases crop yields as more nutrients, namely nitrogen is available in the soil encouraging greater plant growth.  This has consequently led to an increase in nitrous oxide concentrations in the atmosphere, as more nitrogen in the soil means a greater rate of microbe activity, which creates and releases nitrous oxide. This is important, because nitrous oxide has a warming affect on the planet, being a greenhouse gas, but it also according to Crutzen and Ehhalt (1977)  has a depleting affect on the ozone in the stratosphere, which means more UV rays can penetrate through warming the planet.  It is thought that nitrous oxide accounts for 6% of total anthropogenic radiative forcing (Davidson 2009).  Nitrous oxide production only occurs under specific conditions and results from the combination of aerobic and anaerobic processes.  Nitrification is the process of ammonium transforming to nitrate, an aerobic process and denitrification, the formation of nitrogen gas from nitrate reduction, an anaerobic process.  This nitrogen gas is then oxidised to form nitrous oxide (Monteny et al 2005)

Nitrogen fertilisers are used to increase crop yields so it is possible to produce more food from the same amount of space, making food production more efficient.  This consequently led to an increase in population, and is one of the reasons why we have seen the population expand so rapidly over recent years.  But in turn the increasing population needs food so results in higher crop yields being needed.  This means that fertilisers are used at an increased rate having negative effects on the planet.  Not only does it result in increased concentrations of nitrous oxide in the atmosphere but also the use of fertilisers causes loss of soil nutrients, soil acidification and erosion reducing soil quality in the long run.  

The work of Boering et al. is discussed in this article, who looked at samples of Antarctic ice dating between 1940 and 2005, to reconstruct nitrous oxide concentrations in the atmosphere between these times.  They found that it is possible to differentiate between natural and agricultural nitrous oxide, due to its isotopic composition.  This is useful as it allows us to determine when concentrations of nitrous oxide rose significantly as a result of agriculture and see how successful mitigation strategies are at reducing it.  

The use of nitrogen as a fertiliser is important due to the negative affect it has on climate change, as it is the predominant cause of increase in nitrous oxide concentrations. For more information on the effects of nitrous oxide on the atmosphere, I found this link useful. 
The diagram below shows how nitrogen fertilisers are incorporated into the nitrogen cycle.  

rice paddies and methane emissions

Another way domestication has had a significant impact on concentrations of greenhouse gases, is the effect of rice paddies on methane concentrations.  It is thought that rice paddies could contribute to 20% of current methane emissions.  

Rice paddies date back to the beginning of agriculture and archeological evidence shows the first paddy to be in Korea.  Since this time rice cultivation using paddy fields has developed all over the world, in Europe, the USA and across much of Southeast Asia. China is now the greatest producer, accounting for 36% of the worlds rice.  Methane is produced by anaerobic bacteria in the flooded paddy fields. (see picture below)

From 1940-1980 methane emissions have increased by 49% and during this time, the global rice harvest has increased by 41%.  This is as a result of higher yielding crops, expansion of crop areas and increasing use of fertilizers, but this increase in production of rice has meant increase in methane concentrations.  

According to Aselmann and Crutzen (1989) rice paddies cover 1.3x106 km2  of the Earth's surface.  They undertook a study determining methane emissions from both rice paddies and wetlands.  They estimated rice paddy methane emissions to be between 60-140 Teragrams per year.  They found emissions to be highly seasonal, with greatest emission levels to be in the summer in both Hemispheres.           

A paper by Cao et al. (1996), used modelling to predict methane contributions from rice paddies across the world.  They found this difficult as not only does it vary greatly between regions, but they also found great seasonal variation between emissions as well.  They estimated emissions to be 50-60Tg yr.^-1.  Another study by Liu and Wu (2004) uses models again to estimate methane emissions from Taiwanese paddy fields. Here they found temperature to be the most important factor affecting the amount of methane emitted, so the higher the temperature the higher the concentration of methane.  They also found there were seasonal variations in methane emissions.  

Bachelet and Neue (1993), estimated methane emissions from Asia, as this is where 90% of rice is produced.  Here they evaluated different approaches of estimating methane emissions, again they found a major weakness to be that some methods used a constant rate of emissions for all months, which affects the results, as there are seasonal variations.  From their comparisons they concluded that in the past it seemed emissions for this area had been over estimated.  They gave an estimate of methane emissions from Asian rice fields to be 63 Tg yr.^-1, which is different from Cao et al.  

Consequently, this shows that current methane emissions from rice paddies are uncertain, as suggested by Aselmann and Crutzen, due to the complexity of measuring/modelling as a result of spatial and temporal differences.  However, it is important that reliable predictions of methane emissions are calculated as methane has a a warming effect  21 times greater than carbon dioxide.  

Methane emissions are set to increase as rice production increases to supply a growing population.  However Nueu (2007) suggests that we do have sufficient understanding of rice production and its effect on methane emissions to put in place policies to reduce impacts. These are important and I will discuss mitigation strategies surrounding rice cultivation in a later post.  As Nueu says, many people rely on rice as a staple in their diet and at the rate that population is set to increase, at least 880 million tonnes of rice will need to be produced by 2025 to keep up with growth.  Unless changes are made this will lead to huge increase in atmospheric methane concentrations.  

Deforestation

Before agriculture, forest covered 57 million km² of the earth (Malhi et al 2002), however, as a result of domestication much of this forest cover has been lost.  As domestication became more wide spread and intensive, it was necessary to clear land to create space for grazing and crops.  

Deforestation has an impact on global climate through increasing the release of carbon dioxide into the atmosphere.  Clearing often occurs through burning the forests which its self releases high concentrations of carbon dioxide into the atmosphere, but also the vegetation and the soil store large amounts of carbon which is subsequently released into the atmosphere when forests are burnt or logged.  Crops or grasses in grazing areas do not store as much carbon as large forested areas, meaning there is a higher concentration of carbon in the atmosphere.  

Burning of forest from charcoal records can be seen around the beginning of domestication 8000 years ago, and forest clearances seems to accompany the spread of agriculture.  It seems that most temperate forests, for example in Europe and China, have been cleared progressively since the beginning of domestication, a very small amount survived to the industrial era (Malhi et al, 2002).  However, until 1700 only about 7% of global forests had been lost (Goldewijk, 2001).  This has significantly increased in recent years and deforestation has contributed to 45% of the increase in atmospheric carbon dioxide since 1850 (Malhi et al, 2002).  This is as a result of more widespread deforestation and deforestation of more densely forested tropical areas which are thought to be ‘carbon sinks’.  

Carbon sinks are forests which are thought to be fertilised by the recent increase in atmospheric carbon dioxide, leading to increased growth rates meaning the forests store more carbon so there is less in the atmosphere. The Amazon rainforest is an example of this and it is thought to be actually slowing the anthropogenic effect, however if these forests are deforested they then become a huge source of carbon (Laurance 1998).  They are very resilient if they remain intact, however many, like the Amazon are being deforested for agricultural purposes, for example by 2001 837000 km² was cleared for cattle ranching and soya bean production (Malhi et al 2008).  Clearing leads to fragmentation of the environment, limiting regrowth.  Much of the tropical forests in southeast Asia have been lost, the map below shows loss in Borneo, and projections for the future.  

It is likely that if large areas of these tropical forests are deforested, they will become large sources of carbon which will increase atmospheric concentration.  Therefore policies are continually being put in place to limit deforestation and protect forests.  There are many campaigns, such as Green Peace, their video below is useful for seeing the effects of deforestation in the Amazon rainforest.