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.  

   

why domestication intensified and its impact on global climate

In my last post I explained Ruddiman’s ‘early anthropogenic hypothesis’ (2003), as there has been much debate surrounding this hypothesis, in this post I am going to discuss the reasons why it is most likely that humans have only significantly affected global climate through domestication since industrialisation.  

This is the common viewpoint because greenhouse gases in the atmosphere have increased exponentially in recent years (shown in the graph below), as a result of human influence since industrialisation. The time period between 1800-1850 and the present day has been coined the 'anthropocene' by Crutzen in 2002, showing that humans are having more impact on global climate than orbital parameters. Part of this increase is caused by agriculture, especially increases in methane, nitrous oxides and carbon dioxide.  

One of the many reasons agriculture is responsible for an increase in emissions is due to population growth.  The graph below shows global population growth over the last 2000 years, it is possible to see a small increase until around 1800 when the population grew at a much quicker rate.  An increase in population size consequently leads to an increase in domestication as more people means more food.  In the last 4 decades agricultural land gained almost 500 million hectares from other land uses (IPCC, 2007).  As a result this leads to increased deforestation for crops and grazing, for example clearing of the Amazon rainforest for grazing.  Animals themselves increase methane emissions as does the expansion of rice paddies.  Our population is now so large that we artificially produce more nitrogen for fertilising crops than is produced naturally.  With the growth of population, there is also a growth of migration, which leads to more areas becoming domesticated.  For example, large migrations of Western Europeans across the world led to more areas becoming domesticated due to the spread of ideas, which changed other populations from nomadic societies to those more settled.  

The ability of mining and burning fossil fuels was also important in the development of agriculture as it permitted the use of machinery, meaning that greater areas could be domesticated and crops could be grown more easily.  Also, improvements in machinery improved the efficiency of food production, meaning more food could be produced more quickly and cheaply.  Indirectly, fossil fuels allowed greater transport and communication links leading to an increase in trade. This meant food was not just produced for people in the local area, rather it could be produced in one area in a large scale and then transported to another area.  This led to the formation of large cities as people were able to buy food from another area rather than producing it themselves.  

Consequently greenhouse gas emissions as a result of agriculture have increased, in 2005 agriculture accounted for 47% of anthropogenic methane emissions (IPCC, 2007).  Therefore, it is commonly thought that domestication began having significant effect on climate from about 1800 due to population growth, which in turn led to industrialisation, and improved farming productivity. Emissions from agriculture became more significant impact as farming became more intensified to support the growing population.   As to the ‘early anthropocene hypothesis’, I think it possible humans could have affected their local area thousands of years ago, but from archeological evidence, the population was not large enough and domestication was not intense enough for it to have a significant global effect.  

more information can be found about the effects of domestication on climate since industrialisation on the IPCC website Climate Change 2007: Working Group III: Mitigation of Climate Change

'The early anthropogenic hypothesis'

It is commonly thought that humans have significantly affected global climate since industrialisation in the 19^th Century, through increasing levels of greenhouse gases, such as carbon dioxide, methane.  An exponential increase can be seen in these levels since the 1950s, which has led to the warming of the climate on a global scale.  In 2002, Crutzen coined the term ‘anthropocene’ to describe the last 150 years as he and many others believe this the first time humans have significantly altered global climate.  

Contrary to this common opinion, Ruddiman published the ‘early anthropogenic hypothesis’ in 2003.  This is where he states that humans have been affecting global climate for thousands of years due to expansion of domestication of agriculture.  He suggests that humans affected the climate so much thousands of years ago that they prevented the onset of an ice age.  

He focuses the levels of methane and carbon dioxide.  He found through looking at ice cores, that the level of methane is related to that of summer insolation.  He found that summer isolation has been steadily declining for the last 11000 years, so the levels of methane would be expected to do the same.  He states that the level of methane was dropping until about 5000 years ago but started to increase again to a pre-industrial level of 700ppb (parts per billion), whereas previous records of the Earth’s orbit show it should have been 450ppb (Ruddiman, 2003).  He suggests the reason for this discrepancy is the expansion of rice irrigation across South East Asia that began about 5000 years ago increasing levels of atmospheric methane. 

He also looked at levels of carbon dioxide and compared these to records of previous interglacials with similar orbital parameters.  He found that previously carbon dioxide values fell to an average of 240-245ppm (parts per million) whereas during the Holocene levels rose to 280-285ppm (pre-industrial).  He believes this anomaly to again be as a result of domestication as it is necessary to clear land for crops and grazing, increasing atmospheric carbon dioxide.  He found this anomaly started about 8000 years ago and grew in size through the Holocene as more areas were deforested.  

Ruddiman argues that as a result of anthropogenic increases in methane and carbon dioxide, the green house effect was intensified, preventing the onset of the next ice age as global temperatures increased as a result.  He estimated the global mean increase to be about 0.8ºC, roughly 2ºC at the higher latitudes (Ruddiman, 2003).  

Another point Ruddiman made as part of his hypothesis, was that it was possible to see a drop in levels of carbon dioxide during times where there were major pandemics, such as the fall of the Roman civilisation as there would have been less cultivation at this time. 

There have been many challenges to this hypothesis, for example some criticise the analogues Ruddiman has used to compare levels during the Holocene to, as matching it to previous interstadials is difficult due to the variation in orbital parameters (Ruddiman 2007).  The methods used to calculate the anomalous green house gas concentrations have also been criticised.  There is a limited amount of archeological evidence about the size of populations thousands of years ago, and about the rate of spread and the intensity of domestication, so it is difficult to determine how significant an impact they had.  Alternative theories have also been put forward to account for the anomalies Ruddiman found, for example in 2001 Broecker et al. published the ocean chemistry hypothesis, to account for the increase in carbon dioxide levels.  

Despite being an interesting hypothesis, I think, pre-industrialisation, humans did have an impact on their surrounding environment and on climate but not on a global scale, there is too limited amount of archeological evidence to make any definite conclusions on the impacts of humans 5000 years ago, but it is unlikely these affects would not have been significant enough to prevent the onset of an ice age.  This topic has been recently debated in Real Climate, a climate blog, written by climate scientists.  They present interesting comments on the debate and put forward many arguments, they come to the conclusion, that to solve this debate more research will need to be undertaken.  

Where and why domestication happened

Domestication of wild crops and animals began after the last ice age about 10,000 years ago.  It is thought that the warming of the climate both directly and indirectly allowed domestication.  As a direct result of climate there was a change and an expansion in the area wild grains could grow, and indirectly, it is thought that retreating glaciers permitted the migration of species formerly hunted by hunter-gatherer societies, meaning they had to develop a more stable source of food.  

Initially domestication of crops and animals would not have significant effect on the global climate, but domestication allowed the population to expand due to a reliable source of food, in turn leading to to the expansion of cultivation.  It is thought by some, namely Ruddiman that this expansion of cultivation and domestication began to effect climate about 8000 years ago but this hypothesis is debated.  

As the population grew more attention was given to the harvest, as grain was such a stable source of food.  The domestication of animals was a different process, some were much easier to domesticate than others.  It is thought that wolves were tamed as pups as much as 12,000 years ago and herd animals such as sheep, pigs and goats were domesticated about 9000 - 7000 BC and cattle from about 6500 BC.  Cattle were significant as they could be used to pull ploughs and waggons improving and expanding crop cultivation.  The beginning of domestication also led to the building of small villages which was an important societal change.  

Originally it was thought from archaeological evidence, that domestication first began in the Near East, and spread from there to the rest of the world. The exact date of domestication has not been determined.  However there is now some debate as to whether there were other centres of domestication.  Fuller et al. (2011), put forward this idea, arguing that there were multiple centres of domestication (as shown in the map below) which developed simultaneously rather than there being one core area in the Near East from which domestication spread through the migration of people.