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.

Introduction

In this blog I will be looking at the effect domestication of agriculture has had on global climate.  I am going to look at whether these effects are a recent occurrence or whether in the past our ancestors also impacted on climate.  Domestication of agriculture began after the last ice age due to a warmer climate and a growing population size.  Wild grains were cultivated by humans on a regular basis and herd animals such as goats began to be domesticated.  This saw a transition from hunter-gather societies, to those more similar to farming today.  

It has been suggested that societal change from hunter-gatherers to a more modern style of farming using domesticated livestock and crops led to change of the global climate which prevented the onset of another ice age.  I am going to discuss this debate in my forth coming posts, determining whether this hypothesis is plausible or whether it is more likely that humans have not influenced global climate significantly until the more recent intensification we have seen over the last century to support our growing population.  

I am then going to discuss the ways in which domestication has influenced climate, such as through land use change, and how this has affected concentrations of greenhouse gases.  Here I will look at the impacts stemming from the more recent intensification of agriculture, and following on from this, the possible solutions and strategies to reduce the impact on global climate.  

these pictures show the progression from hunter-gatherer societies to early domestication to modern day agriculture.