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
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