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.
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