The UnAustralian

Saturday, January 03, 2004
 
Prehistory and Climate Change

One of the more interesting pieces of research to come out last year was a paper by William Ruddiman from the University of Virginia, titled "The Anthropogenic Greenhouse Era Began Thousands of Years Ago" (Climatic Change, 61: 261-293, 2003).

The paper puts forward a case for human alteration of the earth's climate to have started much earlier than previously assumed. Because of significant amounts of methane (starting approx. 5000 years ago) and carbon dioxide (starting approx. 8000 years ago) emissions, humans counteracting a slight cooling trend, which lead to the holocene being an era of unusual climate stability.

Methane

For the past 350 000 years there has been a correlation between changes in the earth's orbital and methane concentrations captured in the Vostok ice core. Following a 23,000 year cycle, methane concentrations rise and fall in a pattern that matches the earth's orbital. It has been posited that as changes in the earth's orbit increase the amount of sunlight reaching the earth, the frequency of monsoon's increases, which in turn leads to more flooded wetlands, and more methane.

From 350 000 - 5000 years ago, this pattern persisted, but then something odd happened. Methane levels had been dropping as expected (they had peaked 10 000 years ago), when they suddenly started to rise. This rise should have happened at approx. 3000 AD, so it's 9000 years premature. Ruddiman calculates that the methane levels have increased by ~100 parts per billion (up until 300 years ago, then methane levels increased dramatically), whereas they were expected to drop by ~ 150 parts per billion. So, we have to explain where enough methane to cause atmospheric levels to rise ~ 250 ppb, came from.

It had previously been assumed that this unexpected growth came from natural causes. There is considerable evidence that about 6000 years ago, large areas of peat bogs formed in the high latitudes of the Northern Hemisphere. This was presumed to explain that 100 ppb rise in methane levels. However, this explanation fell apart when one looked at Greenland and Vostok ice cores. Because methane has a relatively short atmospheric life time before it is broken down, there is a methane gradient running from the Northern to Southern Hemisphere (more methane is produced in the Northern Hemisphere). Had peat bogs located in the upper Northern Hemisphere been responsible for a significant increase in methane emissions, this methane gradient would be expected to increase. Rather the opposite is observed.

Another natural explanations have been put forward, however these appear unlikely to explain the increase in methane levels.

However, what happens when we examine human contributions to methane levels. Human's produce significant amounts of methane in a number of ways; sewerage, livestock, rice farming, biomass burning etc. By comparing estimates of past human population, we can account for about a 100 ppb increase in methane levels. However, Ruddiman and Thomson have pointed out that early rice farming was probably quite inefficient, leading to early human's producing a much more methane per unit of food, relative to today's farmers. With this in mind, it appears that early human activity can account for a very significant proportion of the unexpected 250 ppb increase in atmospheric methane concentrations.

Wild rice was first cultivated 7500 years ago, and irrigated paddies started appearing in the archeological record around 5000 years.

As Ruddiman concludes "[i]n summary, the 'anomalous' late Holocene CH4 increase cannot be explained by nature forcing, but it coincides closely with innovations in agriculture that produce methane in abundance."

Carbon Dioxide

Carbon dioxide atmospherics concentrations have oscillated in a similar fashion to methane as the earth moves through it's full orbital cycle. Approx. 8000 years ago, a similar event happened. Carbon dioxide levels, which were expected to fall, started to increase. They increased by about 25 parts per million, when they were expected to drop by 15 parts per million (leading to an anomaly of 40 parts per million). Converting this increase into tons of carbon released into the atmosphere, it appears that we need to account for an increase 320 GtC.

There have been various attempts to explain this rise. A loss of terrestrial biomass, and changes in ocean chemistry are the big players here. However, natural loss of biomass can only explain 11% of the increase in CO2, whereas changes in ocean chemistry are unlikely to have occurred (they involve a hypothesised change in chemistry which has never been observed in the ice core records).

If natural causes can't account for this increase, can human activity explain this increase in CO2? Early humans most likely caused significant CO2 emissions by burning down forests (clearing of land, slash and burn agriculture etc).

Ruddiman puts forward three features that must be explained by the forest burning hypothesis; "(1) clearance must begin near 8000 yrs BP (when the CO2 rise began) on a small, yet 'non-negligible' scale; (2) clearance must grow large enough by ~2000 yrs BP to explain ~80% of the pre-industrial CO2 anomaly; and (3) the negative oscillation of 4 to 10 ppm after 2000 years BP also need an explanation."

Previous studies of the spread of grains from the Fertile Crescent suggest that around 8000 years BP, agriculture had spread through significant areas of non-forested European, and was started to spread into the forested areas. Agriculture started in China around 9400 years ago, and significant disruption of forest pollens is observed from around 8000 years ago. Similarly, agriculture reached India by 8500 years BP.

2000 years ago, agriculture had spread considerably further. Complex agriculture was common in Europe, North Africa, China, India, Central America, Southern America and the Middle East. More simpler agriculture was also observed throughout these areas, and throughout North America, large swathes of Africa, PNG and Indonesia. Using previous estimates of deforestation, Ruddiman calculates that societies employing complex agriculture had released approx. 156 GtC, whereas simple agriculture had released between 68 and 93 GtC. These numbers are pretty close to the required 80% of 320 GtC.

There is an explanation for the drops in CO2, however, I'm going to save them for another post.

Conclusions

In conclusion, Ruddiman has presented a fascinating and plausible hypothesis for early human activity causing the introduction of significant amounts of GHG's into the atmosphere. I'll use later posts to talk about how these effected the climate.

Sources

William F. Ruddiman and Jonathan S. Thomson. The case for human causes of increased atmospheric CH4 over the last 5000 years Quaternary Science Reviews 20: 1769–1777, 2001.

William F. Ruddiman. The Anthropogenic Greenhouse Era Began Thousands of Years Ago Climatic Change 61: 261-293, 2003

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