Rivalling the forces of nature

Figure 1. Future Earth? (source)

Global average temperatures throughout the Holocene were relatively stable in comparison to previous warm or interglacial period in the Earth's history. It was originally thought that this was the result of natural climatic drivers and that humans did not influence the climate until two centuries ago during the Industrial Revolution which saw a rapid surge of greenhouse gases into the atmosphere (figure 2). This has been challenged by a hypothesis which states that without the anthropogenic factor, the Holocene should have experienced gradual cooling under a natural state, and that anthropogenic emissions first started altering the climate 8,000 years earlier (figure 2, Ruddiman, 2003). This blogs explores the overdue glaciation hypothesis.  

Figure 2: Changes in atmospheric greenhouse gases and global average temperature over the last 8,000 years (source)

Ruddiman (2003) bases his hypothesis on two key point. Firstly, atmospheric gases stopped following the natural oscillating trend driven by Milankovitch cycles seen throughout most of the Quaternary and instead of falling, CO₂ levels began rising 8,000 years ago and CH₄ around 5,000 years ago. Secondly, numerous archaeological, historical and geological evidence exists which points towards a large social shift in Eurasia 8,000 years ago as the Neolithic Revolution drove hunter-gatherers into settled farmers. This gave rise to the domestication of plants and animals, change in landuse and an increase in population. Bringing both together, evidence shows how the rise of agriculture and extensive deforestation around 8,000 ago coincides with the gradual rise in CO₂ whilst the expanding of rice irrigation occurred around 5,000 years ago coinciding with a rise in CH₄. Together, these changes have contributed to the 0.8°C rise observed above the Holocene baseline which has been enough to stop the natural cooling trend and prevent the Earth entering a glacial period. 

The climate model GENESIS was created to test the hypothesis to see how the Earth would have responded if the greenhouse gases had not increased since the agricultural revolution (figure 3) (Ruddiman, 2005). The model showed how temperatures today would be 2°C colder and the Earth would be one third on the way to maximum glacial conditions. Other studies also support the findings showing how changes in orbital configuration and solar radiation based on oxygen isotope suggest that global ice volumes should have reached minimum 6,000 years ago (Berger and Loutre, 2003). 

Figure 3. An artists impression of the Agriculture Revolution (source)

The overdue glaciation hypothesis has received three main criticism all of which are address by Ruddiman (2007). Firstly, many have criticised Ruddiman's use of the previous three interglacials to compare against the Holocene (MIS1) since the orbital configuration of marine isotope stage 5, 7 and 9 are significantly different to the Holocene. During MIS 5, 7 and 9, eccentricity was higher than for MIS 1 resulting in more extreme solar radiation received on Earth. MIS11 has been proposed as a better analogue for MIS1 due to low eccentricity. The duration of MIS11 was significantly greater than other interglacials with marine sediments used to reveal how it lasted for 28,000 years, and this would therefore suggest that MIS1 has 16,000 years until the current interglacial come to termination (McManuset al., 2003). Ruddiman fires back with an argument that aligning the orbital configurations is flawed and that cooling had began during MIS11 at the isolation level comparable to that of MIS1 (Ruddiman, 2005). 

Secondly, many have question if humans have the capacity to increase greenhouse gases to reach such levels. To account of the 230 ppb anomaly found for methane, studies have suggested the expansion of wetlands across Eurasia but this has been rejected due to differences in the concentration between Greenland and Antarctica (Chappellaz et al. 1997). Ruddiman argues that substantial evidence from sedimentary records from deltas in the tropics show increased sedimentation and provide evidence of extensive deforestation  and widespread erosion (Roberts, 1998). Thirdly, detailed reconstruction of the Holocene temperature reveal small fluctuations around 10ppm in CO₂ in the last 1,000 years whilst human activity continues to increase. Ruddiman provides a possible explanation for this by claiming that three key major pandemics occurred over the last 1,000 years during the Roman Era (Cartwright, 1991), the Black Death pandemic during the Dark Ages, and the arrival of the Europeans in American 1942 (Denevan, 1992). 

Ruddiman provides a strong argument in favour of his overdue glaciation hypothesis. The implication of this provides important for defining the start of the Anthropocene which marks a moment in the Earth's history then humans activity becomes so profound that it rival the geological forces of the Earth and changes the course of the Earth away from its natural variation (Crutzen and Stoermer, 2000). Ruddiman's work shows how the Holocene was not free of human influence as previously believe and this has significant implications with respect to the current impact anthropogenic forcing are having on the current climate and on future climate.