Introduction:
The rising greenhouse gas levels in our atmosphere and change in global climate has lead scientists and world authorities to question whether these changes are natural or induced by humans. An Anthropocene implies that humans are not just responsible for these changes as a result of post-industrial agricultural and technological advances, but that we have caused major impact thousands of years before (Ruddiman 2013). Therefore, this results in another period in the earth’s life that defines humans as a major geological agent. Over the past 8000 year carbon dioxide (CO2) levels in the atmosphere have increased beyond what is predicted following the last glaciation (Broecker 2006). It is theorised that this rise in carbon dioxide emissions can be attributed to human inference as a result of deforestation (Kaplan et al 2010) thus lending to the hypothesis that human are acting as a geological agent. In response, studies are being conducted by Broecker (2006) and Svetik (2013) which look at samples of Antarctic ice to measure isotopes of carbon (C12, C13 and C14) which can determine whether the CO2 emissions are Anthropogenic in nature. Methane (CH4) is an example of another greenhouse gas where global emission levels are showing worrying trends. While methane levels were decreasing during the first half of the Holocene, levels have risen steadily since. Ruddiman et al (2008) believes this could be a by-product of early rice farming in Asia, specifically India and China. Another explanation for global methane emissions lays at the bottom of the ocean – gases once trapped in coaliferous formations have risen up into sediment, eventually dissolving in the ocean and atmosphere (Akulichev 2013). Accumulation of these emission levels lead to greater problems in the global climate, such as a ‘greenhouse effect’ on the planet and rising sea levels (Crutzen 2014). For the concept of an Anthropocene to be accepted it must be proved that humans have indeed been acting as geological agents for thousands of years.
Global CO2 Levels and Anthropogenic Land Cover Change:
Global carbon dioxide levels have risen over the past 8000 years which in turn has scientists worried that the trajectory of our global climate is under influence from unnatural causes from humans during the latter half of the Holocene (Crutzen 2014). CO2 levels have been on the rise for thousands of years so it cannot be merely assumed that post-industrial greenhouse gas emission due to fossil fuel burning is a factor. While this could explain more recent trends, early land clearing from Homo sapiens who have spread across the world is the most likely candidate when considering human accountability. Various models have been derived from the relationship between global population and land cover change from 8000 years ago to the beginning of the industrial revolution (Kaplan et al, 2010). An outdated model shows a linear relationship between human population and an increase in anthropogenic land cover change (ALCC). This linear relationship concludes that there is not sufficient ALCC to impact on CO2 and as a result a human cause cannot be assumed. However, this model is flawed in that is assumes the ALCC is directly linear to a growing human population which is incorrect. ALCC per person is a result of cultural, geographical and generational pressures on that specific person.
Figure 1: Cumulative carbon emissions (Pg C) and Annual carbon emission (Pg C/yr) resulting from ALCC
The HYDE model uses information from climate, human expansion and agriculture, soils and atmospheric levels to create a more accurate depiction of global vegetation. By introducing a Dynamic Global Vegetation Model and running simulations researchers were able to quantify emissions over the Holocene. From this it is clear that both scenarios give drastic depictions of Carbon emissions as result of ALCC with the HYDE scenario showing far less impact than the KK10. They concluded that ALCC could have contributed up to 25ppm of pre-industrial CO2 into our atmosphere. A more recent paper by Lightfoot and Cuthrell (2015) addresses what they consider to be a major bias when developing ALCC. They believe studies such as those in Figure 1 do not put enough emphasis on early deforestation from hunter-gatherer societies. Early land clearance from burning to assist in agrarian practises may extend the time frame past 8000 years and subsequently increase estimates of anthropogenic emissions in pre-industrial times. Using California as a case study, they also state that Global population is too inconsistent with ALCC to be used as a representation for the later. Information from ALCC models so far is inconclusive as to the extent to which humans have been acting as a geological agent. Further research into finding an accurate model which includes hunter-gatherer data and the inconsistent relationship between global human population size and ALCC is require to synthesis quantities on consequential CO2 emissions. When considering the question ‘Are humans influencing global emission levels?’ it could be suggested that damage in fact has been caused to the atmosphere to some degree – it might be premature to consider that humans are entering an Anthropocene.
Global CO2 Levels and Antarctic Carbon Isotope sampling:
The earth undergoes CO2 periods where the atmospheric level rises and falls in correlation with natural glaciation events. Following the last glaciation event the earth’s carbon dioxide level was forecast to drop to 240 ppm in the atmosphere – instead it remained at 280ppm (Broecker, 2006). This increase in carbon dioxide could explain why we are overdue for the next ice age. It is also theorised that this anomaly could be the result of natural causes, for example natural eccentricity cycles, this could explain the high CO2 emisions. In the interest of discovering whether this is a natural event or a result of anthropogenic practises scientists have collected samples of carbon isotopes trapped in Antarctic ice. Broecker (2006) was unable to find sufficient evidence that points to a relationship between human practises and the rise in CO2. Carbon released into the environment by human activities such as the burning of fossil fuels is known as terrestrial carbon. A significant increase in this type of carbon would appear as a decrease in the ratio of C12 and C13 trapped in the ice. While the results did show such a decrease it was determined not adequate enough to conclude that the CO2 rise was anthropogenic in nature. More recently Svetlik et al. (2013) has used similar methods of measuring carbon isotopes in Antarctic ice cores with the intention of mapping the 14C isotope in relation to 14CO2 and carbon dioxide. One of their findings showed a strong negative correlation of approximately 93.1% between CO2 concentration in the air and 14C between 20 to 2 ka BP. They relate this to atmospheric dilution from CO2 sources from deeper ocean layers. This CO2 is likely to have been released by an increase in global temperature. This study, as a result, had a far less focus on anthropogenic causes for trends in CO2 levels, but instead endeavoured to gather further information about 14C levels in the Holocene. They could provide other studies with the data necessary to answer about the possibility of an Anthropocene. Overall there is not sufficient evidence in these papers to attribute the rise in CO2 to an anthropogenic cause. This is why it is important for studies such as the one in Figure 1 to continue to be conducted. By further work into the concept of an Anthropocene through information gathered by studies such as those which have been discussed, the cause(s) for the rise in CO2 need to be examined as well as observe the role humans have played. This is an important and relevant issue because of the current climate change that is occurring – possibly as a result of these atmospheric trends and by extension humans.
Arguments for and against Global Methane Levels:
It is observed that global methane (CH4) levels were decreasing during the first half of the Holocene but about 5000 years ago this trend has reversed and global levels are on the rise. As with CO2 there is much debate as to whether this rise is anthropogenic. One study done by Ruddiman et al (2008) has produced this graph clearly depicting a swing in methane emission 5000 years ago. This study considered the relationship between early rice farming in Asia and methane trends over the past 5000 years. By synthesising data gathered from over 300 archaeological sites they were able to conclude that the compiled data was not sufficient to quantify the amount of methane released due to rice farming.
Although it was sufficient to say that rice farming did have an impact on methane emissions due to the growth in irrigated wetlands and other by-products – and has sustained an increase in population from India and China through to the present – it is important to recognise the large concentration of sites in this study dated between 4000 and 5000 years ago. For example, this could be due to experimental bias – archaeologists only searching for sites dated further back. Nevertheless this study does conclude that humans have acted as a geological agent by influencing methane emissions. In contrast some scientists believe that at least a proportion of the global methane rise comes from natural sources such as coaliferous formations in the ocean (Akulichev et al 2014). Using recently developed seismoacoustic methods and instruments, scientists have been able to examine the amount of CH4 that has migrated from submarine coaliferous formation to Holocene sediment. This has major implications for the amount of methane which then has dissolved into the ocean and atmosphere. This study particularly focuses on the Amur Bay in the Sea of Japan. Therefore it cannot be said with certainty that this is a trend common across coaliferous formation throughout the ocean, nor do they put emphasis on why this is happening. Broecker (2006) explains that a rising global temperature could be to blame for the release of CO2 into the atmosphere from deposits in the ocean. If this was the same cause for the rise in methane than this could represent a positive feedback system where greenhouse gases are released, increasing the planets temperature which in turn releases more greenhouse gases. It seems that humans are projected for a dramatic increase in the earth’s temperature (Crutzen 2014) and it is clear that this increase needs to be addressed whether it is anthropogenic in nature or not.
Conclusion:
An Anthropocene implies that human activities have impacted on the planet far prior to the industrial revolution during the mid-19th century (Ruddiman 2013). According to this theory humans have been acting as a geological agent up to 8000 years. This paper focuses on the impact that human practises might have had on CO2 and CH4 emissions due to anthropogenic land cover change, burning and early rice farming in Asia. Not all the evidence supports such a claim – the debate is split between natural and anthropogenic causes for the increase in greenhouse gas emissions. In Broecker (2006) the question of whether the CO2 rise in the Holocene natural or anthropogenic was raised and there was not sufficient evidence to support an anthropogenic cause. In another paper that focuses on the correlation between ALCC and CO2 emissions, it was concluded that humans can attribute up to 25ppm of preindustrial CO2 towards land clearing and burning following early Homo Sapien movements (Kaplan et al 2010). A later study builds on this, stating the effect of early hunter-gatherer land burning might have been underestimated which would increase anthropogenic emissions further (Lightfoot & Cuthrell 2015). Ruddiman et al (2008) gathered sufficient evidence to support an anthropogenic influence in the rise CH4 caused by early rice farming in India through China. The wetland like environment required for rice farming saw a massive growth around 5000 years ago, which is when methane levels began to rise. Natural causes for methane emissions include naturally occurring wetlands (Ruddiman et al 2008) and the release of trapped methane in coaliferous formations (Akulichev et al 2015). There is evidence for both sides of the debate and both views have been inconclusively researched. More data is required to gain a suitable understanding as to why these greenhouse gases are showing an increase in emissions for thousands of years. However, there seems to be enough information to attribute a proportion of the emissions to human activities. While it is premature to assume that homo sapiens are entering into an era dominated by geological influences such as that described as an Anthropocene, it can be assumed that humans have been acting as geological agents.
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