The Earth’s climate has constantly changed throughout history. In the last 650,000 alone there have been many cycles of glacial retreat and advance with the last ice age ending about 7000 years ago. These changes are fundamental to the global climate system. Crutzen and Stoermer (2000) proposed that the human effect on the global climate system began in the 1800’s with the discovery of fossil fuels which coincided with the beginning of the industrial revolution. They labelled this era of human impact the ‘Anthropocene’. However recently William Ruddiman hypothesised that humans have been acting as a geologic agent since the Holocene nearly 8000 years ago. In this essay I will outline the principle lines of evidence of the early anthropogenic theory proposed by Ruddiman and the associated opposition to the theory. Additionally I will discuss the principle lines of evidence that highlight how humans are acting as a geologic agent in the modern anthropocene.
The early anthropogenic hypothesis proposed by Ruddiman essentially changed how we perceive the timing of the geologic impact that humans have. The theory states that human impact on the global climate system did not begin with the discovery of fossil fuels and the industrial revolution, but rather 8000 years ago. There are two principle lines of evidence for the early anthropogenic theory, the first one being the carbon dioxide and methane anomaly discovered in ice cores and the second being the archaeological evidence showing the intensification of large scale agriculture and land use practices, in particular rice culture. Analysis of ice cores has been able to determine climatic differences between the current interglacial period which we are living in now and previous interglacials. The record of the most recent interglacial atmospheric carbon dioxide and methane is anomalous when compared to the three prior to it. The Greenland ice cores for example show that over the last hundred thousand years the levels of carbon dioxide and methane have rose and fallen according to a twenty three thousand year cycle however this natural rise and fall stopped approximately five thousand years ago and has been steadily increasing since. Ruddiman states that without human influence the atmospheric carbon would be 240ppm (parts per million) but it rose to 280ppm. The 40ppm increase has essentially prevented the onset of another ice age. (Broecker, 2006).
• The graphs shows the cyclical CH4 and CO2 trends represented by the blue line as opposed to the measured CH4 and CO2 represented by the red line. (source Ruddiman, 2007)
With no natural explanation for the CH4 and CO2 anomaly it was determined that it was caused by humans. The second principle line of evidence for the early anthropogenic hypothesis is the historical records that show the increase of large scale agriculture and land use during the Holocene. Generally the world’s leading producer of atmospheric methane is low altitude wetlands however this was not the cause of the Holocene anomaly. Schmidt et al stated that the increase in rice agriculture was the cause for the anomaly. Farming practices such as the construction of irrigated rice paddies would have essentially created a man-made wetland and would have caused the spike in atmospheric methane. Additionally domesticating livestock for rice cultivation would have also contributed to the spike. It is estimated that large scale agriculture began in China approximately 6000-6500 years ago which coincides with the anomalous CO2 and CH4 trends. In China 311 archaeological dig sites were examined, each site contained husks, kernels and rice stalks. This highlighted that large scale rice farming in China was more expansive than originally thought which could explain the anomaly. (Ruddiman et al, 2008).
As is the case with any hypothesis that changes decades of scientific thought the early anthropogenic theory was not without criticisms and doubt. There have been many natural explanations proposed that can potentially explain the CO2 rise during the Holocene. The most notable was proposed by Broecker et al (1999). Following the glacial retreat northern forests grew and extracted billions of tons of carbon form the sea. This transfer of carbon offset the balance of ocean chemistry and allowed the deposition of extra calcium carbonate on the seafloor. When the northern forests stopped growing the calcium carbonate in the ocean dissolved which emitted billions of tons of carbon into the atmosphere. (Kaplan et al, 2010). In addition to this two similar simulations carried out that attempted to construct a history of anthropogenically induced land cover change during the Holocene. The Kaplan Krumhardt (KK10) scenario suggests that humans use land more intensively as population increases. Whereas the Hyde scenario was based on a linear relationship between population and agricultural land use. Although the two simulations were very similar the results were significantly different. (Kaplan et al, 2010) This study highlights the uncertainty involved when attempting to simulate land use trends during the Holocene. The key input for both models is population, as a result of this accurate simulations are impossible to create because there is no population data for the Holocene era.
The early anthropogenic theory states that there was a 40ppm increase over thousands of years whereas in the post-industrial anthropocene there has been a 100pm increase in just over 200 years. When the two theories are compared the early anthropogenic theory is miniscule in scope. The key element in favour of the post-industrial anthropocene is the growing amount of current data.
• The graph highlights the C02 and CH4 increase since the post-industrial period. (source
Man-made emissions of carbon dioxide, more than any other greenhouse gas, have contributed most to the enhancement of Earth’s natural greenhouse effect, about 60% since the late 18th century when man-made greenhouse gas emissions began to increase. Methane, nitrous oxide and the CFCs have contributed about 20%, 4% and 12% respectively (Lerch, 2010). The origins of the enhanced greenhouse coincided with the beginning of the industrial revolution. When it was discovered that coal, oil and natural gas could be harnessed as an abundant, easily accessible and cheap source of energy it led to unimaginable changes in manufacturing, food production and transportation and in the blink of an eye the worlds global climate system transformed rapidly. It was like mankind won the lottery, and like every lottery winner mankind went a little bit crazy with an explosion of human population, consumption, economic activity and environmental destruction and is only now realising the full consequences of this binge. The evidence that suggests that we are truly in an anthropocene is mounting and it is becoming undeniable that humans are indeed acting as a geologic agent in the present day.
The first principle line of evidence to suggest that humans are acting as a geologic agent is the sharp increase in the sea levels over the past few decades. Since the mid-19th century there has been a significant rise in sea levels primarily as a result of human induced climate change. Satellite measurements show that during the 20th century the sea level increased about 15-20cm which is approximately 1.5 millimetres per year. This figure has jumped to approximately 3.1 millimetres per year which is a significant increase (Sahagian et al, 1994). This increase is likely caused by one or a combination of two key causal factors. The first one being thermal expansion. Similar to air and other fluids water expands as its temperature increases, while climate change warms the oceans the water will expand causing the sea level to rise. It is estimated that thermal expansion has contributed to about 2.5 cm of sea level increase during the 20th century. Because this causal factor of rising sea levels depends mainly on the temperature of the ocean it is difficult to predict and consequently was criticised by the public and its reliability is questionable. The IPCC estimated that thermal expansion will lead to a rise of approximately 17-28 cm. This estimation is less than the rise that would occur based on a linear model of the data. This suggests that the IPCC estimation of sea level increase due to thermal expansion is too low which raises questions about the reliability and validity of the evidence. The second causal factor is the loss of ice mass from the Greenland and Antarctic ice sheets. If the entire Greenland ice cap were to melt sea levels would rise approximately seven meters and if the Antarctic ice sheet were to melt sea levels would rise by approximately 55 meters. However the models that are used to calculate the change in ice mass are only capable of factoring in surface processes like evaporation and snowfall (Wigley,1987). ICPP’s estimation of sea level increase has caused significant debate within the scientific community and there has been a lot of criticism of the accuracy of the projections for a number of reasons. Most notably the limitations of the modelling of the ice sheets which do not account for sub surface processes. There are also problems in estimating how much global temperatures will increase and because atmospheric temperatures essentially determine the rate of sea level increase. Perhaps part of the problem is the various types of instruments used to record data as different instruments can create different results.
The second principle line of evidence is the increase in global temperatures. In Australia the climate has warmed has been warming since national records began in 1910 and has especially spiked since 1950. This has most likely been caused by human practices and in particularly the burning of fossil fuels. Australia’s mean air temperature has increased by 0.9 degrees Celsius since 1910. The average daytime maximum temperature has increased by 0.8 degrees Celsius whilst the overnight minimum temperature has increased by 1.1 degrees Celsius. Although these changes may seem relatively minor these are in fact very abrupt changes. The earth’s climate records preserved in proxies like tree rings, ice cores and coral reefs show that the average global temperatures have been relatively stable for a long period of time (Thomas, 2003). This highlights that these recent changes are undeniably caused by human induced greenhouse gas emissions and is not due to natural climate variability. Australia’s climate is usually associated with El Nino and La Nina which create natural climate variability in Australia. This natural variability however has been enhanced in recent decades. 2013 was the warmest year on record in Australia and was 1.2 degrees Celsius above the 1961-1990 average of 21.8 degrees Celsius and 0.17 degrees Celsius above the previous warmest year in 2005. This is interesting because an unusually warm year would normally occur during an El Nino event in Australia but 2013 was a neutral phase for the El Nino cycle. This highlights the fact that there is nothing cyclical about these recent events and this can only mean that this is occurring because of anthropogenically enhanced warming. This data came from the Australian Bureau of Meteorology which presumably are reliable source for evidence.
The third principle line of evidence for anthropogenically enhanced climate change is the acidification of the world’s oceans. Ocean acidification is a much less known line of evidence than others previously mentioned but nonetheless is a direct result of anthropogenic activities. The pre-industrial revolution C02 concentration was measured at 280 ppm and in 2005 it was measured at 379ppm. To fully understand ocean acidification it is firstly important to understand the global carbon cycle. In a very short time span humans used reserves of carbon in the form of fossil fuels which took millions of years to form. This created a huge influx of carbon dioxide and being unable to escape the atmosphere was dissolved in sea water. The carbon dioxide that dissolves in the water has two possible outcomes, one being used by photosynthesis or other biological processes or two which is remain free in its different dissolved forms in the water. The latter is the cause of ocean acidification (James, 2005). Global data collected over several decades indicate that the oceans have absorbed at least half of the anthropogenic carbon dioxide emissions that have occurred since 1750. This highlights the earth’s natural ability to store carbon but unfortunately the rate of human carbon dioxide production has surpassed the earth’s rate of storing it which is having negative effects on the world’s oceans.
It is undeniable that humans have been acting as geologic agents since the Holocene. This is evident through ice cores and historical records of early human land use and agriculture. The early anthropogenic theory does redefine the time scale of human impact on the planet but without accurate records it is difficult to model exactly how serious the effect has been. The post-industrial anthropogenic however has become an undeniable truth because of modern instruments and accurate data. Humans acting as a geologic agent in the present day is evident through increased climate variability, thermal expansion of the oceans and ocean acidification.
Sources
Broecker, W.S. 2006, ‘The Holocene co2 rise: Anthropogenic or natural. American Geophysical union, vol. 87, pages 27-29.
James et al. 2005 “Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms.” Nature 437.7059. 681-686.
Kaplan et al. 2010. ‘Holocene carbon emissions as a result of anthropogenic land cover change’. The Holocene, pages 9-10.
Lerch, 2010. The post carbon reader: managing the 21st century’s sustainability crisis. Pages 5-15
Ruddiman et al, 2008, Early rice farming and anomalous methane trends. Quaternary Science Review, vol. 27, pages 1291-1295.
Sahagian, et al. 1994. “Direct anthropogenic contributions to sea level rise in the twentieth century.” pages54-57.
Thomas, K. 2003 “Modern global climate change.”science 302, no. 5651. Pages1719-1723
Wigley,et al. 1987. . “Thermal expansion of sea water associated with global warming.” Nature 330, no. 6144. Pages127-131.