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.