There is strong evidence that the increase in concentration of carbon dioxide in the past 200 years is the result of human activity. The primary anthropogenic source of carbon dioxide is the combustion of fossil fuels. It is not a coincidence that the start of the current steep increase in atmospheric carbon dioxide levels corresponds with the beginning of the Industrial Revolution, when the combustion of fossil fuels began at a large scale. Before the Industrial Revolution, the concentration of carbon dioxide was approximately 280 parts per million (ppm), while the concentration of carbon dioxide in 2013 is over 390 ppm and rising.
A second piece of evidence for the human signature on our climate comes from isotope ratios (Lesson 1 - Isotopes). The 13C/12C ratio of carbon dioxide in the atmosphere can be analyzed to determine if the carbon dioxide originated from a natural or anthropogenic source. This is possible because natural sources of carbon dioxide increase the 13C/12C ratio in the atmosphere while anthropogenic sources decrease the 13C/12C ratio. Today’s 13C/12C ratio is the lowest it has been in the last 10,000 years, which strongly suggests that the increase in atmospheric CO2 over the last few hundred years is due to anthropogenic sources (For a more in-depth discussion of isotope ratios and carbon sourcing, see the Student Resource on Isotopes).
Carbon dioxide is a very important greenhouse gas and has a significant effect on the climate of the Earth. It is sometimes called Earth's thermostat because carbon dioxide has kept Earth’s temperature within reasonable limits. However, human activity is currently increasing the concentration of carbon dioxide at a much more rapid rate, which is significantly influencing Earth’s climate. This has profound implications for humans and our environment.
Your Turn
Data from ice cores and atmospheric monitoring show that atmospheric carbon dioxide concentrations have been increasing. What evidence suggests that the recent increase in carbon dioxide concentrations is not a natural occurrence but is due to human activity?
First of all, fossil fuel combustion began, in earnest, in the Industrial Revolution and the concentration of atmospheric carbon dioxide has increased greatly since the Industrial Revolution. This suggests that increased carbon dioxide concentrations are due to human activity.
Second, the 13C/12C ratio of atmospheric carbon dioxide suggests that the carbon dioxide in the atmosphere is due to human combustion of fossil fuels, not from the natural process of cellular respiration. This occurs because the carbon dioxide released from fossil fuel combustion contains more carbon-12 than carbon-13, thereby lowering the 13C/12C ratio of carbon dioxide in the atmosphere. Currently, the 13C/12C ratio of carbon dioxide in the atmosphere is very low, suggesting that the carbon dioxide in the atmosphere is mostly due to the combustion of fossil fuels, rather than due to natural processes.