There is overwhelming scientific evidence that the significant climate changes that are currently taking place are due to anthropogenic causes. While the natural causes discussed above can help explain historic climate variations, they cannot account for the dramatic warming that the Earth has been experiencing since the 1950s. As stated in the 2007 IPCC report, “Most of the observed increase in global average temperatures since the mid-twentieth century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.”
To review photosynthesis, go back to the Energy Chapter.
The greenhouse gas emitted into the atmosphere in the highest quantity, due to human activity is carbon dioxide. Carbon is naturally cycled through the Earth’s oceans, land, biosphere, and atmosphere (Figure 13). In the atmosphere, carbon is present primarily as carbon dioxide gas (CO2). Carbon dioxide is released into the atmosphere from natural sources such as plant, microbe, and animal respiration. Alternately, carbon dioxide is absorbed out of the atmosphere through natural sinks, such as the process of photosynthesis used by plants and algae. Through this natural cycle, the amount of CO2 released through respiration balances the amount consumed by photosynthesis. In this way, the atmosphere neither accumulates too much CO2 nor becomes depleted of CO2. Here, the amount of CO2 released through respiration balances the amount consumed by photosynthesis.
However, as you recall from the Energy Chapter, since the Industrial Revolution, humans have extracted large amounts of carbon-based resources, such as wood, peat, and fossil fuels from within the Earth’s crust (Figure 14). Fossil fuels, such as coal, oil, and natural gas are created from buried accumulations of decaying plant and animal organic compounds that have gradually been transformed.
Once extracted, humans burn or combust these carbon-based resources for fuel. The carbon that is emitted as a result of burning these fuels reacts with the oxygen that naturally occurs in the atmosphere and releases both heat and carbon dioxide in a process called a combustion reaction. The amount of carbon dioxide added to the atmosphere through combustion reactions far exceeds the ability of plants and ocean algae to take-up through photosynthesis. This pushes the natural carbon cycle significantly out of balance.
For example, the atmospheric concentration of CO2 rose from 280 ppm (parts per million) in 1850 to over 400 ppm in 2014 (a 30% increase) because carbon dioxide was added to the atmosphere through fossil fuel emissions faster than it was removed by plants and algae. The maximum carbon dioxide level for Earth to maintain balances among its many systems and natural processes is <350 ppm. This is often referred to as the “tipping point”.
Questions to Consider
Imagine you are talking to a friend who is convinced that global climate change has natural causes and that human actions have not significantly contributed to this phenomenon.
What scientific evidence could you offer to challenge your friend’s point of view?
If your friend does not accept your scientific evidence, what point would you make next to try and help your friend understand the problem of global climate change?
The burning of fossil fuels is not the only anthropogenic driver of global climate change. As noted in the discussion of greenhouse gases earlier in this section, climate change has also been accelerated by industrial agricultural practices and deforestation. In fact, industrial agricultural practices produce more greenhouse gas emissions than any other human activity. Additionally, deforestation is largely linked to industrial agriculture to expand land under cultivation.