Why focus on carbon dioxide emitted during the combustion of fossil fuels when plants and animals emit far more carbon dioxide to the atmosphere?
Humans, animals, bacteria, and plants do indeed emit enormous amounts of carbon dioxide to the atmosphere - the land biosphere emits 100 GtC/yr, compared with emissions from fossil fuels of less than 10 GtC/yr. So why should we care about carbon dioxide from fossil fuels? To understand the answer, you need to understand the difference between carbon dioxide coming from fossil fuel combustion and from
respiration by living organisms. Let's begin by imagining that you plant a carrot seed, and over the next few months this seed grows into a carrot. As described by Equation 5.1, the plant grows by absorbing carbon dioxide directly from the atmosphere. So the carrot contains carbon that, just a few months ago, was floating in the atmosphere as carbon dioxide. Now let's assume that the carrot is eaten by a goat. The goat metabolizes the carrot (in a manner approximately following Equation 5.2), which produces energy that is used to power the goat's vital functions. The carbon dioxide is also produced, and it is exhaled back into the atmosphere.
Thus, when an animal exhales carbon dioxide, it is releasing back into the atmosphere the carbon dioxide that was in the atmosphere just a few months before.
Although this can lead to seasonal variations in carbon dioxide, as shown in Figure 5.1, it does not cause long-term increases in carbon dioxide. Figure 5.6 confirms this by showing basically no change in carbon dioxide over the past 10,000 years, before about 1750. Over this time we know that humans, plants, and animals were certainly releasing carbon dioxide to the atmosphere, yet atmospheric carbon dioxide remained approximately constant.
In contrast, when you burn fossil fuels, you are releasing to the atmosphere the carbon dioxide that had been safely sequestered in rocks (e.g., Figure 5.5) for hundreds of millions of years. This is a net addition to the atmosphere, so it does cause a long-term increase in carbon dioxide. Figure 5.6 confirms this: The increase in atmospheric carbon dioxide started with the industrial revolution, when society-wide burning of fossil fuels began.