There are two main reasons for the rapid rise in CO2. First, the human population has exploded in size. The planet now has over 7 billion people, a 700 percent increase over a mere two centuries—a blink of an eye in Earth’s history. But an equally important part of the answer involves resource use. The average per capita (per person) use of fossil fuels is skyrocketing—especially in industrialized countries.
The data in Figure 1 illustrate the consequences of population size and resource use. By multiplying the total population size of a country (left graph) by the quantity of CO2 emissions per person in that country (center graph), the total emissions per country can be calculated (right graph). For example, even though the average person in China causes less than half of the CO2 emissions that an average person from the United States does, the total emissions from China surpass all others due to its enormous population size.
Conversely, although the United States represents less than 5 percent of the world population, Americans produce an astonishing one-sixth of the global CO2 emissions due to their heavy use of fossil fuels. Looking at the data in another way, the population of the United States was about twice that of Nigeria in 2013, but the average American produced 33 times as much CO2 as the average Nigerian that year.
The bottom line is that the recent exponential increase of the human population, plus the steep increase in per capita fossil fuel use—as well as land-use changes such as deforestation— have caused a steep increase in the mission of CO2 and other greenhouse gases. These changes, in turn, have caused the recent and dramatic increase in the average temperatures and climate variability around the globe. The rapid rate of climate change is ultimately rooted in human behavior.