Passage VI

CO₂ is put into Earth's atmosphere by human activities and by natural processes such as diffusion from the oceans.

Atmospheric CO₂ affects Earth's climate. In general, as the atmospheric CO₂ concentration increases, more infrared radiation from Earth's surface is absorbed by CO₂, and the average global temperature rises. The atmospheric CO₂ concentration that existed at any give time over the past 160,000 years can be determined from air trapped in bubbles in Antarctic ice formed at that time. Figure 1 shows the atmospheric CO₂ concentration over the past 1,000 years.

Figure 1 adapted from International Panel on Climate Change, "Climate Change 2001: The Scientific Basis." @2001 by Cambridge University Press.

Figure 2 shows the atmospheric CO₂ concentration and the global temperature deviation over the past 160,000 years. Global temperature deviation equals:

(average global temperature at a given time) - (present-day average global temperature)

Figure 2 adapted from Stephen Schneider, "The Changing Climate." 1989 by Scientific American.

Two scientists discuss why the average global temperature has risen over the past 150 years.

Scientist 1

Global warming is a direct result of the CO₂ put into the air by the widespread burning of fossil fuels. Figure 1 shows that for hundreds of years prior to 1850, the atmospheric CO₂ concentration was relatively constant at around 280 parts per million (ppm). Since 1850, a steady increase in the amount of fossil fuels burned has caused the atmospheric CO₂ concentration to increase, reaching 360 ppm by the year 2000, higher than at any time in the past 160,000 years. Over the same 150 years, average global temperature has risen by almost 1°C. Since the year 2000, the atmospheric CO₂ concentration has been increasing at a constant rate of 1.5% per year.

Scientist 2

Global warming is a natural process that has occurred a number of times during the past 160,000 years, even before the widespread burning of fossil fuels. In general, increases and decreases in average global temperature result from increases and decreases, respectively, in the atmospheric CO₂ concentration (see Figure 2) . The CO₂ put into the atmosphere by human activities is less than 5% of the CO₂ put into the atmosphere by all sources. Therefore, the CO₂ put into the atmosphere by human activities, by itself, has no measurable effect on average global temperature.