Greenhouse Gases

Earlier in this chapter, you read about how the greenhouse effect keeps the lower atmosphere warm. In this process, certain gases in the atmosphere absorb energy that has been reradiated by Earth's surface. Greenhouse gases help insulate Earth's surface, keeping it warm enough to support life.

Three important greenhouse gases are water vapor (H₂O), carbon dioxide (CO₂), and methane (CH₄). Human activity has little effect on the amount of water vapor in the atmosphere. However, activities such as fossil fuel burning and deforestation release billions of tons of CO₂ into the atmosphere each year. In addition, livestock farming releases methane into the air. Many human activities are increasing the levels of greenhouse gases in the atmosphere. This increase is causing global temperatures to rise. Between 1910 and 2010, average global temperatures increased by about 1°C. The process by which emissions from human activities increase the greenhouse effect is known as global warming.

Figure 38 The graph shows how CO₂ levels in the atmosphere have changed over time.
Inferring Why do you think atmospheric CO₂ levels increased dramatically after the year 1800?

d

To see how greenhouse gas levels vary over the long term, scientists can refer to Earth's climate record. For example, ice core samples indicate that atmospheric CO₂ levels over the past 800,000 years typically fluctuated between 170 and 280 parts per million (ppm). But over the last 200 years, CO₂ levels have increased sharply, as shown in Figure 38. Today, the concentration of CO₂ in the atmosphere is 390 ppm—and on the rise.

With the aid of climate models, scientists can predict the extent of global warming. Climate models are sophisticated computer programs that take into account natural and human causes to model both past and future climate changes. Current climate models predict that global temperatures will increase 2 to 5°C over the next century.