Global Warming

Scientists have noticed a relationship between average levels of CO2 in the atmosphere and average global temperatures. As CO2 levels have increased, so have temperatures, as shown in Figure 22-6. This pattern, in which two variables change in similar ways, is called a correlation. However, a correlation can be difficult to interpret. A correlation could result from a cause-and-effect relationship, in which a change in one variable causes a change in another variable. However, a correlation does not necessarily indicate which variable, if any, is the cause. That is, rising CO2 might cause temperature to rise, rising temperature might cause CO2 to rise, or other variables might cause both CO2 and temperature to rise.

Scientists cannot conduct a global experiment to test these hypotheses. However, scientists have strong evidence that CO2 and certain other gases do cause Earth's atmosphere to trap more solar energy. For example, scientists have developed computer models to simulate Earth's climate and predict possible climate changes. Climate models are very complex because they include many factors, such as wind patterns and the effects of oceans on the atmosphere. By varying CO2 levels in climate models, scientists can simulate the possible effects of CO2 on global temperatures. The results of such simulations consistently show that doubling atmospheric CO2 leads to an increase in average global temperatures of 1.0°C to 4.5°C (2°F to 8°F), although the exact temperature change depends on the particular model. Most scientists studying this problem have concluded that increased CO2 levels in the atmosphere cause increases in global temperatures.

The year 2003 was the second-warmest year on record at that time. Average global temperatures were 0.56°C (1.01°F) above the past century's average. The warmest year before 2003 was 1998. An international panel of scientists predicts that temperatures will rise an additional 2°C (4°F) in the next 100 years. This small change could alter rainfall patterns, soil moisture, and sea level around the world. Also, this change could shift agricultural regions and disrupt both terrestrial and aquatic ecosystems. Rising temperatures are already changing biological communities in measurable ways.

Atmospheric Carbon Dioxide

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