Case Study #3: Climate Change (continued)

• Biological Evidence Small changes in climate that humans scarcely notice can be important to other organisms. Remember that each organism's range is determined by factors like temperature, humidity, and rainfall. If those conditions change, the organisms can be affected. If temperature rises, for example, organisms would usually move toward cooler places away from the equator and from warm lowlands to cooler, higher altitudes. In addition, plant flowering and animal breeding are often cued by seasonal changes. If warming is occurring, these organisms should respond as though spring begins earlier.

  The IPCC report summarizes data from 75 studies covering 1700 species of plants and animals. These data confirm that many species and communities are responding as though they are experiencing rising temperatures. The yellow-bellied marmot in Figure 6–31, for example, is coming out of hibernation over a month earlier than it used to.

FIGURE 6–31 Waking Up Too Early

FIGURE 6–32 Greenhouse Gases

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2. Researching the Cause: Models and Questions What is causing global warming? Earth's climate has changed often during its history. So researchers had to determine whether current warming is part of a natural cycle or whether it is caused by human activity or by astronomical and geological changes. As the IPCC report documents, concentrations of carbon dioxide and several other greenhouse gases have increased significantly over the last 200 years, as shown in Figure 6–32. Several kinds of data suggest this increase is due to the burning of fossil fuels, combined with the cutting and burning of forests worldwide. These activities add carbon dioxide to the atmosphere faster than the carbon cycle removes it. Most climate scientists agree that this added carbon dioxide is strengthening the natural greenhouse effect, causing the biosphere to retain more heat.

   • How Much Change? How much warming is expected? For answers, researchers turn to computer models based on data. The models are complex and involve assumptions about climate and human activities. For these reasons, predictions are open to debate. The IPCC reports the result of six different models, which predict that average global temperatures will rise by the end of the twenty-first century from just under 2°C to as much as 6.4°C higher than they were in the year 2000.

   • Possible Effects of Climate Change What does climate change mean? Some changes are likely to threaten ecosystems ranging from tundra and northern forests to coral reefs and the Amazon rain forest. The western United States is likely to get drier. The Sahara Desert, on the other hand, may become greener. Sea level may rise enough to flood some coastal ecosystems and human communities. And some models suggest that parts of North America may experience more droughts during the summer growing season.