Scientists develop mathematically-based climate models to help predict future climate changes. Each model uses different assumptions about the future to predict how atmospheric CO₂ levels and temperatures will change.

The variables in each model include:

• Population growth rate
• Economic development
• Energy use
• Efficiency of energy use
• Mix of energy technologies

The graph below shows the results from three climate models used by the IPCC, with predictions starting in 1990 and ending in the year 2100. In all three, the global population rate rises during the first half of the century, then declines.

• The A1B model assumes rapid economic growth and increased equity—the reduction of regional differences in per-person income. New and more efficient technologies are introduced, without relying heavily on a single energy source.
• The A1F1 model is the same as A1B, but assumes the continued use of fossil fuel-intensive technologies.
• In the B1 model, the world moves rapidly from a producer-consumer economy toward a service and information economy. There is a reduction in the use of raw materials, and an emphasis on clean and efficient technologies and improved equity.

Other models have been developed, each based upon a different set of assumptions.

Although differing in degree, these three climate prediction models show similar trends:

• The projected rate of global warming in the future is much larger than the rate of global warming during the 20th century.
• Predicted rates of global warming are greater than any seen in the past 10,000 years.

5. Why do scientists develop numerous models rather than rely on just one?

6. Based upon all the models shown, what range of temperature increase is expected to occur by 2100?