Answer to Question 1
Computer models can be used to calculate global temperatures based on changes in Earth's energy budget that can be related to both natural processes and human activities. The temperatures calculated from the computer models can then be compared to the actual measured temperatures. If only natural processes are used in the computer models, then the calculated and measured temperatures are similar prior to about 1970. After 1970, however, calculated and measured temperatures match only by including warming attributed to human activities.
Answer to Question 2
To estimate future temperature changes, scientists primarily examine different scenarios for fossil-fuel consumption based on projections of population growth (energy needs grow as population grows) and various expectations for conserving fossil fuels or replacing them with energy sources that do not emit greenhouse gases. These scenarios for fossil-fuel consumption are then used to estimate the changing concentration of greenhouse gases in the atmosphere, which is the most important factor for predicting future temperature change.
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