This figure compares measurements of the Earth’s past temperature variations (shown by the black line) with simulations of past temperature variations (shown by the red and blue lines) in order to determine whether the major changes in temperature were caused by natural or human-caused factors.
The black line shows observed surface temperature variations from the average. The blue and red lines show computer model results when past temperatures are simulated including different drivers of the climate system. Natural drivers include solar radiation and volcanic emissions, while anthropogenic (man-made) drivers include man-made greenhouse gases and sulfate aerosols. The blue line shows variation when natural drivers are included in the calculations, while the red line shows variation when both natural and anthropogenic drivers are included.
This figure shows that the combination of natural and anthropogenic drivers (the red line) provides a better match to the observed temperatures (black line) than only natural drivers (the blue line). Natural drivers alone can explain much of the temperature change in the first half of the century, as demonstrated by the similarity between the black and blue lines during that time period. As can be seen with the close match between the red and black lines, human-produced drivers strongly dominated the temperature change in the latter part of the 20th century.
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