This figure compares measurements of the Earth’s past temperature variations (shown by the black line) with computer model 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.
All lines are shown as variations from the average temperature. Natural drivers include solar radiation and volcanic emissions, while anthropogenic (man-made) drivers include human emitted greenhouse gases and sulfate aerosols. The blue line shows variation when only 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.