Did Humans Cause Warming?
Are Human Activities the Major Cause of Recent Warming?
Probably yes, but comparing climate model projections to actual temperatures shows that both humans and nature have contributed to warming in the 20th century.
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| A climate model projection that includes both natural processes and human activities closely matches actual measurements of 20th-century temperature changes. |
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| The same climate model without human activity (natural processes only) does not match the strong warming that occurred during the past few decades. |
Changes in the Sun and volcanic eruptions were relatively important early in the century. But they do not explain the strong warming in recent decades, when the effects of human-produced greenhouse gases became apparent.
What Are Climate Models?
They are computer-based tools used to make projections about future climate and interpret the causes of past changes. They take into account many of the factors that were illustrated in an earlier section on “causes” and “feedbacks” in the climate system, such as greenhouse gases, ocean circulation, and clouds.
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