Climate change is a natural phenomenon but one that can also be accentuated by human activities. The extent to which temperature change over the last 100 years is due to human activities has been studied by looking at patterns of change across the Earth's surface and in the atmosphere and the ocean. The contributions of natural effects, e.g. variations in the Sun's output and volcanoes, have also been studied. Natural effects were found to be unable to account for all the observed changes. Only when greenhouse gases emitted by human activities are included can the changes be explained.

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