It is very unlikely that the 20th century warming can be explained by natural causes. The late 20th century has been unusually warm. Palaeoclimatic reconstructions show that the second half of the 20th century was likely the warmest 50-year period in the Norther Hemisphere in the last 1300 years. This rapid warming is consistent with the scientific understanding of how the climate should respond to a rapid increase in greenhouse gases like that which has occured over the past century, and the warming is inconsistent with the scientific understanding of how the climate should respond to natural external factors such as the variability in solar output and volcanic activity.
Climate models provide a suitable tool to study the various influences on the Earth's climate. When the effects of increasing levels of greenhouse gases are included in the models, as well as natural external factors, the models produce good simulations of warming that has occured over the past century. The models fail to produce the observed warming when run using only natural factors. When human factors are included, the models also simulate a geographic pattern of temperature change around the globe similar to that which has occured in recent decades. This spatial pattern, which has features such as greater warming at higer northern latitudes, differs from the most important patetrns of natural climate variability that are associated with internal climate processes, such as El Nino.
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