There are very few people left who dispute that earth's climate has changed significantly in the last century -- Can you say “Little Ice Age” or “Medieval Warm Period”?
Similarly, there are few in the scientific community who dispute the conclusions of the Intergovernmental Panel on Climate Change that “Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.” (see my commentary on the cost of climate change for more information.)
The few skeptics who doubt that humans are responsible for recent climate warming have clung to the hope that a recent increase in solar heating or in the emission of greenhouse gases by volcanoes could explain it instead. Wrong! Scientists from the National Center for Atmospheric Research (Boulder, Colorado) and the Division of Climate and Environmental Physics of the Physics Institute (Bern, Switzerland) have shown that “solar and volcanic effects appear to dominate most of the slow climate variations within the past thousand years,” but “the impacts of greenhouse gases have dominated since the second half of the last century.” (source; Open Access article, no subscription required)
In short, the sun and volcanoes played a big role in climate fluctuations before the 20th century, but we have been primarily responsible for global warming for at least the last 50 years.
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