Conservatives point out a handful of INDIVIDUAL scientists who have some doubts (few who say it's a "hoax" though).
However, the vast majority of scientists engaged in climate research support it. Look for the most well established SCIENTIFIC (not political) organizations engaged in climate research and see what they've been saying.
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1) THE AMERICAN GEOPHYSICAL UNION, FOUNDED IN 1919, OVER 45,000 MEMBERS
"Human activities are increasingly altering the Earth's climate. These effects add to natural influences that have been present over Earth's history. Scientific evidence strongly indicates that natural influences cannot explain the rapid increase in global near-surface temperatures observed during the second half of the 20th century."
http://www.agu.org/sci_soc/policy/climate_change_position.html
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