It's who you ask...
If you ask the general public, you get a silly answer. Ask those with a degree, or those with a science degree, or those who publish papers in astronomy and you'll get a better answer. Probably the most striking illustration of this can be seen in Doran and Zimmerman's 2009 paper (see graph in http://tigger.uic.edu/~pdoran/012009_Doran_final.pdf) where they compare belief in Anthropic Global Warming with the respondent's scientific background.
For general public, we get under 60% belief, rising to 82% for "Earth Scientists" in general, rising to nearly 90% for those who actively publish or are climatologists, and a stunning 97.4% of climatologist who are active publishers in climate change (*)
It's all down to who you ask
(*) or involved in the global conspiracy of scientists to destroy capitalism, which ever you prefer :-)