The IPCC is the international group responsible for the best information/science on climate change, but the U.S. has a large federal research initiative independent of the IPCC. The "United States Global Research Program" has hundreds of scientists and policy-makers from many different agencies. Their assessments (last 2009) are published. Here are some links. Note also under Key Findings the following:
"Global warming is unequivocal and primarily human-induced.
Global temperature has increased over the past 50 years. This observed increase is due primarily to human-induced emissions of heat-trapping gases. (p. 13)"
You can download the 2009 report.
Thanks to Leslie Rissler for summary and links.