Of the various petitions on global warming circulated for signatures by scientists, the one by the Petition Project, a group of physicists and physical chemists based in La Jolla, Calif., has by far the most signatures—more than 31,000 (more than 9,000 with a Ph.D.). It was most recently published in 2009, and most signers were added or reaffirmed since 2007. The petition states that “there is no convincing scientific evidence that human release of . . . carbon dioxide, methane, or other greenhouse gases is causing or will, in the foreseeable future, cause catastrophic heating of the Earth’s atmosphere and disruption of the Earth’s climate.”This petition is a deeply troubled and conflicted operation that verges on outright fraud. It's an example of what you might call "push polling on steroids."
Obviously the petition doesn't and can't try to gauge scientific consensus or lack thereof. Unlike surveys that ask questions of verified respondents, the petition promotes a point of view. It comes with a cover letter from a famous climate change denier. It claims over 31,000 signatories, but has no way to verify who they are. Pranksters have submitted fake signatures with made up names. Most of the "more than 9,000" Ph.D.s that Spencer mentions are engineers, not scientists. From the project's own statistics, a minute fraction of signatories are climatologists or closely related specialists. A document included with the petition was deceptively designed to look like a reprint from the National Academy of Sciences, even though it wasn't. The NAS felt the need to publicly distance itself from the project, and to reaffirm its own assessment of scientific opinion.
Bottom line: If you're honestly trying to gauge scientific opinion, you don't include materials making the case against anthropogenic climate change. Respondents, particularly scientists, ought not need to be told by you what they should believe.
Here's a Wikipedia article on the project.