Wait a minute--what happened to that alleged consensus of 2,500 scientists?

Last month, I wrote:

So, Mr Ki-moon, just tear down that wall: just put up a simple web page containing the names, titles and affiliations of the 2,500 scientists that allegedly agree that global warming is a crisis, along with their role (author? reviewer? which chapter?) in creating the most recent IPCC report.

Under the headline "Scientists beg for climate action", Seth Borenstein writes:

A petition from at least 215 climate scientists calls for the world to cut in half greenhouse gas emissions by 2050...It includes many co-authors of the intergovernmental climate change panel reports, directors of major American and European climate science research institutions, a Nobel winner for atmospheric chemistry and a winner of a MacArthur "genius" award.

The actual petition is here.

Here's the key sentence:

The 2007 IPCC report, compiled by several hundred climate scientists, has unequivocally concluded that our climate is warming rapidly, and that we are now at least 90% certain that this is mostly due to human activities.

Questions that popped immediately to my head:

1. We were constantly told that we had a clear consensus of 2,500 scientists. Why did only 215 sign this petition, and what do the other nearly 2,300 scientists think?

2. How many of the 215 signers independently assessed the scientific evidence underlying the "key sentence" above?

If you didn't independently assess the evidence, doesn't your signature on this petition merely indicate your belief that someone else has assessed it?
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3. For extra credit, I'd like to ask the following of each scientist who's actually assessed the evidence: Why are you only "at least 90%" sure? What's your personal estimate of "mostly" (ie, is it 51%? 99%?)