Well, let's look at the sixteen climate scientists who signed this, shall we?
Claude Allegre, former director of the Institute for the Study of the Earth, University of Paris: Sounds reasonable, though it looks like the proper name for the "University of Paris" is the "Paris VI University", or "Pierre and Marie Curie University". Unfortunately, it looks like the man is kind of a crank, and he hasn't been the director of that Institute since 1986, which makes it weird that it's the one thing they list about him.
J. Scott Armstrong, cofounder of the Journal of Forecasting and the International Journal of Forecasting: That's pretty reasonable, but forecasting and climate science aren't exactly the same thing; forecasting is the study of what's going to happen tomorrow or next week in any topic, while climate science is trying to figure out what will happen in the next year or the the next ten years with the weather. Also, Armstrong's professional background seems to be primarily in advertising, not forecasting, and he hasn't actually published any papers on climatology that I can see.
Jan Breslow, head of the Laboratory of Biochemical Genetics and Metabolism, Rockefeller University: I'm not exactly sure what he's doing on this list, since presumably it's a list of climate scientists? I mean, just because he's a researcher in one field doesn't automatically qualify him in others; it's like taking your car to ten mechanics and ignoring what they say, then asking your doctor about it and following his advice.
Roger Cohen, fellow, American Physical Society: This dude seems to be a writer for the NY Times, and I can't seem to find anyone by that name on the list of Fellows of the American Physical Society [aps.org]. Maybe he received his fellowship before 1990? In any case, it doesn't signify much in terms of his ability to evaluate any kind of science; those fellowships are kinda prestigious, but they're handed out for all sorts of things.
Edward David, member, National Academy of Engineering and National Academy of Sciences: What can I say? He's an electrical engineer. Would you trust him to diagnose a heart condition? An expert in one subject is not automatically an expert in all subjects.
William Happer, professor of physics, Princeton: What can I say? Damnit Jim, he's a physicist, not a climatologist! Sure, they're related - but would you trust this guy if he was talking on the way that chemists all over the world are trying to fool us about the mind control properties of fluorine? (as a side note, he's also a Fellow of the American Physical Society - why didn't they mention that?)
Michael Kelly, professor of technology, University of Cambridge, U.K.: This dude is kinda hard to Google because he shares a name with a fairly famous guitar company and a well-respected journalist (who died in 2003); however, it looks like he's done some pretty awesome work on semi-conductors. Unfortunately, that doesn't have anything to do with climate research.
William Kininmonth, former head of climate research at the Australian Bureau of Meteorology: Well, for one thing, he hasn't been the head of the ABM since 1998 (this seems to be a theme, you know?); for another, he's trained as a meteorologist, not a climate scientist. Just because they both deal with the weather doesn't necessarily mean that his word carries extra weight, but I do have to admit that he's one of the better signatories of this list.
Richard Lindzen, professor of atmospheric sciences, MIT: Lindzen is the most impressive name on this list, which makes me really wonder why he's in the middle. He's long been an outspoken skeptic of climate change, and is one of the few who really has a background in the stuff. He really should have been at the top, not the weird French dude who said that asbestos is harmless (even though it killed 22 of his students).
James McGrath, professor of chemistry, Virginia Technical University: Well, he's a chemist. That's not climate science. We've been over this.
Rodney Nichols, former president and CEO of the New York Academy of Sciences: Stopped being the president and CEO of the NYAS in 2001. Is that really the best they can do? The administrator of scientific academy that doesn't even focus on climate?
Burt Rutan, aerospace engineer, designer of Voyager and SpaceShipOne: Look, Burt, you're great and all, but you're an engineer, you're not even a scientist.
Harrison H. Schmitt, Apollo 17 astronaut and former U.S. senator: With a background in geology. That's not climate. It's close, and a lot of geology departments are transitioning over to Earth Systems Science departments that do include things like climate, but Schmitt has a hardcore rocks-and-fossils kind of background.
Nir Shaviv, professor of astrophysics, Hebrew University, Jerusalem: Hey, astrophysics! That's closer than a lot of these other guys. Unfortunately, he's the dude who's been really pushing the solar variation theory of climate change, which has been shot down repeatedly.
Henk Tennekes, former director, Royal Dutch Meteorological Service: Well, he hasn't been the director of the RDMS for a long time, but I can't find out when he left. He also rejected the use of computers in medium-range weather forecasting, and supported this decision by referring to the Bible.
Antonio Zichichi, president of the World Federation of Scientists, Geneva: He's a nuclear physicist! Awesome! But not climate!
In summary: There are “scientists” in here but not with the qualifications that would indicate a peculiar knowledge of climatology. On the spectrum from forerunners of climate science through peripheral ideologues, these individuals would seem to all fall decidedly towards the latter end.