There have been various attempts to survey scientists opinions about climate change. Wiki has an article on this: Scientific opinion on climate change. Check the recent history for another attempt… :-)
All of these attempts have various methodological problems which I’m sure you can think of for yourselves; but they also suffer from a structural problem which is what I’ll discuss here: which is: does it matter?
Of course in terms of public relations it does matter. People like to quote these things one way or the other, and learned societies put out statements either because they feel they ought to or in the spirit of public outreach or for some other reason. If public opinion went with the learned societies then there would be no doubt at all over GW (see the wiki article). This rather suggests that either the PR doesn’t work (or that it would be even worse without them?).
But is this the correct way to assess scientific opinion? I would say mostly no: what counts for science is what is published in the literature (and, I suppose, spoken at conferences etc; but any of that of any value get published eventually (unless you’re JA arguing against the freqentists, of course)). Therefore the correct way to survey “opinion” is not to ask people what they think but to read their papers. Oreskes attempted this with abstracts, and anyone wanting to do more of this should do the same, but more thoroughly.
There are some problems with this argument, of course.
One is that of timeliness: if things are fast-changing, the literature won’t reflect that. Arguably the possibilities of large sea level rise from polar ice sheets comes in this category. But if you are trying to assess agreement or otherwise with the IPCC, which is/should be largely based on pre-2006 literature, thats not so much of a problem.
Another is that you have to assume that the literature is “fair”. Some would say that it unfairly rejects “sceptic” material; others can just as plausibly argue that it unfairly priviledges speculative papers that disagree with the consensus.
But probably the main problem is that if the question you want to ask is The scientific basis for human impacts on climate is well represented by the IPCC WG1 report. The lead scientists know what they are doing. We are warming the planet, with CO2 as the main culprit. At least some of the forecast consequences of this change are based on robust evidence. then you can’t do it easily that way, since people don’t write papers that say this. Instead I suppose you have to pluck out various statements from the SPM and see if you can find people to disagree with them. For example:
The understanding of anthropogenic warming and cooling influences on climate has improved since the TAR, leading to very high confidence7 that the global average net effect of human activities since 1750 has been one of warming, with a radiative forcing of +1.6 [+0.6 to +2.4] W m-2
Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level
or (more controversially :-)
Palaeoclimatic information supports the interpretation that the warmth of the last half century is unusual in at least the previous 1,300 years.
All of those could be testing by looking for papers that disagree.
The other problem is the good old fashioned “science isn’t done by consensus” arguement. Which is true enough, but not very illuminating. You can only go with what we have. In an effort to think about that: Suppose, just at the start of the twentieth century you had attempted to decide scientific opinion on “is the universe essentially Newtonian”? what would you have found? Not being a historian of science, I’m not sure. Had you been a person of the time, likely you would have interpreted things one way and come to the conclusion that there were a few quibbles but most people did feel that way. Looking back now, you might well find all the papers that were beginning to find problems with the conventional classical interpretations. I’m not sure that helps one way or the other.
[Update: of course, you could try asking reality instead -W]