Just ran across this and thought I'd pass it on:
http://www.realclimate.org/index.php...n-attribution/
"How do we know what caused climate to change – or even if anything did?
This is a central question with respect to recent temperature trends, but of course it is much more general and applies to a whole range of climate changes over all time scales. Judging from comments we receive here and discussions elsewhere on the web, there is a fair amount of confusion about how this process works and what can (and cannot) be said with confidence. For instance, many people appear to (incorrectly) think that attribution is just based on a naive correlation of the global mean temperature, or that it is impossible to do unless a change is ‘unprecedented’ or that the answers are based on our lack of imagination about other causes.
In fact the process is more sophisticated than these misconceptions imply and I’ll go over the main issues below. But the executive summary is this:
* You can’t do attribution based only on statistics
* Attribution has nothing to do with something being “unprecedented”
* You always need a model of some sort
* The more distinct the fingerprint of a particular cause is, the easier it is to detect
......"
(see link for complete article)