Here's his response -- quoted with permission -- not exactly reassuring:
There is no unique fingerprint of AGW. Since it is a "radiative forcing" (radiative energy imbalance of the climate system), it cannot be distinguished from circulation-induced changes such as a decrease in albedo of the Earth (e.g. a decrease in cloud cover) or an increase in atmospheric water vapor or high cloud cover
The AGW crowd either do not believe these changes occur, or they do not know enough to realize that is what they are implicitly assuming.
Some will claim stratospheric cooling IS a signature of increasing CO2, which indeed is possible, but the stratosphere is much simpler in behavior than the troposphere, where clouds and other moist processes have such a huge influence.
So, I do not believe there is any way to recognize AGW...short of global temperatures increasing another 5 deg. C. That would probably make even me a believer. :-)
It has occurred to me that it is possible this debate -- and all the ancillary junk that goes along with it -- will never end. Think about it: suppose we drastically refurbish society and get all of our future energy from renewable, non-carbon sources. It's the year 2080 and temperatures are the same as they are today.
Advocates will argue that it was the switch to non-C energy sources that saved the day. Skeptics will argue that CO2 was not such a big problem all along and we spend $5T for nothing.
Hopefully 2080's science will be advanced enough to distinguish between these two scenarios. But I'm not sure that's a given.
(PS: If you're reading this in the year 2080 AD, PLEASE leave a comment.)