But now, idly googling for "Spencer and Christy" I came across S+C's report for 1978-2003. It will probably all have to be revised when the vn5.2 data finally becomes "official", but therein they offer this:
That cold air has very little water vapor in it, so if you add another greenhouse gas you have an opportunity to trap more heat. When you go to the tropics, where there’s lots of water vapor, the extra carbon dioxide doesn’t have as much effect. As a greenhouse gas, carbon dioxide’s greatest effect is in the driest, coldest places
I rather suspect this is wurble, with no analysis behind it: its an explanation in search of an effect. But its its interesting to see someone who (presumably) knows something about atmospheric radiation putting it forward. By contrast, Keith Shine (who very definitely does know about radiation) didn't recognise the idea (pers. comm.).
Another interesting feature of that report is that the second major headline of it is "The atmosphere is warming" in big letters. Oddly enough, the std.spetics don't seem to have picked up on that... in fact, I sometimes wonder if S+C have noticed, either...