Clearing up some confusion over Hansen 1988

A while back, commenter "Nick" made some assertions in the comments to this post about James Hansen's famous 1988 paper that I didn't have specific responses too. Some good recaps elsewhere make that possible. Nick says:

"We take the Scenario A prediction that is in line with what has happened with non human causes for climate change and CO2 output."

This is a little confused, but Scenario A was meant to show exponential growth in emissions, as opposed to Scenario B's more accurate linear growth. "Scenario A, since it is exponential, must eventually be on the high side of reality."

"Well, there are reasons for not using scenario B. Hansen put in a volcanic erruption to show its effects. Over the post Pinatobu period there has not been a major erruption. Scenario B therefore has a downward bias that makes the prediction look better than it should. (The erruption that didn't happen would have depressed temperatures.)"

Hansen in 1988 assumed a major volcanic eruption in 1995, and one actually happened in 1991. In other words, his assumption doesn't bias the outcome (it would have if he assumed two major eruptions when only one occurred).

"The claims are still being made that we have an escalating CO2 problem. That's scenario A."

No. Scenario B also had escalating CO2. Only Scenario C didn't.

More info at RealClimate.