In his testimony last week NCAR’s Jim Hurrell made an interesting comment, “… it should be recognized that mitigation actions taken now mainly have benefits 50 years and beyond now.” I wrote here last week that, “This point would seem to be generally appreciated by experts in climate science and policy but is generally lost in the more general debate.” But after talking to some folks about my post, it seems that this point is worth some further examination.

So let me pose a few questions to the climate science community. (And in particular, if the RealClimate folks would take a crack at it I’d appreciate it.) These are not economics questions or policy questions, but questions of climate science with relevance to consideration of policy options. And if these questions are dealt with by the IPCC or in the published literature, I am unaware of it, so pointers to references are also appreciated.

(a) If anthropogenic CO2 emissions we instantaneously halted on 31 December 2005, how long would it be before the scientific community could detect and attribute a discernable influence of this policy on the climate system? Specifically, please use the following four variables: atmospheric CO2 concentrations, global average temperature, sea level rise, and extreme weather events (heat waves, hurricanes, floods, etc.).

And there are some obvious permutations also worth considering, such as for example (b) halting CO2 emissions in 2020, (b) cutting CO2 to 1990 levels by 2012, and (c) cutting global CO2 emissions by 50% by 2030.

All replies welcomed, Thanks!