moltenmetal - I fully understand the biological carbon cycle. Perhaps my example is a little tongue-in-cheek. However, where do you draw the line with how long said carbon has been sequestered? Less than 1 year for most food? How about burning biomass? Only if it is less than X years old? What is X? Is 100 years too much, or more...
With respect to the idea of risk and risk mitigation, I do this for a living, so I understand the concept fully. You take an examination of the harm, apply a cost to it. Then, you determine the probability of said harm occurring in a particular span of time. Multiply the harm by the probability and you get the risk-based cost of the harm. Now, compare that to the cost of mitigation. Then, divide the mitigation with the probability that the mitigation will succeed in reducing the harm by the stated amount. If the adjusted cost of mitigation is lower than the cost reduction in the harm, then mitigate. If the cost of mitigation is higher, do nothing.
In regards to CAGW, I have found the assessment of the harms to be greatly overstated, while the benefits to be greatly understated. And it is the algebraic sum of the two that determines the cost of the harm. Next, the probability of both the harm actually occurring (do we really believe the 100-year computer models showing the increasing temperature with increasing CO2 content, absent any other natural causes) and the probability of the severity of the harm being at the stated quantity need to be considered. In my opinion, we are nowhere near anything reasonable to even try to quantify these quantities. Compare that with the cost of mitigation - a carbon tax, in your example. The costs will be well understood. However, and this is where I go toe-to-toe with carbon-tax enthusiasts, what is the probability that the mitigation will actually reduce the harms? If I recall correctly, the number that I have calculated in the past, based on a carbon tax of $50/tonne, when based on the emissions of Canada, was valued $36.7 BILLION dollars per year, which will have an impact of 0.000306°C/year. That values the global temperature at $122.876 TRILLION per degrees Celsius. And, that's if you actually believe the IPCC sensitivity values... Are the harms really that expensive?
Like you said, we're engineers. We don't deal in hand-waving, but real numbers. I can provide the back-up calculations for the above number, if you want. Until I have been shown that the cost of the harms (on a per °C basis) multiplied by the probability of the harms occurring is shown to be greater than the number that I have shown above, I will remain convinced that the mitigation is unnecessary.
Of course, we also need the causality to demonstrate that the proposed mitigation will actually work, too... Without that causal link, the cost of the mitigation divided by the probability of the success of the mitigation approaches infinity as the probability of the mitigation working approaches zero...
So, here's the gauntlet:
1) What is the total cost of the sum of the harms due to AGW?
2) What is the total cost of the benefits due to AGW?
3) What is the algebraic sum of 1) and 2) above?
4) What is the probability of those harms actually occurring in a specified time?
5) What is the adjusted cost of the harms?
6) What is the probability of the proposed mitigation (carbon tax) actually reducing CO2 emissions and total atmospheric CO2 content?
7) What is the probability that the above-noted reduction in CO2 emissions and total atmospheric content will reduce the temperature?