The problem with assigning any level of probability for AGW is AGW isn't one thing its a whole range of things which I would assign probabilities ranging from close enough to 100% to practically be treated as certainty (the combination of the Earth has warmed in the last couple of centuries and human emission of CO2 has some impact on that) to close enough to zero percent to practically be totally ignored even by people prone to focus on remote risks (the most extreme alarmist claims like a runaway greenhouse effect making the Earth's climate like Venus's and doing so primarily because of human emission of CO2).
Even ignoring the essentially non-controversial, and the cray implausible sci-fi scenarios you still have a broad range. No its beyond that you have multiple ranges. Most global warming alarmists and many global warming skeptics want to boil it down to two positions you either "believe in global warming" or you don't. There are multiple questions you have to answer to define even a broad position here.
1 - How accurate is the past temperature record? There has been warming since the end of the little ice age, that is beyond all reasonable doubt but how much of that warming, and how much more recent warming is real, as opposed to being data adjustments to fit models, or mismeasurement of the initial data?
2 - How much of the warming is local, at the sites where the temperature is measured, from urban heat island effects and other local changes, and how much is really global.
3 - How much of global warming is because of human activity, and how much of that portion is from human CO2 emission?
4 - With the various positive and negative feedbacks on global warming how much extra warming are you going to get for each increase in CO2 concentration (also other green house gas emissions, and other activities causing warming besides gas emissions, but CO2 dominates most scenarios) beyond the warming that CO2 itself will cause (in direct terms it almost certainly won't be enough to deserve the level of attention it gets, if there is any sort of crisis it will be from positive feedback).
5 - To the extent human actions are going to increase global temperatures going forward by any particular amount (the amount determined partially by the direct effect of human action on tempertures, but more by the answer to question 4, at least if AGW actually is a serious concern), what will the positive and negative effects be. What will the balance between them be? If its negative (as is often assumed) how negative will it be?
6 - What can be done to reduce the warming and at what cost?
7 - What can be done to mitigate the impact of warming, and again at what cost?
8 - Is there any major random scenario entirely outside not just the models but the considerations that are thought of to put in to the models. For example are we going to be turning towards another ice age, with global warming possibly stopping or reducing it (or in at least one scenario that's in some of the models causing it).
-------
What will I give 95% certainty to? That the Earth has gotten warmer, that some portion of that change (ranging from insignificant to near total) was because of human activity, and that going forward human activity will put some upward pressure on temperatures (which may or may not be significant).
What would I give assess at 99+% probability? That climate has changed and will continue to change. |
