John, I think physicists (and climate scientist) often open themselves up for trouble by overstating confidence in areas that are either uncertain or outside their expertise. This erodes confidence so that the general public has less confidence when we do speak from expertise.
In this thread you were confident in proclaiming that 500 GT of carbon (about 50 years) will lead not just to some range of warming, but that it will lead to "utter catastrophe". Then you backpedal to "I am not an expert in this area" and give some plausible (but hardly certain) arguments.
*"The #1 most powerful greenhouse gas is water vapor."
More water also leads to more clouds, which is a cooling effect. Are you ready to confidently say which one is stronger?
*"There are already "dead zones" in the ocean"
Attributed to excessive nutrient pollution, not global warming.
* "Collapse of antarctic ice sheets may already have passed the tipping point. This process will play out over many decades"
1) You say "may". 2) this will occur over *centuries*, not decades.
* "There's a lot of frozen peat in the high latitudes. "
Yep. I agree with you there. That could be very important.
* "The last time there was a sudden warming, 75% of the large animal species died out. Do you really want to gamble that humans are going to be in the lucky 25%?"
The decline has been attributed to climate change & getting killed off by humans. Since people are adaptable and can live about anywhere, I am not worried about extinction due to climate change. I would worry more about extinction at the hands of humans.
* "The last 20 years of data is systematically /above/ the 100-year trend line,
which tells me the warming is most likely accelerating."
My quick analysis of the GISS data says 2 of the last 20 were below the 100 year trendline. another 3 were pretty close. Furthermore, for the last 30 years, there was a NEGATIVE acceleration (although only at the p=0.12 level). While CO2 levels were accelerating, the temperature rise was decelerating.
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It is getting late. The point is that there is a lot of important and complicated science involved that needs to be understood so that we as a species can make good decisions. This process is not helped by hyperbole.