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One of the weakest points in the anthropogenic global warming argument is the heavy reliance on positive feedback assumptions.  CO₂ is a weak greenhouse gas, and can be projected to cause rapid, catastrophic warming only if we assume that it will increase water vapor, which is turn is a much stronger greenhouse gas.  The problem is that there is little evidence that the positive feedback mechanism exists, and even some reason to suppose that the feedback may be negative.  New evidence from NASA's water vapor project highlights the uncertainty:

Climate models predict upper atmosphere moistening which triples the greenhouse effect from man-made carbon dioxide emissions.  The new satellite data from the NASA water vapor project shows declining upper atmosphere water vapor during the period 1988 to 2001. . . .  The cooling effect of the water vapor changes on OLR [outgoing longwave radiation] is 16 times greater than the warming effect of CO₂ during the 1990 to 2001 period.  Radiosonde data shows that upper atmosphere water vapor declines with warming. . . .  Both satellite data and radiosonde data confirm the absence of any tropical upper atmosphere temperature amplification, contrary to IPCC theory.  Four independent data sets demonstrate that the IPCC theory is wrong.  CO₂ does not cause significant global warming.