1: Global mean temperatures have increased 0.5C-0.7C since 1900.
2: CO2 has increased from about 280ppm to 383ppm since pre-industrial times
3: Human CO2 emissions were about 24 billion metric tons in 2002
4: 24 billion tonnes would be about 0.8% of the 2.996×10^12 tonnes of CO2 in the atmosphere, or about a 383 ppm * 0.008 = 3ppm annual increase
5. The actual annual increase in atmospheric CO2 is about 2ppm, less than #4, so a “CO2 following temperature” can probably be ruled out for the current trend.
6: All other variables being fixed, every doubling of CO2 concentration would increase global temperatures by a fixed amount. (aka saturation of the infrared absorption of the CO2)
7. Because of #2 and #6, some part of the temperature increase in 1 should be caused by CO2 forcing.
8: Climate feedbacks can reduce or increase the affect of any given temperature forcing
9: According to Hansen et al in 2003 (missing my citation, sorry), the CO2*forcing is 3 degrees C per doubling of CO2 concentration
10: At the rate in #9 and a 3ppm rate of change (higher than current levels) in CO2 levels, in 2157 CO2 will be 832ppm and the temperature delta would be 4.7C.
11: The temperature delta in #10 would cause significant changes to ecosystems, food supplies, and economies.
12: Major changes are usually catastrophic, QED.
IMO, The so called “consensus” of climate scientists covers items #1-8 above. The part that starts to get really really iffy is item #9. Hansen himself revises his figure every few years, and has rather large error bars on it. Also, if Hansen’s numbers were correct for historical data, we would have seen a temperature delta of 1.35C, where we have only seen 0.5-0.7C—and that is ignoring that temperature does indeed correlate to solar forcing very well for about half of that temperature increase.
The only non-debatable issue is that there is some temperature increase due to human CO2 emission. The magnitude of CO2’s historical contribution to temperature is debatable, and the magnitude of future CO2 emissions is a step above guesswork (maybe two steps). Add to that the question of whether warming is bad in the first place (which is probably a function of how much warming we’re talking about, which is guesswork), and the question of what we can do about it anyway (not much without nuclear or draconian measures to change our energy consumption), and I think I can safely say that debate is still called for here. (emph. added)