(Continued) The “3°C” you mention depends not only on CO2 concentration but also on atmospheric water vapour acting as a POSITIVE feedback amplifier mechanism. Without an assumed POSITIVE feedback amplifier mechanism atmospheric CO2, on its own, cannot produce the 3°C you quote at current concentration levels or with future projected increases in atmospheric CO2.

“Climate sensitivity can not be as high as the IPCC range because otherwise we would have had runaway climate change at the end of the last ice age. This is exactly backwards.”

No, I said “by giving dominance to the water vapour feedback mechanism” (upon which climate sensitivity related to CO2 forcing is based) then “at the end of the last glaciation, when atmospheric CO2 was increasing, we would expect to see runaway climate change – this clearly did not occur.”

“It was 5K colder at the last Glacial Maximum with CO2 concentrations half what they are to-day.”

At that time atmospheric CO2 concentrations were around 180-200ppm so the CO2 warming component contributed something around the 5K figure you quote.

“If sensitivity was as low as you suggest, there is no way that there could have been an ice age. The fact that we have ice ages is one of the strongest pieces of evidence that climate sensitivity is around the IPCC value.”

I do not suggest sensitivity per se, only the increasingly diminishing contribution of atmospheric CO2 relative to concentration, which around 200ppm is a few degrees while a doubling from the current approximate 380ppm to 760ppm is around 0.12°C. Climate sensitivity due to CO2 forcing of the order you quote (3°C) is based on the atmospheric CO2 concentration’s relationship to temperature in ADDITION to there being a POSITIVE feedback amplifier mechanism – it is not just atmospheric CO2 concentration on its own.

I should have added the “but” Raredog.

The concentration of CO2 in the atmosphere is not saturated and will continue to absorb (and irradiate) energy as its concentration increases, albeit at a diminishing rate - as both you and Graham state.

It is the change (and rate of change) and the relationship between "temperature change" and "radiative forcing" that most people don’t seem to fully grasp.

http://folk.uio.no/gunnarmy/paper/myhre_grl98.pdf

http://www.ipcc.ch/publications_and_data/ar4/wg1/en/ch2s2-2.html

As you say; water vapour is "assumed" to be a positive feedback mechanism - I would add for very good reasons.

Can you provide a robust reason why it should not, or why water vapour should be considered a forcing, rather than a feedback?

You may also find these helpful:

http://www.giss.nasa.gov/research/news/20101014/

http://pubs.giss.nasa.gov/docs/2010/2010_Schmidt_etal_1.pdf

Cheers

The link was just emailed to me. Some people with an aversion to the site won't even go there:

http://tinyurl.com/more-on-feedbacks

They should if they are a real "sceptic", agnostic even.

Thanks Bonmot and Raredog for these edifying exchanges. But Bonmot, your last link doesn't appear to work.

I would just add that whatever the outcome of the atmospheric Co2 debate, the acidification of the world's oceans, and their saturation point as carbon sinks, is surely every bit as catastrophic as warming, even if it does stay below three degrees.

Squeers, tinyurl site was down, now up - try again.

"Catastrophic" warming won't happen anytime soon, this is taken way out of context by "alarmists" and by those most to gain from the FUD, the "sceptics".

The oceans are becoming more acidic, they are not acidic per se and are extremely unlikely to become acidic, ever. Nevertheless, the "acidification" is having serious repercussions at the bottom of the ocean's food chain - that in itself is cause for concern.

3 degrees C per doubling [CO2] is worrisome, it will take a while (time doesn't stop at 2100). It is going to be hard enough to limit it to 2 degrees this century - we should try. In any case, it won't happen overnight, CO2 is a long lived greenhouse gas..