Steve wrote:
One more attempt at clarification: do you believe the AGW argument?
I am not convinced that the observed increase in global temperature is caused by the observed increase in atmospheric carbon dioxide. But I am not able to exclude the possibility that it is.
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Do you agree your airport analogy is irrelevant? If not, why not and how is my reasoning false?
I introduce the analogy to counter jomouk's argument that the amount of man made carbon dioxide being introduced into the atmosphere is small compared with the natural equilibrium amount and thus cannot be responsible for a change in climate. The analogy is relevant to that argument and that argument alone. It certainly doesn't address your point that temperatures have been lower with higher carbon dioxide equilibriums, nor was it intended to
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- If the facts around the said stumbling block is true, does it cast serious doubt on a rapidly approaching AGW "tipping point"?
AIUI, and I am happy to be corrected if I am wrong, the "tipping point" would be the rapidly increasing positive feedback which would occur if the global temperature rose to the point where methane held in frozen ground was released. I give no credence to any aargument that there is a tipping point determined by the amount of carbon dioxide in the atmosphere
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- Does the concentration of atmospheric water vapour (a much stronger contributor to the 'greenhouse' effect) also naturally fluctuate?
- Do you agree the relative effect will be large even if that fluctuation of water vapur is relatively small (compared to CO2)?
The water vapour concentration does fluctuate but I think that the average level must be substantially constant rather than continually rising like the carbon dioxide. But I don't have a reference for that yet.
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- Is the other main emission of fossil fuel burning was water vapour? So why has water vapour been discounted?
I don't know. That must go to the top of my reading list
This is from the Wikipedia article on Greenhouse Gases
"Water vapor accounts for the largest percentage of the greenhouse effect, between 36% and 66% for water vapor alone, and between 66% and 85% when factoring in clouds.[8] However, the warming due to the greenhouse effect of cloud cover is, at least in part, mitigated by the change in the Earth's albedo. According to NASA, "The overall effect of all clouds together is that the Earth's surface is cooler than it would be if the atmosphere had no clouds." (cf. NASA Clouds and Radiation) Water vapor concentrations fluctuate regionally, but human activity does not significantly affect water vapor concentrations except at local scales, such as near irrigated fields. According to the Environmental Health Center of the National Safety Council, water vapor constitutes as much as 2% of the atmosphere.[31]
The Clausius-Clapeyron relation establishes that air can hold more water vapor per unit volume when it warms. This and other basic principles indicate that warming associated with increased concentrations of the other greenhouse gases also will increase the concentration of water vapor.
When a warming trend results in effects that induce further warming, the process is referred to as a "positive feedback"; this amplifies the original warming. When the warming trend results in effects that induce cooling, the process is referred to as a "negative feedback"; this reduces the original warming. Because water vapor is a greenhouse gas and because warm air can hold more water vapor than cooler air, the primary positive feedback involves water vapor. This positive feedback does not result in runaway global warming because it is offset by other processes that induce negative feedbacks, which stabilizes average global temperatures. The primary negative feedback is the effect of temperature on emission of infrared radiation: as the temperature of a body increases, the emitted radiation increases with the fourth power of its absolute temperature.[32]
Other important considerations involve water vapor being the only greenhouse gas whose concentration is highly variable in space and time in the atmosphere and the only one that also exists in both liquid and solid phases, frequently changing to and from each of the three phases or existing in mixes. Such considerations include clouds themselves, air and water vapor density interactions when they are the same or different temperatures, the absorption and release of kinetic energy as water evaporates and condenses to and from vapor, and behaviors related to vapor partial pressure. For example, the release of latent heat by rain in the ITCZ drives atmospheric circulation, clouds vary atmospheric albedo levels, and the oceans provide evaporative cooling that modulates the greenhouse effect down from estimated 67 °C surface temperature.[5][33]"