GW Johnson wrote:Look, in the real world, there are a lot of effects going on. It's not linear cause and effect from one piece of physics. But one at a time, we can understand the effects of the pieces.
There is such a thing as the greenhouse effect, which you can demonstrate for yourself with glass bell jars, thermometers, and bottled gases on a sunny day outside. The bell jar with methane in it gets hottest, followed by carbon dioxide, followed by plain dry air as the coolest of the hot bell jars (yeah, any greenhouse gets hot!). The other two gases absorb-as-UV and re-emit-as-IR more energy than the oxygen-nitrogen mix that is air. Who cares exactly what the absorption science details are, when you see that effect? it is a very real effect. It says that in a period of climate warming (whatever else might be causing it), adding more CO2 or CH4 is the wrong thing to be doing. That's just plain old common sense from observations and elementary experiments.
Water vapor is the odd one. It's difficult to keep a pure steam atmosphere under that bell jar at ordinary conditions, but water vapor does appear to be the most potent greenhouse gas of all. Unlike the others, however, it also changes phase easily at ordinary conditions. This seems to play out in a way that makes water vapor an amplifier, not a source, of these greenhouse effects. Warm the climate (by whatever means), there's more humidity, which acts to warm the climate further. Cool the climate (by whatever means), there's less humidity, which acts to cool the climate further. Positive (unstable) feedback.
If you think that's complicated, try understanding the real weather and climate. I don't think anybody really understands either, but we can at least try. We won't succeed, but we can learn some things, at least. The computer models are only slightly better than looking out the window to predict the weather, after all. But, you do better if you do both! Engineers know that, and some (but not all) scientists, too.
Whatever is causing the glaciers to melt, some fertile lands to desertify, and sea levels to rise, I don't care. The real questions are (1) "what can we do to stave this off or at least buy some more time?", and (2) "how are we going to cope with massive changes like this when (not if) we fail?" Pessimism, now that's the engineer in me talking. Since the climate has been getting warm enough of late to cause desertification and sea level rise problems, it's clear we ought not do those things that make the problem worse! So, we cut back emitting carbon dioxide and methane as much as we can.
That's easy to say, but hard to do. I know. But until the climate starts cooling again, that's what we need to do. And, we need to figure out how to cope with massive human migrations when sea levels rise a meter to six meters. There's around a billion people living within a meter or two of sea level now, and around half of us live within 6 to 12 meters of current sea level. If we fail to stave off the land ice melting (which seems like a likely outcome), what are we going to do with billions on the march looking for new homes, right where the rest of us live? Do you smell a looming disaster in that scenario? I do!
Who cares what caused it? What do we do, that's the real question! Assigning blame and making decisions based on blame are pointless. Playing politics with this issue is a good way to commit cultural suicide, it looks like to me.
It seems like you've got a partial understanding of what i'm trying to say. You just aren't thinking it through far enough. According to theory, water vapor OUGHT to be a powerful and dominant positive feedback effect. As you pointed out, diffusion into the air increases with temperature. Increased humidity absorbs more energy, causing increased diffusion.
According to the greenhouse theory and the spectral absorption characteristics of water vapor, the planet ought to be a raging inferno like Venus. Why is it not? If water (making up 3/4ths of the earth surface) has positive feedback characteristics the planet temperature ought to be so high that life cannot exist.
The answer is simple. Water is NOT a Net positive feedback effect. It is a Net NEGATIVE feedback effect.
How is it a NEGATIVE feedback effect? Clouds. As humidity increases, cloud cover increases as well causing a greater reflectivity of solar radiation before it even has a chance to warm the lower atmosphere.
The whole thing is pretty simple really. Water vapor in air heats the atmosphere and increases diffusion. A positive feedback effect. Enough water in the air forms clouds which increases reflectivity of the planet and thereby lowering the temperature. A Negative Feedback effect.
Which effect dominates? The Negative effect. How do I know? Because we're alive, which we wouldn't be if the Positive effect dominated.
If anyone can punch a hole in that explanation, i'd like to see it.
As for those "other" greenhouse gases? They are inconsequential in comparison to water, both in terms of scope and scale.
Long live King Water Vapor.
