Given the IR absorption spectra of CO2 and H2O, why is it that we are concerned about CO2?

CO2 here:

H2O here:

Notice that CO2 has a relatively small absorbance in two regions of the spectra (4.2 um and ~14 - 16 um). Whereas, water absorbs across a broad portion of the IR spectrum with a particularly large spike at about 3.1 um.

Wouldn't this dictate that most IR would be absorbed by H2O, except that at 4.2 um? Especially given the fact that the atmosphere has far more H2O than CO2.

Based on this simple data, it looks like CO2 should not be much of a concern given its relative scarcity and its relatively weak absorption profile in comparison to H2O.

What is the IR emission profile of the earth? Is it evenly distributed through all wavelengths, or are there regions of greater emission that coincide with CO2 absorption lines?

What am I missing? Why CO2?

Well, I hope you can wade through some of the "stuff" here.
Nonsense such as

computer models have "fully interactive" clouds

water vapor is not a significant factor in heating because of precipitation

water vapor as feedback to anthropogenic greenhouse gases.

But to answer your question, yes you are noticing what many stubbornly refuse to acknowledge - that water vapor has a MUCH greater potential for absorbing infrared than does CO2. Not only does it have a broader absorption spectra and exist in tremendously higher concentrations as you pointed out, it has a higher specific heat than does CO2. Combine this with the scientific principle that the IR emissions are more or less at the same wavelengths as absorption AND 75% of the Earth's surface is thus emitting at the optimal absorption spectrum of water, then you may become even more suspicious of some of the claims pointing to CO2 as THE culprit.

But, you're not the first to raise questions. Here is an interesting piece:

The range of Earth's IR emissions are put at 5um - 60um, relegating CO2's effective absorption band at 14-16 um, well within the absorption range of water.

And to address the "nonsense":

"Fully interactive" clouds is as scientifically meaningful as the "new and improved" claims of household cleanser manufacturers. For one, atmospheric physicists and meteorologists have been working on understanding clouds for far longer than climatologists. But we are to believe that the knowledge of cloud dynamics has mysteriously been imparted on climatologists like some Promethean gift? Why have they not shared their algorithms with meteorologists? And why has there been no significant shift in global warming predictions from the time when models DID NOT incorporate clouds? Are we to believe that they made a lucky guess and got it right the first time.

As long as there is a net global increase in temperatures, then precipitation has no effect on the overall water vapor content of the atmosphere. For every amount of water that precipitates, cooling the Earth, you would have have an equal or greater increase of evaporation elsewhere...otherwise you would have net cooling. You can't have it both ways. If anything, increased rates of precipitation when the overall concentration in the atmosphere is increasing, results in decreased atmospheric CO2, since the gas readily dissolves in the rain (that's why rainwater is acidic relative to boiled water).

Water vapor will act as a feedback to ANY temperature increase regardless of cause - including any forcing it creates independent of all other forcings except the Sun.
Good question! The problem is that there is a water circulation system in place on Earth to both put water into (through evaporation) and get water out of (through precipitation) the atmosphere. The equivalent system for CO2 relies on organic processes to pull CO2 out of the air and keep it out (in the form of biomass / fossil fuel deposits / etc.) Once you take CO2 out of wherever it's hiding, it takes a long time for it to get re-deposited again.

In sum: the quantity of H2O in the air is roughly stable, and most of the water in the world moves through the water cycle. C02 is far more variable, and thus more of a concern.
Because the natural "water cycle" reacts rapidly to remove excess water vapor from the air by precipitation, keeping it constant.

The natural "carbon cycle" reacts much more slowly and we're overwhelming by burning huge amounts of fossil fuels.

Look at this graph.

The little squiggles are nature doing its' thing. CO2 falls a bit during summer when plants are active, and rises during the winter. The huge increase is us, burning fossil fuels. The scientists can actually show that the increased CO2 in the air comes from burning fossil fuels by using "isotopic ratios" to identify that CO2. The natural carbon cycle buried carbon in fossil fuels over a very long time, little bit by little bit. We dig them up and burn them, real fast. That's a problem.

Man is upsetting the balance of nature. We need to fix that.

More here:
Water vapor is the biggest contributor to the greenhouse effect. However, atmospheric water vapor concentrations are dependent on atmospheric temperature.

Current state-of-the-art climate models include fully interactive clouds. They show that an increase in atmospheric temperature caused by the greenhouse effect due to anthropogenic gases will in turn lead to an increase in the water vapor content of the troposphere, with approximately constant relative humidity. The increased water vapor in turn leads to an increase in the greenhouse effect and thus a further increase in temperature; the increase in temperature leads to still further increase in atmospheric water vapor; and the feedback cycle continues until equilibrium is reached. Thus water vapor acts as a positive feedback to the forcing provided by human-released greenhouse gases such as CO2
You are right on the money. The whole issue is strictly political. If the libs have a issue and convince enough morons, they win elections. Face it, the only gas causing global warming comes from the likes of green piece.,, and their choir.
I'm assuming the reason CO2 is more of a concern than the extra water in the atmosphere precipitates back down to earth rather sooner than the CO2, so doesn't accumulate. CO2 is extracted from the atmosphere by biological organisms and doesn't just rain or condense back down to earth.

Anyway, I'm not convinced that global warming is or isn't due to man. The earth has always naturally gone through climate changes. There have been many ice ages come and go before man ever existed.
As usual, the stuff you quote only shows the ignorance of the people who posted it.

Here's why--what these idiots you've been gullible enough to take seriously don't grasp is the basic concept of a system in equilibrium. For the global temperature, this means there are a variety of components--water vabor, solar enrgy input, CO2, etc. All in a balance at which a temperature equilibrium was established.

Change oneof the components--any of them--and you change the equiblibrium. In this case, all the other components have held essentially constant (including solar, just to forstall that straw man arguement). The only changed factor is a substantial increase in CO2-.

And--as long as we keep adding CO2 to the atmosphere, we keep changingthe mix--so the equilibriumtemperature keeps climbing.

I know scientists don't usually explain this so explicitly in scientific reports. That's ibecause they are writing with the assumption that their audience (primarily other scientists, engineers, and policy makers) aready know these simple basics--so they don't waste the reaader's time repeating waht any educated person already knows.
The water cycle is much faster than the CO2 cycle, water goes up, it rains, water comes down; cloud cover changes constantly. CO2 goes up and hangs around up there unseen for a century.

Of course the warmer it gets the more cloud cover forms. Due to their colour clouds also act to reflect some of the sun's radiation as well as trapping some of the heat; so although H2O vapour acts as a green house gas, it also prevents some radiation from reaching the biosphere.

CO2 doesn't reflect much of the incoming radiation, but it does reflect the lower wavelength heat back to the Earth's surface.
Why indeed?

This fact is one of the principle reasons why I’m sceptical about the whole issue. The simple fact is, no one can answer the fundamental question;

By how much can the temperature be expected to rise as a result of a given additional amount of greenhouse gas in the atmosphere?

It has been established by laboratory experiment that increased CO2 concentrations can cause additional scattering of outgoing longwave radiation at the tropopause, but not at or near the surface, and only at the fringes of one of the three principal absorption bands of CO2.

This would suggest that you are correct in your statement “it looks like CO2 should not be much of a concern given its relative scarcity and its relatively weak absorption profile in comparison to H2O.”

dana, above, claims that…

“Current state-of-the-art climate models include fully interactive clouds.”

Firstly, the phrase “state-of-the-art” is meaningless. If the current *state* of the art is ‘a bit rubbish’, as is the case with climate models, then a “state-of-the-art” climate model is also ‘a bit rubbish’. Remember, these models, with a minor tweak to one of their thousands of parameters, will show cooling!

And as for their “fully interactive clouds”, the IPCC states in Chapter 8 of its WG1 report (the part that dealt with climate prediction and climate models)…

“…it is not yet possible to determine which estimates of the climate change cloud feedbacks are the most reliable”

So are these “fully interactive clouds” reliable? The IPCC doesn’t think so.

Given this uncertainty, I am of the opinion that it is far too early to start spending billions on fixing a problem that we’re not even certain *is* a problem. If the Global Warming Alarmists wish to spend *their* money, then they can go ahead – it’s a free country after all – but they’ll have to be a lot more certain about all this before they’ll get me to willingly part with mine.

As ever with global warming - don't believe the hype.

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