The ratio of carbon dioxide to air is running at about .033 parts per hundred.
I can’t seem to find a reference to the ratio of water vapour in air in the same terms, but I sense that under most conditions water vapour is way more prevalent in the atmosphere than carbon dixide. Both water vapour and carbon dioxide are green house gases.
So isn’t an increase in a miniscule component of the atmosphere of one green house gas insignificant when compared to the wild swings in concentration of a far more prevalent green house gas?
Nothing other than increasing temperature itself (which increases surface evaporation rates) can cause an increase in the amount of water vapor in the air, at least in the average over long periods of time. This is part of the ‘runaway greenhouse’ scenario; the increase in CO2 causes a slight increase in overall air temperature, which causes H2O evaporation and increases the water vapor %, which causes more of an increase in air temperature …
But which also results in an increase in cloud cover, which raises the albedo of the Earth and reflects more heat back into space, thus cooling the Earth…
Despite all the models and best guesses, I don’t think anyone really knows what feedback processes will occur as carbon dioxide levels increase.
Plants will bring water up from the ground via their root system where through various metabolic processes water vapor leaves the plants. The rate at which water in moist ground gets into the air can be greatly changed by plant cover. Unless the atmosphere has reached the limit of how much water vapor it can hold there are many things other than temperature that affect the amount of moister in the air.
The commonly accepted percentages of atmospheric gases- 78% nitrogen, 21% oxygen, 0.033% carbon dioxide, etc.-are a measurement of what we call “dry air.” This means those are the percentages of air if we completely ignored all of the water vapor. The amount of water vapor in the atmosphere is so incredibly variable that there is virtually no value to be determined that can be considered accurate for Earth’s atmosphere. For one, it is different everywhere- this is why some places have humid climates while others are dry. For two, it is inifinitely changing due to the hydrologic cycle. Water evaporates adding to the amount of water vapor, then condenses to form clouds. The amount of water vapor in our atmosphere is, by nature, eternally elusive. If you wanted to know the amount of water vapor present in a given location at a given point in time, you can figure it out by using the relative humidity and temperature from a local weather report.
The main thing about CO[sub]2[/sub] and H[sub]2[/sub]O is that the former stores a lot more energy per molecule than the latter, and is being steadily produced, so it’s a much bigger deal.
Not necessarily. A common misconception is that it will always rain if the air becomes saturated, i.e. a relative humidity of 100%, and that just isn’t true. The air generally becomes saturated in the evening when the temperature drops, which is how we get dew. Not to mention, the amount of precipitation that falls worldwide from year to year is not constant, so during sometimes the atmosphere definitely has more water vapor in it than it does at others. In a more “big picture” way what you say is true- the water from the oceans would not infinitely evaporate, at some point clouds would definitely form, and eventually precipitation. Another common misconception is that clouds are water vapor, and that’s not true either.
The air in proximity to the ground becomes saturated. A few hundred feet up, the air temperature is relatively stable, and saturation is less common. When it happens you have fog/drizzle.
I guess this begs the question: Why historically have global temperatures closely matched the CO2 concentration when H2O is the better IR absorber?
I can think of two possibilities. One is that historically, the global H2O concentration is not independent of the CO2 concentration. The other is that the global H2O concentration has not varied significantly.
