CO2 is not the only factor at play in global warming, but it determines, among other things, the concentration of water vapor in the atmosphere. CO2 is the trigger, and H2O is the bullet.
In any discussion about climate change, there is a definite chance that someone will ask, “Why CO2 only?” or, equivalently, “Why isn’t H2O taken into account in the warming mechanism?” The implication is that it is all a conspiracy of the PTB to demonize the oil industry and force us to freeze in darkness.
Now, it is true that the focus on CO2 in the public debate tends to obscure some of the many facets of the climate question. And it is also true that the biophysical effects of water vapor as a cooling agent are often misunderstood and go unmentioned in the discussion. That leads, sometimes, to silly proposals such as cutting forests to cool the planet.
But it is simply not true that water vapor is neglected in climate science. Not at all. It is just one of the many legends that surround the issue. If you want to delve into this matter, you may look at this review or this one, and you’ll see that the role of water vapor is known, amply considered, and extensively discussed.
I don’t claim to be a specialist in atmospheric physics, but I did my homework, and I think I can summarize the main points about the false dichotomy “CO2 vs. H2O.” I’ll mention only CO2 since it is the main non-condensable greenhouse gas. The others, mainly CH4, change little to the overall picture.
First, CO2 does basically ONE thing. It is a greenhouse gas that traps infrared radiation emitted from Earth’s surface and partly reflects it back, warming the atmosphere. This is the standard explanation of global warming.
Then, H2O vapor is no less important than CO2, and it does at least THREE things.
It is a greenhouse gas, and it traps heat just as CO2 does. It warms the atmosphere.
It forms clouds that may have warming and cooling effects by reflecting visible and infrared light. Overall, clouds are believed to have a modest overall cooling effect.
It transports heat from low to high atmospheric levels by a process of evaporation and condensation. The heat transported at high altitudes is then more easily beamed out to space. The result is atmospheric cooling. You may also say that water vapor makes the atmospheric “lapse rate” (how temperature varies with height) less steep.
A very important element of the CO2/H2O question is that water vapor, unlike CO2, is a condensable gas. Hence, the higher layers of the atmosphere act as a “cold trap” that causes H2O to condense and make the upper layers dry.
This is the situation as it is now and how it has been during the past few billion years. But if the atmosphere's temperature were to increase over a certain limit, then the cold trap effect could disappear. Then, the system would switch to a “Moist Greenhouse” state, with the atmosphere saturated with H2O vapor. Bad, because it would lead to a “Runaway Greenhouse” effect that would boil the oceans and turn Earth into a rocky desert. It is expected to happen in the future due to the gradual increase in solar irradiation. Fortunately, that won’t be soon. (or so it is believed, anyway…).
Another consequence of the condensable/non-condensable story is that the water vapor concentration in the atmosphere is determined mainly by temperature. Using the terminology of system dynamics, we can say that water vapor is not a “forcing” but a “feedback.” It means we cannot change the atmosphere’s temperature by pumping extra water vapor into it. The excess would simply condense and return in the form of rain in times of the order of a few days.
Then, if the water vapor concentration is controlled by temperature, it means that it is controlled by CO2 and, in a smaller measure, by other non-condensable gases. More CO2 means more atmospheric water vapor (the reverse doesn’t happen). It is an “enhancing feedback” effect that increases the warming effect that CO2 would have if it were alone in the atmosphere (or if there were only non-condensable gases in it).
There is much more than that. The matter is hugely interesting and fantastically complicated, but I think these are the main points to keep in mind when discussing the respective roles of CO2 and H2O.
In all cases, never forget that in complex systems there are no causes and no effects. There are only forcings and feedback. So, in the climate system, CO2 is the forcing and H2O is the feedback. In other words, CO2 is the trigger, and H2O is the bullet.
Thank you for this explanation. It makes sense on a global level, and my understanding is models have been accounting for this feedback for a long time. But the local and regional effects of water cycles still seem left out. For example, in the Western Mediterranean Basin, Millan Millan showed that, due to water-cycle damage, water vapor that should have been converted to rain in afternoon summer storms, instead recirculated back out over the Mediterranean Sea, creating a local greenhouse effect that warmed the water and supercharged storm in central Europe. The models "can't see it" was his steady lament.
When I began studies (biology) CO² was at 333ppm. Now it is about 440ppm.
Parts per million.
And we learned that the sun was primarily responsible for the energy contained in the atmosphere.
And, too, that CO² concentration in the air was supoptimal for plant growth, being thus one of the limiting factors. One reason for gassing greenhouses for bigger harvest in crops.
I don't think this knowledge has since changed a lot. Climate is ever changing. Globally and locally. Earth has seen oceans rising and falling, and were are still living in a warm period of an ice age. Cities have been submerged in the past and coastal lines have drastically changed.
This will happen again. Perhaps we'll see some tiny bit of it, and then we'll have to adapt.
And this is the gist of it all: we have to adapt to our planet earth, we can not adapt the earth to our biddings.