Saturday, February 20, 2010

We Live On a Water Planet

I’m still working on that lecture on climate change.

I’ve been trying to track down the issues on water in the climate models. Water is funny stuff, and its funny thermodynamic properties are responsible for the kind of world we live in.

Washington has been suffering from solid, particulate water that will eventually melt into liquid. It’s a mixed blessing for us that water’s melting point is within the normal range of temperatures where most of us live. Its boiling point is considerably above, but if you have a heat source, water is a great heat transfer medium for cooking food and can become part of that food, as in soup.

Even when it’s not boiling, though, water vaporizes. In our dry Santa Fe climate, snow and ice disappear without ever melting. And Washingtonians know well how much water the air can hold on a hot day.

Water is a greenhouse gas, and there is more of it in the atmosphere than any other, including carbon dioxide. But, from what I read, it is not included in the climate models in the same way as carbon dioxide; it is a feedback rather than an input. I include the qualification because I have not yet found an explanation of how water is treated that makes sense to me.

I have concluded that water concentrations in air are not included as an input to the models because water is everywhere on earth and it is so volatile. By volatile, I mean that it rapidly establishes an equilibrium among its liquid, solid, and gaseous forms. So if you include temperature as an input along with water’s thermodynamic properties, the model will calculate the correct concentration of water in the atmosphere. When added carbon dioxide increases the temperature, the additional water in the atmosphere is automatically calculated.

That’s my bottom line. Let’s see what a few of the climate blogs and websites have to say.

Skeptical Science seems to be the closest to my interpretation:
Unlike external forcings such as CO2 which can be added to the atmosphere, the level of water vapour in the atmosphere is a function of temperature. Water vapour is brought into the atmosphere via evaporation - the rate depends on the temperature of the ocean and air, being governed by the Clausius-Clapeyron relation. If extra water is added to the atmosphere, it condenses and falls as rain or snow within a week or two. Similarly, if somehow moisture was sucked out of the atmosphere, evaporation would restore water vapour levels to 'normal levels' in short time.
A bit more here, but not substantially different.

Scholars and Rogues has a similar explanation.
Basically, water vapor is a more important greenhouse gas than CO2, but because CO2 will cause heating independently of water vapor, as man-made CO2 increases global heating, water vapor will increase too, boosting the amount of warming with a positive feedback loop. How much exactly is up for debate, and there’s not a long enough data series on water vapor in the atmosphere to know everything. But just because humans can’t increase or decrease water vapor in the air directly doesn’t mean that CO2 heating of the air won’t do so indirectly.
But they precede it with an explanation I find confusing.

RealClimate’s argument is impossible to summarize in a short quote. This seems to be the center of it:
While water vapour is indeed the most important greenhouse gas, the issue that makes it a feedback (rather than a forcing) is the relatively short residence time for water in the atmosphere (around 10 days). To demonstrate how quickly water reacts, I did a GCM experiment where I removed all the water in the atmosphere and waited to see how quickly it would fill up again (through evaporation from the ocean) . The result is shown in the figure. It’s not a very exciting graph because the atmosphere fills up very quickly. At Day 0 there is zero water, but after only 14 days, the water is back to 90% of its normal value, and after 50 days it’s back to within 1%.
This comes after a calculation of the percentage that the various greenhouse gases contribute by removing them from model calculations. If someone is skeptical about models, I don’t think that the way to prove anything to them is through model calculations.

I also don’t like the use of the word forcing. It doesn’t have an immediately obvious meaning to the general public; it means any factor that raises or lowers the earth’s temperature. I’m not sure where the word came from. I haven’t seen it in chemistry or physics. I’m usually pretty good at figuring out meanings from context, but I didn’t feel confident about this one until I found an explicit definition. It's not going to go away, though; too implanted in the modeling community.

I’m not convinced that my interpretation of why water vapor is treated as a feedback rather than an input is correct, although writing this post has made me more confident of my logic. And it doesn’t conflict with any of the explanations I’ve quoted.


MT said...

My sense is that a "forcing" input is incompressible or inelastic like a bowling ball dropped on a car. A weakly forcing input would be like dropping a sponge of the same mass. Input tons of water into the atmosphere and it will fall out as rain and be just drops in the ocean. That's not forcing. A ton of methane isn't falling out. About "feedback" I'm not sure what usage we're talking about, but obviously the water of the world is in cycle--evaporating from lake, sea and plant surfaces, then returning via rain and rivers, which invites use of the word.

Cheryl Rofer said...

Thanks, MT. I've had the same problems.

"Feedback" is in the sense of modeling, not the real world. You can treat the amount of water in the atmosphere as an input and specify it, or you can make it a function of temperature. The second is what the climate models do.

I thought about mentioning the coercive connotations of "forcing," which you have done. As far as I can tell from the definition I found (will have to check on where it was and link it), it's just any parameter that makes the earth's temperature go up or down.

So carbon dioxide is a forcing and so is water vapor and haze from volcanoes. I would be more comfortable with "parameter" or "input", but I'm not sure they would be any more generally understandable.

MT said...

I tend to trust Real Climate knows whereof they speak, and though I didn't read your link at first, I skimmed it now and this strikes me as the relevant pith--
"To first approximation, the water vapour adjusts to maintain constant relative humidity. It’s important to point out that this is a result of the models, not a built-in assumption. Since approximately constant relative humidity implies an increase in specific humidity for an increase in air temperatures, the total amount of water vapour will increase adding to the greenhouse trapping of long-wave radiation. This is the famed ‘water vapour feedback’. "

Same as I said, but adding that per simple linear relation between humidity and temperature, any ostensible increment in temperature that would result from an input of interest--say annual anthropogenic C02--yields a formulaic additional temperature increase from the solar heat gathering power of the extra water in the air. That's the feedback by which the temp change is bigger than what you'd see without water. Because water behaves in a formulaic way, it's hardwired into the model, so to speak, and not an input. There's so much liquid water on Earth, there's no need to keep tabs on it.

Cheryl Rofer said...

The problem I have with RealClimate and some of the other climate blogs is that they seem to start in the middle, assuming more than I know or that of course I could have no questions.

I trust they know whereof they speak, but I don't always understand what they're saying.

Cheryl Rofer said...

Here's a definition of "forcings" from RealClimate: anything that is imposed from the outside that causes a model’s climate to change.

Phila said...

Would it make sense to you, as a chemist, to deemphasize forcing vs. feedback in favor of "synergy"? Or would that just be muddying the waters further?

No opinion on my's just what came to mind while reading your post.