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Ecology 110 – Water Vapour, January 6, 2058
Occasionally I am caught flat-footed by questions from students. It is one of the hidden benefits of teaching — being kept on your toes. Theories of education differ, but generally I aim for the broad middle band. I know there are some impatient, high percentile types who think my lectures slow and pedantic. Just as there is a segment of determined plodders who get through more by hard work than brainpower. Between them ranges the middle group who will understand ideas and data, if you present them in a clear, concrete manner.
I was giving the Ecology 110 class a quick update on the Group 6 cloudmakers. The ironic fleet is how I think of them — the old oil tankers outfitted with wind and solar power to sail the polar regions creating clouds.
With a generic description of water and the hydrosphere, I started. “One of the trickiest topics in meteorology is water vapour. Students are often amazed when I have them calculate the amount of water present in a fluffy white cloud. The thought of tons of water floating overhead can be disconcerting at first.”
“Just look at the wealth of terms we use to describe this substance: water vapour, steam, fog, mist, dew, hoar frost, drizzle, ice pellets, rain, snow, ice. I’m sure you can think of others. The properties of this peculiar and beguiling material are central to climatology, perhaps even to life itself. Our cells are, after all, mainly bags of water.
“Let’s take a quick overview of the hydrosphere.” I put up a diagram of the water cycle showing the major repositories of water in the environment with the statistics listed at the bottom. Oceans, fresh water, glaciers, groundwater, lakes, river and the atmosphere.
“You will note the entire hydrosphere consists of some 1.36 x 10^9 cubic kilometers of water. Converted to weight that is 1.36 x 10^21 kg. To put that into context, the mass of the entire planet is 5.97 x 10^24 kg.
“For our purposes, consider only the water in the atmosphere. At any given time, some 1.3 x 10^4 cubic km. of water permeates the atmosphere. It is a dynamic process with water evaporating and precipitation falling somewhere around the world continually.
“What is the average lifetime of a water molecule in the atmosphere?”
I put up a diagram showing the lifetime curve. “The average is ten days, with some 97% precipitating between three and twenty-five days.
“And what is the behaviour of water vapour in the atmosphere?
“Water vapour is a strong greenhouse gas. However, H2O cannot be treated like a simple gaseous molecule, because it is more complicated than that. Water has relatively high melting and boiling points, so it exists naturally in our environment in the gaseous, liquid and solid states. Water that evaporates is liable to fall as snow and snow will change the percentage of light reflected by the earth, ie. the albedo. Further water is a primary constituent of clouds which have yet more complicated effects. Clouds will both reflect incoming sunlight and hold in heat coming from the earth.
“And what is the other constituent of clouds I hear you ask. On what does the water condense? Dust, pollen, particulates — generically referred to as aerosols. These aerosols complicate the picture by sometimes absorbing, sometimes reflecting radiant energy themselves. And further sometimes aerosols trigger precipitation. You may have heard the term ‘cloud seeding’.
“Because of all these different modes, water vapour’s behaviour is complex and difficult to capture in a model and definitely not to be treated as a simple one dimensional forcing like other GHGs which are purely gaseous in terrestrial conditions.”
I had got that far, and I was starting to describe how the presence of salt in the sprayed water made the clouds brighter and more reflective, when one of my new students put up his hand.
“I don’t understand why, if water is a greenhouse gas, you want to put more of it in the atmosphere,” he said.
It stopped me cold for a second. Where to begin? I wasn’t sure where he was coming from, so I started asking qualifying questions.
“You understand about albedo?” I asked.
My questioner nodded his head.
“Okay, what is the albedo of a fluffy white cloud?”
He shook his head.
“Anyone?” I raised my eyes to the take in the entire class.
“0.8 to 0.9,” called out one of the bright impatient students near the front.
“And what is the albedo of ocean water?” I asked.
Again he shook his head.
“0.05 to 0.3 depending upon local conditions,” said the same guy at the front.
“What conditions?” I asked.
“The angle of the light, the presence of bubbles and froth, the waviness of the water.”
“Okay,” I said, “For the sake of argument, let’s take the low number, 0.8 for the clouds and a median number 0.2 for open water.”
“Now the other effect to consider is the absorbtion of incoming light energy in the atmosphere. What proportion is absorbed there?”
No one had an answer for that, so I had to fill in. “It is about 19%. Now of that 19% about 55% is due to water vapour. Other GHGs such as carbon dioxide and methane absorb the rest. That means that the water vapour is absorbing about 10.5% of the energy.
“The effect of raising the humidity is minor, because even at relatively low humidity the absorbtion spectrum is well populated. At most, the effect will be to increase that percentage to about 12%.
“Now let’s unpack this. The insolation, that is the incoming solar radiation, varies with the distance from the equator. Let us for the sake of argument say the insolation in the area where the ships sail is 350 watts per square meter [w/m2].
“On the one hand, we have clouds which will reflect 80% or 280 w/m2 versus ocean water which will reflect 20% or 70 w/m2 . A diffference of 210 w/m2 . While on the other hand we have the absorbtion of the atmosphere which will rise from 10.5% or 36.7 w/m2 to 12% or 42 w/m2. A difference of 5.3 w/m2.”
I looked right at my questionner. “Does this make sense to you?”
I got a dubious nod from him. He clearly had reservations.
It was getting to be toward the end of the class, but I didn’t want him to feel I was fobbing him off.
“Look, we’re running out of time. We can talk about this after class if you like. For those of you who may be interested in following up this line of inquiry, I would recommend Simior et al. 2054 and McCarty et al. 2047 from the reading list.”
There were noises from the hall as other students bustled about between classes.
“Good questions,” I said looking at my diligent student. The class broke up quickly when I said “Until next time.”
My questionner did not stick around.
I sighed in relief. I had got through the entire explanation without once intimating how useless I thought the whole enterprise.
Excerpted from _The Bottleneck Years_ by H.E. Taylor
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Last modified September 17, 2013