Water vapor feedback

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Water vapor is responsible for more greenhouse heat trapping on Earth than CO2 is, and yet your impression is that global warming is primarily about CO2, not water vapor.

Water Phase Diagram Mars

Vapor

Fig. 7.2 A phase diagram for water demonstrating that the water vapor feedback on Earth and Mars is limited, while Venus is free to have a runaway greenhouse effect.

Vapor

Fig. 7.2 A phase diagram for water demonstrating that the water vapor feedback on Earth and Mars is limited, while Venus is free to have a runaway greenhouse effect.

No one screams about global warming when you run your lawn sprinkler. Why not? The answer is that, at some given temperature, the amount of water vapor is controlled by negative feedbacks with the rate of evaporation and with rain. Our lawn sprinkler may be depleting our fresh water supply, but it is not going to lead to global warming because any water we drive into the atmosphere will just rain out next week. A negative feedback stabilizes the water vapor concentration at any given temperature (Fig. 7.1d).

The positive feedback arises when we start to think about water vapor affecting a state variable of temperature (Fig. 7.1c). An initial increase in temperature will allow more water to evaporate (Fig. 5.6). Water vapor is a greenhouse gas, so its increase tends to warm the planet still further.

Where does this end, you may wonder to yourself. Does a positive feedback ever stop, or does it explode? The water vapor feedback can in principle; this scenario is called a runaway greenhouse effect. It happened on Venus, but we are in no immediate danger of it happening on Earth. The easiest way to visualize the water vapor feedback stopping or not stopping is to draw water vapor feedback trajectories on a phase diagram for water (Fig. 7.2). The phases of water are solid, liquid, and vapor. The phase diagram for water shows us which phase or phases of water you will find, if you subject water to some pressure and temperature. At high temperature and low pressure, you expect to find vapor. Vapor likes it hot. As you decrease the temperature and increase the pressure, you get liquid, and when it gets cold enough you get ice. On the boundaries between the regions you get two phases, like vapor and water, or ice and water. At the triple point, a certain pressure and temperature combination, you get all three phases coexisting together, a glass of boiling ice water. Of course, you can throw ice cubes in a pot of boiling water on your stove any time you like, but the ice will quickly melt because it is not at equilibrium. The phase plot in Fig. 7.2 represents equilibrium conditions.

To understand the water vapor feedback, we are going to imagine what happens if we suddenly introduce water to a planet that initially had no water. The starting position on

Fig. 7.2 is all the way at the bottom of the figure, where the pressure of water vapor in the air would be low. From this starting point, what will happen is that water will evaporate, and the vapor pressure in the atmosphere will rise. Water vapor is a greenhouse gas, so as the water vapor content of the atmosphere increases, the temperature warms. Therefore, as the condition of the planet moves upward on the figure (higher water vapor pressure), it also moves somewhat to the right (higher temperature).

The middle curve on Fig. 7.2, labeled Earth, moves up and to the right until it intersects the stability field of water. At this point, the atmosphere is holding as much water vapor as it can carry. Any further evaporation just makes it rain. The water vapor feedback has increased the temperature of the planet above what it would have been if it were dry, but the positive feedback did not run away as it was limited by a tendency to rain. The curve on the left on Fig. 7.2 represents something like the situation on Mars. Here the water vapor feedback path intersects the stability field of ice. The water vapor feedback does not lead to a runaway greenhouse effect on Mars or Earth because the feedback is limited by the stability field of water or ice.

We only see the runaway greenhouse effect in the path on the right, labeled Venus. If water were introduced on Venus it would evaporate, increasing the temperature, as shown. The difference here is that this path never intersects the stability fields of either liquid or solid water. Planetary scientists presume that Venus originally had about as much water as Earth, but the high solar heat flux associated with orbiting so close to the Sun forced that water to evaporate into the atmosphere, rather than condense into oceans as it has on Earth.

Evaporation of a planet's water is a one-way street because if water vapor reaches the upper atmosphere, its chemical bonds will get blown apart by the intense ultraviolet (UV) light in the upper atmosphere. The hydrogen atoms, once they are disconnected from oxygen, are small enough and light enough that they are able to escape into space. The water is lost for good. This is the presumed fate of Venus' water, but Earth has retained its water because the air is so cold in the tropopause, the layer of atmosphere that separates the troposphere from the stratosphere (Fig. 5.1). The troposphere contains lots of water, but the stratosphere is dry because of the cold trap of the tropopause. The oceans, viewed from space, look heartbreakingly vulnerable, but they are protected, and apparently have been for billions of years, by a thin layer of air. Marvelous!

Theoretically, if the Sun were to get hotter, or if CO2 concentrations were high enough, the Earth could move to the right on Fig. 7.2, sufficiently far that it could escape the liquid water stability field and hence run away. But don't worry, there is not enough fossil fuel carbon on Earth to do this. The Sun is heating up over geologic time, but only very gradually, and the runaway greenhouse effect from this is nothing to worry about for billions of years. Interestingly, if the equator were isolated from the poles, blocked from any heat transport and forced to balance its energy fluxes using outgoing IR only, the tropics would be a runaway greenhouse. It is a lucky thing that we have the high latitudes to act as cooling fins!

The role that the water vapor feedback plays in the global warming forecast can be drawn on Fig. 7.2, although global warming climate change is tiny compared with Venus' climate, so I've drawn it horribly exaggerated so we can see it. An initial

Low High relative humidity Low

Low High relative humidity Low

Dry Rain Dry

Fig. 7.3 The Hadley circulation and its effect on the humidity of the air in the troposphere.

Dry Rain Dry

Fig. 7.3 The Hadley circulation and its effect on the humidity of the air in the troposphere.

temperature perturbation, say from fossil fuel CO2 release, moves the water vapor feedback path a bit toward the right, toward Venus. The path ends as it hits the liquid saturation line, as for the unperturbed Earth but at a higher temperature. The temperature difference between the two raining, moist Earths (where they intersect liquid water) is greater than the temperature difference between the two dry Earths (at the bottom of the plot). The water vapor feedback amplifies the temperature change that you would get from increasing CO2 on a dry planet.

On the real Earth, mercifully, the relative humidity is not everywhere 100%. One mechanism which controls a lot of the atmospheric water vapor franchise is called Hadley circulation (Fig. 7.3). Warm air at the equator rises convectively. Water condenses as the air rises and cools. This column of air has a lot of water vapor in it, in general. The air spreads out at high altitude, then begins to subside in the subtropics, about 30° latitude north and south. That air has been through the wringer, the cold tropopause, and there is not much water vapor left in it. The great deserts of the world are located under these dry air blowers. The air flows back equatorward along the surface, picking up water vapor as it goes. Globally, the average humidity of surface air is about 80%.

One could imagine changing the winds, the Hadley circulation for example, and changing the humidity of the surface air. Maybe if the Hadley circulation were stronger, the surface air would dry out a bit. Could a mechanism like this counteract the warming effect of rising CO2? This is a difficult question to answer for sure because it hinges on issues ofturbulence and fluid flow, which are impossible to model perfectly (Chapter 6). Models predictions tend to agree, however, that the amount of water vapor in the atmosphere tends to increase with warming from rising CO2 because warm air holds more water vapor than cold (Fig. 5.6). The water vapor feedback is predicted to be extremely important for future climate change, doubling or tripling the warming that we would expect if Earth were a dry planet. If it weren't for the water vapor feedback, maybe there would be no need to worry about rising CO2 concentrations!

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