[MUSIC] The water vapor feedback is a positive feedback that has an important impact on Earth's climate with rising CO2 concentrations. In fact, if it weren't for the water vapor feedback, it really wouldn't be as much of a global warming problem at all. The water vapor feedback something like doubles the sensitivity of the Earth to changes in greenhouse gas concentrations. At the heart of it, there's actually a negative feedback that controls the amount of water vapor in the atmosphere. So the state variable here is the water vapor, and if the water vapor concentration is perturbed by some external thing, like if you run a garden sprinkler, it changes the rate of evaporation someplace else, or else it just rains out someplace else, because you put more water in the air. So the hydrological cycle leads to a negative feedback that stabilizes the amount of water vapor in the air. That's why running a garden sprinkler doesn't lead to global warming. [SOUND] But the vapor leads to a positive feedback up rating on a state variable now of temperature. So the way this works is that if you have some (UNKNOWN) to the temperature, say by making the sun brighter or adding more greenhouse gases, it changes how much water vapor you want to hold in equilibrium. It redefines 100% relative humidity essentially. And so, if you warm things up by some external means that leads to an increase in the amount of vapor pressure in the in the air, and then water as you know is a greenhouse gas, because it's got three atoms, you know it's a greenhouse gas and it's asymmetrical. So having more water vapor pressure means that there's more greenhouse gas forcing to warm the Earth even further. [SOUND] So the way this could go is you add some greenhouse gas, or you make the sun warmer, and then that makes the temperature go up by itself, which causes the amount of water vapor in the air to go up, because the warmer air can hold more water vapor moving in this direction on that pressure curve. And then, that's a greenhouse gas so that causes the temperature to go up even more, it amplifies the temperature. So you may wonder you have this loop of cause and effect. You may wonder where does this end? Well, the answer can be seen on a diagram such as this one, which is a phase diagram for water. So it's got dimensions of temperature and pressure of water vapor here. And then, it has these different regions on the graph. So up here, where the temperature is fairly high and there's a lot of water vapor, you can have liquid plus vapor. If you use to have a lot of water vapor pressure but it's very cold, you're gonna have vapor plus ice. Or if the vapor pressure is very low, then you may not have any other phase other than vapor. You'll only have vapor in this region of the phase diagram. So we're gonna do sort of a thought experiment. Where we start out a planet and we'll say, let's not put any water vapor in the atmosphere yet. And we'll just have it be dry. And then, we'll calculate what the temperature that planet would be. And so, for Earth that would lead us to a point sort of right here. But now, if its got liquid water it's going to evaporate, and once the liquid water is gonna wanna evaporate until it starts to be along this line here. So as the water vapor evaporates it causes the temperature of the planet to rise, because water vapor is a greenhouse gas. But ultimately, this trajectory hits the phase boundary to liquid. And at this point, it rains. And so, the water vapor feedback is limited on Earth by intersecting the phase boundary for liquid water. The hydrological cycle is stabilizing, and it limits the extent of the water vapor feedback. If we back to this one, this is Mars, it starts out colder and you add water vapor and it goes up. And maybe it hits the phase diagram, phase boundary of ice plus vapor. So on Mars it would snow. But again, the water vapor feedback would be limited by this phase boundary. Now, Venus is a different story. Venus originally had water. It doesn't anymore, because Venus suffered a runaway greenhouse effect. If you start from a condition that's warmer, and you raised the water vapor concentration and raised the temperature, it can curve over and miss the phase boundary for liquid or solid entirely, and this is called a runaway greenhouse effect. So all of the water on the planet would go into the atmosphere. It would all boil. [SOUND] And this is a one-way street. If this ever happens on a planet, it never can go back, because if you get too much water vapor up in the upper atmosphere where it's exposed to the ultraviolet light from the Sun, this ultraviolet light can split the water molecule into hydrogens and oxygens. And these hydrogens are so small, they travel quickly with their allotted kinetic energy just put into that small mass they go very fast, and so they can be lost to space. So a runaway greenhouse effect is the end of a biosphere for a planet. So looking at a picture of Earth from space is always astonishing to me, because the atmosphere is so thin. And the ocean is just right there, spread out through the whole surface of the planet. And what keeps this from happening on Earth is sort of a cold trap. The fact that it gets so much colder in the upper atmosphere, the water vapor can't get through it. It all rains or snows out. So it's like this very, very thin layer of cold air that has protected and guarded all of Earths of water for these billions of years, it looks so fragile to me looking at a picture of Earth from space. [MUSIC]
