 In this lecture, we'll be looking at the greenhouse gas water vapor. It's actually one of the reasons why our climate is so sensitive to small changes. Water vapor amplifies a small amount of warming and makes it a big warming. Let's explore why. Water vapor and carbon dioxide are both greenhouse gases, meaning they both absorb Earth's outgoing heat, raising global average temperatures through a blanketing effect called the greenhouse effect. For example, maybe you've experienced water vapor as a greenhouse gas because on a humid summer night, temperatures stay hot because the water vapor doesn't allow the heat to escape. Now, there's a lot of water vapor in the air, but the amount varies based on location and height. Carbon dioxide concentrations are small, only making up about 0.04% of the air. Carbon dioxide is emitted through burning fossil fuels and deforestation, both caused by humans, while water vapor comes from evaporation of liquid water off of bodies of water, mainly the oceans. The amount of evaporation depends on the global temperature. A warmer Earth has a more humid atmosphere. The warmer it gets, the more evaporation off the surface of the ocean, putting more water vapor into the atmosphere. Another reason for higher humidity is that warmer air can hold more water than cold air. As the globe warms, evaporation from the oceans causes water vapor to build up. But water vapor is a greenhouse gas, so this excess water vapor causes the atmosphere to warm even further. This allows even more water to evaporate into the atmosphere, causing an even further warming due to a stronger greenhouse effect. This is an amplifying feedback, a self-reinforcing loop. In fact, this feedback is the most important of many fascinating feedbacks in our climate system. It takes a small warming, amplifies it, and makes it a big warming. Water vapor feedback play a big part in making the Earth's climate sensitive to small changes. If the global climate and the global average temperatures stayed relatively the same, the water vapor feedback wouldn't be kicked into gear. But because we're increasing carbon dioxide from burning fossil fuels and deforestation, the Earth is getting slightly warmer, kicking this feedback into playing a large role in the climate system. It's like having a skateboarder at the top of a hill. Let's call him water vapor. He's sitting comfortably at the peak, stably sitting steadily at rest in equilibrium. His buddy, let's call him carbon dioxide, comes along and gives him a little nudge. Well, this gets him rolling down the hill faster and faster as he goes down. Without carbon dioxide pushing water vapor just a little bit and getting him started down the hill, he would have stayed comfortably at rest at the top of the hill. This is how amplifying feedbacks work. They amplify changes once they are set into motion. The same thing happens in the atmosphere. Carbon dioxide acts as the trigger for the water vapor feedback. Humans don't control how much water vapor is in the atmosphere. That's up to the weather. We can control carbon dioxide concentrations, though. And by letting our carbon dioxide levels get high enough to warm the atmosphere, we are therefore responsible for triggering the water vapor feedback, amplifying the warming from carbon dioxide. If carbon dioxide warms the atmosphere one degree Celsius, then water vapor can cause the temperature to go up another one degree. But some climate myths take this science out of context. Let's look at an example. One myth related to water vapor claims, forget carbon dioxide, water vapor is the most important greenhouse gas. It controls the earth's temperature. This implies if water vapor is a strong greenhouse gas, and we have it in large concentrations in the atmosphere, then adding a small amount of the greenhouse gas carbon dioxide can't make a big difference in the global average temperatures. The fallacy here is jumping to false conclusions. It is true that water vapor is a greenhouse gas, but that doesn't mean that carbon dioxide is not. It ignores the fact that they play very different roles in the climate system. In other words, this myth is trying to use information about water vapor to distract you to imply that an increase in carbon dioxide isn't a problem. It also oversimplifies the science. But now you know that water vapor doesn't control the earth's temperature, but instead is controlled by earth's temperature. Carbon dioxide is important because it controls the earth's thermostat. If carbon dioxide levels go up, the temperature gets warmer. As a consequence, water vapor levels increase. Water vapor is the most dominant greenhouse gas, but the only way we can keep levels under control is by keeping temperatures low through lowering our carbon dioxide emissions.