Hello, in an earlier video, we learned about greenhouse gases, water vapor, carbon dioxide, methane, and nitrous oxide. In this video, we will learn how nature plays a role in either increasing or decreasing the amount of greenhouse gases in the atmosphere. We will look specifically at the carbon cycle and what we call sources and sinks. Nature plays a role in the greenhouse effects, because trees, soil, and oceans all release greenhouse gases into the atmosphere. Nature also stores elements or compounds of greenhouse gases in different parts of the earth. Any process or activity that releases a greenhouse gas into the atmosphere is called a source. Let's look at some of the natural sources of carbon dioxide. One source is animal and plant respiration. What's that? Well, for animals and people that's breathing. We take in oxygen and we breathe out carbon dioxide. For plants, it's not breathing the same way humans do, but it does involve taking in oxygen and letting out carbon dioxide in order to release stored energy. Another natural source of carbon dioxide to the atmosphere is soil respiration and decomposition. What do we mean by soil respiration or soil breathing? Well, when we mention soil respiration, the dirt itself doesn't breathe, but all the living things in the dirt do, things like plant roots, or tiny, little insects, bugs and bacteria. When they are alive and respire in the soil, we have soil respiration. When they die in the soil, they begin decomposition. To decompose is to be destroyed or broken down into simpler compounds by microbes, worms and insects. One of the products of decomposition is carbon dioxide. Animal and plant respiration account for almost 29% of carbon dioxide in the atmosphere. Soil respiration and decomposition also account for about 29% of the carbon dioxide in the atmosphere. But the largest natural source of carbon dioxide in the atmosphere, at about 43%, is the ocean atmosphere exchange. Exchange is when you give one thing and receive another in return for it. In the ocean atmosphere exchange, the oceans and the atmosphere continuously exchange carbon dioxide and oxygen with each other. That means they both give carbon dioxide and oxygen to each other and take carbon dioxide and oxygen from each other. In the oceans, like in the soil, a lot of carbon dioxide is produced from the plants and animals that live beneath the waters. So those are the natural sources of carbon dioxide. What are the sinks? A sink is something that absorbs or takes in the greenhouse gases from the atmosphere and stores it. One example of a sink would be trees absorbing carbon dioxide and then releasing oxygen. It is the reverse of respiration. We call this photosynthesis. As you can see, things can be sources and sinks. We mentioned earlier that trees and plants were sources. They are, they are both sources and sinks. They put some carbon dioxide into the atmosphere at night, but they absorb much more of it during the day. You can say the same thing about the oceans, they are both sources and sinks. As we explained in describing the ocean atmosphere exchange, the oceans not only produce some of the carbon dioxide that goes into the atmosphere, they also store a massive amount of carbon dioxide in the water in the plants that live in it. This exchange of carbon dioxide between the sources and sinks is called the carbon cycle. The climate that we and all other living things on Earth are now comfortable with is created in part by this exchange. But now experts say the climate is changing because humans are putting more carbon dioxide into the atmosphere than what occurs naturally between the sources and the sinks. We will learn that even small changes to these natural cycles can cause large impacts. In this video, we learned how nature plays a role in both increasing or decreasing the amount of greenhouse gases in the atmosphere. We specifically looked at the carbon cycle in what we call sources and sinks. In the next video, we will look at how the human activity of burning fossil fuels can threaten the balance of the carbon cycle.