 Earth's energy budget is the balance between the amount of energy coming into the Earth's system and the amount of energy moving out of the Earth's system. This energy budget is complicated, and there are a lot of things that affect how much energy goes in and how much energy goes out. For Earth's energy budget to be stable, the incoming energy and the outgoing energy must be equal. In a previous video, we talked about some of the factors that determine how much energy gets to us from the Sun. In this video, we'll be talking about what happens after solar energy strikes the Earth or parts of the Earth's atmosphere. We'll also be talking about how human activity is changing the Earth's energy balance. So what happens to the incoming energy from the Sun when it reaches the Earth? Well, there are a few things that can happen to it. The energy can be reflected back into space. The energy can be absorbed by the Earth or the clouds or the atmosphere. The energy can be moved around from one of Earth's spheres to another. Or the energy that has been absorbed by the atmosphere or the water or the land can be re-emitted as infrared radiation. Let's take a look at some of these in more detail. This diagram from NASA gives us a nice overview, and this is where we'll get some of our numbers from. Note, however, that the exact percentages of incoming and outgoing radiation vary a little bit from textbook to textbook. Let's start with the Sun, which is transmitting energy to the Earth. About 30% of this energy is reflected right back into space, and we can break it down like this. About 6% of this energy is reflected by gases and particles in the Earth's atmosphere. Another 20% is reflected back by clouds, and about 4% is reflected back by the Earth's surface, specifically by bright surfaces on the ground, like sea ice and snow and desert sand. The fraction of light reflected by a surface is called albedo. Light-colored surfaces like clouds and snow and deserts reflect back a lot of light. But dark-colored surfaces like the ocean and forests tend to absorb light. The amount of sunlight reflected back into space depends a lot on how much ice and snow there is, as well as how much cloud cover there is because areas with a lot of clouds reflect a lot of light. This is going to be very important when we talk about feedback loops in a future lesson. So if 30% of the incoming light is reflected, what happens to the rest of the light? Well, about 16% of the energy that reaches the Earth is absorbed by gases, dust, and particles in the atmosphere. Another 3% is absorbed by clouds. The remaining 51% is absorbed at the Earth's surface, and by this we mean both the land and the water, including the oceans. When energy is absorbed by a substance, it makes the particles in that substance move faster, and the temperature increases. Now, if the land and the oceans and the atmosphere could only absorb energy, we'd have a big problem. The Earth would just keep getting hotter and hotter and hotter. Fortunately, there are a few ways that the energy that is not reflected can leave both the Earth's surface and the atmosphere so that it can radiate back into space and keep Earth's energy budget balanced. About 23% of the energy absorbed by Earth's surface is moved to the atmosphere through the evaporation of water, and this also includes transpiration of water from trees and other plants. Evaporation is an endothermic process, which means that water needs to take in energy in order to evaporate. This energy is solar energy that was absorbed, especially energy that was absorbed by water particles in the ocean. After it evaporates, water vapor condenses into liquid water droplets in the clouds. Condensation is an exothermic process, and energy is released into the atmosphere. Another 7% of the absorbed solar energy leaves the surface through convection. The surface gets heated and it transfers energy to the air right above it, and the air rises. And this is what fuels the global circulation and winds, but that's a story for another lesson. The remaining 15% of the incoming solar energy is radiated back as infrared radiation. We can't see infrared radiation, but we can feel it as heat. Although some of this infrared radiation goes right back into space, a lot of it is absorbed by the atmosphere, especially by greenhouse gases. Greenhouse gases can radiate this energy back towards the Earth, and this is one of the things that keeps our planet a lot warmer than it would be if there were no greenhouse gases. We'll talk about how greenhouse gases work in another video. Now, when we're talking about Earth's energy budget, for the most part, energy comes in as ultraviolet and visible light, and it goes back out as infrared radiation. As long as the same amount of energy is coming in as is going out, the Earth's energy budget stays balanced. But one problem we're facing right now is that some human activities, especially the burning of fossil fuels, are shifting the balance of Earth's energy budget. If there's more energy coming into the Earth's system than there is going back out into space, this will result in the Earth getting warmer. A recent study by NASA and NOAA has shown that this is happening, and if we don't work together to slow this rate of change, this will not be good for the future of our planet. In summary, the Earth's energy budget is the balance of energy coming into and going out of the Earth system. Of the energy that comes from the Sun, about 30% is reflected back into space. Of the remaining 70% that is absorbed by the atmosphere, clouds, land and ocean, this is eventually radiated back into space as infrared radiation. As long as the energy budget is balanced, the Earth's temperature will remain stable. But if something alters this balance, the Earth could get warmer or cooler.