 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. In this video, we will be talking about the factors that affect how much energy comes into the Earth's system from the Sun. We'll also talk about how this energy affects climate. Most of the radiation coming into the Earth's system is in the form of visible light which we can see and ultraviolet light which we cannot see. This light hits the surface of the Earth as well as the atmosphere and the clouds and when it does it can get absorbed or reflected or re-emitted and we'll talk about some of these things in future videos. The Sun's brightness varies and the amount of energy that reaches our Earth's system can change over minutes to millions of years. There are several factors that can affect the energy coming from our Sun. Some of these can affect our long-term climate over tens of thousands of years but most of these natural variations are not responsible for the quickly rising global temperatures we've been seeing over the last several decades. Still, these things are important to know about if we want to understand Earth's energy budget as well as why these natural cycles in the energy we get from the Sun are not responsible for the recent increases in global temperature. First, let's talk about sunspots. Sunspots are dark areas on the Sun caused by increased magnetic activity. Sunspots occur in an 11-year cycle where the number decreases to a minimum or small amount and then gradually increases to a maximum or larger amount and then the number returns to a minimum. The Sun releases more energy when there are a lot of sunspots and less when there are fewer but when there is a solar maximum Sun's energy output increases by only about 0.15% and that's a tiny bit. Most scientists agree that these small fluctuations in the Sun's output cannot explain the large increase in global temperature that we are seeing today. Indeed, since the 1970s, the solar output has remained stable or has even been decreasing as we can see in this graph and yet our average global temperatures have continued to rapidly increase. So even if sunspots can affect climate, they cannot explain the climate change we are seeing today. Sunspots can affect the Sun's energy output over short periods of time. However, there are several other things that can affect its energy output over thousands of years. About a century ago, a Serbian scientist named Milutin Milankovic proposed that changes in Earth's position relative to the Sun can affect Earth's long-term climate over tens of thousands of years. We call these changes Milankovic cycles and there are three main ones you need to know about. First, the shape of the Earth's orbit changes over time and we call this eccentricity. Sometimes the orbit is more circular and sometimes it's more elliptical. This cycle is about 100,000 years long. Depending on the shape of the orbit and depending on whether the Earth is farther or closer to the Sun, the Earth gets more or less energy. When the orbit is circular, the seasons are milder. When the orbit is elliptical, the seasonal differences are greater. However, although eccentricity plays a role in long-term climate change, in other words, over tens of thousands of years, it cannot explain the rapid rise in average global temperature that we are seeing today. The second of these cycles relates to changes in the tilt of the Earth's axis. The angle of tilt ranges from 22.1 degrees to 24.5 degrees. We call this obliquity. When the Earth is more tilted, there tends to be more melting of glaciers and ice sheets and the difference in temperatures between summer and winter is greater than it is when the Earth is less tilted. This cycle of changes in Earth's tilt happens over a very long time, about 41,000 years, so it cannot be responsible for the rapid rise in global temperature that we see today. The third cycle relates to the changes in the Earth's wobble. The Earth wobbles on its axis of rotation, kind of like a spinning top does. We call this axial procession. This results in changes in where the Earth's axis is pointing. Sometimes the Northern Hemisphere points toward the Sun when it's farthest from the Sun in its orbit, and sometimes it points toward the Sun when it's closest to it. Depending on where the Earth's axis is pointing, this can determine whether seasons are milder or more extreme. The Earth's wobble changes over a cycle that's about 24,000 years long, so it can affect climate in the long term, but it's not responsible for the rapid changes in climate we are seeing today. In addition to the Milankovitch cycles, one more thing affects the amount of energy we get from the Sun, and that is that the Sun is gradually getting brighter. However, this is occurring over millions of years, much, much too slowly to be responsible for the temperature changes we are seeing today. In summary, the energy that enters the Earth system comes from the Sun. There are variations in how much solar energy the Earth gets. Some of these changes are short-term variations like sunspots, but there are also longer-term variations like the shape of the Earth's orbit, the tilt of the Earth's axis, and the way the Earth wobbles about its axis, as well as the fact that the Sun is gradually getting brighter. But none of these are thought to be responsible for the rapid increases in temperature that we are seeing today.