Earth's energy budget refers to the tracking of how much energy is flowing into and out of the Earth's climate, where the energy is going, and if the energy in balances with the energy out. Understanding the Earth's energy budget can help to predict future effects of global warming, as well as helping to understand flows of energy on the Earth. Additionally, knowing how Earth's energy budget balances can provide insight into how the energy from the Sun interacts with the atmosphere - specifically with the greenhouse gases in the atmosphere - to create conditions that make the Earth habitable. For the energy budget to balance, all that needs to occur is:
The two major components that must be investigated to determine if the Earth's energy budget balances is the incoming energy from the Sun and the outgoing infrared radiation from the Earth and its atmosphere. Looking more closely at the interactions that occur in the atmosphere and on the surface of the Earth can help to understand more of how the Earth's energy budget balances by investigating the energy flows. Figure 1 shows the current understanding of how energy flows on the Earth look in broad terms.
Earth's energy budget is vital in establishing the Earth's climate. When the energy budget balances, the temperature on the Earth stays relatively constant, with no overall increase or decrease in average temperature. The energy coming in to the Earth comes from the Sun, and over the surface of the planet this incoming radiation has a rate of transport of . A thorough explanation of how this value is determined can be found here.
Not all of this energy reaches the Earths atmosphere or surface however, as some is reflected by clouds or the atmosphere. The energy that does pass through is absorbed by the atmosphere or the surface, and then moves around through convection, evaporation, or in the form of latent heat. Finally, when the energy exits the Earth it can do so by emission from the surface of the Earth, by clouds, or by the atmosphere. Some of the energy that is radiated by the surface of the Earth is absorbed by clouds and greenhouse gases in the atmosphere and then re-emitted downwards, which is how the surface of the Earth is heated and kept at a habitable temperature. This process of heating is known as the greenhouse effect. Overall, the energy that exits the Earth in different forms, when added together is equal to the energy that is absorbed by different parts of the Earth.
The incoming energy to the Earth and the outgoing energy from the Earth do not actually balance. This imbalance is partially caused by the incoming energy from the Sun - which varies with the seasons - but due also to changes in the composition of the Earth's atmosphere. Changes in the composition of Earth's atmosphere changes the values of energy absorbed and reflected by the atmosphere, shown in Figure 1. Changing factors such as these result in a very small, but significant energy imbalance on the Earth.
As human activities increase the amount of carbon dioxide in the atmosphere, the energy imbalance continues to grow. Today, the energy imbalance amounts to approximately more energy incoming than is leaving the Earth. Compared to flow values in the hundreds of watts per meter squared, this imbalance seems negligible. However, to account for this imbalance, the Earth's temperature will increase in response. As well, since the amounts of carbon dioxide and other greenhouse gases in our atmosphere are increasing, this value is projected to increase at a rate of per decade, contributing even more to increasing temperatures. It is this imbalance in the energy budget that results in increasing temperatures on the Earth, one of the most significant effects of climate change.