The balance between heat entering and leaving an entity is its heat budget. This term can be used in many contexts, but often refers specifically to the Earth, which receives heat from the sun and loses heat in a variety of ways to maintain equilibrium. If the Earth lost more than it gained, it would cool down, and if it retained heat, it would warm up. Some researchers are interested in the heat budget of specific areas of the Earth, like the oceans, because it plays an important role in the climate.
Much of the Earth's heat comes from insolation, the incoming radiation from the sun. Some of this bounces off before reaching the surface of the planet, where it is absorbed by air, water, and soil. Over the course of the year, incoming insolation can vary by region because the Earth moves in space. There are also secondary sources of heat at work, like biological and chemical processes on Earth that generate heat. Radioactive decay, for example, produces warmth.
Heat doesn't remain in place when it hits the Earth; if it did, some areas of the Earth would get extremely hot, while others would be quite cold. It circulates through a variety of mechanisms. The oceans, for example, radiate the heat they collect, which warms the air above them. Warmed air masses start to move, bringing warmth with them. The circulation of these air masses can create a variety of weather conditions which distribute the heat and allow it to slowly radiate back out into space.
Imbalances from region to region equalize over time with the circulation of heat through the atmosphere. Researchers with an interest in climate science use the heat budget in their calculations and projections to learn more about current weather patterns and to predict what may emerge in the future. As researchers learn more, sometimes they disprove previous hypotheses, or uncover new information about the origins of the Earth's climate. The Earth's heat budget was not always perfectly balanced, and this shifted as conditions changed and contributed to the formation of the atmopshere.
Numerous tools are available to measure incoming heat, sources of warmth on Earth, and various means for the planet to lose heat to keep conditions stable. Heat budget research can include satellite observations from space as well as lab experiments and observations to learn more about physical, chemical, and biological processes associated with heat. Researchers can also use computer modeling for activities like predicting what could happen as the area of the polar ice caps decreases.