8.2.2. Heat Budget of the Planet Earth

The earth as a whole does not accumulate or lose heat. It maintains its temperature. This can happen only if the amount of heat received in the form of insolation equals the amount lost by the earth through terrestrial radiation.

The heat budget refers to the balance between the incoming solar energy received by the Earth and the energy that is released back into space. If the incoming and outgoing energy are equal, the Earth's temperature remains constant. Essentially, if the sun provides a certain amount of energy (100% insolation), some of that energy is reflected or absorbed as it passes through the atmosphere, and only the remaining amount reaches the Earth's surface. This balance is crucial for maintaining the overall climate and temperature.

You can think of the Earth's heat budget like a bathtub. If you fill the tub with water (incoming solar radiation) at the same rate that you drain it (terrestrial radiation), the water level (or temperature) will stay constant. If you add water faster than it drains, the level will rise (heating), and if it drains faster than you add, the level will fall (cooling).

As explained earlier, there are variations in the amount of radiation received at the earth’s surface. Some part of the earth has surplus radiation balance while the other part has deficit.

Different regions of the Earth receive varying amounts of radiation due to factors such as latitude and land-sea distribution. The idea here is that some areas (like the tropics) may receive more energy than they lose, resulting in a surplus, while polar regions may lose more energy than they gain, leading to a deficit. This imbalance in energy leads to the redistribution of heat across the planet, which is a significant driver of weather patterns and climate.

Imagine a store that sells ice cream in summer. If more ice cream is sold than received due to hot weather (surplus), there will be a rush to restock (redistribution of resources) from other parts of the store. Conversely, in winter, the store may have more ice cream than it can sell (deficit), leading to a need to lower prices or find other ways to balance the stock. Similarly, energy moves from areas of surplus (like the tropics) to areas of deficit (like the poles) to help balance the planet's temperatures.

The surplus heat energy from the tropics is redistributed polewards and as a result the tropics do not get progressively heated up due to the accumulation of excess heat or the high latitudes get permanently frozen due to excess deficit.

The Earth does not allow for permanent heating in tropical regions or permanent freezing in polar regions. This is due to the dynamic processes of wind and ocean currents which transport heat from the equator toward the poles. The mechanisms include atmospheric circulation patterns that move warm air from the tropics toward higher latitudes and ocean currents that redistribute thermal energy throughout the world's oceans.

Think about how warm water in a pot will rise and cooler water will sink—creating convection currents. Similarly, the warm air from hot regions rises and moves towards the cooler areas, distributing heat throughout the planet. Just like you wouldn't let a single area of soup boil over while leaving other parts cold, Earth's energy distribution helps prevent extreme heat or cold in any one region.

In summary, the total radiation returning from the earth and the atmosphere respectively is 17+48=65 units which balance the total of 65 units received from the sun. This is termed the heat budget or heat balance of the earth.

The figure mentioned indicates that there is a careful balance achieved between what is received and what is lost. A total of 100 units of solar energy is received, with 35 units being reflected and 65 units absorbed. Of the absorbed energy, 51 units are re-radiated as terrestrial radiation, leading to a balance of total energy—a concept known as the heat budget. When this budget is disrupted (for example, by an increase in greenhouse gases), it can lead to climate change.

Imagine a bank account where you deposit and withdraw money. If you deposit $100 into your account (insolation) and spend$65 (terrestrial radiation) while leaving $35 untouched (reflection), your account stays balanced. If you regularly spend more than you deposit, your account could go empty (akin to climate change), just as the Earth's heat budget can be unbalanced by excess heating or cooling.

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