Question

The rate of solar energy striking Earth averages 169 watts per square meter. The rate of energy radiated from Earth's surface averages 390 watts per square meter. Comparing these numbers, one might expect that the planet would cool quickly, yet it does not. Why not?

Short answer

Earth does not cool quickly despite the difference between incoming solar energy (169 W/m²) and outgoing radiated energy (390 W/m²) due to the presence of its atmosphere and the greenhouse effect. Greenhouse gases absorb and reemit some of the outgoing infrared radiation, reflecting it back to the surface, which compensates for the energy rate difference and maintains Earth's energy balance. This energy equilibrium helps in sustaining Earth's relatively stable temperature.

Step-by-step solution

Understanding the energy equilibrium

Earth's temperature remains relatively stable because it is in a state of energy balance. Energy equilibrium occurs when the incoming solar energy (169 watts per square meter) is balanced by the outgoing energy radiated from Earth's surface (390 watts per square meter). If there were no processes in place to help achieve this balance, the Earth would indeed cool quickly.

Role of the atmosphere and the greenhouse effect

The main reason why Earth does not cool quickly is due to the presence of its atmosphere and the greenhouse effect. The greenhouse effect helps maintain Earth's energy balance by trapping a portion of the outgoing energy radiated from Earth's surface. Greenhouse gases such as carbon dioxide, water vapor, and methane present in the atmosphere absorb and reemit some of the outgoing infrared radiation. This reemitted energy is then partially reflected back to the surface, keeping it warmer than it would be without the atmosphere. This process compensates for the difference between incoming and outgoing energy rates.

Energy balance explanation

The numbers we are given, 169 watts per square meter for incoming solar energy and 390 watts per square meter for outgoing radiated energy, represent average values. In actuality, the incoming and outgoing energy rates vary depending on location, time of day, and season. Even though these numbers appear to show an imbalance, the overall energy balance for the Earth is maintained through the greenhouse effect. It implies that on a global scale, the 169 watts per square meter of solar energy absorbed by the Earth must be equal to the 390 watts per square meter of energy radiated from the Earth's surface, after taking into account the portion that is trapped and reemitted by the greenhouse effect. In conclusion, Earth does not cool quickly because of the processes in place, such as the atmosphere and the greenhouse effect, that maintain its energy balance. This energy equilibrium prevents the Earth from cooling too rapidly and helps maintain its relatively stable temperature.