Balance of Energy on Earth Yumna Sarah Maria

The global energy balance is the balance between incoming energy from the sun and outgoing heat from the Earth. The main form of energy is heat.

It regulates the state of the Earth’s climate. Modifications, man-made or natural climate forcing, result in changes in the global climate.

The term “balance” suggests that the system is at equilibrium. This means that there is equal action between opposing forces. Equilibrium basically tells us: ENERGY IN = ENERGY OUT

If the amount of radiation entering the system is unequal to the amount leaving, the system would continually heat up or cool down. This changes the equation: ENERGY IN = ENERGY OUT + CHANGE IN STORED ENERGY

Not all of this energy is radiative (giving off radiation); some of it is sensible (capable of being perceived by the senses) and latent (not visible but present).

The Sun Sunlight is the source of energy for the Earth’s oceans, atmosphere, land and biosphere. Sun deposits 324 watts of energy into every square meter of the Earth.

A lot of the sun’s heat is focused into the tropics of the earth. Tilt of Earth’s axis is nearly perpendicular to plane of Earth’s orbit. If tilt was completely perpendicular, there would be no seasons (climate would stay moderately same throughout year.) Earth’s rotational axis tilts about 23.5 degrees away from perpendicular.

During one orbit of sun, the North Pole will be tilted at the angle (23.5 degrees towards the sun) and would result in sunlight 24 hours a day and vice versa. During the equinoxes, tilt is parallel to sun, thus allowing same amount of light. Radiation is at most at true equator.

The amount of solar heating for polar latitudes can fluctuate through the year. Summer receives most heat; winter receives almost no heat. The solar energy we see with our eyes heats up the planet as well as radiation emitted at longer wavelengths called thermal infrared radiation.

Thermal infrared radiation = solar energy we cannot see with our eyes. Shortest wavelengths of solar radiation are absorbed by molecules in upper and middle atmosphere. They are most dangerous to life on Earth.

An average of 49% of solar radiation striking earth is absorbed at surface. Atmosphere absorbs 20%. Remaining 30% is reflected back to space. Different surfaces have albedos. Refers to the percent of radiation returning from a surface compared to the incident radiation. Earth reflects 30% of solar radiation, so average is 0.3. Albedo = the amount of reflectivity and is represented as a fraction. It affects the ability to absorb sunlight.

The Atmosphere When considering the atmosphere alone, it experiences radiative cooling. However, it does not cool down due to the additional energy of sensible and latent heating.

If the Earth had no atmosphere, the average surface temperature would be approximately - 18 o C. This temperature is known as the radiative equilibrium temperature. Since it does have an atmosphere, the average surface temperature is around 15 o C. The atmosphere has a warming effect on the Earth, known as the atmospheric greenhouse effect.

Most of the sun’s radiation that hits the surface is in the visible part of the spectrum. The Earth’s radiation that escape the atmosphere is in the infrared band between microns (a very small unit of pressure). This region of the spectrum is called the atmospheric window.

Surface Absorption and Reflection Snow and ice are one of the things that can change Earth’s energy balance. When the surface of the earth becomes too cold, snow and ice cover the surface and increase the amount of sunlight reflected back to space.

The amount of snow and ice changes the global energy balance; therefore the global temperature also changes. On the other hand, as the planet warms, there is a tremendous decrease in snow and ice which causes further heating of the planet.

When vegetation is cleared from land surfaces (such as in deforestation, or agriculture burning), the bare surface reflects more sunlight back into space. Because of this there is an increase of carbon dioxide being released.

Atmospheric Aerosols Aerosols are tiny particles suspended in the air. Some occur naturally originating from volcanoes, dust storms, forest and grass land fires, living vegetation, and sea spray. The aerosols can be cooling or warming depending on how much solar radiation they absorb verses how much of it they disperse it back into space.

Clouds that are close to a burning area can look nearly black which shows strong absorption. On the other hand, clouds that are not close to a burning area look nearly white and they have weak or no absorption. We do not really know the exact amount of absorption from each aerosol.

Clouds Block much of the solar energy and reflect it back into space before absorbed to Earth. The height of clouds change the amount of thermal infrared blocking. Therefore there is a decrease in temperature with higher altitude.

High clouds are cooler so they are effective at absorbing surface-emitted heat in the atmosphere and emit very little into space. Overall, clouds can cool or warm the planet depending on their altitude, how thick they are and how much of the Earth they cover.

Terra Spacecraft A flagship of EOS (Earth Observing System) - a series of spacecrafts that observe the earth from unique points in space. Scientists of EOS research Earth’s land, ocean, air, ice and life systems which help us better understand and protect our home planet.

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