Climate Change Science – Energy Budget of the Earth

Australia: The Land Where Time Began

Climate Change Science – Energy Budget of the Earth – the Basics

The energy balance of the Earth is the balance between the amount of the solar energy that is absorbed by the Earth and the amount that is radiated back to space as heat. The Earth warms when the amount of energy absorbed by the Earth is greater than the amount radiated to space and cools when the heat radiated from the Earth is greater than that being absorbed. According to the authors¹ the energy imbalance of the Earth is the single most crucial measure of the status of the climate of the Earth, also defining expectations for climate change in the future. In this chapter of his book¹ the energy budget of the Earth is explained, and the fact the Earth retains more of the electromagnetic radiation from the Sun that is incident upon it than is radiated back to space. The authors¹ discuss the solar constant and aspects of solar electromagnetic radiation and discuss and illustrate the electromagnetic spectrum. Weather is distinguished from climate and calculations are made of the temperature with and without an atmosphere. They also define the radiation laws affecting the Earth, and the outgoing spectral radiation and the absorption of specific frequencies by greenhouse gases are illustrated.

Climate

The climate of an area has been defined as weather over a long time period, usually taken to be at least 30 years, according to the World Meteorological Organisation (WMO).

Weather

Weather is what happens over a period of days, and has been defined as the state of the atmosphere at any given time. It is necessary to make the distinction between weather and climate before climate change can be considered. Climate cannot be spoken of in terms of global weather, as at any given time the weather varies from place to place throughout the world, but global climate can be spoken of. And throughout most of the world the global climate has been getting warmer.

Solar and Heat Energy

Heat is energy and most of the energy of the Earth comes from the Sun. Some of the solar energy is reflected back to space by the top of the atmosphere (TOA), by clouds, solid particles suspended in the atmosphere (aerosols), glaciers, oceans, and by the solid Earth. Solar energy travels from the Sun to the Earth as electromagnetic radiation (EMR) to impinge on the Earth. EMR is the energy form that is emitted and absorbed by charged particles, and it exhibits wave-like behaviour as it travels through space.

Wavelengths of light vary from long wavelengths (low energy) to very short wavelengths, high energy, which together comprise the electromagnetic spectrum. The length of a wave is measured from crest to crest or from trough to trough. Light is a type of radiation that behaves as both waves of light and particles of light, photons, hence the “duality of light.” Part of the radiation emitted from the Sun, light bulbs, LED devices, and fires, is visible light. Visible light waves ranges from 0.4-0.7 μm (4,000-7,000Å), though an atom is only a few ångströms in size.

The motion of electrically charged particles produce electromagnetic waves and these waves are also called “electromagnetic radiation” as they radiate from electrically charged particles, and they travel through empty space as well as air and other substances.

As well as acting like waves, electromagnetic radiation behaves like a stream or packets of particles, the photons, that have no mass. The highest energy photons correspond to the shortest wavelengths. Electromagnetic radiation travels at the speed of light, which is believed to be a universal constant on the order of 3 x 10⁸m/s or 300,000,000 m/s, or to be more accurate, 299,792,458 m/s (or 186,000 miles/s or 300,000 km/s).

Light produces heat, hence lasers, and the most common light sources are thermal (heat); a characteristic spectrum is emitted by a body at a given temperature, blackbody radiation. Examples of this are Sunlight (radiation at about 6,000 K). The main source of the heat of the Earth is about 40% of the Sunlight that is in the visible range. Other sources of heat are from the interior of the Earth and incandescent light bulbs and the glowing solid particles in flames.

A blackbody is an object absorbing all incident radiation and radiates heat that depends on the temperature of the blackbody.

For objects that are relatively cool, such as humans, the peak of the blackbody spectrum is in the infrared. The peak shifts to shorter wavelengths with a red glow being produced at first, which changes to white and as the peak moves out of the visible light range of the spectrum it changes to blue section of the spectrum than to ultraviolet energies. This is no longer detectable by humans though some animals such as the mantis shrimp can also detect this frequency of light. When metal objects are heated they glow red, then white, though the next step, blue thermal emissions, are only rarely seen.

The amount of solar radiation, short-wave or ultraviolet, is absorbed by the Earth and atmosphere over time is balanced by the same amount of outgoing longwave radiation, which is heat energy or infrared radiation. Of the incoming solar radiation about half is absorbed by the Earth’s surface. Warming of the air that is in contact with the surface transfers energy to the atmosphere, by evapotranspiration and by longwave radiation which is absorbed by clouds and greenhouse gases, though some of this longwave radiation escapes back to space. Longwave radiation is trapped by certain atmospheric greenhouse gases and re-radiated back to the surface of the Earth. The surface of the Earth is about 30^oC warmer than it would otherwise be as a result of this re-radiation; therefore it is the presence of these gases in the atmosphere that has allowed the development of life on Earth and its continued existence. According to the authors¹ the average global temperature of the Earth would be between -15^oC and -19^oC without these gases in the atmosphere.

Sources & Further reading