Thermodynamics

THE SCIENCE OF thermodynamics, a branch of physics, aims to describe transformations in energy. Thermodynamics comprises three laws. The first holds that energy can neither be created nor destroyed. Energy in various forms may be transformed into heat (thermal energy) and heat may be transformed into another form of energy so long as the total energy in the system remains constant. The second law states that entropy, a measure of the amount of energy dissipated as heat, increases over time in a closed system. The conversion of energy into heat increases the entropy of a system and the dissipation of heat likewise increases the entropy of a system. The third law states that as temperature approaches absolute zero, the theoretical minimum temperature in the universe, entropy approaches a maximum.

The first law of thermodynamics accounts for the relative constancy of the climate, averaged over long durations. Were Earth simply a reservoir nergy in the form of sunlight, it would heat up to a very high but finite temperature. Earth does not heat up to this magnitude because it radiates heat back into space. The dissipation of energy as heat, according to the second law of thermodynamics, describes the Earths shedding of radiant energy received from the sun as heat. This law, functioning as a heat accountant, is at the heart of understanding the role of heat in determining the climate. The third law of thermodynamics does not operate as long as the Sun generates energy. Rather, the third law anticipates the end of the universe. The Sun will one day burn out. Bereft of its heat, Earths climate will be eternally cold, as its temperature approaches absolute zero. Not only will the Sun be extinguished, but all stars in the universe will one day burn out. The heat from these stars will dissipate in all directions in the universe, bringing the temperature, uniform throughout the universe, near absolute zero.

The science of thermodynamics traces the origin of energy in the solar system to the Sun. Energy from the Sun is the basis of Earths climate, but not all sunlight reaches Earth. The thermosphere lies 190 mi. (306 km.) above Earths surface, and is the outermost layer of the atmosphere. It absorbs ultraviolet light so efficiently that its temperature rises as high as 570 degrees F (299 degrees C). This conversion of the sun's radiant energy into thermal energy obeys the second law of thermodynamics. The next layer of the atmosphere, the mésosphère, is 50 mi. (80 km.) above earth. Its temperature, cooler than the thermosphere, is 200 degrees F (93 degrees C). Carbon dioxide (CO2) in the mésosphère absorbs infrared light as heat, and that light radiates from Earth back into space. CO2 molecules absorb a portion of this light before it reaches space. The larger the number of CO2 molecules, the more heat they will absorb. The heating of the atmosphere by the absorption of infrared light causes the Greenhouse Effect, the warming of Earths climate. Beneath the mésosphère is the ozone rich stratosphere, roughly 15 mi. (24 km.) above Earth. The ozone in the stratosphere blocks some 90 percent of sunlight from reaching Earth. Ozone, like the thermosphere, absorbs ultraviolet light. Beneath the ozone layer is the troposphere, a variable layer 5 mi. (8 km.) thick at the poles and 20 mi. (32 km.) thick at the equator. The troposphere holds water vapor, which absorbs both infrared and ultraviolet light, heating the atmosphere. These layers of the atmosphere both absorb and radiate heat. The heat that they radiate either scatters into space or reaches Earth.

...