Thermodynamics

The science of thermodynamics, a branch of physics, aims to describe transformations in energy. Thermodynamics is comprised of 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.

Three Laws of Thermodynamics

The first law of thermodynamics accounts for the relative constancy of the climate, averaged over long durations. Were Earth simply a reservoir for energy 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 Earth’s 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, Earth’s 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 Earth’s climate, but not all sunlight reaches Earth. The thermosphere lies 190 mi. (306 km) above Earth’s 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 mesosphere, 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 mesosphere 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 Earth’s climate. Beneath the mesosphere 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. Earth absorbs sunlight, chiefly at the equator. This sunlight, in the form of heat, moves to the poles through the currents of the oceans and air. This distribution of heat from an area of greater concentration (the equator) to a region of lesser concentration (the poles) obeys the second law of thermodynamics. 

Heat supplies the energy for the movement of the oceanic and air currents, which in turn transform the potential energy of stasis into the kinetic energy of motion. On an idealized Earth on which the oceans and air distributed heat evenly throughout the planet, heat would reach thermodynamic equilibrium, the point at which entropy would be at a maximum. Earth is much less efficient than this idealized model. For all the motion of the oceanic and air currents, heat nevertheless concentrates at the equator, which is always warmer than the poles. The waters at the equator hold enormous amounts of heat. Because the oceans liberate their heat slowly, heat accumulates at the equator and is slowly transferred toward the poles.

In accord with the second law of thermodynamics, entropy would increase as heat moves from equator to poles, but the sun continuously adds heat to Earth, keeping the equator warmer than the poles. Entropy does not increase because the equator remains warmer than the poles. Without the oceanic and air currents, heat would accumulate at the equator and would not circulate to cooler regions of Earth. The currents therefore perform an important function in carrying heat from the equator to temperate and cold latitudes.

Earth and the atmosphere reflect roughly onethird of the sunlight they receive and radiate the other two-thirds into space. Earth sheds the same amount of heat as it receives, keeping Earth on average at 60 degrees F (16 degrees C). By contrast, outer space, which has no atmosphere to absorb heat, is much colder, at minus 454 degrees F (minus 270 degrees C). Earth absorbs sunlight as ultraviolet and visible light and continually radiates it back into space as infrared light.

Earth also reflects light back into space. The oceans reflect half the sunlight they receive, whereas ice and fresh snow reflect 90 percent. In accord with the second law of thermodynamics, entropy decreases when Earth absorbs heat, and increases when the oceanic and air currents diffuse heat to other regions of the planet. Similarly, entropy increases when Earth reflects light back into space, thereby dissipating heat. 

Entropy is least in equatorial waters because they retain heat and slowly liberate it to other regions of Earth. Heat is not evenly distributed in equatorial waters, as thermodynamic equilibrium would suggest. In holding heat, the oceans at the equator moderate the climate, keeping lands near them warmer than inland stretches of territory. The land warms four times faster than the oceans; the air warms faster still. Land and air also radiate heat faster than the oceans. The climate of a desert underscores the rapidity of heating and cooling on land. Temperatures in a desert rise rapidly during the day, often surpassing 100 degrees F (38 degrees C). At night, a desert cools with equal speed, dipping as low as freezing. In accord with the second law of thermodynamics, entropy decreases as a desert absorbs heat and increases as it dissipates heat.

Warm climates hold heat not only in water and land, but also in air. Warm air holds more moisture than cool air in the form of water vapor, a greenhouse gas. Water vapor holds more heat than CO2, methane, or other greenhouse gases. Water in all three phases absorbs and emits heat. Ice absorbs the least heat and reflects the most sunlight back into space. Liquid water and water vapor are efficient reservoirs of heat.

The laws of thermodynamics work because Earth and its atmosphere absorb and radiate heat. The absorption and radiation of heat give Earth its distinctive characteristics and its ability to sustain life.