Tropopause

The tropopause is the boundary region dividing the troposphere, the lowest layer of the atmosphere, from the overlying stratosphere. Since the tropospheric and stratospheric air masses have rather distinct features, in correspondence to each surface location, the tropopause height is the level in the vertical where abrupt changes in the physical and chemical properties of the atmosphere are observed.

Three different definitions are typically adopted. The thermal tropopause is related to the change of the sign of the vertical derivative of the temperature (lapse rate), which is negative in the troposphere

and positive in the stratosphere. The World Meteorological Organization defines the tropopause as the lowest level where the absolute value of the temperature lapse rate decreases to 2K/km or less, with the average lapse rate between this level and all higher levels within 1.2 mi. (2 km) not exceeding 2K/km. The dynamical tropopause is defined in terms of sharp changes in the potential vorticity (much higher in the stratosphere), which measures stratification and rotation of the air masses. An abrupt increase (decrease) with height of the ozone (water vapor) mixing ratio indicates the presence of the chemical tropopause. In spite of the necessity of choosing phenomenological thresholds, the three definitions of the tropopause are quite consistent.

Tropical Tropopause

Typically, the tropopause height decreases with latitude, at around 3.7 mi. (6 km) near the poles and 11 mi. (18 km) near the equator. Whereas radiative and convective processes with time scale of the order of one week to one month basically determine the properties of the tropical tropopause; in the midlatitudes, a relevant role is also played by baroclinic-fuelled extra-tropical cyclones, having a typical time scale of a few days, where the tropopause readjusts its height to effectively act as a stabilizing mechanism limiting the growth of the weather perturbations. The tropopause is not a hard boundary: Exchanges of tropospheric and stratospheric air occur through various mechanisms, including vigorous thunderstorms and midlatitude perturbations.

The globally averaged tropopause height tends to increase if the troposphere warms up and/ or the stratosphere cools down, and the height change is approximately proportional to the difference between the tropospheric and stratospheric temperature changes. Therefore, the mean tropopause height can act as a robust indicator of climate change. Recent climate simulations have shown that the estimated increase after 1979 of about 492 ft. (150 m) may be primarily explained by anthropogenic causes, namely the stratospheric cooling driven by ozone depletion and the tropospheric warming driven by increases in greenhouse gas concentration. Considering natural processes, episodic short-lived and strong reductions of the globally averaged mean tropopause height are caused by large explosive volcanic eruptions, which warm the troposphere and cool the stratosphere.

Triassic Period

The Triassic period is the geologic time period that extends from about 251 to 199 million years ago. This is the first period of the Mesozoic era, following the Permian and preceding the Jurassic period. Both the start and end of the Triassic are marked by major extinction events. During the Triassic period, both marine and continental life showed an adaptive radiation, beginning from the starkly impoverished biosphere that followed the Permian–Triassic extinction. The first flowering plants may have evolved during the Triassic, as did the first flying vertebrates, the pterosaurs. The Triassic period is further separated into Early, Middle, and Late Triassic epochs.

During the Triassic period, almost all the Earth’s land mass was concentrated into a single supercontinent centered more or less on the equator, known as Pangaea. This supercontinent began to rift during the Triassic period but had not yet separated.

The Triassic climate was generally hot and dry, forming typical red bed sandstones and evaporites. There is no evidence of glaciation at or near either pole. The polar regions were moist and temperate— a climate suitable for reptile-like creatures. Pangaea’s continental climate was highly seasonal, with very hot summers and cold winters. It probably had strong, cross-equatorial monsoons. The interior of Pangaea was hot and dry during the Triassic period. This may have been one of the hottest times in Earth history. Rapid global warming at the very end of the Permian may have created a super hothouse world that caused the great Permo-Triassic extinction.

The Permian–Triassic extinction event, also known as the Great Dying, was an extinction event that occurred 251.4 mya (million years ago). This was the Earth’s most severe extinction event, with up to 96 percent of all marine species and 70 percent of all terrestrial vertebrate species becoming extinct. There are several proposed mechanisms for the extinction event, including both catastrophic and gradualistic processes, similar to those theorized for the Cretaceous extinction event. The former include large or multiple impact events, increased volcanism, or sudden release of methane hydrates from the seafloor. The latter include sea-level change, anoxia, and increasing aridity. Evidence that an impact event caused the Cretaceous–Tertiary extinction event has led naturally to speculation that an impact may have been the cause of other extinction events, including the Permian–Triassic extinction. Several possible impact craters have been proposed as possible causes of this extinction event, including the Bedout structure off the northwest coast of Australia and the Wilkes Land crater of east Antarctica. 

In each of these cases, the idea that an impact was responsible has not been proven and has been widely criticized. If impact was a major cause of this extinction event, it is possible or even likely that the crater no longer exists. Seventy percent of the Earth’s surface is sea, so an asteroid or comet fragment is over twice as likely to hit sea as to hit land. There is evidence that the oceans became anoxic toward the end of the Permian. There was a noticeable and rapid onset of anoxic deposition in marine sediments around east Greenland near the end of the Permian. The most likely causes of the global warming that drove the anoxic event was a severe anoxic event at the end of the Permian, causing sulphate-reducing bacteria to dominate the oceanic ecosystems and causing massive emissions of hydrogen sulfide, which poisoned plant and animal life on both land and sea. These massive emissions of hydrogen would have severely weakened the ozone layer, exposing much of the life that remained to fatal levels of ultraviolet radiation.

Pangaea’s formation would also have altered both oceanic circulation and atmospheric weather patterns, creating seasonal monsoons near the coasts and an arid climate in the vast continental interior. Marine life suffered very high, but not catastrophic rates of extinction after the formation

of Pangaea—rates almost as high as in some of the Big Five mass extinctions. The formation of Pangaea seems not to have caused a significant rise in extinction levels on land, and in fact, most of the advance of Therapsids and the increase in their diversity seems to have occurred in the late Permian, after Pangaea was almost complete.

Thus, it seems likely that Pangaea initiated a long period of severe marine extinctions, but was not directly responsible for the Great Dying and the end of the Permian. The possible causes, which are supported by strong evidence, appear to describe a sequence of catastrophes, each one worse than the previous. The resultant global warming may have caused perhaps the most severe anoxic event in the oceans’ history. The oceans became so anoxic that anaerobic sulfur-reducing organisms dominated their chemistry.

Trade Winds

The trade winds are a large-scale component of Earth circulation, occupying most of the tropics, straddling the equator between approximately latitude 30 degrees N and latitude 30 degrees S, with a seasonal shift of the entire trade wind belt system about 5 degrees of latitude northward during summer (July) and southward during winter (December).

In the Northern Hemisphere, warm equatorial air rises and flows north toward the pole, the Coriolis effect (caused by the Earth’s rotation) deflects the current, and as the air cools, it descends, blowing southwestward from the northeast. In the Southern Hemisphere, warm equatorial air rises and flows south toward the pole, the Coriolis effect deflects the current, and as the air cools, it descends, blowing northwestward from the southeast. The rising air is associated with deep atmospheric convection, heavy precipitation, and weak wind speeds, with an influence on global weather patterns. Air heated by the sun rises and releases moisture through rain and thunderstorms.

Once the air cools, it descends as drier air. In the equatorial low, the air rises and travels aloft to the subtropical highs, where it then sinks. Mariners called these reliable wind currents for sailing the trade winds, or westerlies. The name trade winds comes from an old sailing term meaning that the winds could be counted on to blow steadily from the same direction at a constant speed. The trade winds, or easterlies, carried air from east to west at low latitudes and are less regular over land areas than they are over the oceans. The trade winds meet at the Intertropical Convergence Zone. The doldrums (downward branch of the Hadley Cell, named for George Hadley, whose 1735 paper linked rising air and the Earth’s rotation in causing the trade winds) are the calm winds at the Intertropical Convergence Zone in the area between latitude 5 degrees N and latitude 5 degrees S, where a sailing ship might not move because of the calm winds. In satellite imaging, the Intertropical Convergence Zone appears as a band of clouds. The strength and position of the Intertropical Convergence Zone influences tropical and global weather patterns.

Air temperature differences across the Earth’s surface (both land and water) create winds, with warm air being lighter than cold air. Near the equator, the sun heats the sea surface, causing the warm air at the surface to rise and be replaced by the trade winds blowing from subtropical high pressure systems into equatorial low-pressure troughs. The trade winds blow steadily for days and are among the most consistent on Earth. When trade winds move over warm tropical waters, they pick up moisture and bring heavy rainfall to the windward-facing slopes of mountainous areas, contrasting with the downward motion of dry air that creates desert areas on land. Because the area of Earth between the Tropic of Cancer and Tropic of Capricorn, lying at approximately 23 degrees latitude on either side of the equator, receives more solar heat than the rest of the Earth, the warm air creates clouds and rain, with thundershowers there almost every day.

The influence of the trade winds on weather and climate is seen with El Niño, La Niña, and the development of hurricanes and cyclones. The differences in pressure and temperature between the two sides of the Pacific are caused by the trade winds; air blowing from east to west pushes water, making the sea level higher in the western Pacific, and makes cold water rise toward the surface, making the eastern Pacific approximately 14 degrees F (7.7 degrees C) cooler than the western Pacific. During El Niño years, the eastern Pacific sea surface is warmer, and the Intertropical Convergence Zone is closer to the equator, causing rainfall over the Pacific. The warm surface temperature is associated with reversed air pressure patterns and decreasing strength of trade winds, so more water stays in the eastern Pacific off the coast of South America. With the rain pattern shift eastward, the western Pacific can become drier over India and much of southeast Asia. A similar pattern sets up in the Atlantic, resulting in extreme drought in the eastern United States and reduced tropical storm development in the Atlantic Ocean.

During La Niña years, the trade winds are stronger than normal, causing more cold water to rise to the ocean surface. The cooler surface temperature is associated with a rain pattern shift westward. The eastern areas thus become drier, with an increased probability of flooding from monsoons in both India and much of southeast Asia. Hurricanes (Atlantic) and cyclones (Indian Ocean) are tropical storms of low-pressure cells. Formation of hurricanes in the Atlantic comes from solar heating of water off the West African coast along the Intertropical Convergence Zone, with high cumulus cloud formation in the lowpressure area along the edge. These systems are pushed westward by the trade winds, and the rotation is set in motion by the Coriolis effect. A similar pattern sets up in the Pacific, causing cyclones.

Thunderstorms

A thunderstorm is a localized storm that is produced by a cumulonimbus cloud and always contains thunder and lightning. Thunderstorms form in conditionally unstable environments, meaning there is cold, dry air aloft over warm, moist surface air. This causes the air to become buoyant and allows for rising air motion. A lifting mechanism is also needed to start the air moving. Such lifting mechanisms include surface heating, surface convergence, lifting due to mountains, or lifting along frontal boundaries.

The heat and the humidity of the summertime can often produce what are called ordinary thunderstorms or air mass thunderstorms. These are the type of the thunderstorms that seem to suddenly “pop up,” last less than an hour, and are rarely severe. A severe thunderstorm is defined by the National Weather Service as having 3/4-inchdiameter hail and/or surface winds exceeding 58 mph and/or producing a tornado. Ordinary thunderstorms also do not usually have excessive vertical

wind shear, meaning that the wind speed or direction does not change greatly with height. 

Stages of a Thunderstorm

Thunderstorms usually go through a series of stages, from birth to decay. The first stage is known as the cumulus stage, which is dominated by updrafts. The updrafts bring in warm, moist air, which then cools and condenses as it rises. When the clouds further develop and precipitation starts to fall, a downdraft is produced. This marks the beginning of the mature stage, which is the most intense. During this stage, the strong updraft is still present, supplying the warm, moist air, but the strong downdraft is also evident. The gust front is located at the boundary of the updraft and the downdraft. This is an area where the wind velocity rapidly changes. Eventually, the downdraft will cut off the supply of warm, moist air in the updraft. When this occurs, typically 15–30 minutes after the mature stage, the thunderstorm will start to weaken and enter the dissipating stage due to the deprivation of energy from the updraft.

If the vertical wind shear increases, this allows for the thunderstorm to tilt. Therefore, the downdraft is less likely to cut off the updraft, which allows the thunderstorm to persist for a longer period of time. Sometimes, the downdraft can slide underneath an updraft, which can produce multiple-cell thunderstorms, or simply multicell storms. If the vertical wind shear becomes extremely strong, the shear can produce a large rotational thunderstorm known as a supercell, thunderstorms that last longer than an hour, are often severe, and can produce tornadoes. The strong wind shear creates horizontal spin, which can then rotate vertically when the updraft encounters the vortex.

Thunderstorms can occur as a line of multiplecell thunderstorms known as squall line thunderstorms. These usually form along or slightly ahead of a cold front. The line of thunderstorms can extend over 500 mi. (800 km) and often exhibit severe characteristics. When thunderstorms occur in a large circular pattern, they are known as a Mesoscale Convective Complex (MCC), a large, convectively driven system that usually lasts more than 12 hours and covers more than 386,000 sq. mi. (100,000 sq. km). Many thunderstorms are embedded within the MCC and often form during the summer in the Great Plains. As warm, moist air is brought in from the Gulf of Mexico, the tops of the very high clouds cool rapidly by emitting radiation into space. This makes the atmosphere very unstable and allows for the MCCs to generate and persist. Since MCCs are usually located underneath weak, upper-level winds, they tend to travel very slowly, which can cause locally heavy rains and flooding events.

All thunderstorms have lightning, the electrical discharge, thunder, and resulting shockwave produced by extreme heating. Lightning has a temperature of approximately 54,000 degrees F (30,000 degrees C), which is five times hotter than the surface of the sun. Lightning occurs during the mature stages of thunderstorms and can appear within a cloud, connect from one cloud to another, or travel from cloud to ground. Most lightning strikes are within a cloud.

Worldwide, it is estimated that 50,000 thunderstorms occur every day and over 18 million occur per year. Thunderstorm frequency is the most common in the tropics, especially near the Intertropical Convergence Zone (ITCZ), an area of low pressure near the equator. Lower frequencies occur in drier regions near 30 degrees north/south, which is dominated by the subtropical high pressure, and also in the polar regions. In the United States, thunderstorm activity is predominantly in the southeast, with a maximum located over Florida. Florida has over 90 days of thunderstorms per year due to the convergence of wind from the Gulf of Mexico and the Atlantic Ocean.

Thermosphere

The Earth is surrounded by a blanket of air called the atmosphere. The atmosphere is a thin layer of gases that envelope the Earth. The gases are held close to the Earth by gravity and the thermal movement of air molecules. Life on Earth is supported by the atmosphere, solar energy, and the magnetic fields. Five layers have been identified in the atmosphere, using thermal characteristics, chemical composition, movement, and density.

The atmosphere is divided into the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. The thermosphere, from the Greek word for heat (thermos), is the fourth atmospheric layer from Earth, separated from the mesosphere by the mesopause. It begins about 50 mi. (80 km) above the Earth and is the layer of the atmosphere directly above the mesosphere and below the exosphere. The lower part of the thermosphere, from 50 to 342 mi. (80 to 550 km) above the Earth’s surface, contains the ionosphere, which is the region of the atmosphere that is filled with charged particles. Beyond the ionosphere, extending out to perhaps 6,214 mi. (10,000 km), is the exosphere.

Hottest Layer in the Atmosphere

The Earth’s thermosphere is the layer of the atmosphere that is first exposed to the sun’s radiation, and so is first heated by the sun; it is the hottest layer of the atmosphere. Within the thermosphere, temperatures rise continually to well beyond 1,832 degrees F (1,000 degrees C). In the thermosphere, ultraviolet radiation causes ionization. At these high altitudes, the residual atmospheric gases sort into strata according to their molecular mass. Thermospheric temperatures increase with altitude as a result of the absorption of highly energetic solar radiation by the small amount of residual oxygen present. Temperatures in the thermosphere are highly dependent on solar activity. Radiation causes the air particles in this layer to become electrically charged, enabling radio waves to bounce off and be received beyond the horizon.

The few molecules that are present in the thermosphere receive extraordinary amounts of energy from the sun, causing the layer to warm to high temperatures. Air temperature, however, is a measure of the kinetic energy of air molecules—not of the total energy stored by the air. The air is so thin that a small increase in energy can cause a large increase in temperature. Because the air is so thin within the thermosphere, such temperature values are not comparable to those of the troposphere or stratosphere. Again, because of the thin air in the thermosphere, scientists cannot measure the temperature directly. Instead, they measure the density of the air by how much drag it puts on satellites, and then use the density to determine the temperature.

Although the measured temperature is very hot, the thermosphere would actually feel very cold to humans because the total energy of the few air molecules residing there would not be enough to transfer any appreciable heat to our skin. In addition, it is so near vacuum that there is not enough contact with the few atoms of gas to transfer much heat. A normal thermometer would read significantly below 32 degrees F (0 degree C). The dynamics of the lower thermosphere are dominated by the atmospheric tide, which is driven in part by the very significant diurnal heating.

The atmospheric tide dissipates above this level because molecular concentrations do not support the coherent motion needed for fluid flow. The International Space Station has a stable orbit within the upper part of the thermosphere, between 199 and 236 mi. (320 and 380 km). The northern lights also occur in the thermosphere.

Thermohaline Circulation

The warm surface waters of the tropical oceans occupy a very shallow layer, approximately 328 ft. (100 m) deep that floats on the far colder water of the deep ocean that reaches to depths of 3.1 mi. (5 km). The temperature and salinity of the deep ocean is so uniform that its water must originate in cold, high latitudes where surface waters sink into the deep ocean and then spread across the globe. Ultimately, this water must rise back to the surface and return to the regions of sinking, thus constituting a conveyor belt. Because the motion has strong north–south and up–down components, it is referred to as a meridional overturning cell. For the water to sink, it must be dense, which means that it must be cold and saline. Hence, the circuit away from and then back to the regions of sinking, depending on density and salinity gradients, is known as the thermohaline circulation. Its depiction in documentary films such as Al Gore’s An Inconvenient Truth has brought the oceanic conveyor belt to the public’s attention.

At first, oceanographers speculated that the thermohaline circulation is symmetrical about the equator, with water sinking in polar regions and rising to the surface in lower latitudes. In reality, the circulation is very asymmetrical. Sinking in the northern hemisphere is limited to the northern Atlantic Ocean and is absent from the Pacific and Indian oceans because those two oceans are insufficiently saline at the surface. Confirmation of this asymmetry is available in carbon-14 measurements of the age of a parcel of seawater, the time since it was last at the ocean surface. The results show that the deep water is youngest in the northern Atlantic and oldest in the northern Pacific, where its age approaches 1,000 years. The surface flow toward the northern Atlantic affects a northward transport of heat that contributes to the temperate climate of western Europe.

Oceanic density’s dependence on both temperature and salinity means that the increase in density associated with low temperatures in high latitudes could be countered by a flux of freshwater onto the ocean’s surface, for example, when glaciers melt. If the water becomes too buoyant, it no longer sinks, the conveyor belt stops, and oceanic circulation experiences radical changes.

Such changes, which may have altered climate in

Earth’s very distant past, are shown vividly in the movie The Day After Tomorrow. Geochemical measurements provide a wealth of information about the thermohaline circulation. Unfortunately, there is also much misinformation concerning the role of that circulation in Earth’s climate. The reason is a lack of information about key aspects of the thermohaline circulation.

Although it is known where the surface waters sink into the deep ocean, where that water returns to the surface is at present a puzzle. The water has to be heated as it rises into warmer layers close to the surface. The required rate of heating is far greater than that which measurements show to be available in much of the oceans. Hence, the cold water probably rises in relatively small regions of strong turbulent mixing, for example over ridges on the ocean floor. It is possible that the waters that sink in high latitudes rise back to the surface in high latitudes where they can do so without requiring significant heat. 

Once in the surface layers, the motion of the water is strongly under the influence of the winds that drive intense currents shallow, swift currents, which would be present even in the absence of a thermohaline circulation, are also involved in the transport of heat to high latitudes, and therefore in the maintenance of temperate climates in high latitudes. Do fluctuations in the intensity of the Gulf Stream indicate imminent changes in the climate of western Europe? What is the relative importance of the wind-driven and thermohaline circulations in the poleward transport of heat today? Did changes in that transport contribute to climate changes in Earth’s distant past, and contribute to the recurrent ice ages, for example? 

The disagreements among scientists regarding answers to these questions reflect the uncertainties in what is known about the thermohaline circulation. This circulation is of central importance to the global climate because it maintains remarkably uniform conditions in the deep ocean, which has a high concentration of the greenhouse gas carbon dioxide. It is so slow that water particles take approximately 1,000 years to travel from the North Atlantic to the deep northern Pacific; measurements of current conditions provide only limited information about possible changes in that circulation. Observations of past climates similarly provide limited information. As a result, predictions of future changes in this circulation that will accompany global warming are also uncertain.

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.