Encyclopedia of Energy
Earth’s Energy Balance
Kevin E. Trenberth
NationalCenter for Atmospheric Research[1]
P.O. Box 3000
Boulder, CO80307-3000
U.S.A.
Phone: (303) 497 1318
Fax: (303) 497 1333
email:
June 2002
Reformatted August 2002
CONTENTS:
I. The Earth and climate system
II. The global energy balance
A. The greenhouse effect
B. Effects of clouds
III. Regional patterns
IV. The atmosphere
V. The hydrological cycle
VI. The oceans
VII. The land
VIII. Ice
IX. The role of heat storage
X. Atmosphere-ocean interaction: El Niño
XI. Anthropogenic climate change
A. Human influences
B. The enhanced greenhouse effect
C. Effects of aerosols
XII. Observed and projected temperatures
Bibliography
Figure Captions
Glossary
Aerosol: Microscopic particles suspended in the atmosphere, originating from either a natural source (e.g., volcanoes) or human activity (e.g., coal burning).
Anthropogenic climate change: Climate change arising from human influences.
Albedo: The reflectivity of the earth.
Anticyclone: A high pressure weather system. The wind rotates around these in a clockwise sense in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. They usually give rise to fine settled weather.
Convection: In weather, the process of warm air rising rapidly while cooler air usually subsides more gradually over broader regions elsewhere to take its place. This process often produces cumulus clouds and may result in rain.
Cyclone: A low-pressure weather system. The wind rotates around cyclones in a counterclockwise direction in the Northern Hemisphere and clockwise in the Southern Hemisphere. Cyclones are usually associated with rainy, unsettled weather and may include warm and cold fronts.
Dry static energy: The sum of the atmospheric sensible heat and potential energy.
El Niño: The occasional warming of the tropical Pacific Ocean from the west coast of South America to the central Pacific that typically lasts a year or so and alters weather patterns around the world.
El Niño-Southern Oscillation (ENSO): El Niño and the Southern Oscillation together; the warm phase is El Niño and the cold phase is La Niña.
Enthalpy: The heat content of a substance per unit mass. Used to refer to sensible heat in atmospheric science (as opposed to latent heat).
Greenhouse gas: Any gas that absorbs infrared radiation in the atmosphere
Greenhouse effect: The effect produced as certain atmospheric gases allow incoming solar radiation to pass through to the Earth's surface, but reduce the outgoing (infrared) radiation, which is reradiated from Earth, escaping into outer space. The effect is responsible for warming the planet.
Hadley circulation: The large-scalemeridional overturning in the atmosphere in the tropics.
La Niña: A substantial cooling of the central and eastern tropical Pacific Ocean lasting 5 months or longer.
Longwave radiation: Infrared radiation in the longwave part of the electromagnetic spectrum, corresponding to wavelengths of 0.8 microns to 1,000 microns. For the Earth, it also corresponds to the wavelengths of thermal emitted radiation.
Shortwave radiation: Radiation from the sun, most of which occurs at wavelengths shorter than the infrared.
Southern Oscillation: A global-scale variation in the atmosphere associated with El Niño and La Niña events.
Total solar irradiance: The solar radiation received at the top of the Earth’s atmosphere on a surface oriented perpendicular to the incoming radiation and at the mean distance of the Earth from the sun.
Thermocline: The region of vertical temperature gradient in the oceans lying between the deep abyssal waters and the surface mixed layer.
Troposphere: The part of the atmosphere in which we live, ascending to about 15 km above the Earth's surface; in which temperatures generally decrease with height. The atmospheric dynamics we know as "weather" take place within the troposphere.
Urban heat island: The region of warm air over built-up cities associated with the presence of city structures, roads, etc.
I. THE EARTH AND CLIMATE SYSTEM
The distribution of solar radiation absorbed on Earth is very uneven and largely determined by the geometry of the Sun-Earth orbit and its variations. This incoming radiant energy is transformed into various forms (internal heat, potential energy, latent energy, and kinetic energy) moved around in various ways primarily by the atmosphere and oceans, stored and sequestered in the ocean, land, and ice components of the climate system, and ultimately radiated back to space as infrared radiation. The requirement for an equilibrium climate mandates a balance between the incoming and outgoing radiation and further mandates that the flows of energy are systematic. These drive the weather systems in the atmosphere, currents in the ocean, and fundamentally determine the climate. And they can be perturbed, causing climate change. Here we examine the processes involved and follow the flows, storage and release of energy.
Our planet orbits the sun at an average distance of 1.50 × 1011 m once per year. It receives from the sun an average radiation of 1368 W m-2 at this distance, and this value is referred to as the total solar irradiance and used to be called the “solar constant” even though it does vary by small amounts with the sunspot cycle and related changes on the sun. The Earth’s shape is close to that of an oblate spheroid, with an average radius of 6371 km. It rotates on an axis with a tilt relative to the ecliptic plane of 23.5 around the sun once per year in a slightly elliptical orbit that brings the Earth closest to the sun on January 3rd (called perihelion). The shape of the Earth means that incoming solar radiation varies enormously with latitude. Moreover, tilt of the axis and the rotation of the Earth around the sun give rise to the seasons, as the Northern Hemisphere points more toward the sun in June while the Southern Hemisphere points toward the sun in late December. The Earth also turns on its axis once per day, resulting in the day-night cycle (Fig. 1). A consequence of the Earth’s roughly spherical shape and the rotation is that the average energy in the form of solar radiation received at the top of the Earth’s atmosphere is the total solar irradiance divided by 4, which is the ratio of the Earth’s surface area (4a2, where a is the mean radius) to that of the cross section (a2).
[FIG. 1 NEAR HERE]
The Earth system can be altered by effects or influences from outside the planet usually regarded as “externally” imposed. Most important are the sun and its output, the Earth’s rotation rate, sun-Earth geometry and the slowly changing orbit, the physical make up of the Earth system such as the distribution of land and ocean, the geographic features on the land, the ocean bottom topography and basin configurations, and the mass and basic composition of the atmosphere and ocean. These components affect the absorption and reflection of radiation, the storage of energy, and the movement of energy around, all of which determine the mean climate, which may vary from natural causes.
On time-scales of tens of thousands of years, the Earth’s orbit slowly changes, the shape of the orbit is altered, the tilt changes, and the Earth precesses on its axis like a rotating top, all of which combine to alter the annual distribution of solar radiation received at the Earth. Similarly, a change in the average net radiation at the top of the atmosphere due to perturbations in the incident solar radiation from the changes internal to the sun or the emergent infrared radiation from changes in atmospheric composition leads to a change in heating. Changes in atmospheric composition arise from natural events such as volcanoes, which can create a cloud of debris that blocks the sun. Or they may arise from human activities such as the burning of fossil fuels that creates visible particulate pollution and carbon dioxide, which is a greenhouse gas. The greatest variations in the composition of the atmosphere involve water in various phases, as water vapor, clouds of liquid water, ice crystal clouds, and hail, and these affect the radiative balance of the Earth.
The climate system has several internal interactive components. The atmosphere does not have very much heat capacity but is very important, as the most volatile component of the climate system with wind speeds in the jet stream often exceeding 50 m s-1, in moving heat and energy around. The oceans have enormous heat capacity and, being fluid, also can move heat and energy around in important ways. Ocean currents may be >1 m s-1 in strong currents like the Gulf Stream, but are more typically a few cm s-1 at the surface. Other major components of the climate system include sea ice, the land and its features (including the vegetation, albedo (reflective character), biomass, and ecosystems), snow cover, land ice (including the semi-permanent ice sheets of Antarctica and Greenland and glaciers), and rivers, lakes and surface and subsurface water. Their role in energy storage and the energy balance of the Earth is addressed below.
Changes in any of the climate system components, whether internal and thus a part of the system, or from the external forcings, cause the climate to vary or to change. Thus climate can vary because of alterations in the internal exchanges of energy or in the internal dynamics of the climate system. An example is El Niño-Southern Oscillation (ENSO) events, which arise from natural coupled interactions between the atmosphere and the ocean centered in the tropical Pacific. Such interactions are also briefly discussed below from the standpoint of energy.
II. THE GLOBAL ENERGY BALANCE
The incoming energy to the Earth system is in the form of solar radiation and roughly corresponds to that of a black body at the temperature of the Sun of about 6000 K. The Sun’s emissions peak at a wavelength of about 0.6 m and much of this energy is in the visible part of the electromagnetic spectrum although some extends beyond the red into the infrared and some extends beyond the violet into the ultraviolet. As noted earlier, because of the roughly spherical shape of the Earth, at any one time half the Earth is in night (Fig. 1) and the average amount of energy incident on a level surface outside the atmosphere is one quarter of the total solar irradiance, or 342 W m-2. About 31% of this energy is scattered or reflected back to space by molecules, tiny airborne particles (known as aerosols) and clouds in the atmosphere, or by the Earth’s surface, which leaves about 235 W m-2 on average to warm the Earth’s surface and atmosphere (Fig. 2).
To balance the incoming energy, the Earth must radiate on average the same amount of energy back to space (Fig. 2). The amount of thermal radiation emitted by a warm surface depends on its temperature and on how absorbing it is. For a completely absorbing surface to emit 235 W m-2 of thermal radiation, it would have a temperature of about -19C (254 K). Therefore the emitted thermal radiation occurs at about 10 m which is in the infrared part of the electromagnetic radiation spectrum. Near 4 m, radiation from both the sun and the Earth is very small, and hence there is a separation of wavelengths which has led to the solar radiation being referred to as shortwave radiation, while the outgoing terrestrial radiation is referred to as longwave radiation. Note that -19C is much colder than the conditions that actually exist near the Earth’s surface where the annual average global mean temperature is about 14C. However, because the temperature in the lower atmosphere (troposphere) falls off quite rapidly with height, a temperature of -19C is reached typically at an altitude of 5 km above the surface in mid-latitudes. This provides a clue about the role of the atmosphere in making the surface climate hospitable.
A. The greenhouse effect
Some of the infrared radiation leaving the atmosphere originates near the Earth’s surface and is transmitted relatively unimpeded through the atmosphere; this is the radiation from areas where there is no cloud and which is present in the part of the spectrum known as the atmospheric “window” (Fig. 2). The bulk of the radiation, however, is intercepted and re-emitted both up and down. The emissions to space occur either from the tops of clouds at different atmospheric levels (which are almost always colder than the surface), or by gases present in the atmosphere which absorb and emit infrared radiation. Most of the atmosphere consists of nitrogen and oxygen (99% of dry air), which are transparent to infrared radiation. It is the water vapor, which varies in amount from 0 to about 3%, carbon dioxide and some other minor gases present in the atmosphere in much smaller quantities that absorb some of the thermal radiation leaving the surface and re-emit radiation from much higher and colder levels out to space. These radiatively-active gases are known as greenhouse gases because they act as a partial blanket for the thermal radiation from the surface and enable it to be substantially warmer than it would otherwise be, analogous to the effects of a greenhouse. Note that while a real greenhouse does work this way, the main heat retention in a greenhouse actually comes through protection from the wind. In the current climate, water vapor is estimated to account for about 60% of the greenhouse effect, carbon dioxide 26%, ozone 8% and other gases 6% for clear skies.
[FIG. 2 NEAR HERE]
B. Effects of clouds
Clouds also absorb and emit thermal radiation and have a blanketing effect similar to that of greenhouse gases. But clouds are also bright reflectors of solar radiation and thus also act to cool the surface. While on average there is strong cancellation between the two opposing effects of shortwave and longwave cloud heating, the net global effect of clouds in our current climate, as determined by space-based measurements, is a small cooling of the surface. A key issue is how clouds will change as climate changes. This issue is complicated by the fact that clouds are also strongly influenced by particulate pollution, which tends to make more smaller cloud droplets, and thus makes clouds brighter and more reflective of solar radiation. These effects may also influence precipitation. If cloud tops get higher, the radiation to space from clouds is at a colder temperature and so this produces a warming. However, more extensive low clouds would be likely to produce cooling because of the greater influence on solar radiation.
III. REGIONAL PATTERNS
The annual mean absorbed solar radiation (ASR) and outgoing longwave radiation (OLR) are shown in Fig. 3. Most of the atmosphere is relatively transparent to solar radiation with the most notable exception being clouds. At the surface, snow and ice have a high albedo and consequently absorb little incoming radiation. Therefore the main departures in the ASR from what would be expected simply from the Sun-Earth geometry are the signatures of persistent clouds. Bright clouds occur over Indonesia and Malaysia, across the Pacific near 10N, and over the Amazon in the southern summer, contributing to the relatively low values in these locations, while dark oceanic cloud-free regions along and south of the equator in the Pacific and Atlantic and in the subtropical anticyclones absorb most solar radiation.
The OLR, as noted above, is greatly influenced by water vapor and clouds, but is generally more uniform with latitude than ASR in Fig. 3. Nevertheless, the signature of high-top, and therefore cold clouds is strongly evident in the OLR. Similarly, the dry cloud-free regions are where the most surface radiation escapes to space. There is a remarkable cancellation between much of the effects of clouds on the net radiation (Fig. 3). In particular, the high convective clouds are bright and reflect solar radiation but are also cold and hence reduce OLR. The main remaining signature of clouds in the net radiation from Earth is seen from the low stratocumulus cloud decks that persist above cold ocean waters most notably off the west coasts of California and Peru. Such clouds are also bright but, as they have low tops they radiate at temperatures close to those at the surface, resulting in a cooling of the planet. Note that the Sahara desert has a high OLR, consistent with dry cloud-free and warm conditions, but it is also bright and reflects solar radiation, and it stands out as a region of net radiation deficit.
[FIG. 3 NEAR HERE]
For the Earth, on an annual mean basis, there is an excess of solar over outgoing longwave radiation in the tropics and the deficit at mid to high latitudes (Fig. 1) that sets up an equator-to-pole temperature gradient. These result, with the Earth’s rotation, in a broad band of westerlies and a jet stream in each hemisphere in the troposphere. Embedded within the mid-latitude westerlies are large-scale weather systems which, along with the ocean, act to transport heat polewards to achieve an overall energy balance, as described below.
IV. THE ATMOSPHERE
In the atmosphere, phenomena and events are loosely divided into the realms of “weather” and “climate.” Climate is usually defined to be average weather and thus is thought of as the prevailing weather, which includes not just average conditions but also the range of variations. Climate involves variations in which the atmosphere is influenced by and interacts with other parts of the climate system, and the external forcings. The large fluctuations in the atmosphere from hour-to-hour or day-to-day constitute the weather but occur as part of much larger-scale organized weather systems that arise mainly from atmospheric instabilities driven by heating patterns from the sun.