Earth's climate and energy balance (2023)

Earth's climate and energy balance

Von Libessie LindsayJanuary 14, 2009

The sun does not heat the earth evenly. Because the Earth is a sphere, the sun heats the equatorial regions more than the polar regions. The atmosphere and oceans constantly work to correct the imbalance in solar heating through surface water evaporation, convection, precipitation, wind and ocean circulation. This coupled cycle of atmosphere and oceans is known as the Earth's heat engine.

A climate heat engine would need to redistribute the sun's heat not only from the equator to the poles, but also from the Earth's surface and lower atmosphere back into space. Otherwise the earth will keep warming. The Earth's temperature cannot rise indefinitely because both the Earth's surface and atmosphere radiate heat into space. This net flow of energy into and out of the Earth system is the Earth's energy balance.

The energy received by the Earth from the sun's rays is balanced by an equal amount of energy radiated into space. The energy escapes in the form of thermal infrared radiation: just like the energy from a heat lamp. (NASA illustration by Robert Simmons.)

incident sunlight

All matter in the universe above absolute zero (the temperature at which all atomic or molecular motion ceases) radiates energy in a range of wavelengths across the electromagnetic spectrum. The hotter the object, the shorter the peak wavelength of the radiated energy. The hottest objects in the universe primarily emit gamma rays and X-rays. Cooler objects emit primarily longer-wavelength radiation, including visible light, thermal infrared, radio and microwaves.

The surface temperature of the sun is 5500°C, and the peak radiation is in the visible wavelength range of light. The Earth's effective temperature - as seen from space - is -20 °C, and the energy it radiates peaks in the thermal infrared wavelength range. (image adapted fromRobert Lord.)

Incandescent lamps radiate between 40 and 100 watts. The sun provides 1360 watts of energy per square meter. The area an astronaut faces toward the sun is about 0.85 square meters, so the energy he or she receives is equivalent to1960 watt light bulb. (Photo © 2005Paul Watson.)

The temperature on the surface of the sun is about 5800 Kelvin (about 5500 degrees Celsius or about 10,000 degrees Fahrenheit). At this temperature, most of the energy emitted by the sun is visible light and infrared light. At the average distance between Earth and the sun (about 150 million kilometers), the average intensity of solar energy reaching the top of the atmosphere directly facing the sun is about 1360 watts per square meter, according to measurements from NASA's latest satellite mission. This energy is called total solar radiation. (Total solar radiation was sometimes called the "solar constant" until scientists discovered that it varied slightly during the sunspot cycle.)

Watts are a measure of the power or energy that something produces or consumes over time. How much power is 1360 watts? A light bulb consumes 40 to 100 watts. Microwave ovens use about 1000 watts. If, in just one hour, you could capture and re-use all the solar energy in a square meter of the top of the atmosphere facing the sun—an area no larger than the outstretched arm of an adult human—you'd have enough to run a refrigerator for a day.

Solar radiation is the maximum possible power the sun can deliver to a planet at the average distance from the earth to the sun; the basic geometry limits how much solar energy the earth can actually absorb. The sun can only illuminate half of the earth at a time, halving the total solar radiation.

The energy of sunlight is unevenly distributed on the earth. One hemisphere is always dark and receives no solar radiation at all. On the sun side, only points directly below the sun receive full solar radiation. From the equator to the poles, the sun's rays hit the Earth at smaller and smaller angles, and the rays are spread over larger and larger areas (red lines). (NASA illustration by Robert Simmons.)

The amount of solar radiation received by the Earth's surface varies with time and latitude. This chart illustrates the relationship between latitude, time, and solar energy during the equinoxes. The insets show how the time of day (A-E) affects the angle of incoming sunlight (identifiable by the length of the shadow) and the intensity of the light. On an equinox, the sun rises at 6:00 am. When the sun is directly over the equator (casting no shadows), the intensity of sunlight increases from sunrise to noon. After noon, the intensity of the sun gradually decreases until the sun sets at 18:00. Compared to the equator, the tropics (from 0 to 23.5° latitude) receive about 90% of the energy, the intermediate geographic zone (45°) receives about 70%, and the polar and Antarctic circles receive about 40%. (NASA illustration by Robert Simmons.)

On average, the amount of sunlight reaching the Earth's atmosphere for the entire planet is only a quarter of the total solar radiation, or about 340 watts per square meter.

When the incoming solar flux is balanced by the uniform heat flux into space, the Earth is in radiative equilibrium and the global temperature is relatively stable. Anything that increases or decreases the energy going in and out disrupts the Earth's radiative balance; in response, global temperatures must rise or fall.

unbalanced heating

The global average of incident solar energy per square meter is 340 watts; sunlight varies with space and time. The annual incident solar energy varies significantly between tropical and polar latitudes (as discussed on page 2). In the middle and high latitudes, the four seasons also change greatly.

thistopThe energy received at different latitudes varies throughout the year. This graph shows how the solar energy received at noon each day of the year varies with latitude. At the equator (gray line), peak energy varies little throughout the year. Seasonal variation is extreme in the high-latitude north (blue line) and south (green). (NASA illustration by Robert Simmons.)

thisIn the summerThe daily energy absorbed by the top of the atmosphere depends on latitude. The highest daily energy levels (pale pink) occur at high latitudes where summer days are longer, not at the equator. In winter, some polar latitudes get no light at all (black). The Southern Hemisphere receives more energy in December (Southern Summer) than the Northern Hemisphere in June (Northern Summer) because the Earth's orbit is not a perfect circle and the Earth is slightly closer to the Sun during this part of its orbit. The total energy absorbed is between 0 (polar winter) and about 50 (polar summer) megajoules per square meter per day.

The amount of sunlight absorbed by the Earth depends on the reflectivity of the atmosphere and the Earth's surface. This satellite map shows the amount of reflected solar radiation (watts per square meter) in September 2008. Clouds along the equator reflect most of the sunlight, while the light-colored sands of the Sahara Desert in North Africa provide high reflectivity. At this time of year, the poles don't get much sunlight, so they reflect very little energy, despite being covered in ice. (NASA map by Robert Simmons, based onCeresdata. )

This map of net radiation (incoming sunlight minus reflected light and outgoing heat) shows the global energy imbalance for the month of the September 2008 equinox. Areas around the equator absorb, on average, 200 watts more energy per square meter (orange and red) than they reflect or radiate. Areas near the poles reflect and/or radiate approximately 200 watts more energy per square meter (green and blue) than they absorb. The average latitude is roughly balanced. (NASA map by Robert Simmons, based onCeresdata. )

Earth's Energy Balance

Note: Determining accurate values ​​of energy fluxes in the Earth system is an area of ​​ongoing climate research. Estimates vary, and all estimates involve a degree of uncertainty. Estimates come from satellite observations, ground observations and numerical weather models. The numbers in this article are based primarily on direct satellite observations of reflected sunlight and infrared heat energy radiated from the atmosphere and surface.

The Earth's heat engine doesn't just move heat from one part of the Earth's surface to another; it also carries heat from the Earth's surface and lower atmosphere back into space. This flow of energy in and out is the energy balance of the planet. In order for the Earth's temperature to remain stable over time, the incoming and outgoing energy must be equal. In other words, the energy balance at the top of the atmosphere must be balanced. This state of equilibrium is called radiation equilibrium.

About 29 percent of solar energy reaching the upper atmosphere is reflected back into space by clouds, atmospheric particles, or bright surfaces such as sea ice and snow. This energy plays no role in the Earth's climate system. About 23% of incident solar energy is absorbed by water vapor, dust, and ozone in the atmosphere, and 48% passes through the atmosphere and is absorbed at the surface. Therefore, approximately 71% of incident solar energy is absorbed by the Earth system.

Of the 340 watts per square meter of solar energy that falls on Earth, 29 percent is reflected back into space, mostly by clouds but also by other bright surfaces and the atmosphere itself. About 23% of the incident energy is reflected by atmospheric gases, dust and other particles in the atmosphere. The remaining 48% is absorbed on the surface. (NASA illustration by Robert Simmons. Astronaut photoISS013-E-8948。)

When matter absorbs energy, the atoms and molecules that make it up become animated; they move faster. As the motion increases, the temperature of the material increases. If matter could only absorb energy, then the temperature of the earth would be like the water level in a sink with no drain and the faucet running all the time.

However, the temperature does not rise indefinitely because the atoms and molecules on Earth not only absorb sunlight but also radiate infrared thermal energy (heat). The heat radiated by a surface is proportional to the fourth power of its temperature. If the temperature doubles, the radiant energy increases by a factor of 16 (2 to the 4th power). As Earth heats up, the planet rapidly releases more and more heat into space. A relatively small increase in temperature results in a large increase in heat loss (known as radiative cooling), which is the main mechanism preventing the planet from warming unexpectedly.

The absorbed sunlight is balanced by the heat radiated from the Earth's surface and atmosphere. This satellite map shows the distribution of thermal infrared radiation emitted by Earth in September 2008. Most of the heat escapes from regions north and south of the equator, where the surface is warm but clouds are sparse. Persistent cloud cover along the equator prevents heat loss. Likewise, the cold pole also dissipates some heat. (NASA map by Robert Simmons, based onCeresdata. )

Calculation of Surface Energy

To understand how Earth's climate system balances energy, we need to consider processes that occur at three levels: at the Earth's surface, where most of the solar heating occurs; the Earth's atmospheric edge, where sunlight enters the system; and the atmosphere in between. At each level, the incoming and outgoing energy or net flow must be equal.

Remember that about 29% of incoming sunlight is reflected back into space by bright particles in the atmosphere or bright ground, so about 71% is absorbed by the atmosphere (23%) and the ground (48%). To balance the energy balance at the Earth's surface, terrestrial processes must remove 48% of the incident solar energy absorbed by the ocean and land surface. Energy leaves the surface through three processes: evaporation, convection, and emission of infrared heat energy.

The surface absorbs approximately 48% of incident sunlight. Three processes extract equal amounts of energy from the Earth's surface: evaporation (25%), convection (5%), and thermal infrared radiation or heat (a net 17%). (NASA illustration by Robert Simmons. Photo © 2006Salem.)

Cumulus towers divert energy away from the Earth's surface. Solar heating facilitates evaporation. Warm, moist air gains buoyancy and rises, transferring energy from high above the surface into the atmosphere. When water vapor condenses into liquid water or freezes into ice crystals, energy is released back into the atmosphere. (astronaut photoISS006-E-19436。)

Another 5% of the incident solar energy leaves the surface by convection. The air in direct contact with the sun-heated ground becomes warm and vibrant. In general, the atmosphere is warmer near the surface and cooler at higher altitudes. Under these conditions, warm air rises, carrying heat away from the surface.

the last oneInner imageAbout 17% of incident solar energy leaves the surface as infrared thermal energy (heat) radiated by surface atoms and molecules. This net upward flux is the result of two large but opposite fluxes: heat flowing up from the surface to the atmosphere (117%) and heat flowing down from the atmosphere to the ground (100%). (These competing streams are part ofgreenhouse effect,well describedpage 6.) Remember that the peak wavelength of energy radiated from a surface depends on its temperature. Solar radiation peaks in the visible and near-infrared ranges. The Earth's surface is much colder, averaging only about 15 degrees Celsius. Peak radiation from the surface is at a thermal infrared wavelength of approximately 12.5 microns.

Atmospheric Energy Balance

Just as energy input and output from the Earth's surface must be balanced, the flow of energy into the atmosphere must be balanced by the flow of energy out of the atmosphere and back into space. Satellite measurements show that the atmosphere radiates thermal infrared energy equivalent to 59% of the incident solar energy. If the atmosphere radiates this much, it must also absorb this much. Where does this energy come from?

Clouds, aerosols, water vapor, and ozone directly absorb 23% of incident solar energy. Evaporation and convection transfer 25% and 5% of incident solar energy from the surface to the atmosphere, respectively. These three processes transmit the equivalent of 53% of incident solar energy into the atmosphere. If the total energy input has to match the outgoing thermal infrared radiation observed at the top of the atmosphere, where does the rest (~5-6%) come from? The rest of the energy comes from the Earth's surface.

natural greenhouse effect

Just as the main gases in the atmosphere (oxygen and nitrogen) are transparent to incoming sunlight, they are also transparent to outgoing thermal infrared light. However, water vapor, carbon dioxide, methane, and other trace gases are opaque to many wavelengths of infrared thermal energy. Remember that the net energy emitted by this surface in the form of thermal infrared is equivalent to 17% of the incoming solar energy. However, the energy that escapes directly into space is only about 12% of the incoming solar energy. The remainder (5-6% of net incident solar energy) is transferred to the atmosphere as the greenhouse gas molecules absorb the infrared heat energy radiated from the surface.

The atmosphere radiates the equivalent of 59 percent of incoming sunlight back into space as infrared thermal energy, or heat. Where does the atmosphere get its energy from? The atmosphere directly absorbs approximately 23% of incoming sunlight, with the remaining energy transferred from the Earth's surface through evaporation (25%), convection (5%), and thermal infrared radiation (net 5–6%). The remaining infrared heat energy (12%) from the Earth's surface passes through the atmosphere and escapes into space. (NASA illustration by Robert Simmons. Astronaut photoISS017-E-13859。)

When greenhouse gas molecules absorb infrared heat energy, their temperature increases. Like coals that are warm but not glowing, greenhouse gases emit more infrared heat in all directions. The upwardly radiated heat continues to encounter greenhouse gas molecules; these molecules absorb heat, their temperature increases and the amount of heat they radiate increases. At an altitude of about 5-6 km, the concentration of greenhouse gases in the surrounding atmosphere is low and heat can radiate unimpeded into space.

As the greenhouse gas molecules radiate heat in all directions, some of them diffuse downward and eventually make contact with the Earth's surface and are absorbed. The surface temperature becomes higher than if heated by direct sunlight alone. This additional heating of the Earth's surface by the atmosphere is a natural greenhouse effect.

Effect on surface temperature

Due to the natural greenhouse effect, the Earth's surface temperature has risen to around 15 degrees Celsius on average - more than 30 degrees higher than it would be without the atmosphere. The heat radiated from the atmosphere to the surface (sometimes called "back radiation") is equal to 100% of the incident solar energy. The Earth's surface responds to "extra" energy (other than direct solar heating) by increasing its temperature.

On average, 340 watts of solar energy enter the atmosphere per square meter. Earth returns an equal amount of energy to space by reflecting some of the incoming light and radiating heat (infrared heat). Most of the solar energy is absorbed at the surface, while most of the heat is radiated back into space from the atmosphere. Earth's average surface temperature is maintained by two major opposing energy flows between the atmosphere and the ground (right) - the greenhouse effect. NASA illustration by Robert Simmons, adapted from Trenberth et al. 2009, CERES fluxes estimated using Norman Loeb. )

Why doesn't the natural greenhouse effect cause a sudden rise in surface temperature? Remember that the energy radiated from a surface always increases faster than its temperature - energy output increases with temperature to the fourth power. At the same time, as the sun heats up and the atmosphere "reflects" to increase the surface temperature, the surface emits more and more heat—equivalent to 117% of the incoming solar energy. Therefore, the net upward heat flux is 17% of the incident sunlight (117% upward - 100% downward).

Some of the heat goes directly into space, and the rest is transferred higher and higher into the atmosphere, until the energy leaving the upper atmosphere no longer matches the incoming solar energy. Since the maximum possible amount of incoming sunlight is determined by the solar constant (which depends only on the Earth's distance from the Sun and very small fluctuations in the solar cycle), the natural greenhouse effect does not cause a sudden increase in temperature on the Earth's surface.

Climate Impacts and Global Warming

Any change in Earth's climate system that affects energy entering or leaving the system changes the planet's radiative balance and can cause temperatures to rise or fall. These destabilizing effects are called climate effects. Natural influences on climate include variations in the brightness of the sun, Milankovitch cycles (small changes over thousands of years in the shape of the Earth's orbit and its axis of rotation), and advection of large volcanic eruptions that project reflective particles downward onto the planet layer. Human-caused impacts include particle pollution (aerosols) that absorb and reflect incoming sunlight; deforestation, which changes the way the surface reflects and absorbs sunlight; and rising concentrations of carbon dioxide and other greenhouse gases in the atmosphere, reducing the amount of heat radiated into space. Enforcement can trigger feedback loops that reinforce or weaken initial enforcement. The loss of ice at the poles, making them less reflective, is an example of a feedback loop.

A thing is something that changes the balance between input and output energy in the climate systemstrength.Natural impacts include volcanic eruptions. Human influences include air pollution and greenhouse gases. Climate effects, such as increases in greenhouse gases, lead to feedbacks, such as the loss of ice that reflects sunlight. (Image © 2008Antonio,©2008Haglund, and friendlyMike Embree/National Science Foundation.)

Carbon dioxide disrupts the Earth's energy balance by absorbing infrared thermal energy (heat) radiated from the Earth's surface. It absorbs infrared thermal energy at wavelengths in parts of the energy spectrum that other gases (such as water vapor) do not absorb. Although water vapor strongly absorbs many wavelengths of infrared heat energy, it is nearly transparent to other wavelengths. The transparency of these wavelengths acts like a window, opening up the atmosphere to radiative cooling of the Earth's surface. The most important of these "water vapor windows" is thermal infrared light with a wavelength of around 10 microns. (Maximum transparency occurs at 10 microns, but partial transparency occurs at wavelengths between about 8 and about 14 microns.)

Carbon dioxide is a very strong absorber of infrared heat energy with wavelengths greater than 12-13 microns, which means that increasing concentrations of carbon dioxide partially "close" the atmospheric window. In other words, the wavelengths of infrared heat energy that allow the most common greenhouse gas in our atmosphere -- water vapor -- to escape into space are absorbed by carbon dioxide.

All atmospheric gases have unique energy absorption patterns: they absorb energy at certain wavelengths but are transparent to others. The absorption modes of water vapor (blue peak) and carbon dioxide (pink peak) overlap at certain wavelengths. Carbon dioxide is not a potent greenhouse gas like water vapor, but it absorbs energy at wavelengths (12-15 microns) that water vapor does not, thereby partially closing the "window" through which heat radiated from surfaces would normally pass "Escape into space." (Image adapted fromRobert Lord.)

The absorption of outgoing thermal infrared radiation by carbon dioxide means that the Earth still absorbs about 70% of the incoming solar energy, but no longer escapes the corresponding heat. The exact magnitude of the energy imbalance is difficult to measure, but seems to be a little over 0.8 watts per square meter. This imbalance is measured by a combination of satellite and ocean observations of sea level rise and warming.

When a forcing such as an increase in greenhouse gas concentrations throws the energy balance off balance, it does not immediately change the global mean surface temperature. It may take years or even decades to feel the full effects of OCD. This delay between the onset of the imbalance and the full onset of surface temperature effects is largely due to the enormous heat capacity of the global oceans. The heat content of the oceans gives the climate thermal inertia, which can cause the surface to warm or cool more slowly, but cannot stop the change.

Climate change observed so far is only part of the overall response we can expect to the current energy imbalance caused only by the greenhouse gases we have emitted so far. Global average surface temperature has risen by 0.6 to 0.9 degrees Celsius over the past century and is projected to rise by at least 0.6 degrees to address existing energy imbalances.

As the surface temperature increases, the amount of heat radiated from the surface increases rapidly (see description of radiative cooling on page 4). As concentrations of greenhouse gases stabilize, the Earth's climate will return to equilibrium, although the "thermostat" -- the global average surface temperature -- will be set at a higher temperature than it was before the Industrial Revolution.

However, as long as greenhouse gas concentrations continue to rise, the amount of solar energy absorbed will continue to exceed the amount of thermal infrared energy that can escape into space. Energy imbalances will continue to increase, and surface temperatures will continue to rise.

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