Earth's energy budget

Earth's climate is largely determined by the planet's power budget, i.e., the balance of incoming and outgoing radiation. It is measured by satellites. The units shown here are in W/m². A watt is a unit of power, or energy per unit time. A square meter, or m², is a unit area. W/m² is then power per unit area. This can be called "energy per unit time per unit area".

Earth's energy budget could also be called Earth's power budget, as power is energy per unit time. This is an accounting of how much Solar power comes into the Earth's climate system from the Sun, how much power is lost to space, and the net gain or loss of energy for the remainder on Earth and in its atmosphere. Quantifying changes in these amounts is required to accurately model climate.

Incoming, top-of-atmosphere (TOA) shortwave flux radiation, shows energy received from the sun (Jan 26–27, 2012).

Outgoing, longwave flux radiation at the top-of-atmosphere (Jan 26–27, 2012). Heat energy radiated from Earth (in watts per square metre) is shown in shades of yellow, red, blue and white. The brightest-yellow areas are the hottest and are emitting the most energy out to space, while the dark blue areas and the bright white clouds are much colder, emitting the least energy.

Received radiation is unevenly distributed over the planet, because the Sun heats equatorial regions more than polar regions. Solar energy is absorbed by the atmosphere and hydrosphere (oceans, lakes, rivers). This received solar energy is then distributed through the evaporation of surface water, convection, rainfall, winds, and ocean circulation. When incoming solar power is balanced by an equal flow of heat to space, Earth is in radiative equilibrium and global temperatures stabilize.

Disturbances of Earth's radiative equilibrium change global temperatures in response. The global warming theory proposes that greenhouse gases can affect Earth's power budget. Earth's power balance and heat flows depend on many factors, such as atmospheric composition, the albedo (reflectivity) of surfaces, cloud cover, vegetation, and land use patterns. Changes in surface temperature due to Earth's power budget do not occur instantaneously, due to the thermal inertia of the oceans and the cryosphere. The net gain or loss of heat energy is primarily stored in the ocean's heat content until a new equilibrium state is established.

Energy budget

A Sankey diagram illustrating the Earth's energy budget described in this section — line thickness is linearly proportional to relative amount of energy.

In spite of the enormous transfer of energy into and from Earth, it maintains a constant temperature because, as a whole, there is no net gain or loss: Earth receives the same amount of energy via insolation (all the energy from the Sun) as it emits via atmospheric and terrestrial radiation (long-wave or infrared radiation).

To quantify Earth's heat budget or heat balance, let the insolation received at the top of the atmosphere be 100 units, as shown in the accompanying illustration. Called the albedo of Earth, around 35 units are reflected back to space: 27 from the top of clouds, 2 from snow and ice-covered areas, and 6 by other parts of the atmosphere. The 65 remaining units are absorbed: 14 within the atmosphere and 51 by the Earth’s surface. These 51 units are radiated back in the form of terrestrial radiation: 17 directly radiated to space and 34 absorbed by the atmosphere (19 through latent heat of condensation, 9 via convection and turbulence, and 6 directly absorbed). The 48 units absorbed by the atmosphere (34 units from terrestrial radiation and 14 from insolation) are finally radiated back to space. These 65 units (17 from the ground and 48 from the atmosphere) balance the 65 units absorbed from the sun.

Incoming radiant energy (shortwave)

The total amount of energy received per second (i.e. the power) at the top of Earth's atmosphere (TOA) is measured in watts and is given by the solar constant times the cross-sectional area of the Earth. Because the surface of a sphere is four times the cross-sectional area of a sphere, the average TOA flux is one quarter of the solar constant and so is approximately 340 W/m². Since the absorption varies with location as well as with diurnal, seasonal and annual variations, numbers quoted are long-term averages, typically averaged from multiple satellite measurements.

Of the ~340 W/m² of solar radiation received by the Earth, an average of ~77 W/m² is reflected back to space by clouds and the atmosphere and ~23 W/m² is reflected by the surface albedo, leaving ~240 W/m² of solar power input to the Earth's power budget. This gives the earth a mean net albedo of 0.29.

Earth's internal heat and other small effects

The geothermal heat flux from the Earth's interior is estimated to be 47 terawatts. This comes to 0.087 watt/square metre, which represents only 0.027% of Earth's total energy budget at the surface, which is dominated by 173,000 terawatts of incoming solar radiation.

Other minor sources of energy are usually ignored in these calculations, including accretion of interplanetary dust and solar wind, light from distant stars and the thermal radiation of space. Earlier, Joseph Fourier had claimed that deep space radiation was significant in a paper often cited as the first on the greenhouse effect.

Longwave radiation

Main article: Outgoing longwave radiation

Longwave radiation is usually defined as outgoing infrared energy, leaving the planet. However, the atmosphere absorbs parts initially, or cloud cover can reflect radiation. Generally, heat energy is transported between the planet's surface layers (land and ocean) to the atmosphere, transported via evapotranspiration and latent heat fluxes or conduction/convection processes. Ultimately, energy is radiated in the form of longwave infrared radiation back into space.

Recent satellite observations indicate additional precipitation, which is sustained by increased energy leaving the surface through evaporation (the latent heat flux), offsetting increases in longwave flux to the surface.

Earth's energy imbalance

If the incoming energy flux (power per unit area) is not equal to the outgoing thermal radiation, the result is a power imbalance, resulting in net heat energy added to or lost from the planet (if the incoming flux is larger or smaller than the outgoing). Earth's energy imbalance measurements provided by Argo floats detected accumulation of ocean heat content (OHC). The estimated imbalance was measured during a deep solar minimum of 2005-2010 at 0.58 ± 0.15 W/m². Later research estimated the surface energy imbalance to be 0.60 ± 0.17 W/m².

Measurement

Several satellites indirectly measure the energy absorbed and radiated by Earth and by inference the energy imbalance. The NASA Earth Radiation Budget Experiment (ERBE) project involves three such satellites: the Earth Radiation Budget Satellite (ERBS), launched October 1984; NOAA-9, launched December 1984; and NOAA-10, launched September 1986.

Today NASA's satellite instruments, provided by CERES, part of the NASA's Earth Observing System (EOS), are designed to measure both solar-reflected and Earth-emitted radiation.

Natural greenhouse effect

Diagram showing the energy budget of Earth's atmosphere, which includes the greenhouse effect

See also: Greenhouse effect

The major atmospheric gases (oxygen and nitrogen) are transparent to incoming sunlight and are also transparent to outgoing thermal infrared. However, water vapor, carbon dioxide, methane and other trace gases are opaque to many wavelengths of thermal infrared energy. The Earth's surface radiates the net equivalent of 17 percent of incoming solar energy as thermal infrared. However, the amount that directly escapes to space is only about 12 percent of incoming solar energy. The remaining fraction—a net 5-6 percent of incoming solar energy—is transferred to the atmosphere when greenhouse gas molecules absorb thermal infrared energy radiated by the surface.

Atmospheric gases only absorb some wavelengths of energy but are transparent to others. The absorption patterns of water vapor (blue peaks) and carbon dioxide (pink peaks) overlap in some wavelengths. Carbon dioxide is not as strong a greenhouse gas as water vapor, but it absorbs energy in wavelengths (12–15 micrometres) that water vapor does not, partially closing the "window" through which heat radiated by the surface would normally escape to space. (Illustration NASA, Robert Rohde)

When greenhouse gas molecules absorb thermal infrared energy, their temperature rises. Like coals from a fire that are warm but not glowing, greenhouse gases then radiate an increased amount of thermal infrared energy in all directions. Heat radiated upward continues to encounter greenhouse gas molecules; those molecules absorb the heat, their temperature rises and the amount of heat they radiate increases. At an altitude of roughly 5–6 kilometres, the concentration of greenhouse gases in the overlying atmosphere is so small that heat can reach space.

Because greenhouse gas molecules radiate infrared energy in all directions, some of it spreads downward and ultimately returns to Earth's surface, where it is absorbed. Surface temperature is thus higher than it would be if it were heated only by direct solar heating. This supplemental heating is the natural greenhouse effect.

Climate sensitivity

Main article: Radiative forcing

A change in the incident or radiated portion of the energy budget is referred to as a radiative forcing.

Climate sensitivity is the steady state change in the equilibrium temperature as a result of changes in the energy budget.

Climate forcings and global warming

Expected Earth energy imbalance for three choices of aerosol climate forcing. Measured imbalance, close to 0.6 W/m², implies that aerosol forcing is close to −1.6 W/m². (Credit: NASA/GISS)

Climate forcings are energy balance changes that affect that cause temperatures to rise or fall. Natural climate forcings include changes in the Sun's brightness, Milankovitch cycles (small variations in the shape of Earth's orbit and its axis of rotation that occur over thousands of years) and volcanic eruptions that inject light-reflecting particles as high as the stratosphere. Man-made forcings include particle pollution (aerosols) that absorb and reflect incoming sunlight; deforestation, which changes how the surface reflects and absorbs sunlight; and the rising concentration of atmospheric carbon dioxide and other greenhouse gases, which decrease heat radiated to space.

A forcing can trigger feedbacks that intensify (positive feedback) or weaken (negative feedback) the original forcing. For example, loss of ice at the poles, which makes them less reflective, is an example of a positive feedback.

The observed planetary energy imbalance during the recent solar minimum shows that solar forcing of climate, although significant, is overwhelmed by anthropogenic climate forcing.

Eliminating the anthropogenic forcing would require that atmospheric CO₂ content be reduced to about 350 ppm. The impact of anthropogenic aerosols has not been quantified, but individual aerosol types are thought to have substantial heating and cooling effects.

See also

• Planetary equilibrium temperature
• Clouds and the Earth's Radiant Energy System

External links

• NASA: The Atmosphere's Energy Budget
• Clouds and Earth's Radiant Energy System (CERES)
• NASA/GEWEX Surface Radiation Budget (SRB) Project