Earth's energy budget accounts for the balance between the energy that Earth receives from the Sun and the energy the Earth radiates back into outer space. Smaller energy sources, such as Earth's internal heat, are taken into consideration, but make a tiny contribution compared to solar energy. The energy budget also accounts for how energy moves through the climate system. Because the Sun heats the equatorial tropics more than the polar regions, received solar irradiance is unevenly distributed. As the energy seeks equilibrium across the planet, it drives interactions in Earth's climate climate system, i.e., Earth's water, ice, atmosphere, rocky crust, and all living things. The result is Earth's climate.
Earth's energy budget depends on many factors, such as atmospheric aerosols, greenhouse gases, the planet's surface albedo (reflectivity), clouds, vegetation, land use patterns, and more. When the incoming and outgoing energy fluxes are in balance, Earth is in radiative equilibrium and the climate system will be relatively stable. Global warming occurs when earth receives more energy that it gives back to space, and global cooling takes place when the outgoing energy is greater.
When the energy budget changes, there is a delay before average global surface temperature changes significantly. This is due to the thermal inertia of the oceans and the cryosphere. Accurate modeling of Earth's climate requires quantification of these amounts.
In spite of the enormous transfers of energy into and from the Earth, it maintains a relatively constant temperature because, as a whole, there is little net gain or loss: Earth emits via atmospheric and terrestrial radiation (shifted to longer electromagnetic wavelengths) to space about the same amount of energy as it receives via insolation (all forms of electromagnetic radiation).
The total amount of energy received per second 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 corresponded to the radiation. Because the surface area of a sphere is four times the cross-sectional area of a sphere (i.e. the area of a circle), 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, the 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 energy input to the Earth's energy budget. This amount is called the absorbed solar radiation (ASR). It implies a mean net albedo for Earth (specifically, its Bond albedo) of 0.306.
Outgoing longwave radiation (OLR) 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.
The geothermal heat flux from the Earth's interior is estimated to be 47 terawatts and split approximately equally between radiogenic heat and heat leftover from the Earth's formation. 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.
Human production of energy is even lower, at an estimated 18 TW.
Photosynthesis has a larger effect: photosynthetic efficiency turns up to 2% of the sunlight striking plants into biomass. 100 to 140 TW (or around 0.08%) of the initial energy gets captured by photosynthesis, giving energy to plants.[clarification needed]
Other minor sources of energy are usually ignored in the calculations, including accretion of interplanetary dust and solar wind, light from stars other than the Sun and the thermal radiation from space. Earlier, Joseph Fourier had claimed that deep space radiation was significant in a paper often cited as the first on the greenhouse effect.
In simplest terms, Earth's energy budget is balanced when absorbed incoming solar (shortwave) radiation equals outgoing longwave radiation:
To further analyze the internal flows within the balance, let the insolation received at the top of the atmosphere be 100 units (=340 watts per square meter), as shown in the accompanying illustration. Called the albedo of Earth, around 35 units in this example 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 (ASR=220 watts per square meter) are absorbed: 14 within the atmosphere and 51 by the Earth's surface.
The 51 units reaching the surface are radiated to space 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 then finally radiated back to space.
Ultimately the 65 units (17 from the ground and 48 from the atmosphere) are emitted as OLR. They approximately balance the 65 units (ASR) absorbed from the sun in order to maintain a net-zero gain of energy by the Earth.
If Earth's incoming energy flux is larger or smaller than the outgoing energy flux, then the planet will gain or lose net heat energy in accordance with the law of energy conservation. When the imbalance in global energy fluxes shifts by a sufficiently large amount, it is directly measurable by orbiting satellite-based radiometric instruments. Imbalances which fail to reverse over time will also drive long-term temperature changes in the atmospheric, oceanic, terrestrial, and ice components of the climate system. Temperature changes and their related effects may thus provide indirect measures of the imbalance. From mid-2005 to mid-2019, satellite and ocean temperature observations have each independently shown an approximate doubling of Earth's energy imbalance (EEI).
Several satellites directly measure the energy absorbed and radiated by Earth, and thus 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.
NASA's Clouds and the Earth's Radiant Energy System (CERES) instruments are part of the NASA's Earth Observing System (EOS) since 1998. CERES is designed to measure both solar-reflected (short wavelength) and Earth-emitted (long wavelength) radiation. Analysis of CERES data by its principal investigators showed a linearly increasing trend in EEI, from +0.42 W m-2 (+/-0.48 W m-2) in 2005 to +1.12 W m-2 (+/-0.48 W m-2) in 2019.
Other researchers have used data from CERES, AIRS, CloudSat, and other EOS instruments to look for trends of radiative forcing embedded within the EEI data. Their data analysis showed a rise of +0.53 W m-2 (+/-0.11 W m-2) from years 2003 to 2018. 80% of the increase was associated with the rising concentration of greenhouse gases which reduced the outgoing long-wavelength radiation.
It is noteworthy that radiometric calibration uncertainties limit the capability of the current generation of satellite-based instruments. As a result, relative changes in EEI are quantifiable with an accuracy which is not also achievable for any single measurement of the absolute imbalance.
Global surface temperature (GST) is calculated by averaging atmospheric temperatures measured over the surface of the sea along with temperatures measured over land. Reliable data extending to at least 1880 shows that GST has undergone a steady increase of about 0.18°C per decade since about year 1970.
Ocean waters are especially effective absorbents of solar energy and have far greater total heat capacity than the atmosphere. Research vessels and stations have sampled sea temperatures around the globe since before 1960. Additionally after year 2000, an expanding network of over 3000 Argo robotic floats has measured the temperature anomaly, or equivalently the change in ocean heat content (OHC). Since at least 1990, OHC has increased at a steady or accelerating rate. Changes in OHC provide the most robust indirect measure of EEI since the oceans take up 90% of the excess heat.
The extent of floating and grounded ice is measured by satellites, while the change in mass is then inferred from measured changes in sea level in concert with computational models that account for thermal expansion and other factors. Observations since 1994 show that ice has retreated from every part of Earth at an accelerating rate.
The major atmospheric gases (oxygen and nitrogen) are transparent to incoming sunlight but are also transparent to outgoing thermal (infrared) radiation. However, water vapor, carbon dioxide, methane and other trace gases are opaque to many wavelengths of thermal radiation. The Earth's surface radiates the net equivalent of 17 percent of the incoming solar energy in the form of thermal infrared. However, the amount that directly escapes to space is only about 12 percent of incoming solar energy. The remaining fraction, 5 to 6 percent, is absorbed by the atmosphere by greenhouse gas molecules. 
When greenhouse gas molecules absorb thermal infrared energy, their temperature rises. Those gases then radiate an increased amount of thermal infrared energy in all directions. Heat radiated upward continues to encounter greenhouse gas molecules; those molecules also absorb the heat, and their temperature rises and the amount of heat they radiate increases. The atmosphere thins with altitude, and at roughly 5-6 kilometres, the concentration of greenhouse gases in the overlying atmosphere is so thin that heat can escape to space.
Because greenhouse gas molecules radiate infrared energy in all directions, some of it spreads downward and ultimately returns to the Earth's surface, where it is absorbed. The Earth's 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. It is as if the Earth is covered by a blanket that allows high frequency radiation (sunlight) to enter, but slows the rate at which the low frequency infrared radiant energy emitted by the Earth leaves.
Climate sensitivity is the steady state change in the equilibrium temperature as a result of changes in the energy budget.
Climate forcings are changes that cause temperatures to rise or fall, disrupting the energy balance. 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 decreases the rate at which heat is 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, causes greater absorption of energy and so increases the rate at which the ice melts, is an example of a positive feedback.
In 2012, NASA scientists reported that to stop global warming atmospheric CO2 content would have to be reduced to 350 ppm or less, assuming all other climate forcings were fixed. The impact of anthropogenic aerosols has not been quantified, but individual aerosol types are thought to have substantial heating and cooling effects.