Earth's energy budget
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 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.
Earth's energy flowsEdit
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 solar insolation (all forms of electromagnetic radiation).
Incoming radiant energy (shortwave)Edit
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 radiationEdit
Outgoing longwave radiation (OLR) is usually defined as outgoing energy leaving the planet, most of which is in the infrared band. Generally, absorbed solar energy is converted to different forms of heat energy. Some of this energy is emitted as OLR directly to space, while the rest is first transported through the climate system as radiant and other forms of thermal energy. For example, indirect emissions occur following heat transport from the planet's surface layers (land and ocean) to the atmosphere via evapotranspiration and latent heat fluxes or conduction/convection processes. Ultimately, all of outgoing energy is radiated in the form of longwave radiation back into space.
Despite multiple other influences, the Stefan-Boltzmann law of radiation describes the fundamental dependence of OLR upon Earth's surface skin temperature (Tskin):
Tskin has been globally measured from satellite observations of OLR in the infrared and microwave bands. The strong (fourth-power) temperature sensitivity acts to maintain a near-balance of the outgoing energy flow to the incoming flow via small changes in absolute temperature.
Earth's internal heat sources and other small effectsEdit
The geothermal heat flow from the Earth's interior is estimated to be 47 terawatts (TW) and split approximately equally between radiogenic heat and heat leftover from the Earth's formation. This corresponds to an average flux of 0.087 W/m2 and represents only 0.027% of Earth's total energy budget at the surface, being dwarfed by the 173,000 TW of incoming solar radiation.
Photosynthesis has a larger effect: An estimated 140 TW (or around 0.08%) of incident energy gets captured by photosynthesis, giving energy to plants to produce biomass. A similar flow of thermal energy is released over the course of a year when plants are used as food or fuel.
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 the incoming flow equals the outgoing flow. Since a portion of incoming energy is directly reflected, the balance can also be stated as absorbed incoming solar (shortwave) radiation equal to outgoing longwave radiation:
Internal flow analysisEdit
To describe some of the internal flows within the budget, let the insolation received at the top of the atmosphere be 100 units (=340 W/m2), as shown in the accompanying Sankey diagram. Called the albedo of Earth, around 35 units in this example are directly 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 W/m2) are absorbed: 14 within the atmosphere and 51 by the Earth's surface.
The 51 units reaching and absorbed by the surface are emitted back to space through various forms of terrestrial energy: 17 directly radiated to space and 34 absorbed by the atmosphere (19 through latent heat of vaporisation, 9 via convection and turbulence, and 6 as absorbed infrared by greenhouse gases). The 48 units absorbed by the atmosphere (34 units from terrestrial energy 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.
Role of the greenhouse effectEdit
The major atmospheric gases (oxygen and nitrogen) are transparent to incoming sunlight but are also transparent to outgoing longwave (thermal/infrared) radiation. However, water vapor, carbon dioxide, methane and other trace gases are opaque to many wavelengths of thermal radiation.
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 longwave infrared radiation leaves.
Ultimately, the surface temperature rises until the ASR = OLR balance is restored.
Heating/cooling rate analysisEdit
More generally speaking, changes to Earth's energy balance are the result of internal system variability, external forcings (both radiative and non-radiative), and system feedbacks. These are primarily expressed as observable changes in temperature (T), clouds (C), water vapor (W), aerosols (A), trace greenhouse gases (G), land/ocean/ice surface reflectance (S), and as minor shifts in insolaton (I) among other possible factors. Earth's heating/cooling rate (ΔE) then follows as the net change in energy associated with these attributes:
Absolute and relative rates inferred from measurements of the imbalances are also compared to those obtained from Earth system simulations in order to further improve climate models and understanding. Climate forcing is complex since it can produce feedbacks that intensify (positive feedback) or weaken (negative feedback) the original forcing. For example, the loss of Arctic ice makes the region less reflective, leading to greater absorption of energy and even faster ice melt rates, is known as the positive ice-albedo feedback.
Earth's energy imbalanceEdit
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=ΔE).
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. An effort to simulate the behavior with climate models concluded there was a less than 1% chance that internal climate variability caused the trend.
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 forcing rise of +0.53 W m−2 (+/-0.11 W m−2) from years 2003 to 2018. About 80% of the increase was associated with the rising concentration of greenhouse gases which reduced the outgoing longwave radiation.
Satellite observations have also indicated additional precipitation, which is sustained by increased energy leaving the surface through evaporation (the latent heat flux), offsetting some of the increase in the longwave greenhouse flux to the surface.
It is noteworthy that radiometric calibration uncertainties limit the capability of the current generation of satellite-based instruments, which are otherwise stable and precise. 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.
Importance as a climate change metricEdit
Long-time climate scientist James Hansen and colleagues have identified the monitoring of Earth's energy imbalance as a scientific imperative. EEI provides the most direct measure of heating that is "in the pipeline". In 2012, NASA scientists reported that to stop global warming atmospheric CO2 concentration would have to be reduced to 350 ppm or less, assuming all other climate forcings were fixed. As of 2020, atmospheric CO2 increased to exceed 415 ppm due to the sustained growth in emissions.
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Additional bibliography for cited sourcesEdit
- IPCC AR5 Working Group I Report
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- IPCC (2013). "Annex III: Glossary" (PDF). IPCC AR5 WG1 2013. pp. 1447–1465.
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