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{{Short description|Concept for energy flows to and from Earth}} | |||
]es and the exchange that occurs between the ] and the atmosphere or any other exchanges.)<!-- click on image for more reference info -->]] | |||
{{About|energy flows at and above Earth's surface|Earth's internal heat|Earth's internal heat budget}} | |||
{{Use dmy dates|date=October 2020}} | |||
]es in the atmosphere, leading to Earth's energy imbalance of about 460 TW.<ref name="Trenberth2022" /> The percentage going into each domain of the ] is also indicated.]] | |||
'''Earth's energy budget''' (or '''Earth's energy balance''') is the balance between the ] that ] receives from the ] and the energy the Earth loses back into ]. 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 takes into account how energy moves through the ].<ref name="IPCC AR6 glossary" />{{rp|2227}} The Sun heats the equatorial ] more than the ]. Therefore, the amount of ] received by a certain region is unevenly distributed. As the energy seeks equilibrium across the planet, it drives interactions in Earth's climate system, i.e., Earth's ], ], ], ], and all ].<ref name="IPCC AR6 glossary" />{{rp|2224}} The result is Earth's ]. | |||
Earth's energy budget depends on many factors, such as atmospheric ], ], surface ], ], and ] patterns. When the incoming and outgoing ]es are in balance, Earth is in ] and the climate system will be ''relatively'' stable. ] occurs when earth receives more energy than it gives back to space, and ] takes place when the outgoing energy is greater.<ref name="WEB-NASA-EnergyBalance" /> | |||
The ''']''' can be considered as a physical system with an ] that includes all gains of incoming energy and all losses of outgoing energy. The planet is approximately in equilibrium, so the sum of the gains is approximately equal to the sum of the losses. All energy use results in waste heat. In order to use more total energy than presently and without raising the Earth's average temperature, that additional energy must be compensated for by using only sources that would otherwise have been wasted. | |||
Multiple types of measurements and observations show a warming imbalance since at least year 1970.<ref name="JOC20161015" /><ref name="EarthSysSciData_20230417" /> The rate of heating from this human-caused event is without precedent.<ref>Allen, M.R., O.P. Dube, W. Solecki, F. Aragón-Durand, W. Cramer, S. Humphreys, M. Kainuma, J. Kala, N. Mahowald, Y. Mulugetta, R. Perez, M.Wairiu, and K. Zickfeld, 2018: . In: . Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 49-92. https://doi.org/10.1017/9781009157940.003.</ref>{{rp|54}} The main origin of changes in the Earth's energy is from human-induced changes in the composition of the atmosphere.<ref name="Trenberth2022" /> During 2005 to 2019 the ] averaged about 460 ] or globally {{val|0.90|0.15|u=W/m<sup>2</sup>}}.<ref name="Trenberth2022" /> | |||
== The energy budget == | |||
=== Incoming energy === | |||
The total rate at which the energy enters the ] is estimated at 174 ]]s{{Citation needed|date=December 2012}}. This flux consists of: | |||
*] (99.97%, or nearly 173 petawatts) | |||
**This is equal to the product of the ], about 1,366 watts per square meter, and the area of the Earth's disc as seen from the Sun, about 1.28 × 10<sup>14</sup> square meters, averaged over the Earth's surface, which is four times larger. (That is, the area of a disc with the Earth's diameter, which is effectively the target for solar energy, is 1/4 the area of the entire surface of the Earth.) The solar flux averaged over just the sunlit half of the Earth's surface is about 680 W m<sup>−2</sup> | |||
**This is the incident energy. The energy actually absorbed by the earth is lower by a factor of the albedo; this is discussed in the next section. | |||
**Note that the solar constant varies (by approximately 0.1% over a solar cycle); and is not known absolutely to within better than about one watt per square meter. Hence geothermal, tidal, and waste heat contributions are less uncertain than solar power.{{Citation needed|date=July 2011}} | |||
*] (0.025%; or about 44<ref name=pollack_et_al>{{Cite news | |||
| last = Pollack | first = H.N. | |||
| coauthors = S. J. Hurter, and J. R. Johnson | |||
| year = 1993 | |||
| title = Heat Flow from the Earth's Interior: Analysis of the Global Data Set | |||
| url = http://www.agu.org/pubs/crossref/1993/93RG01249.shtml | |||
| volume = 30 | |||
| issue = 3 | |||
| pages = 267–280 | |||
| periodical = Rev. Geophys. | |||
| postscript = <!--None--> | |||
}}</ref> to 47<ref>J. H. Davies and D. R. Davies, "Earth’s Surface heat flux," ''Solid Earth, 1'', 5–24 (2010), available in here (accessed 8 October 2010)</ref> terawatts) | |||
**This is produced by stored heat and heat produced by ] leaking out of the Earth's interior. | |||
*] (0.002%, or about 3 terawatts) | |||
**This is produced by the interaction of the Earth's mass with the gravitational fields of other bodies such as the Moon and Sun. | |||
*] from ] consumption (about 0.007%, or about 13 terawatts)<ref>http://mustelid.blogspot.com/2005/04/global-warming-is-not-from-waste-heat.html</ref> The total energy used by commercial energy sources from 1880 to 2000 (including fossil fuels and nuclear) is calculated to be 17.3x10<sup>21</sup> joules.<ref>{{Cite book | |||
| last = Nordell | |||
| first = Bo | |||
| coauthors = Bruno Gervet | |||
| title = Global energy accumulation and net heat emission | |||
| accessdate = 2009-12-23 | |||
| url = http://www.ltu.se/polopoly_fs/1.5035!nordell-gervet%20ijgw.pdf | |||
}}</ref> | |||
It takes time for any changes in the energy budget to result in any significant changes in the ]. This is due to the ] of the ]s, land and ].<ref name="JOURNAL-Previdi-2013" /> Most ]s make accurate calculations of this inertia, energy flows and storage amounts. | |||
There are other minor sources of energy that are usually ignored in these calculations: accretion of interplanetary dust and solar wind, light from distant stars, the thermal radiation of space. Although these are now known to be negligibly small, this was not always obvious: ] initially thought radiation from deep space was significant when he discussed the Earth's energy budget in a paper often cited as the first on the greenhouse effect.<ref>{{cite web|title=William M. Connolley's page about Fourier 1827: MEMO IRE sur les temperatures du globe terrestre et des espaces planetaires|url=http://www.wmconnolley.org.uk/sci/fourier_1827/|last=Connolley|first=William M.|authorlink=William Connolley|publisher=William M. Connolley|date=18 May 2003|accessdate=5 July 2010}}</ref> | |||
== Definition == | |||
Earth's energy budget includes the "major energy flows of relevance for the climate system".<ref name="IPCC AR6 glossary" >IPCC, 2021: . In . Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 2215–2256, doi:10.1017/9781009157896.022.</ref> These are "the top-of-atmosphere energy budget; the surface energy budget; changes in the global energy inventory and internal flows of energy within the climate system".<ref name="IPCC AR6 glossary" />{{rp|2227}} | |||
The average ] (reflectivity) of the Earth is about 0.3, which means that 30% of the incident solar energy is reflected into space, while 70% is absorbed by the Earth and '''reradiated''' as ]. The planet's albedo varies from month to month and place to place, but 0.3 is the average figure. The contributions from geothermal and tidal power sources are so small that they are omitted from the following calculations. | |||
== Earth's energy flows == | |||
30% of the incident energy is '''reflected''', consisting of | |||
] measurements (26–27 Jan 2012). Brightest white areas show the highest reflectivity (least absorption) of solar energy, while darkest blue areas show the greatest absorption.]] | |||
*6% reflected from the ] | |||
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 ] (all forms of electromagnetic radiation). | |||
*20% reflected from ]s | |||
*4% reflected from the ground (including land, water and ice) | |||
The main origin of changes in the Earth's energy is from human-induced changes in the composition of the atmosphere, amounting to about 460 TW or globally {{val|0.90|0.15|u=W/m<sup>2</sup>}}.<ref name="Trenberth2022" /> | |||
] intensity, from clouds, atmosphere and ground]] | |||
=== Incoming solar energy (shortwave radiation) === | |||
The remaining 70% of the incident energy is '''absorbed''': | |||
{{Main|Solar irradiance}} | |||
*51% is absorbed by land and water, and then emerges in the following ways: | |||
The total amount of energy received per second at the top of ] (TOA) is measured in ]s and is given by the ] 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 globally and yearly averaged TOA flux is one quarter of the solar constant and so is approximately 340 watts per square ] (W/m<sup>2</sup>).<ref name="WEB-NASA-EnergyBudgetPoster" /><ref name="JOURNAL-Wild-2013" /> Since the absorption varies with location as well as with diurnal, seasonal and annual variations, the numbers quoted are multi-year averages obtained from multiple satellite measurements.<ref name="WEB-NASA-EnergyBudgetPoster" /> | |||
**23% is transferred back into the atmosphere as ] by the ] of water, called ] | |||
**7% is transferred back into the atmosphere by heated rising air, called ] | |||
**15% is transferred into the atmosphere by radiation | |||
**6% is radiated directly into space | |||
*19% is absorbed by the atmosphere (16% by the air, 3% by clouds). | |||
Of the ~340 W/m<sup>2</sup> of solar radiation received by the Earth, an average of ~77 W/m<sup>2</sup> is reflected back to space by clouds and the atmosphere and ~23 W/m<sup>2</sup> is reflected by the surface ], leaving ~240 W/m<sup>2</sup> of solar energy input to the Earth's energy budget. This amount is called the absorbed solar radiation (ASR). It implies a value of about 0.3 for the mean net albedo of Earth, also called its ] (A):<ref name="WEB-NASA-EnergyBudgetPoster" /> | |||
The Earth and its atmosphere are also radiant energy sources themselves. The atmosphere absorbs 90% of the energy radiated by the Earth, and radiates its own energy, 50% back towards the ground and 50% into space. | |||
: <math> ASR = (1-A) \times 340~\mathrm{W}~\mathrm{m}^{-2} \simeq 240~\mathrm{W}~\mathrm{m}^{-2}.</math> | |||
=== Outgoing longwave radiation === | |||
When the Earth is at thermal equilibrium, the absorbed and radiated energy are equal: 70% of the incident solar energy = 50% of the atmosphere's radiation + 11% of the land+water radiation + 9% of the cloud's radiation. | |||
] | |||
{{Main|Outgoing longwave radiation|Greenhouse effect}} | |||
Thermal energy leaves the planet in the form of ''outgoing longwave radiation'' (OLR). Longwave radiation is electromagnetic ] emitted by Earth's surface and atmosphere. Longwave radiation is in the ] ]. But, the terms are not synonymous, as infrared radiation can be either ''shortwave'' or ''longwave''. Sunlight contains significant amounts of ] infrared radiation. A threshold wavelength of 4 microns is sometimes used to distinguish longwave and shortwave radiation. | |||
==Natural greenhouse effect== | |||
Generally, absorbed solar energy is converted to different forms of heat energy. Some of the solar energy absorbed by the surface is converted to thermal radiation at wavelengths in the "]"; this radiation is able to pass through the atmosphere unimpeded and directly escape to space, contributing to OLR. The remainder of absorbed solar energy is transported upwards through the atmosphere through a variety of heat transfer mechanisms, until the atmosphere emits that energy as thermal energy which is able to escape to space, again contributing to OLR. For example, heat is transported into the atmosphere via ] and ] fluxes or ]/] processes, as well as via radiative heat transport.<ref name="WEB-NASA-EnergyBudgetPoster" /> Ultimately, all outgoing energy is radiated into space in the form of longwave radiation. | |||
] | |||
The transport of longwave radiation from Earth's surface through its multi-layered atmosphere is governed by radiative transfer equations such as ] (or more complex equations if scattering is present) and obeys ]. | |||
{{See also|greenhouse effect}} | |||
A ] produces an approximate description of OLR which yields temperatures at the surface (T<sub>s</sub>=288 ]) and at the middle of the ] (''T''<sub>a</sub>=242 K) that are close to observed average values:<ref>{{cite web |url=https://www.acs.org/content/acs/en/climatescience/atmosphericwarming/singlelayermodel.html |title=ACS Climate Science Toolkit – Atmospheric Warming – A Single-Layer Atmosphere Model |publisher=American Chemical Society |accessdate=30 September 2022|archive-url=https://web.archive.org/web/20230525201203/https://www.acs.org/climatescience/atmosphericwarming/singlelayermodel.html |archive-date=2023-05-25}}</ref> | |||
<!-- paragraph based on the following reference --><ref name="lindsey greenhouse effect p6">Edited quote from public-domain source: {{citation | |||
: <math> OLR \simeq \epsilon \sigma T_\text{a}^4 + (1-\epsilon) \sigma T_\text{s}^4.</math> | |||
| author=Lindsey, R. | |||
In this expression ''σ'' is the ] and ''ε'' represents the ] of the atmosphere, which is less than 1 because the atmosphere does not emit within the wavelength range known as the ]. | |||
| date=January 14, 2009 | |||
| url=http://earthobservatory.nasa.gov/Features/EnergyBalance/page6.php | |||
| title=The Atmosphere’s Energy Budget (page 6), in: Climate and Earth’s Energy Budget: Feature Articles | |||
| publisher=Earth Observatory, part of the EOS Project Science Office, located at NASA Goddard Space Flight Center | |||
}} | |||
</ref> The major ] gases (] and ]) are transparent to incoming sunlight, and are also transparent to outgoing thermal infrared. However, ], ], ], and other trace gases are opaque to many ]s of thermal ] energy. The Earth's surface radiates the net equivalent of 17 percent of incoming ] 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 ] molecules absorb thermal infrared energy radiated by the surface. | |||
Aerosols, clouds, water vapor, and trace greenhouse gases contribute to an effective value of about {{nowrap|1=''ε'' = 0.78}}. The strong (fourth-power) temperature sensitivity maintains a near-balance of the outgoing energy flow to the incoming flow via small changes in the planet's ]s.] (2000–2022) based on satellite data.]] | |||
<!-- paragraph based on the following reference --><ref name="lindsey greenhouse effect p6"/> When greenhouse gas molecules absorb thermal infrared energy, their temperature rises. Like ]s from a ] 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 ] of roughly 5-6 ]s, the concentration of greenhouse gases in the overlying atmosphere is so small that heat can radiate freely to space. | |||
As viewed from Earth's surrounding space, greenhouse gases influence the planet's atmospheric ] (''ε''). Changes in atmospheric composition can thus shift the overall radiation balance. For example, an increase in heat trapping by a growing concentration of greenhouse gases (i.e. an ''enhanced ]'') forces a decrease in ] and a warming (restorative) energy imbalance.<ref>{{cite web |title=ACS Climate Science Toolkit - Radiative Forcing - How Atmospheric Warming Works |url=https://www.acs.org/content/acs/en/climatescience/atmosphericwarming/radiativeforcing.html |accessdate=30 September 2022 |publisher=American Chemical Society}}</ref> Ultimately when the amount of greenhouse gases increases or decreases, in-situ surface temperatures rise or fall until the absorbed solar radiation equals the outgoing longwave radiation, or ASR equals OLR. | |||
=== Earth's internal heat sources and other minor effects === | |||
<!-- paragraph based on the following reference --><ref name="lindsey greenhouse effect p6"/> Because greenhouse gas molecules radiate heat in all directions, some of it spreads downward and ultimately comes back into contact with the Earth’s surface, where it is absorbed. The temperature of the surface becomes warmer than it would be if it were heated only by direct solar heating. This supplemental heating of the Earth’s surface by the atmosphere is the natural greenhouse effect. | |||
{{See also|Earth's internal heat budget|Anthropogenic heat}} | |||
The ] from the Earth's interior is estimated to be 47 ]s (TW)<ref name="JOURNAL-DAVIES-2010" /> and split approximately equally between ] heat and heat left over from the Earth's formation. This corresponds to an average flux of 0.087 W/m<sup>2</sup> and represents only 0.027% of Earth's total energy budget at the surface, being dwarfed by the {{val|173,000|u=TW}} of incoming ].<ref name="BOOK-Archer-2012" /> | |||
===Greenhouse warming of the Earth's surface=== | |||
] is even lower at an average 18 TW, corresponding to an estimated 160,000 TW-hr, for all of year 2019.<ref name="owdprime"/> However, consumption is growing rapidly and energy production with fossil fuels also produces an increase in atmospheric greenhouse gases, leading to a more than 20 times larger ] in the incoming/outgoing flows that originate from solar radiation.<ref name="20220112_SciAm">{{cite web |url= https://www.scientificamerican.com/article/oceans-break-heat-record-for-third-year-in-a-row/ |title=Oceans break heat record for third year in a row |author=Chelsea Harvey |date=2022-01-12 |publisher=]}}</ref> | |||
<!-- paragraph based on the following reference --><ref name="lindsey greenhouse effect p6"/> The natural greenhouse effect raises the Earth’s surface temperature to about 15 degrees Celsius on average—more than 30 degrees warmer than it would be if it didn’t have an atmosphere. Earth maintains a balance between incoming solar energy in the form of ultraviolet and visible light that it absorbs and the amount of heat in the | |||
form of infrared radiation that it redirects back toward space. Variables that cause this energy | |||
balance to change and affect the global average temperature are called forcings because they force | |||
the temperature up or down. Forcings include changes in the sun’s brightness and other | |||
influences that operate on roughly 11-year and longer length cycles; aerosols and particulate | |||
matter originating from the oceans, volcanic eruptions, and man-made air pollution such as heat-trapping CO2.<ref>http://pubs.acs.org/cen/coverstory/87/8751cover.html</ref> | |||
] also has a significant effect: An estimated 140 TW (or around 0.08%) of incident energy gets captured by photosynthesis, giving energy to plants to produce ].<ref name="WEB-Donev-EnergyFlow" /> A similar flow of thermal energy is released over the course of a year when plants are used as food or fuel. | |||
<!-- paragraph based on the following reference --><ref name="lindsey greenhouse effect p6"/> The amount of energy a surface radiates always increases faster than its temperature rises. The amount of heat a surface radiates is proportional to the fourth power of its temperature (the ]).<ref>Edited quote from public-domain source: {{citation | |||
| author=Lindsey, R. | |||
| date=January 14, 2009 | |||
| url=http://earthobservatory.nasa.gov/Features/EnergyBalance/page4.php | |||
| title=Earth’s Energy Budget (page 4), in: Climate and Earth’s Energy Budget: Feature Articles | |||
| publisher=Earth Observatory, part of the EOS Project Science Office, located at NASA Goddard Space Flight Center | |||
}} | |||
</ref> As solar heating and “back radiation” from the atmosphere raise the surface temperature, the surface simultaneously releases an increasing amount of heat—equivalent to about 117 percent of incoming solar energy. The net upward heat flow from the Earth's surface is equivalent to 17 percent of incoming sunlight (117 percent up minus 100 percent down). | |||
Other minor sources of energy are usually ignored in the calculations, including accretion of ] and ], light from stars other than the Sun and the thermal radiation from space. Earlier, ] had claimed that deep space radiation was significant in a paper often cited as the first on the ].<ref name="JOURNAL-Fleming-1999" /> | |||
<!-- paragraph based on the following reference --><ref name="lindsey greenhouse effect p6"/> Some of the heat escapes directly to space, and the rest is transferred to higher and higher levels of the atmosphere, until the energy leaving the top of the atmosphere matches the amount of incoming solar energy. This includes solar radiation reflected from the Earth and outgoing heat radiation. Because the maximum possible amount of incoming sunlight is fixed by the ] (which depends only on Earth’s distance from the Sun and very small variations during the solar cycle), the natural greenhouse effect does not cause a runaway increase in surface temperature on Earth. | |||
== Budget analysis == | |||
] illustrating a balanced example of Earth's energy budget. Line thickness is linearly proportional to relative amount of energy.<ref name="BOOK-Sharma-2008" />]] | |||
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: | |||
: <math> ASR = OLR.</math> | |||
=== Internal flow analysis === | |||
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/m<sup>2</sup>), 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/m<sup>2</sup>) 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 ], 9 via convection and turbulence, and 6 as absorbed infrared by ]). The 48 units absorbed by the atmosphere (34 units from terrestrial energy and 14 from insolation) are then finally radiated back to space. This simplified example neglects some details of mechanisms that recirculate, store, and thus lead to further buildup of heat near the surface. | |||
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 Earth.<ref name="BOOK-Sharma-2008" /> | |||
=== Heat storage reservoirs === | |||
] Land, ice, and oceans are active material constituents of Earth's climate system along with the atmosphere. They have far greater mass and ], and thus much more ]. When radiation is directly absorbed or the surface temperature changes, ] will flow as ] either into or out of the bulk mass of these components via conduction/convection ] processes. The transformation of water between its solid/liquid/vapor states also acts as a source or sink of ] in the form of ]. These processes buffer the surface conditions against some of the rapid radiative changes in the atmosphere. As a result, the daytime versus nighttime difference in surface temperatures is relatively small. Likewise, Earth's climate system as a whole shows a ] to shifts in the atmospheric radiation balance.<ref>{{cite web |url=https://earthobservatory.nasa.gov/features/HeatBucket/heatbucket.php |title=Earth's Big Heat Bucket |publisher=NASA Earth Observatory |author=Michon Scott |date=2006-04-24}}</ref> | |||
The top few meters of Earth's oceans harbor more thermal energy than its entire atmosphere.<ref name=nvsohc>{{cite web |url=https://climate.nasa.gov/vital-signs/ocean-heat/ |title=Vital Signs of the Plant: Ocean Heat Content |publisher=NASA |accessdate=2021-11-15}}</ref> Like atmospheric gases, ]ic ocean waters transport vast amounts of such energy over the planet's surface. Sensible heat also moves into and out of great depths under conditions that favor ] or ].<ref>{{cite web |url=https://www.ametsoc.org/ams/index.cfm/education-careers/education-program/k-12-teachers/project-atmosphere/training-opportunities/project-atmosphere-peer-led-training/project-atmosphere-peer-training-resources/air-sea/ |title=Air-Sea interaction: Teacher's guide |publisher=] |year=2012 |access-date=2021-11-15}}</ref><ref>{{cite web|url=http://oceanmotion.org/html/background/upwelling-and-downwelling.htm|title=Ocean Motion : Definition : Wind Driven Surface Currents - Upwelling and Downwelling|access-date=2021-11-15}}</ref> | |||
Over 90 percent of the extra energy that has accumulated on Earth from ongoing global warming since 1970 has been ].<ref name=nvsohc/> About one-third has propagated to depths below 700 meters. The overall rate of growth has also risen during recent decades, reaching close to 500 TW (1 W/m<sup>2</sup>) as of 2020.<ref name=GeophysResLtrs_20210615 /><ref name=EarthSysSciData_20230417 /> That led to about 14 ] (ZJ) of heat gain for the year, exceeding the 570 ] (=160,000 TW-hr<ref name="owdprime">{{Cite journal |url=https://ourworldindata.org/grapher/global-primary-energy?country=~OWID_WRL |title=Global Direct Primary Energy Consumption |author1=Hannah Ritchie |author1-link=Hannah Ritchie |author2=Max Roser |author2-link=Max Roser |journal=Our World in Data |publisher=Published online at OurWorldInData.org. |year=2020 |access-date=2020-02-09}}</ref>) of ] consumed by humans by a factor of at least 20.<ref name="20220112_SciAm" /> | |||
=== Heating/cooling rate analysis === | |||
Generally speaking, changes to Earth's energy flux balance can be thought of as being the result of external ] (both natural and anthropogenic, radiative and non-radiative), system ], and ].<ref>{{cite book |title=Radiative Forcing of Climate Change: Expanding the Concept and Addressing Uncertainties |author=National Research Council |publisher=The National Academic Press |year=2005 |doi=10.17226/11175|isbn=978-0-309-09506-8 }}</ref> Such changes are primarily expressed as observable shifts 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 can then be analyzed over selected timeframes (Δt) as the net change in energy (ΔE) associated with these attributes: | |||
:<math> | |||
\begin{align} | |||
\Delta E / \Delta t &= ( \ \Delta E_T + \Delta E_C + \Delta E_W + \Delta E_A + \Delta E_G + \Delta E_S + \Delta E_I +... \ ) / \Delta t \\ | |||
\\ | |||
&= ASR - OLR. | |||
\end{align}</math> | |||
Here the term ΔE<sub>T</sub>, corresponding to the ], is negative-valued when temperature rises due to its strong direct influence on OLR.<ref name=Climate_20181215 /><ref name=GeophysResLtrs_20210615 /> | |||
The recent increase in trace greenhouse gases produces an enhanced greenhouse effect, and thus a positive ΔE<sub>G</sub> forcing term. By contrast, a large volcanic eruption (e.g. ], ] 1982) can inject sulfur-containing compounds into the upper atmosphere. High concentrations of ] may persist for up to a few years, yielding a negative forcing contribution to ΔE<sub>A</sub>.<ref name=RevGeophys20000501 /><ref name=GeophysResLtrs_20140718 /> Various other types of anthropogenic aerosol emissions make both positive and negative contributions to ΔE<sub>A</sub>. ]s produce ΔE<sub>I</sub> smaller in magnitude than those of recent ΔE<sub>G</sub> trends from human activity.<ref name=JOGR20120316 /><ref>{{cite web |url=https://climate.nasa.gov/blog/2910/what-is-the-suns-role-in-climate-change/ |title=What Is the Sun's Role in Climate Change? |publisher=NASA |date=2019-09-06}}</ref> | |||
Climate forcings are complex since they can produce direct and indirect ]s that intensify (]) or weaken (]) the original forcing. These often follow the temperature response. Water vapor trends as a positive feedback with respect to temperature changes due to evaporation shifts and the ]. An increase in water vapor results in positive ΔE<sub>W</sub> due to further enhancement of the greenhouse effect. A slower positive feedback is the ]. For example, the ] due to rising temperatures makes the region less reflective, leading to greater absorption of energy and even faster ice melt rates, thus positive influence on ΔE<sub>S</sub>.<ref name="WEB-NASA-LindseyPg7" /> Collectively, feedbacks tend to amplify global warming or cooling.<ref>Arias, P.A., N. Bellouin, E. Coppola, R.G. Jones, G. Krinner, J. Marotzke, V. Naik, M.D. Palmer, G.-K. Plattner, J. Rogelj, M. Rojas, J. Sillmann, T. Storelvmo, P.W. Thorne, B. Trewin, K. Achuta Rao, B. Adhikary, R.P. Allan, K. Armour, G. Bala, R. Barimalala, S. Berger, J.G. Canadell, C. Cassou, A. Cherchi, W. Collins, W.D. Collins, S.L. Connors, S. Corti, F. Cruz, F.J. Dentener, C. Dereczynski, A. Di Luca, A. Diongue Niang, F.J. Doblas-Reyes, A. Dosio, H. Douville, F. Engelbrecht, V. Eyring, E. Fischer, P. Forster, B. Fox-Kemper, J.S. Fuglestvedt, J.C. Fyfe, et al. 2021: . In . Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 33−144. doi:10.1017/9781009157896.002.</ref>{{rp|94}} | |||
Clouds are responsible for about half of Earth's ] and are powerful expressions of internal variability of the climate system.<ref name=RevGeophys_20150125 /><ref name=AGUAdv_20210811 /> They may also act as feedbacks to forcings, and could be forcings themselves if for example a result of ] activity. Contributions to ΔE<sub>C</sub> vary regionally and depending upon cloud type. Measurements from satellites are gathered in concert with simulations from models in an effort to improve understanding and reduce uncertainty.<ref>{{cite web |url=https://earthobservatory.nasa.gov/images/44250/clouds-and-global-warming |title=Clouds and Global Warming |publisher=NASA Earth Observatory |date=2010-06-10}}</ref> | |||
== Earth's energy imbalance (EEI) == | |||
] and can be measured by satellites. The ] is the "net absorbed" energy amount and grew from +0.6 W/m<sup>2</sup> (2009 est.<ref name="WEB-NASA-EnergyBudgetPoster" />) to above +1.0 W/m<sup>2</sup> in 2019.<ref name="GeophysResLtrs_20210615" />]]{{See also|Radiative forcing}} | |||
The Earth's energy imbalance (EEI) is defined as "the persistent and positive (downward) net top of atmosphere energy flux associated with greenhouse gas forcing of the climate system".<ref name="IPCC AR6 glossary" />{{rp|2227}} | |||
If Earth's incoming energy flux (ASR) is larger or smaller than the outgoing energy flux (OLR), then the planet will gain (warm) or lose (cool) net heat energy in accordance with the ]: | |||
: <math>EEI \equiv ASR - OLR</math>. | |||
Positive EEI thus defines the overall rate of planetary heating and is typically expressed as ]s per square ] (W/m<sup>2</sup>). During 2005 to 2019 the Earth's energy imbalance averaged about 460 ] or globally 0.90 ± 0.15 W per m<sup>2</sup>.<ref name="Trenberth2022" /> | |||
When Earth's energy imbalance (EEI) shifts by a sufficiently large amount, the shift is measurable by orbiting satellite-based instruments.<ref name=GeophysResLtrs_20140718 /><ref name=GeophysResLtrs_20090909 /> Imbalances that fail to reverse over time will also drive long-term temperature changes in the atmospheric, oceanic, land, and ice components of the climate system.<ref name=OpinEnvSus20091001 /> Temperature, sea level, ice mass and related shifts thus also provide measures of EEI.<ref name=EarthSysSciData_20230417 /> | |||
The biggest changes in EEI arise from changes in the composition of the atmosphere through human activities, thereby interfering with the natural flow of energy through the climate system.<ref name="Trenberth2022" /> The main changes are from increases in carbon dioxide and other greenhouse gases, that produce heating (positive EEI), and pollution. The latter refers to atmospheric aerosols of various kinds, some of which absorb energy while others reflect energy and produce cooling (or lower EEI). | |||
{|class=wikitable style="text-align:center;" | |||
|+Estimates of the Earth Energy Imbalance (EEI)<ref>{{cite journal |last1=Forster |first1=Piers M. |last2=Smith |first2=Chris |last3=Walsh |first3=Tristram |last4=Lamb |first4=William F. |last5=Lamboll |first5=Robin |display-authors=et al. |title=Indicators of Global Climate Change 2023: annual update of key indicators of the state of the climate system and human influence |journal=Earth System Science Data |volume=16 |issue=6 |pages=2625–2658 |year=2024 |doi=10.5194/essd-16-2625-2024 |doi-access=free }}</ref> | |||
|- | |||
! Time Period !! EEI (W/m<sup>2</sup>) | |||
'''Square brackets show 90% confidence intervals''' | |||
|- | |||
! 1971-2006 | |||
| 0.50 | |||
|- | |||
! 1971-2018 | |||
| 0.57 | |||
|- | |||
! 1976-2023 | |||
| 0.65 | |||
|- | |||
! 2006-2018 | |||
| 0.79 | |||
|- | |||
! 2011-2023 | |||
| 0.96 | |||
|} | |||
It is not (yet) possible to measure the absolute magnitude of EEI directly at top of atmosphere, although ''changes over time'' as observed by satellite-based instruments are thought to be accurate. The only practical way to estimate the absolute magnitude of EEI is through an inventory of the changes in energy in the climate system. The biggest of these ] is the ocean.<ref name="Trenberth2022" /> | |||
=== Energy inventory assessments === | |||
The planetary heat content that resides in the climate system can be compiled given the heat capacity, density and temperature distributions of each of its components. Most regions are now reasonably well sampled and monitored, with the most significant exception being the deep ocean.<ref>{{cite web |url=https://argo.ucsd.edu/expansion/deep-argo-mission/ |title=Deep Argo Mission |publisher=Scripps Institution of Oceanography, UC San Diego |access-date=26 November 2023}}</ref> | |||
] | |||
Estimates of the absolute magnitude of EEI have likewise been calculated using the measured temperature changes during recent multi-decadal time intervals. For the 2006 to 2020 period EEI was about {{val|+0.76|0.2|u=W/m2}} and showed a significant increase above the mean of {{val|+0.48|0.1|u=W/m2}} for the 1971 to 2020 period.<ref name=EarthSysSciData_20230417 /> | |||
EEI has been positive because temperatures have increased almost everywhere for over 50 years. ] (GST) is calculated by averaging temperatures measured at the surface of the sea along with air 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.<ref name="nasa temperatures">{{cite web |title=Global Annual Mean Surface Air Temperature Change |url=https://data.giss.nasa.gov/gistemp/graphs_v4/ |publisher=NASA |access-date=23 February 2020}}</ref> | |||
Ocean waters are especially effective absorbents of solar energy and have a far greater total ] than the atmosphere.<ref>{{cite web |url=https://www.climate.gov/news-features/understanding-climate/climate-change-ocean-heat-content |title=Climate Change: Ocean Heat Content |author=LuAnn Dahlman and Rebecca Lindsey |publisher=NOAA |date=2020-08-17}}</ref> Research vessels and stations have sampled sea temperatures at depth and around the globe since before 1960. Additionally, after the year 2000, an expanding network of nearly 4000 ] has measured the temperature anomaly, or equivalently the ] change (ΔOHC). Since at least 1990, OHC has increased at a steady or accelerating rate. ΔOHC represents the largest portion of EEI since oceans have thus far taken up over 90% of the net excess energy entering the system over time (Δt):<ref name=JClim20220715 /><ref name=RevGeophys20130819 /> | |||
: <math>EEI \gtrsim \Delta OHC / \Delta t</math>. | |||
Earth's outer crust and thick ice-covered regions have taken up relatively little of the excess energy. This is because excess heat at their surfaces flows inward only by means of ], and thus penetrates only several tens of centimeters on the daily cycle and only several tens of meters on the annual cycle.<ref>Lowrie, W. (2007). ''Fundamentals of geophysics''. Cambridge: CUP, 2nd ed.</ref> Much of the heat uptake goes either into melting ice and permafrost or into evaporating more water from soils. | |||
=== Measurements at top of atmosphere (TOA) === | |||
Several ]s measure the energy absorbed and radiated by Earth, and thus by inference the energy imbalance. These are located top of atmosphere (TOA) and provide data covering the globe. The ] ] (ERBE) project involved three such satellites: the ] (ERBS), launched October 1984; NOAA-9, launched December 1984; and NOAA-10, launched September 1986.<ref name="WEB-NASA-1997-satellites" /> | |||
] measurements (2005–2019). A rate of +1.0 W/m<sup>2</sup> summed over the planet's surface equates to a continuous heat uptake of about 500 ]s (~0.3% of the incident solar radiation).<ref name=GeophysResLtrs_20210615 /><ref name=em_20210622 />]] | |||
NASA's ] (CERES) instruments are part of its ] (EOS) since March 2000. CERES is designed to measure both solar-reflected (short wavelength) and Earth-emitted (long wavelength) radiation.<ref name="JOURNAL-Wielicki-1996" /> The CERES data showed increases in EEI from {{val|+0.42|0.48|u=W/m2}} in 2005 to {{val|+1.12|0.48|u=W/m2}} in 2019. Contributing factors included more water vapor, less clouds, increasing greenhouse gases, and declining ice that were partially offset by rising temperatures.<ref name=GeophysResLtrs_20210615/><ref name=em_20210622>{{cite web |url=https://earthobservatory.nasa.gov/blogs/earthmatters/2021/06/22/earths-radiation-budget-is-out-of-balance/ |title=Earth Matters: Earth's Radiation Budget is Out of Balance |author=Joseph Atkinson |publisher=NASA Earth Observatory |date=22 June 2021}}</ref> Subsequent investigation of the behavior using the ] CM4/AM4 ] concluded there was a less than 1% chance that internal climate variability alone caused the trend.<ref name=NatCliChg20210728 /> | |||
Other researchers have used data from CERES, ], ], and other EOS instruments to look for trends of ] embedded within the EEI data. Their analysis showed a forcing rise of {{val|+0.53|0.11|u=W/m2}} 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.<ref name="GeophysResLtrs_20210325" /><ref>{{cite web |url=https://news.umbc.edu/umbcs-ryan-kramer-confirms-human-caused-climate-change-with-direct-evidence-for-first-time/ |title=UMBC's Ryan Kramer confirms human-caused climate change with direct evidence for first time |publisher=University of Maryland, Baltimore County |date=12 April 2021 |author=Sarah Hansen}}</ref><ref>{{cite web |url=https://phys.org/news/2021-03-humans-earth-energy.html |title=Direct observations confirm that humans are throwing Earth's energy budget off balance |website=phys.org |date=26 March 2021}}</ref> | |||
Further satellite measurements including ] and ] data have 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.<ref name="JOURNAL-Stephens-2012" /> | |||
It is noteworthy that ] uncertainties limit the capability of the current generation of satellite-based instruments, which are otherwise stable and ]. As a result, relative changes in EEI are quantifiable with an ] which is not also achievable for any single measurement of the absolute imbalance.<ref name=NatGeo_20120122 /><ref name=JOC_20180115 /> | |||
=== Geodetic and hydrographic surveys === | |||
{{See also|Geodesy}} | |||
].<ref name="Marti21" />]] | |||
Observations since 1994 show that ice has retreated from every part of Earth at an accelerating rate.<ref name=Cryosphere20210125 /> ] has likewise risen as a consequence of the ice melt in combination with the overall rise in ocean temperatures.<ref>{{cite journal |author=WCRP Global Sea Level Budget Group |year=2018 |title=Global sea-level budget 1993–present |journal=Earth System Science Data |volume=10 |issue=3 |pages=1551–1590 |bibcode=2018ESSD...10.1551W |doi=10.5194/essd-10-1551-2018 |doi-access=free}}</ref> | |||
These shifts have contributed measurable changes to the geometric shape and gravity of the planet. | |||
Changes to the mass distribution of water within the hydrosphere and cryosphere have been deduced using ] observations by the ] satellite instruments. These data have been compared against ] and further ] observations using computational models that account for thermal expansion, salinity changes, and other factors. Estimates thereby obtained for ΔOHC and EEI have agreed with the other (mostly) independent assessments within uncertainties.<ref name="Marti21">{{cite journal |last1=Marti |first1=Florence |last2=Blazquez |first2=Alejandro |last3=Meyssignac |first3=Benoit |last4=Ablain |first4=Michaël |last5=Barnoud |first5=Anne |last6=Fraudeau |first6=Robin |last7=Jugier |first7=Rémi |last8=Chenal |first8=Jonathan |last9=Larnicol |first9=Gilles |last10=Pfeffer |first10=Julia |last11=Restano |first11=Marco |last12=Benveniste |first12=Jérôme |display-authors=5 |title=Monitoring the ocean heat content change and the Earth energy imbalance from space altimetry and space gravimetry |journal=Earth System Science Data |year=2021 |doi=10.5194/essd-2021-220 |doi-access=free}}</ref><ref>{{cite journal |last1=Hakuba |first1=M.Z. |last2=Frederikse |first2=T. |last3=Landerer |first3=F.W. |title=Earth's Energy Imbalance From the Ocean Perspective (2005–2019) |journal=Geophysical Research Letters |volume=48 |issue=16 |date=28 August 2021 |doi=10.1029/2021GL093624 |doi-access=free}}</ref> | |||
=== Importance as a climate change metric === | |||
Climate scientists ], ], and colleagues have identified the monitoring of Earth's energy imbalance as an important metric to help policymakers guide the pace for ] and ] measures. Because of ], longer-term EEI (Earth's energy imbalance) trends can forecast further changes that are "in the pipeline".<ref name=OpinEnvSus20091001 /><ref name=NatCliChg20160127 /><ref name=AtmosChemPhys20111222 /> | |||
Scientists found that the EEI is the most important metric related to climate change. It is the net result of all the processes and feedbacks in play in the climate system.<ref name="Trenberth2022">{{Cite journal |last1=Trenberth |first1=Kevin E |last2=Cheng |first2=Lijing |date=2022-09-01 |title=A perspective on climate change from Earth's energy imbalance |journal=Environmental Research: Climate |volume=1 |issue=1 |pages=013001 |doi=10.1088/2752-5295/ac6f74 |issn=2752-5295 |doi-access=free}} ] Text was copied from this source, which is available under a </ref> Knowing how much extra energy affects weather systems and rainfall is vital to understand the increasing weather extremes.<ref name="Trenberth2022" /> | |||
In 2012, ] scientists reported that to stop ] atmospheric CO<sub>2</sub> concentration would have to be reduced to 350 ppm or less, assuming all other climate forcings were fixed.<ref name="WEB-Hansen-2012-EnergyImbalance" /> As of 2020, atmospheric CO<sub>2</sub> reached 415 ppm and all ] exceeded a 500 ppm ] concentration due to continued growth in human emissions.<ref>{{cite web |url=https://gml.noaa.gov/aggi/ |title=NOAA's Annual Greenhouse Gas Index (An Introduction) |publisher=NOAA |access-date=2021-08-04}}</ref> | |||
== See also == | == See also == | ||
*] | * ] | ||
* ] | |||
*], for comparisons with other quantities of power. | |||
*] | * ] | ||
* ] | |||
* {{Portal-inline|Climate change}} | |||
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Latest revision as of 22:04, 11 November 2024
Concept for energy flows to and from Earth This article is about energy flows at and above Earth's surface. For Earth's internal heat, see Earth's internal heat budget.
Earth's energy budget (or Earth's energy balance) is the balance between the energy that Earth receives from the Sun and the energy the Earth loses 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 takes into account how energy moves through the climate system. The Sun heats the equatorial tropics more than the polar regions. Therefore, the amount of solar irradiance received by a certain region 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, surface albedo, clouds, and land use patterns. 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 than it gives back to space, and global cooling takes place when the outgoing energy is greater.
Multiple types of measurements and observations show a warming imbalance since at least year 1970. The rate of heating from this human-caused event is without precedent. The main origin of changes in the Earth's energy is from human-induced changes in the composition of the atmosphere. During 2005 to 2019 the Earth's energy imbalance (EEI) averaged about 460 TW or globally 0.90±0.15 W/m.
It takes time for any changes in the energy budget to result in any significant changes in the global surface temperature. This is due to the thermal inertia of the oceans, land and cryosphere. Most climate models make accurate calculations of this inertia, energy flows and storage amounts.
Definition
Earth's energy budget includes the "major energy flows of relevance for the climate system". These are "the top-of-atmosphere energy budget; the surface energy budget; changes in the global energy inventory and internal flows of energy within the climate system".
Earth's energy flows
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).
The main origin of changes in the Earth's energy is from human-induced changes in the composition of the atmosphere, amounting to about 460 TW or globally 0.90±0.15 W/m.
Incoming solar energy (shortwave radiation)
Main article: Solar irradianceThe 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 globally and yearly averaged TOA flux is one quarter of the solar constant and so is approximately 340 watts per square meter (W/m). Since the absorption varies with location as well as with diurnal, seasonal and annual variations, the numbers quoted are multi-year averages obtained from multiple satellite measurements.
Of the ~340 W/m of solar radiation received by the Earth, an average of ~77 W/m is reflected back to space by clouds and the atmosphere and ~23 W/m is reflected by the surface albedo, leaving ~240 W/m of solar energy input to the Earth's energy budget. This amount is called the absorbed solar radiation (ASR). It implies a value of about 0.3 for the mean net albedo of Earth, also called its Bond albedo (A):
Outgoing longwave radiation
Main articles: Outgoing longwave radiation and Greenhouse effectThermal energy leaves the planet in the form of outgoing longwave radiation (OLR). Longwave radiation is electromagnetic thermal radiation emitted by Earth's surface and atmosphere. Longwave radiation is in the infrared band. But, the terms are not synonymous, as infrared radiation can be either shortwave or longwave. Sunlight contains significant amounts of shortwave infrared radiation. A threshold wavelength of 4 microns is sometimes used to distinguish longwave and shortwave radiation.
Generally, absorbed solar energy is converted to different forms of heat energy. Some of the solar energy absorbed by the surface is converted to thermal radiation at wavelengths in the "atmospheric window"; this radiation is able to pass through the atmosphere unimpeded and directly escape to space, contributing to OLR. The remainder of absorbed solar energy is transported upwards through the atmosphere through a variety of heat transfer mechanisms, until the atmosphere emits that energy as thermal energy which is able to escape to space, again contributing to OLR. For example, heat is transported into the atmosphere via evapotranspiration and latent heat fluxes or conduction/convection processes, as well as via radiative heat transport. Ultimately, all outgoing energy is radiated into space in the form of longwave radiation.
The transport of longwave radiation from Earth's surface through its multi-layered atmosphere is governed by radiative transfer equations such as Schwarzschild's equation for radiative transfer (or more complex equations if scattering is present) and obeys Kirchhoff's law of thermal radiation.
A one-layer model produces an approximate description of OLR which yields temperatures at the surface (Ts=288 Kelvin) and at the middle of the troposphere (Ta=242 K) that are close to observed average values:
In this expression σ is the Stefan–Boltzmann constant and ε represents the emissivity of the atmosphere, which is less than 1 because the atmosphere does not emit within the wavelength range known as the atmospheric window.
Aerosols, clouds, water vapor, and trace greenhouse gases contribute to an effective value of about ε = 0.78. The strong (fourth-power) temperature sensitivity maintains a near-balance of the outgoing energy flow to the incoming flow via small changes in the planet's absolute temperatures.
As viewed from Earth's surrounding space, greenhouse gases influence the planet's atmospheric emissivity (ε). Changes in atmospheric composition can thus shift the overall radiation balance. For example, an increase in heat trapping by a growing concentration of greenhouse gases (i.e. an enhanced greenhouse effect) forces a decrease in OLR and a warming (restorative) energy imbalance. Ultimately when the amount of greenhouse gases increases or decreases, in-situ surface temperatures rise or fall until the absorbed solar radiation equals the outgoing longwave radiation, or ASR equals OLR.
Earth's internal heat sources and other minor effects
See also: Earth's internal heat budget and Anthropogenic heatThe geothermal heat flow from the Earth's interior is estimated to be 47 terawatts (TW) and split approximately equally between radiogenic heat and heat left over from the Earth's formation. This corresponds to an average flux of 0.087 W/m and represents only 0.027% of Earth's total energy budget at the surface, being dwarfed by the 173000 TW of incoming solar radiation.
Human production of energy is even lower at an average 18 TW, corresponding to an estimated 160,000 TW-hr, for all of year 2019. However, consumption is growing rapidly and energy production with fossil fuels also produces an increase in atmospheric greenhouse gases, leading to a more than 20 times larger imbalance in the incoming/outgoing flows that originate from solar radiation.
Photosynthesis also has a significant 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.
Budget analysis
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 analysis
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/m), 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/m) 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. This simplified example neglects some details of mechanisms that recirculate, store, and thus lead to further buildup of heat near the surface.
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 Earth.
Heat storage reservoirs
Land, ice, and oceans are active material constituents of Earth's climate system along with the atmosphere. They have far greater mass and heat capacity, and thus much more thermal inertia. When radiation is directly absorbed or the surface temperature changes, thermal energy will flow as sensible heat either into or out of the bulk mass of these components via conduction/convection heat transfer processes. The transformation of water between its solid/liquid/vapor states also acts as a source or sink of potential energy in the form of latent heat. These processes buffer the surface conditions against some of the rapid radiative changes in the atmosphere. As a result, the daytime versus nighttime difference in surface temperatures is relatively small. Likewise, Earth's climate system as a whole shows a slow response to shifts in the atmospheric radiation balance.
The top few meters of Earth's oceans harbor more thermal energy than its entire atmosphere. Like atmospheric gases, fluidic ocean waters transport vast amounts of such energy over the planet's surface. Sensible heat also moves into and out of great depths under conditions that favor downwelling or upwelling.
Over 90 percent of the extra energy that has accumulated on Earth from ongoing global warming since 1970 has been stored in the ocean. About one-third has propagated to depths below 700 meters. The overall rate of growth has also risen during recent decades, reaching close to 500 TW (1 W/m) as of 2020. That led to about 14 zettajoules (ZJ) of heat gain for the year, exceeding the 570 exajoules (=160,000 TW-hr) of total primary energy consumed by humans by a factor of at least 20.
Heating/cooling rate analysis
Generally speaking, changes to Earth's energy flux balance can be thought of as being the result of external forcings (both natural and anthropogenic, radiative and non-radiative), system feedbacks, and internal system variability. Such changes are primarily expressed as observable shifts 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 can then be analyzed over selected timeframes (Δt) as the net change in energy (ΔE) associated with these attributes:
Here the term ΔET, corresponding to the Planck response, is negative-valued when temperature rises due to its strong direct influence on OLR.
The recent increase in trace greenhouse gases produces an enhanced greenhouse effect, and thus a positive ΔEG forcing term. By contrast, a large volcanic eruption (e.g. Mount Pinatubo 1991, El Chichón 1982) can inject sulfur-containing compounds into the upper atmosphere. High concentrations of stratospheric sulfur aerosols may persist for up to a few years, yielding a negative forcing contribution to ΔEA. Various other types of anthropogenic aerosol emissions make both positive and negative contributions to ΔEA. Solar cycles produce ΔEI smaller in magnitude than those of recent ΔEG trends from human activity.
Climate forcings are complex since they can produce direct and indirect feedbacks that intensify (positive feedback) or weaken (negative feedback) the original forcing. These often follow the temperature response. Water vapor trends as a positive feedback with respect to temperature changes due to evaporation shifts and the Clausius-Clapeyron relation. An increase in water vapor results in positive ΔEW due to further enhancement of the greenhouse effect. A slower positive feedback is the ice-albedo feedback. For example, the loss of Arctic ice due to rising temperatures makes the region less reflective, leading to greater absorption of energy and even faster ice melt rates, thus positive influence on ΔES. Collectively, feedbacks tend to amplify global warming or cooling.
Clouds are responsible for about half of Earth's albedo and are powerful expressions of internal variability of the climate system. They may also act as feedbacks to forcings, and could be forcings themselves if for example a result of cloud seeding activity. Contributions to ΔEC vary regionally and depending upon cloud type. Measurements from satellites are gathered in concert with simulations from models in an effort to improve understanding and reduce uncertainty.
Earth's energy imbalance (EEI)
See also: Radiative forcingThe Earth's energy imbalance (EEI) is defined as "the persistent and positive (downward) net top of atmosphere energy flux associated with greenhouse gas forcing of the climate system".
If Earth's incoming energy flux (ASR) is larger or smaller than the outgoing energy flux (OLR), then the planet will gain (warm) or lose (cool) net heat energy in accordance with the law of energy conservation:
- .
Positive EEI thus defines the overall rate of planetary heating and is typically expressed as watts per square meter (W/m). During 2005 to 2019 the Earth's energy imbalance averaged about 460 TW or globally 0.90 ± 0.15 W per m.
When Earth's energy imbalance (EEI) shifts by a sufficiently large amount, the shift is measurable by orbiting satellite-based instruments. Imbalances that fail to reverse over time will also drive long-term temperature changes in the atmospheric, oceanic, land, and ice components of the climate system. Temperature, sea level, ice mass and related shifts thus also provide measures of EEI.
The biggest changes in EEI arise from changes in the composition of the atmosphere through human activities, thereby interfering with the natural flow of energy through the climate system. The main changes are from increases in carbon dioxide and other greenhouse gases, that produce heating (positive EEI), and pollution. The latter refers to atmospheric aerosols of various kinds, some of which absorb energy while others reflect energy and produce cooling (or lower EEI).
Time Period | EEI (W/m)
Square brackets show 90% confidence intervals |
---|---|
1971-2006 | 0.50 |
1971-2018 | 0.57 |
1976-2023 | 0.65 |
2006-2018 | 0.79 |
2011-2023 | 0.96 |
It is not (yet) possible to measure the absolute magnitude of EEI directly at top of atmosphere, although changes over time as observed by satellite-based instruments are thought to be accurate. The only practical way to estimate the absolute magnitude of EEI is through an inventory of the changes in energy in the climate system. The biggest of these energy reservoirs is the ocean.
Energy inventory assessments
The planetary heat content that resides in the climate system can be compiled given the heat capacity, density and temperature distributions of each of its components. Most regions are now reasonably well sampled and monitored, with the most significant exception being the deep ocean.
Estimates of the absolute magnitude of EEI have likewise been calculated using the measured temperature changes during recent multi-decadal time intervals. For the 2006 to 2020 period EEI was about +0.76±0.2 W/m and showed a significant increase above the mean of +0.48±0.1 W/m for the 1971 to 2020 period.
EEI has been positive because temperatures have increased almost everywhere for over 50 years. Global surface temperature (GST) is calculated by averaging temperatures measured at the surface of the sea along with air 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 a far greater total heat capacity than the atmosphere. Research vessels and stations have sampled sea temperatures at depth and around the globe since before 1960. Additionally, after the year 2000, an expanding network of nearly 4000 Argo robotic floats has measured the temperature anomaly, or equivalently the ocean heat content change (ΔOHC). Since at least 1990, OHC has increased at a steady or accelerating rate. ΔOHC represents the largest portion of EEI since oceans have thus far taken up over 90% of the net excess energy entering the system over time (Δt):
- .
Earth's outer crust and thick ice-covered regions have taken up relatively little of the excess energy. This is because excess heat at their surfaces flows inward only by means of thermal conduction, and thus penetrates only several tens of centimeters on the daily cycle and only several tens of meters on the annual cycle. Much of the heat uptake goes either into melting ice and permafrost or into evaporating more water from soils.
Measurements at top of atmosphere (TOA)
Several satellites measure the energy absorbed and radiated by Earth, and thus by inference the energy imbalance. These are located top of atmosphere (TOA) and provide data covering the globe. The NASA Earth Radiation Budget Experiment (ERBE) project involved 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 its Earth Observing System (EOS) since March 2000. CERES is designed to measure both solar-reflected (short wavelength) and Earth-emitted (long wavelength) radiation. The CERES data showed increases in EEI from +0.42±0.48 W/m in 2005 to +1.12±0.48 W/m in 2019. Contributing factors included more water vapor, less clouds, increasing greenhouse gases, and declining ice that were partially offset by rising temperatures. Subsequent investigation of the behavior using the GFDL CM4/AM4 climate model concluded there was a less than 1% chance that internal climate variability alone 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 analysis showed a forcing rise of +0.53±0.11 W/m 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.
Further satellite measurements including TRMM and CALIPSO data have 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.
Geodetic and hydrographic surveys
See also: GeodesyObservations since 1994 show that ice has retreated from every part of Earth at an accelerating rate. Mean global sea level has likewise risen as a consequence of the ice melt in combination with the overall rise in ocean temperatures. These shifts have contributed measurable changes to the geometric shape and gravity of the planet.
Changes to the mass distribution of water within the hydrosphere and cryosphere have been deduced using gravimetric observations by the GRACE satellite instruments. These data have been compared against ocean surface topography and further hydrographic observations using computational models that account for thermal expansion, salinity changes, and other factors. Estimates thereby obtained for ΔOHC and EEI have agreed with the other (mostly) independent assessments within uncertainties.
Importance as a climate change metric
Climate scientists Kevin Trenberth, James Hansen, and colleagues have identified the monitoring of Earth's energy imbalance as an important metric to help policymakers guide the pace for mitigation and adaptation measures. Because of climate system inertia, longer-term EEI (Earth's energy imbalance) trends can forecast further changes that are "in the pipeline".
Scientists found that the EEI is the most important metric related to climate change. It is the net result of all the processes and feedbacks in play in the climate system. Knowing how much extra energy affects weather systems and rainfall is vital to understand the increasing weather extremes.
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 reached 415 ppm and all long-lived greenhouse gases exceeded a 500 ppm CO2-equivalent concentration due to continued growth in human emissions.
See also
- Lorenz energy cycle
- Planetary equilibrium temperature
- Climate sensitivity
- Tipping points in the climate system
- Climate change portal
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External links
- NASA: The Atmosphere's Energy Budget
- Clouds and Earth's Radiant Energy System (CERES)
- NASA/GEWEX Surface Radiation Budget (SRB) Project
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