Albedo change inGreenland: the map shows the difference between the amount of sunlightGreenland reflected in the summer of 2011 versus the average percent it reflected between 2000 and 2006. Some areas reflect close to 20 percent less light than a decade ago.[1]
Albedo (/ælˈbiːdoʊ/ⓘal-BEE-doh; from Latin albedo'whiteness') is the fraction ofsunlight that isdiffusely reflected by a body. It is measured on a scale from 0 (corresponding to ablack body that absorbs all incident radiation) to 1 (corresponding to a body that reflects all incident radiation).Surface albedo is defined as the ratio ofradiosityJe to theirradianceEe (flux per unit area) received by a surface.[2] The proportion reflected is not only determined by properties of the surface itself, but also by the spectral and angular distribution of solar radiation reaching the Earth's surface.[3] These factors vary with atmospheric composition, geographic location, and time (seeposition of the Sun).
While directional-hemisphericalreflectance factor is calculated for a single angle of incidence (i.e., for a given position of the Sun), albedo is the directional integration of reflectance over all solar angles in a given period. The temporal resolution may range from seconds (as obtained from flux measurements) to daily, monthly, or annual averages.
Unless given for a specific wavelength (spectral albedo), albedo refers to the entire spectrum of solar radiation.[4] Due to measurement constraints, it is often given for the spectrum in which most solar energy reaches the surface (between 0.3 and 3 μm). This spectrum includesvisible light (0.4–0.7 μm), which explains why surfaces with a low albedo appear dark (e.g., trees absorb most radiation), whereas surfaces with a high albedo appear bright (e.g., snow reflects most radiation).
Ice–albedo feedback is apositive feedback climate process where a change in the area ofice caps,glaciers, andsea ice alters the albedo and surface temperature of a planet.Ice is very reflective, therefore it reflects far more solar energy back to space than the other types of land area or open water. Ice–albedo feedback plays an important role in globalclimate change.[5] Albedo is an important concept inclimate science.
The albedo in visible light ranges from about 0.9 to 0.95 for fresh snow to about 0.04 for charcoal, one of the darkest substances. Deeply shadowed cavities can achieve an effective albedo approaching the zero of ablack body. When seen from a distance, the ocean surface has a low albedo, as do most forests, whereas desert areas have some of the highest albedos among landforms. Most land areas are in an albedo range of 0.1 to 0.4.[14] The average albedo ofEarth is about 0.3.[15] This is far higher than for the ocean primarily because of the contribution of clouds.
Earth's surface albedo is regularly estimated viaEarth observation satellite sensors such asNASA'sMODIS instruments on board theTerra andAqua satellites, and the CERES instrument on theSuomi NPP andJPSS. As the amount of reflected radiation is only measured for a single direction by satellite, not all directions, a mathematical model is used to translate a sample set of satellite reflectance measurements into estimates ofdirectional-hemispherical reflectance and bi-hemispherical reflectance (e.g.,[16]). These calculations are based on thebidirectional reflectance distribution function (BRDF), which describes how the reflectance of a given surface depends on the view angle of the observer and the solar angle. BDRF can facilitate translations of observations of reflectance into albedo.[17]
Earth's average surface temperature due to its albedo and thegreenhouse effect is currently about 15 °C (59 °F). If Earth were frozen entirely (and hence be more reflective), the average temperature of the planet would drop below −40 °C (−40 °F).[18] If only the continental land masses became covered by glaciers, the mean temperature of the planet would drop to about 0 °C (32 °F).[19] In contrast, if the entire Earth was covered by water – a so-calledocean planet – the average temperature on the planet would rise to almost 27 °C (81 °F).[20]
In 2021, scientists reported that Earth dimmed by ~0.5% over two decades (1998–2017) as measured by earthshine using modern photometric techniques. This may have both been co-caused byclimate change as well as a substantial increase in global warming. However, the link to climate change has not been explored to date and it is unclear whether or not this represents an ongoing trend.[21][22]
with being the proportion of direct radiation from a given solar angle, and being the proportion of diffuse illumination, the actual albedo (also called blue-sky albedo) can then be given as:
This formula is important because it allows the albedo to be calculated for any given illumination conditions from a knowledge of the intrinsic properties of the surface.[23]
Earth's albedo as monitored by theCERES satellite system shows a darkening of Earth that has caused 1.7W/m2 warming since 2010.[24] That amount, only some of which isclimate forcing, is equivalent to a 138 ppm increase of atmospheric carbon dioxide.[24]Greenhouses of El Ejido, Almería, Spain
Human activities (e.g., deforestation, farming, and urbanization) change the albedo of various areas around the globe.[25]Human impacts to "the physical properties of the land surface can perturb the climate by altering the Earth's radiative energy balance" even on a small scale or when undetected by satellites.[26]
Urbanization generally decreases albedo (commonly being 0.01–0.02 lower than adjacentcroplands), which contributes toglobal warming. Deliberately increasing albedo in urban areas can mitigate theurban heat island effect. An estimate in 2022 found that on a global scale, "an albedo increase of 0.1 in worldwide urban areas would result in a cooling effect that is equivalent to absorbing ~44Gt of CO2 emissions."[27]
Intentionally enhancing the albedo of the Earth's surface, along with its daytimethermal emittance, has been proposed as asolar radiation management strategy to mitigateenergy crises and global warming known aspassive daytime radiative cooling (PDRC).[28][29][30] Efforts toward widespread implementation of PDRCs may focus on maximizing the albedo of surfaces from very low to high values, so long as a thermal emittance of at least 90% can be achieved.[31]
The tens of thousands ofhectares of greenhouses inAlmería, Spain form a large expanse of whitened plastic roofs. A 2008 study found that this anthropogenic change lowered the local surface area temperature of the high-albedo area, although changes were localized.[26] A follow-up study found that "CO2-eq. emissions associated to changes in surface albedo are a consequence of land transformation" and can reduce surface temperature increases associated with climate change.[32]
Albedo is not directly dependent on the illumination because changing the amount of incoming light proportionally changes the amount of reflected light, except in circumstances where a change in illumination induces a change in the Earth's surface at that location (e.g. through melting of reflective ice). However, albedo and illumination both vary by latitude. Albedo is highest near the poles and lowest in the subtropics, with a local maximum in the tropics.[33]
The intensity of albedo temperature effects depends on the amount of albedo and the level of localinsolation (solar irradiance); high albedo areas in theArctic andAntarctic regions are cold due to low insolation, whereas areas such as theSahara Desert, which also have a relatively high albedo, will be hotter due to high insolation.Tropical andsub-tropicalrainforest areas have low albedo, and are much hotter than theirtemperate forest counterparts, which have lower insolation. Because insolation plays such a big role in the heating and cooling effects of albedo, high insolation areas like the tropics will tend to show a more pronounced fluctuation in local temperature when local albedo changes.[34]
Arctic regions notably release more heat back into space than what they absorb, effectively cooling theEarth. Since arctic ice andsnow have been melting at higher rates due to higher temperatures, creating regions in the arctic that are notably darker (being water or ground which is darker color), there is concern because less heat is reflected back into space. Thisfeedback loop results in a reduced albedo effect.[35]
Albedo affectsclimate by determining how muchradiation a planet absorbs.[38] The uneven heating of Earth from albedo variations between land, ice, or ocean surfaces can driveweather.[39]
When an area's albedo changes due to snowfall, a snow–temperaturefeedback results. A layer of snowfall increases local albedo, reflecting away sunlight, leading to local cooling. In principle, if no outside temperature change affects this area (e.g., a warmair mass), the raised albedo and lower temperature would maintain the current snow and invite further snowfall, deepening the snow–temperature feedback. However, because localweather is dynamic due to the change ofseasons, eventually warm air masses and a more direct angle of sunlight (higherinsolation) cause melting. When the melted area reveals surfaces with lower albedo, such as grass, soil, or ocean, the effect is reversed: the darkening surface lowers albedo, increasing local temperatures, which induces more melting and thus reducing the albedo further, resulting in still more heating.
Snow albedo is highly variable, ranging from as high as 0.9 for freshly fallen snow, to about 0.4 for melting snow, and as low as 0.2 for dirty snow.[42] OverAntarctica, snow albedo averages a little more than 0.8. If a marginally snow-covered area warms, snow tends to melt, lowering the albedo, and hence leading to more snowmelt because more radiation is being absorbed by the snowpack (referred to as theice–albedopositive feedback).
InSwitzerland, the citizens have been protecting their glaciers with large white tarpaulins to slow down the ice melt. These large white sheets are helping to reject the rays from the sun and defecting the heat. Although this method is very expensive, it has been shown to work, reducing snow and ice melt by 60%.[43]
Just as fresh snow has a higher albedo than does dirty snow, the albedo of snow-covered sea ice is far higher than that of sea water. Sea water absorbs moresolar radiation than would the same surface covered with reflective snow. When sea ice melts, either due to a rise in sea temperature or in response to increased solar radiation from above, the snow-covered surface is reduced, and more surface of sea water is exposed, so the rate of energy absorption increases. The extra absorbed energy heats the sea water, which in turn increases the rate at which sea ice melts. As with the preceding example of snowmelt, the process of melting of sea ice is thus another example of a positive feedback.[44] Both positive feedback loops have long been recognized as important forglobal warming.[citation needed]
Cryoconite, powdery windblowndust containing soot, sometimes reduces albedo on glaciers and ice sheets.[45]
The dynamical nature of albedo in response to positive feedback, together with the effects of small errors in the measurement of albedo, can lead to large errors in energy estimates. Because of this, in order to reduce the error of energy estimates, it is important to measure the albedo of snow-covered areas throughremote sensing techniques rather than applying a single value for albedo over broad regions.[46]
Albedo works on a smaller scale, too. In sunlight, dark clothes absorb more heat and light-coloured clothes reflect it better, thus allowing some control over body temperature by exploiting the albedo effect of the colour of external clothing.[47]
Albedo can affect theelectrical energy output of solarphotovoltaic devices. For example, the effects of a spectrally responsive albedo are illustrated by the differences between the spectrally weighted albedo of solar photovoltaic technology based on hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si)-based compared to traditional spectral-integrated albedo predictions. Research showed impacts of over 10% for vertically (90°) mounted systems, but such effects were substantially lower for systems with lower surface tilts.[48] Spectral albedo strongly affects the performance ofbifacial solar cells where rear surface performance gains of over 20% have been observed for c-Si cells installed above healthy vegetation.[49] An analysis on the bias due to the specular reflectivity of 22 commonly occurring surface materials (both human-made and natural) provided effective albedo values for simulating the performance of seven photovoltaic materials mounted on three common photovoltaic system topologies: industrial (solar farms), commercial flat rooftops and residential pitched-roof applications.[50]
This section needs to beupdated. The reason given is: the references used are quite old; there must be more updated information available in theIPCC Sixth Assessment Report. Please help update this article to reflect recent events or newly available information.(March 2023)
Forests generally have a low albedo because the majority of the ultraviolet andvisible spectrum is absorbed throughphotosynthesis. For this reason, the greater heat absorption by trees could offset some of the carbon benefits ofafforestation (or offset the negative climate impacts ofdeforestation). In other words: Theclimate change mitigation effect ofcarbon sequestration by forests is partially counterbalanced in thatreforestation can decrease the reflection of sunlight (albedo).[51]
In the case of evergreen forests with seasonal snow cover, albedo reduction may be significant enough for deforestation to cause a net cooling effect.[52] Trees also impact climate in extremely complicated ways throughevapotranspiration. The water vapor causes cooling on the land surface, causes heating where it condenses, acts as strong greenhouse gas, and can increase albedo when it condenses into clouds.[53] Scientists generally treat evapotranspiration as a net cooling impact, and the net climate impact of albedo and evapotranspiration changes from deforestation depends greatly on local climate.[54]
Mid-to-high-latitude forests have a much lower albedo during snow seasons than flat ground, thus contributing to warming. Modeling that compares the effects of albedo differences between forests and grasslands suggests that expanding the land area of forests in temperate zones offers only a temporary mitigation benefit.[55][56][57][58]
In seasonally snow-covered zones, winter albedos of treeless areas are 10% to 50% higher than nearby forested areas because snow does not cover the trees as readily.Deciduous trees have an albedo value of about 0.15 to 0.18 whereasconiferous trees have a value of about 0.09 to 0.15.[9] Variation in summer albedo across both forest types is associated with maximum rates of photosynthesis because plants with high growth capacity display a greater fraction of their foliage for direct interception of incoming radiation in the upper canopy.[59] The result is that wavelengths of light not used in photosynthesis are more likely to be reflected back to space rather than being absorbed by other surfaces lower in the canopy.
Studies by theHadley Centre have investigated the relative (generally warming) effect of albedo change and (cooling) effect ofcarbon sequestration on planting forests. They found that new forests in tropical and midlatitude areas tended to cool; new forests in high latitudes (e.g., Siberia) were neutral or perhaps warming.[52]
Research in 2023, drawing from 176 flux stations globally, revealed a climate trade-off: increased carbon uptake fromafforestation results in reduced albedo. Initially, this reduction may lead to moderate global warming over a span of approximately 20 years, but it is expected to transition into significant cooling thereafter.[60]
Reflectivity of smooth water at 20 °C (68 °F) (refractive index=1.333)
Water reflects light very differently from typical terrestrial materials. The reflectivity of a water surface is calculated using theFresnel equations.
At the scale of the wavelength of light even wavy water is always smooth so the light is reflected in a locallyspecular manner (notdiffusely). The glint of light off water is a commonplace effect of this. At smallangles of incident light,waviness results in reduced reflectivity because of the steepness of the reflectivity-vs.-incident-angle curve and a locally increased average incident angle.[61]
Although the reflectivity of water is very low at low and medium angles of incident light, it becomes very high at high angles of incident light such as those that occur on the illuminated side of Earth near theterminator (early morning, late afternoon, and near the poles). However, as mentioned above, waviness causes an appreciable reduction. Because light specularly reflected from water does not usually reach the viewer, water is usually considered to have a very low albedo in spite of its high reflectivity at high angles of incident light.
Note that white caps on waves look white (and have high albedo) because the water is foamed up, so there are many superimposed bubble surfaces which reflect, adding up their reflectivities. Fresh 'black' ice exhibits Fresnel reflection.Snow on top of this sea ice increases the albedo to 0.9.[62]
Cloud albedo has substantial influence over atmospheric temperatures. Different types of clouds exhibit different reflectivity, theoretically ranging in albedo from a minimum of near 0 to a maximum approaching 0.8. "On any given day, about half of Earth is covered by clouds, which reflect more sunlight than land and water. Clouds keep Earth cool by reflecting sunlight, but they can also serve as blankets to trap warmth."[63]
Albedo and climate in some areas are affected by artificial clouds, such as those created by thecontrails of heavy commercial airliner traffic.[64] A study following the burning of the Kuwaiti oil fields during Iraqi occupation showed that temperatures under the burning oil fires were as much as 10 °C (18 °F) colder than temperatures several miles away under clear skies.[65]
Aerosols (very fine particles/droplets in the atmosphere) have both direct and indirect effects on Earth's radiative balance. The direct (albedo) effect is generally to cool the planet; the indirect effect (the particles act ascloud condensation nuclei and thereby change cloud properties) is less certain.[66]
Another albedo-related effect on the climate is fromblack carbon particles. The size of this effect is difficult to quantify: theIntergovernmental Panel on Climate Change estimates that the global meanradiative forcing for black carbon aerosols from fossil fuels is +0.2 W m−2, with a range +0.1 to +0.4 W m−2.[67] Black carbon is a bigger cause of the melting of the polar ice cap in the Arctic than carbon dioxide due to its effect on the albedo.[68][failed verification]
The moonTitan is darker thanSaturn even though they receive the same amount of sunlight. This is due to a difference in albedo (0.22 versus 0.499 ingeometric albedo).
In astronomy, the termalbedo can be defined in several different ways, depending upon the application and the wavelength of electromagnetic radiation involved.
The albedos ofplanets,satellites andminor planets such asasteroids can be used to infer much about their properties. The study of albedos, their dependence on wavelength, lighting angle ("phase angle"), and variation in time composes a major part of the astronomical field ofphotometry. For small and far objects that cannot be resolved by telescopes, much of what we know comes from the study of their albedos. For example, the absolute albedo can indicate the surface ice content of outerSolar System objects, the variation of albedo with phase angle gives information aboutregolith properties, whereas unusually high radar albedo is indicative of high metal content inasteroids.
Enceladus, a moon of Saturn, has one of the highest known optical albedos of any body in the Solar System, with an albedo of 0.99. Another notable high-albedo body isEris, with an albedo of 0.96.[69] Many small objects in the outer Solar System[70] andasteroid belt have low albedos down to about 0.05.[71] A typicalcomet nucleus has an albedo of 0.04.[72] Such a dark surface is thought to be indicative of a primitive and heavilyspace weathered surface containing someorganic compounds.
The overall albedo of theMoon is measured to be around 0.14,[73] but it is strongly directional and non-Lambertian, displaying also a strongopposition effect.[74] Although such reflectance properties are different from those of any terrestrial terrains, they are typical of theregolith surfaces of airless Solar System bodies.
Two common optical albedos that are used in astronomy are the (V-band)geometric albedo (measuring brightness when illumination comes from directly behind the observer) and theBond albedo (measuring total proportion of electromagnetic energy reflected). Their values can differ significantly, which is a common source of confusion.
In detailed studies, the directional reflectance properties of astronomical bodies are often expressed in terms of the fiveHapke parameters which semi-empirically describe the variation of albedo withphase angle, including a characterization of the opposition effect ofregolith surfaces. One of these five parameters is yet another type of albedo called thesingle-scattering albedo. It is used to define scattering of electromagnetic waves on small particles. It depends on properties of the material (refractive index), the size of the particle, and the wavelength of the incoming radiation.
An important relationship between an object's astronomical (geometric) albedo,absolute magnitude and diameter is given by:[85]where is the astronomical albedo, is the diameter in kilometers, and is the absolute magnitude.
In planetaryradar astronomy, a microwave (or radar) pulse is transmitted toward a planetary target (e.g. Moon, asteroid, etc.) and the echo from the target is measured. In most instances, the transmitted pulse iscircularly polarized and the received pulse is measured in the same sense of polarization as the transmitted pulse (SC) and the opposite sense (OC).[86][87] The echo power is measured in terms ofradar cross-section,,, or (total power, SC + OC) and is equal to the cross-sectional area of a metallic sphere (perfect reflector) at the same distance as the target that would return the same echo power.[86]
Those components of the received echo that return from first-surface reflections (as from a smooth or mirror-like surface) are dominated by the OC component as there is a reversal in polarization upon reflection. If the surface is rough at the wavelength scale or there is significant penetration into the regolith, there will be a significant SC component in the echo caused by multiple scattering.[87]
For most objects in the solar system, the OC echo dominates and the most commonly reported radar albedo parameter is the (normalized) OC radar albedo (often shortened to radar albedo):[86]
where the denominator is the effective cross-sectional area of the target object with mean radius,. A smooth metallic sphere would have.
The values reported for the Moon, Mercury, Mars, Venus, and Comet P/2005 JQ5 are derived from the total (OC+SC) radar albedo reported in those references.
In the event that most of the echo is from first surface reflections ( or so), the OC radar albedo is a first-order approximation of the Fresnel reflection coefficient (aka reflectivity)[87] and can be used to estimate the bulk density of a planetary surface to a depth of a meter or so (a few wavelengths of the radar wavelength which is typically at the decimeter scale) using the following empirical relationships:[91]
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^Henderson-Sellers, A.; Wilson, M. F. (1983). "The Study of the Ocean and the Land Surface from Satellites".Philosophical Transactions of the Royal Society of London A.309 (1508):285–294.Bibcode:1983RSPTA.309..285H.doi:10.1098/rsta.1983.0042.JSTOR37357.S2CID122094064.Albedo observations of the Earth's surface for climate research
^Wang, Tong; Wu, Yi; Shi, Lan; Hu, Xinhua; Chen, Min; Wu, Limin (2021)."A structural polymer for highly efficient all-day passive radiative cooling".Nature Communications.12 (365): 365.doi:10.1038/s41467-020-20646-7.PMC7809060.PMID33446648.Accordingly, designing and fabricating efficient PDRC with sufficiently high solar reflectance (𝜌¯solar) (λ ~ 0.3–2.5 μm) to minimize solar heat gain and simultaneously strong LWIR thermal emittance (ε¯LWIR) to maximize radiative heat loss is highly desirable. When the incoming radiative heat from the Sun is balanced by the outgoing radiative heat emission, the temperature of the Earth can reach its steady state.
^Chen, Meijie; Pang, Dan; Chen, Xingyu; Yan, Hongjie; Yang, Yuan (October 2021)."Passive daytime radiative cooling: Fundamentals, material designs, and applications".EcoMat.4 e12153.doi:10.1002/eom2.12153.S2CID240331557.Passive daytime radiative cooling (PDRC) dissipates terrestrial heat to the extremely cold outer space without using any energy input or producing pollution. It has the potential to simultaneously alleviate the two major problems of energy crisis and global warming.
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