The albedo effect and global warming. What is albedo? Albedo of sand

When astronomers talk about the reflective properties of the surfaces of planets and satellites, they often use the term albedo. However, turning to reference books and encyclopedias for an explanation of this concept, we learn that there are many different types of albedo: true, visible, normal, flat, monochromatic, spherical, and so on. There is something to be sad about. Therefore, let's try to understand this cycle of terms.

The word "albedo" itself comes from the Latin albedo - whiteness. In its most general form, this is the name for the fraction of incident radiation reflected by a solid surface or scattered by a semitransparent body. Since the magnitude of the reflected radiation cannot exceed the magnitude of the incident radiation, this ratio, that is, the albedo, is always in the range from 0 to 1. The higher its value, the more incident light will be reflected.

The visibility of all non-self-luminous bodies is completely determined by their albedo, that is, their reflectivity. We could even say that we simply would not see non-self-luminous objects if they could not reflect light. Thanks to this property, we "by eye" determine the shape of the body, the nature of the material, its hardness and other characteristics. However, a skillfully selected albedo can hide an object from us - remember military camouflage or the stealth aircraft. When studying the bodies of the solar system, measuring the albedo helps to find out the nature of the material on the surface of a celestial body, its structure and even its chemical composition.

We easily distinguish snow from asphalt because snow almost completely reflects light, and asphalt almost completely absorbs it. However, we can also easily distinguish snow from a sheet of polished aluminum, although both of them almost completely reflect light. This means that just knowing the fraction of reflected light is not enough to judge the nature of the material. Snow scatters light diffusely in all directions, while aluminum reflects specularly. To take into account these and other features of reflection, several types of albedo are distinguished.

True (absolute) albedo coincides with the so-called diffuse reflection coefficient: this is the ratio of the flux scattered by a flat surface element in all directions to the flux incident on it.

To measure the true albedo, laboratory conditions are required, because it is necessary to take into account the light scattered by the body in all directions. For "field" conditions, it is more natural apparent albedo is the ratio of the brightness of a flat surface element illuminated by a parallel beam of rays to the brightness of an absolutely white surface located perpendicular to the rays and having a true albedo equal to unity.

If a surface is illuminated and observed at an angle of 90 degrees, then its apparent albedo is called normal... The normal albedo of pure snow is close to 1.0, and that of charcoal is about 0.04.

Astronomy often uses geometric (flat) albedo - the ratio of the illumination on Earth created by the planet in full phase to the illumination that would be created by a flat absolutely white screen of the same size as the planet, referred to its place and located perpendicular to the line of sight and the sun's rays. Astronomers usually express the physical concept of "illumination" by their word "brilliance" and measure it in stellar magnitudes.

It is clear that the albedo value affects the brilliance of celestial objects as strongly as their size and position in the solar system. For example, if the asteroids Ceres and Vesta were placed side by side, then their brightness would be almost the same, although the diameter of Ceres is twice that of Vesta. The fact is that the surface of Ceres reflects light much worse: the albedo of Vesta is about 0.35, while Ceres has only 0.09.

The albedo value depends both on the surface properties and on the spectrum of the incident radiation. Therefore, the albedo is measured separately for different spectral ranges (optical, ultraviolet, infrared, and so on) or even for individual wavelengths (monochromatic albedo). Studying the change in albedo with wavelength and comparing the obtained curves with the same curves for terrestrial minerals, soil samples and various rocks, one can draw some conclusions about the composition and structure of the surface of planets and their satellites.

To calculate the energy balance of the planets, we use spherical albedo (Bond albedo), introduced by the American astronomer George Bond in 1861. This is the ratio of the radiation flux reflected by the entire planet to the flux incident on it. In order to accurately calculate the spherical albedo, generally speaking, it is necessary to observe the planet at all possible phase angles (angle Sun-planet-Earth). Previously, this was only possible for the inner planets and the Moon. With the advent of artificial satellites, astronomers were able to calculate the spherical albedo of the Earth, and interplanetary spacecraft made it possible to do this for the outer planets. The Bond albedo of the Earth is about 0.33, and the reflection of light from clouds plays a very important role in it. It is 0.12 for the moon without an atmosphere, and 0.76 for Venus, covered with a powerful cloudy atmosphere.

Naturally, different parts of the surface of celestial bodies, which have different structure, composition, and origin, have different albedos. You can see for yourself by looking at the moon. The seas on its surface have extremely low albedo, in contrast to, say, the ray structures of some craters. By the way, observing ray structures, you will easily notice that their appearance strongly depends on the angle at which the Sun illuminates them. This is precisely due to a change in their albedo, which takes on a maximum value when the rays fall perpendicular to the surface of the Moon, where these formations are located.

And one more experiment. Look at the Moon through a telescope (or a planet, preferably Mars or Jupiter) with different light filters. And you will see that, for example, in red rays, the surface of the moon looks slightly different than in blue. This suggests that radiation of different wavelengths are reflected from its surface in different ways.

But what kind of albedo you need to talk about in the examples described above, try to guess for yourself.

The long-term albedo trend is towards cooling. Satellite measurements show little trend in recent years.

Changing the Earth's albedo is potentially a powerful climate change. As albedo, or reflectivity, increases, more sunlight is reflected back into space. This has a cooling effect on global temperatures. On the contrary, a decrease in albedo heats up the planet. A change in albedo of only 1% gives a radiation effect of 3.4 W / m2, comparable to the effect of CO2 doubling. How has albedo affected global temperatures in recent decades?

Albedo trends before 2000

The albedo of the Earth is determined by several factors. Snow and ice reflect light well, so when they melt, the albedo goes down. Forests have a lower albedo than open areas, so deforestation increases albedo (let's make a reservation that the destruction of all forests will not stop global warming). Aerosols have a direct and indirect effect on albedo. The direct influence is the reflection of sunlight into space. An indirect effect consists in the action of aerosol particles as centers of moisture condensation, which affects the formation and lifetime of clouds. Clouds, in turn, affect global temperatures in several ways. They cool the climate by reflecting sunlight, but they can also have a heating effect by trapping the outgoing infrared radiation.

All of these factors should be taken into account when adding up the various radiative forcings that determine climate. Land-use changes are calculated from historical reconstructions of changes in the composition of arable land and pasture. Observations from satellites and from the ground make it possible to determine trends in aerosol level and cloud albedo. It can be seen that the cloud albedo is the strongest factor among the various albedos. The long-term trend is directed towards the cooling, the impact is -0.7 W / m2 from 1850 to 2000.

Fig. 1 Average annual total radiative forcing(Chapter 2 of the IPCC AR4).

Albedo trends after 2000.

One of the ways to measure the albedo of the Earth is the ash light of the Moon. This is sunlight, first reflected by the Earth and then reflected by the Moon back to Earth at night. The moon's ash light has been measured by the Big Bear Solar Observatory since November 1998 (a number of measurements were also made in 1994 and 1995). Fig. 2 shows albedo changes from satellite data reconstruction (black line) and from measurements of the moon's ash light (blue line) (Palle 2004).

Fig. 2 Albedo changes reconstructed from ISCCP satellite data (black line) and from changes in the moon's ash light (dark line). The right vertical bar shows the negative radiative forcing (ie, cooling) (Palle 2004).

The data in Figure 2 is problematic. Black line, ISCCP satellite data reconstruction " is a purely statistical parameter and has little physical meaning, since it does not take into account the nonlinear relationships between cloud-surface properties and planetary albedo, and also does not include aerosol albedo changes, for example, associated with Mount Pinatubo or anthropogenic sulfate emissions"(Real Climate).

Even more problematic is the albedo peak around 2003, visible on the moon's blue ash light line. It strongly contradicts the satellite data showing a slight trend at this time. For comparison, we can recall the eruption of Pinatubo in 1991, which filled the atmosphere with aerosols. These aerosols reflected sunlight, creating a negative radiative forcing of 2.5 W / m2. This has dramatically lowered global temperatures. The ash light data then showed an exposure of almost -6 W / m2, which should have meant an even greater drop in temperature. No similar events happened in 2003. (Wielicki 2007).

In 2008, the reason for the discrepancy was discovered. The Big Bear Observatory installed a new telescope to measure the moon's ash light in 2004. With the new and improved data, they re-calibrated their old data and revised their albedo estimates (Palle 2008). Figure: 3 shows the old (black line) and updated (blue line) albedo values. The abnormal 2003 peak has disappeared. However, the trend of increasing albedo from 1999 to 2003 persisted.

Figure: 3 Change in the albedo of the Earth according to the measurements of the ash light of the Moon. Black line - albedo changes from 2004 publication (Palle 2004). Blue line - updated albedo changes after improved data analysis procedure, data for a longer period of time are also included (Palle 2008).

How accurately is albedo determined from the ash light of the moon? The method is not global in scope. It affects about a third of the Earth in each observation, some areas always remain "invisible" from the observation site. In addition, measurements are infrequent and are made in a narrow wavelength range of 0.4-0.7 µm (Bender 2006).

In contrast, satellite data such as CERES are a global measurement of the Earth's shortwave radiation, incorporating all the effects of surface and atmospheric properties. Compared to ash light measurements, they cover a wider range (0.3-5.0 µm). Analysis of the CERES data shows no long-term albedo trend from March 2000 to June 2005. Comparison with three independent datasets (MODIS, MISR and SeaWiFS) shows a "remarkable fit" of all 4 results (Loeb 2007a).

Figure: 4 Monthly changes in mean values \u200b\u200bCERES SW TOA flux and MODIS cloud fraction ().

Albedo has been affecting global temperatures - mostly cooling in the long term. In terms of recent trends, ashlight data show albedo increases from 1999 to 2003, with little change since 2003. The satellites show little change since 2000. The radiative forcing from albedo changes in recent years is minimal.

Lambertian (true, flat) albedo

True or flat albedo - the coefficient of diffuse reflection, that is, the ratio of the luminous flux scattered by a flat surface element in all directions to the flux incident on this element.
In the case of illumination and observation normal to the surface, the true albedo is called normal .

The normal albedo of pure snow is ~ 0.9, of charcoal ~ 0.04.

Geometric albedo

Geometric optical albedo of the Moon - 0.12, Earth - 0.367.

Bond (spherical) albedo

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See what "Albedo" is in other dictionaries:

ALBEDO, the fraction of light or other radiation reflected from a surface. An ideal reflector has an albedo of 1, while a real reflector has a lower albedo. The snow albedo ranges from 0.45 to 0.90; albedo of the Earth, from artificial satellites, ... ... Scientific and technical encyclopedic dictionary

- (Arabic). A term in photometry that indicates how much of a given surface reflects light rays. Dictionary of foreign words included in the Russian language. Chudinov AN, 1910. albedo (Latin albus light) is a value characterizing ... ... Dictionary of foreign words of the Russian language

ALBEDO - (late Lat. albedo, from Lat. albus white), a value characterizing the ratio between the flow of solar radiation falling on various objects, soil or snow cover, and the amount of such radiation absorbed or reflected by them; ... ... Ecological Dictionary

- (from late lat. albedo whiteness) a value that characterizes the ability of a surface to reflect a flux of electromagnetic radiation or particles falling on it. Albedo is equal to the ratio of the reflected flux to the incident flux. An important characteristic in astronomy ... ... Big Encyclopedic Dictionary

albedo - not. albédo m. lat. albedo. white. 1906. Lexis. Inner white layer of citrus peel. Food industry. Lex. Brockg .: albedo; SIS 1937: Albe / before ... Historical Dictionary of Russian Gallicisms

albedo - Characteristic of the reflectivity of the body surface; is determined by the ratio of the luminous flux reflected (scattered) by this surface to the luminous flux falling on it [Terminological dictionary for construction in 12 languages \u200b\u200b... ... Technical translator's guide

albedo - The ratio of solar radiation reflected from the earth's surface to the intensity of radiation falling on it is expressed as a percentage or decimal fractions (the average albedo of the Earth is 33%, or 0.33). → Fig. five … Geography Dictionary

- (from late lat. albedo whiteness), a value that characterizes the surface ability to. l. body to reflect (scatter) the incident radiation. Distinguish between true, or Lambert, A., coinciding with coeff. diffuse (scattered) reflection, and ... ... Physical encyclopedia

Sush., Number of synonyms: 1 characteristic (9) ASIS synonym dictionary. V.N. Trishin. 2013 ... Synonym dictionary

A value that characterizes the reflectivity of any surface; is expressed by the ratio of radiation reflected by the surface to solar radiation arriving at the surface (for chernozem 0.15; sand 0.3 0.4; average A. Earth 0.39; Moon 0.07) ... ... Business Glossary

The total radiation that reaches the earth's surface is partially absorbed by soil and water bodies and turns into heat, in the oceans and seas it is spent on evaporation, and is partially reflected into the atmosphere (reflected radiation). The ratio of the absorbed and reflected radiant energy depends on the nature of the land, on the angle of incidence of the rays on the water surface. Since the absorbed energy is practically impossible to measure, the value of the reflected energy is determined.

The reflectivity of land and water surfaces is called their albedo... It is calculated in% of the reflected radiation from the incident on a given surface, soaring with the angle (more precisely, the sine of the angle) of incidence of the rays and the amount of optical masses of the atmosphere that they pass through, is one of the most important planetary factors of climate formation.

On land, albedo is determined by the color of natural surfaces. All radiation is capable of assimilating an absolutely black body. The mirror surface reflects 100% of the rays and is not able to heat up. Pure snow has the highest albedo of real surfaces. Below are the albedos of land surfaces by natural zone.

The climatic value of the reflectivity of different surfaces is extremely high. In ice zones of high latitudes, solar radiation, already weakened by the passage of a large number of optical masses of the atmosphere and falling onto the surface at an acute angle, is reflected by eternal snows.

The albedo of the water surface for direct radiation depends on the angle at which the sun's rays fall on it. Vertical rays penetrate deeply into the water, and it absorbs their heat. Oblique rays from the water are reflected as from a mirror, and it is not heated: the albedo of the water surface at a Sun height of 90 ″ is equal to 2%, at a Sun height of 20 ° - 78%.

Surface Views and Zonal Landscapes Albedo

Fresh dry snow …………………………………………… 80-95

Wet snow ……………………………………………… .. 60-70

Sea ice …………………………………………………… .. 30-40

Tundra without snow cover ………………………… .. 18

Persistent snow cover in temperate latitudes 70

The same unstable ……………………………………… .. 38

Coniferous forest in summer …………………………………………. 10-15

The same, with a stable snow cover ……… .. 45

Deciduous forest in summer ……………………………………. 15-20

The same, with yellow leaves in autumn ……………… .. 30-40

Meadow ………………………………………………………………… 15-25

Steppe in summer …………………………………………………… .. 18

Sand of different colors …………………………………… .. 25-35

Desert ………………………………………………………… .. 28

Savannahin dry season ……………………………………… 24

The same, in the rainy season ………………………………………. 18

The entire troposphere ………………………………………………… 33

The Earth as a whole (planet) ………………………………… .. 45

For scattered radiation, the albedo is slightly lower.
Since 2/3 of the world's area is occupied by the ocean, the assimilation of solar energy by the water surface acts as an important climate-forming factor.

The oceans in subpolar latitudes absorb only a small fraction of the sun's heat that reaches them. Tropical seas, on the other hand, absorb almost all solar energy. The albedo of the water surface, like the snow cover of the polar countries, deepens the zonal differentiation of climates.

In the temperate zone, the reflectivity of surfaces increases the difference between seasons. In September and March, the Sun is at the same height above the horizon, but March is colder than September, as the sun's rays are reflected from the snow cover. The appearance of yellow leaves in the fall, and then frost and temporary snow, increases albedo and lowers the air temperature. Persistent snow cover caused by low temperatures accelerates cooling and further decreases in winter temperatures.

The predominant part of direct radiation reflected by the earth's surface and the upper surface of the clouds goes beyond the atmosphere into world space. Also, about one third of the scattered radiation goes into world space. The ratio of all reflected and scatteredsolar radiation to the total amount of solar radiation entering the atmosphere is called planetary albedo of the Earth.The planetary albedo of the Earth is estimated at 35 - 40%. Its main part is the reflection of solar radiation by clouds.

Table 2.6

Dependence of magnitude TO n from the latitude of the place and the time of year

Table 2.7

Dependence of magnitude TO b + s from the latitude of the place and the season

(after A.P. Braslavsky and Z.A. Vikulina)