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Sea water absorbs more solar 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.
Both positive feedback loops have long been recognized as important for global warming.
Cryoconite, powdery windblown dust containing soot, sometimes reduces albedo on glaciers and ice sheets.
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 through remote sensing techniques rather than applying a single value for albedo over broad regions.
Small-scale effects 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.
Solar photovoltaic effects Albedo can affect the electrical energy output of solar photovoltaic 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%.
More recently, the analysis was extended to the effects of spectral bias due to the specular reflectivity of 22 commonly occurring surface materials (both human-made and natural) and analyzes the albedo effects on the performance of seven photovoltaic materials covering three common photovoltaic system topologies: industrial (solar farms), commercial flat rooftops and residential pitched-roof applications.
Trees Because forests generally have a low albedo, (the majority of the ultraviolet and visible spectrum is absorbed through photosynthesis), some scientists have suggested that greater heat absorption by trees could offset some of the carbon benefits of afforestation (or offset the negative climate impacts of deforestation).
In the case of evergreen forests with seasonal snow cover albedo reduction may be great enough for deforestation to cause a net cooling effect.
Trees also impact climate in extremely complicated ways through evapotranspiration.
The water vapor causes cooling on the land surface, causes heating where it condenses, acts a strong greenhouse gas, and can increase albedo when it condenses into clouds.
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.
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 whereas coniferous trees have a value of about 0.09 to 0.15.
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.
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 the Hadley Centre have investigated the relative (generally warming) effect of albedo change and (cooling) effect of carbon 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.
Water Water reflects light very differently from typical terrestrial materials.
The reflectivity of a water surface is calculated using the Fresnel equations.
At the scale of the wavelength of light even wavy water is always smooth so the light is reflected in a locally specular manner (not diffusely).
The glint of light off water is a commonplace effect of this.
At small angles 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.
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 the terminator (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.
Clouds 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."
Albedo and climate in some areas are affected by artificial clouds, such as those created by the contrails of heavy commercial airliner traffic.
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 colder than temperatures several miles away under clear skies.
Aerosol effects 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 as cloud condensation nuclei and thereby change cloud properties) is less certain.
As per Spracklen et al. the effects are: Aerosol direct effect.
Aerosols directly scatter and absorb radiation.
The scattering of radiation causes atmospheric cooling, whereas absorption can cause atmospheric warming.
Aerosols modify the properties of clouds through a subset of the aerosol population called cloud condensation nuclei.
Increased nuclei concentrations lead to increased cloud droplet number concentrations, which in turn leads to increased cloud albedo, increased light scattering and radiative cooling (first indirect effect), but also leads to reduced precipitation efficiency and increased lifetime of the cloud (second indirect effect).
In extremely polluted cities like Delhi, aerosol pollutants influence local weather and induce an urban cool island effect during the day.
Black carbon Another albedo-related effect on the climate is from black carbon particles.
The size of this effect is difficult to quantify: the Intergovernmental Panel on Climate Change estimates that the global mean radiative 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.
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.
Human activities Human activities (e.g., deforestation, farming, and urbanization) change the albedo of various areas around the globe.
However, quantification of this effect on the global scale is difficult, further study is required to determine anthropogenic effects.
Albedo in Astronomy In astronomy, the term albedo can be defined in several different ways, depending upon the application and the wavelength of electromagnetic radiation involved.
Optical or Visual Albedo The albedos of planets, satellites and minor planets such as asteroids 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 of photometry.
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 outer Solar System objects, the variation of albedo with phase angle gives information about regolith properties, whereas unusually high radar albedo is indicative of high metal content in asteroids.
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 is Eris, with an albedo of 0.96.
Many small objects in the outer Solar System and asteroid belt have low albedos down to about 0.05.
A typical comet nucleus has an albedo of 0.04.
Such a dark surface is thought to be indicative of a primitive and heavily space weathered surface containing some organic compounds.
The overall albedo of the Moon is measured to be around 0.14, but it is strongly directional and non-Lambertian, displaying also a strong opposition effect.
Although such reflectance properties are different from those of any terrestrial terrains, they are typical of the regolith 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 the Bond 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 five Hapke parameters which semi-empirically describe the variation of albedo with phase angle, including a characterization of the opposition effect of regolith surfaces.
One of these five parameters is yet another type of albedo called the single-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: where is the astronomical albedo, is the diameter in kilometers, and is the absolute magnitude.
Radar Albedo In planetary radar 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 is circularly polarized and the received pulse is measured in the same sense of polarization as the transmitted pulse (SC) and the opposite sense (OC).
The echo power is measured in terms of radar 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.
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.
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): where the denominator is the effective cross-sectional area of the target object with mean radius, .
A smooth metallic sphere would have .
Radar Albedos of Solar System Objects 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.
Relationship to Surface Bulk Density 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) 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: .
See also Cool roof Daisyworld Emissivity Exitance Global dimming Irradiance Kirchhoff's law of thermal radiation Opposition surge Polar see-saw Radar astronomy Solar radiation management References External links Albedo Project Albedo – Encyclopedia of Earth NASA MODIS BRDF/albedo product site Ocean surface albedo look-up-table Surface albedo derived from Meteosat observations A discussion of Lunar albedos reflectivity of metals (chart) Land surface effects on climate Climate change feedbacks Climate forcing Climatology Electromagnetic radiation Radiometry Scattering, absorption and radiative transfer (optics) Radiation 1760s neologisms
Autism is a neurodevelopmental disorder characterized by difficulties with social interaction and communication, and by restricted and repetitive behavior.
Parents often notice signs during the first three years of their child's life.
These signs often develop gradually, though some autistic children experience regression in their communication and social skills after reaching developmental milestones at a normal pace.
Autism is associated with a combination of genetic and environmental factors.
Risk factors during pregnancy include certain infections, such as rubella, toxins including valproic acid, alcohol, cocaine, pesticides, lead, and air pollution, fetal growth restriction, and autoimmune diseases.
Controversies surround other proposed environmental causes; for example, the vaccine hypothesis, which has been disproven.
Autism affects information processing in the brain and how nerve cells and their synapses connect and organize; how this occurs is not well understood.
The Diagnostic and Statistical Manual of Mental Disorders (DSM-5) combines forms of the condition, including Asperger syndrome and pervasive developmental disorder not otherwise specified (PDD-NOS) into the diagnosis of autism spectrum disorder (ASD).
Several interventions have been shown to reduce symptoms and improve the ability of autistic people to function and participate independently in the community.
Behavioral, psychological, education, and/or skill-building interventions may be used to assist autistic people to learn life skills necessary for living independently, as well as other social, communication, and language skills.
Therapy also aims to reduce challenging behaviors and build upon strengths.
Some autistic adults are unable to live independently.
An autistic culture has developed, with some individuals seeking a cure and others believing autism should be accepted as a difference to be accommodated instead of cured.
Globally, autism is estimated to affect 24.8 million people .
In the 2000s, the number of autistic people worldwide was estimated at 1–2 per 1,000 people.
In the developed countries, about 1.5% of children are diagnosed with ASD , up from 0.7% in 2000 in the United States.