What type of surface reflects radiation?

Figure 1. Albedos of different surfaces.[1] An albedo of 1 would mean a perfect reflector and an albedo of 0 would absorb all light striking it.

Albedo is the amount of sunlight (solar radiation) reflected by a surface, and is usually expressed as a percentage or a decimal value, with 1 being a perfect reflector and 0 absorbing all incoming light. When talking about albedo, the surface is almost always the surface of a planet like Earth.[2] White objects, such as a snowy hill have a high albedo. Conversely, dark objects, like pavement, have a low albedo.[3] The term can also refer to human made structures. In fact, energy efficient building design takes albedo into consideration whether a wall or ceiling warms the building or keeps it cool - resulting in a choice of a dark or light coating respectively.

For a given area, albedo is determined by more than just the composition of soil, it's impacted by soil moisture, type of vegetation, and levels of urbanization. Different surfaces on the Earth have different albedos (shown in Figure 1) and that albedo varies with time. Changes in albedo occur as the amount of cloud cover changes. Likewise, changes in any surface cover, like snow, ice, and vegetation, shift the albedo. Albedo is one of the major properties that controls how much energy is absorbed by the surface of the Earth, the cloudiness and ground cover are important factors in climate models.

The Earth's surface doesn't have a single albedo, rather a number of different albedos that are combined into a single number to accurately describe how the Earth reflects and absorbs solar energy as a whole. Figures 2 and 3 below illustrate different albedos around the world, and how those albedos change depending on the time of year. For both images, high albedos are represented by the colour white while lower albedos are a dark blue.

Figure 2. Albedo values for the Earth for February 2015.[4]

Figure 3. Albedo values for the Earth for July 2015.[4]

Climate Change

The Earth's climate depends on a balance of incoming and outgoing energy from the sun, which is determined by albedo. The overall albedo of the Earth - measured to be 0.30 - has a significant effect on the equilibrium temperature of the Earth as it changes how much solar energy is reflected by the Earth as opposed to how much is absorbed.[5] This changes how Earth's energy budget balances and thus changes Earth's heat balance.

As the world warms the Earth's albedo shifts. The amount of ice covering the planet is dropping as a result of increased temperatures from global warming. This causes a decrease in the area of white surfaces, leading to less energy to be reflected and more to be absorbed. This process warms the Earth even more. The melting of Arctic ice is especially concerning as it triggers a positive feedback (a feedback cycle that drives the climate out of control). In addition to melting ice, which lowers albedo, the conditions caused as a result of the greenhouse effect can also change the albedo of the Earth. The greenhouse effect can lower the albedo of the Earth by trapping in more infrared radiation with the increased number of greenhouse gases in the atmosphere, allowing less energy to bounce off of the atmosphere. Since the greenhouse effect can lower the albedo of the Earth and a changing albedo can result in a change in the equilibrium temperature of the Earth, a change in albedo can contribute to climate change.[6]

For information on how the Earth's albedo is measured please see NASA's site, measuring the Earth's albedo.

For Further Reading

References

1. ↑ Wikimedia Commons. (September 26, 2015). Roof Albedo [Online]. Available: https://upload.wikimedia.org/wikipedia/commons/6/6a/Roof-albedo.gif
2. ↑ ESR. (September 26, 2015). Albedo [Online]. Available: https://www.esr.org/outreach/glossary/albedo.html
3. ↑ IPCC, 2012: Glossary of terms. In: Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation [Field, C.B., V. Barros, T.F. Stocker, D. Qin, D.J. Dokken, K.L. Ebi, M.D. Mastrandrea, K.J. Mach, G.-K. Plattner, S.K. Allen, M. Tignor, and P.M. Midgley (eds.)]. A Special Report of Working Groups I and II of the Intergovernmental Panel on Climate Change (IPCC). Cambridge University Press, Cambridge, UK, and New York, NY, USA, pp. 555-564.
4. ↑ 4.0 4.1 NASA Earth Observatory. (September 26, 2015). Albedo [Online]. Available: http://neo.sci.gsfc.nasa.gov/view.php?datasetId=MCD43C3_M_BSA
5. ↑ Measuring the Earth's Albedo, Accessed March 30th, 2019. https://earthobservatory.nasa.gov/images/84499/measuring-earths-albedo
6. ↑ HyperPhysics. (March 30, 2019). Albedo of the Earth [Online]. Available: http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/albedo.html

The Sun provides the Earth with most of its energy. Today, about 71% of the sunlight that reaches the Earth is absorbed by its surface and atmosphere. Absorption of sunlight causes the molecules of the object or surface it strikes to vibrate faster, increasing its temperature. This energy is then re-radiated by the Earth as longwave, infrared radiation, also known as heat. The more sunlight a surface absorbs, the warmer it gets, and the more energy it re-radiates as heat. This re-radiated heat is then absorbed and re-radiated by greenhouse gases and clouds, and warm the atmosphere through the greenhouse effect.

Earth’s surfaces are better at absorbing solar radiation than air, especially surfaces that are dark in color. You can feel this on a cold winter day when the sunshine warms your face and the air around you remains cold. Your skin and your clothes also absorb solar radiation and convert it to heat. If you wear a black jacket, it will absorb more radiation and make you feel warmer than if you wear a white or light-colored jacket. Similarly, Earth’s different surfaces and parts of the atmosphere absorb solar radiation at different rates.

The Earth is unevenly heated because it is a sphere.

Because Earth is a sphere, not all part of the Earth receives the same amount of solar radiation.  More solar radiation is received and absorbed near the equator than at the poles. Near the equator, the Sun’s rays strike the Earth most directly, while at the poles the rays strike at a steep angle. This means that less solar radiation is absorbed per square cm (or inch) of surface area at higher latitudes than at lower latitudes, and that the tropics are warmer than the poles. This temperature difference shapes global atmospheric and ocean circulation patterns. Additionally, Earth’s tilt affects how much sunlight is received and absorbed by different parts of the Earth at various times of the year, and is why we experience the seasons. The amount of solar radiation received and absorbed also influences process in the biosphere by directly affecting plants and other organisms that photosynthesize and are the primary food source in most ecosystems (see species interactions).

If light is not absorbed by a surface, it is mostly reflected. Reflection occurs when incoming solar radiation bounces back from an object or surface that it strikes in the atmosphere, on land, or water, and is not transformed into heat. The proportion of incoming solar radiation that is reflected by the Earth is known as its albedo. Overall, Earth reflects about 29% of the incoming solar radiation, and therefore, we say the Earth’s average albedo is 0.29.

Snow and ice, airborne particles, and certain gases have high albedos and reflect different amounts of sunlight back into space. Low, thick clouds are reflective and can block sunlight from reaching the Earth’s surface, while high, thin clouds can contribute to the greenhouse effect.

The proportion of sunlight that’s reflected vs. absorbed, the re-radiation of heat, and the intensity of the greenhouse effect influence the amount of energy in the Earth system and global processes such as the water cycle and atmospheric and ocean circulation.

This diagram shows the percentage of sunlight that is reflected by different Earth surfaces or clouds.

Earth system models about the absorption and reflection of sunlight

This Earth system model is one way to represent the essential processes and interactions related to the absorption and reflection of sunlight. Hover over the icons for brief explanations; click on the icons to learn more about each topic. Download the Earth system models on this page.

This model shows some of the changes to Earth’s surface and atmosphere that can affect the amount of sunlight that is absorbed or reflected. These changes influence the amount of heat that is re-radiated, and can also greatly influence the biosphere by altering the amount of sunlight available for photosynthesis.

How human activities influence the absorption and reflection of sunlight

The Earth system model below includes some of the ways that human activities directly affect the amount of sunlight that is absorbed and reflected by Earth’s surface. The development and spread of urban areas, especially using asphalt and other dark colored materials, can dramatically increase the absorptivity of the surface. This creates urban heat islands, where cities experience higher temperatures than surrounding areas. Hover over or click on the icons to learn more about these human causes of change and how they influence the absorption and reflection of sunlight.

The Earth system model below includes additional ways that human activities directly affect the amount of sunlight that is absorbed and reflected by Earth’s atmosphere. Hover over or click on the icons to learn more about these human causes of change and how they influence the absorption and reflection of sunlight.

The Earth system model below shows how human pollutants and waste affect the ozone layer and the amount of ultraviolet sunlight that is absorbed by Earth’s upper atmosphere (the stratosphere). Hover over or click on the icons to learn more about these human causes of change and how they influence the absorption and reflection of sunlight.

Explore the Earth System

Click the icons and bolded terms (e.g. re-radiation of heat, airborne particles, etc.) on this page to learn more about these process and phenomena. Alternatively, explore the Understanding Global Change Infographic and find new topics that are of interest and/or locally relevant to you.

To learn more about teaching the absorption and reflection of sunlight, visit the Teaching Resources page.

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