Powered by Climate Lab

Climate Lab is a collaborative information platform on climate change. Click here to edit this page.

<< Back to Wiki Index

Background

 Albedo is measured on a scale of 0 to 1. '0' indicates no reflectivity whatsoever; an object with an albedo of 0 absorbs all incoming light. An albedo of 1 indicates the opposite: the surfaces reflects all light and absorbs none. Dark surfaces tend to have low albedos, whereas bright and shiny surfaces have high albedos.²

The global mean albedo of the Earth is approximately 0.3. Meaning, the earth reflects 30% of incoming solar energy back into space. The primary sources of this reflectivity are ice, snow, clouds, aerosols, and deserts.  Atmospheric elements, particularly clouds, contribute a much higher percentage to planetary albedo than the surface.  Clouds reflect 20% of total incoming solar energy, and aerosols reflect 6%.  The surface reflects the remaining 4%.  Ice and snow are the largest source of surface albedo.  The average reflectivity of various surfaces can be found in the table on the right.³

Albedo Values for Various Surfaces

Source: Wiki CommonsPerm: Free Use

Aerosols, including sulfates and black carbon, have various affects on global albedo.  Black carbon (soot), as its name implies, has a low albedo and absorbs large amounts of solar energy.  It does so both when suspended in air and once it settles on the ground.  Of particular concern is black carbon that settles onto snow and ice, as this darkens these surfaces, lowering surface reflectivity.⁴  Sulfates, generally light in color, produce the opposite effect of black carbon and act as a global coolant.  They are highly reflective themselves, but more likely important is their effect on clouds.  Evidence suggests sulfate particles lead to increased cloud cover, which would increase planetary albedo.⁵

Albedo and climate change

Albedo is an important part of Earth's energy balance. Changes in reflectivity lead to changes in the amount of solar energy absorbed by the planet - directly impacting Earth's average temperature.   A lower albedo (less reflectivity) leads to more retained solar energy and higher temperatures.  Greenhouse gases do not affect sunlight, so incoming solar energy that is reflected back to space is not trapped by the atmosphere.  When solar energy is absorbed by the Earth's surface, the energy is reemited in the form of long-wave infrared radiation. Greenhouse gases do affect this energy, trapping a portion of it in the atmosphere. This leads to an increase in atmospheric temperatures. Thus, energy that would have otherwise entered and left the earth's system with no affect now contributes to both surface and atmospheric heating.⁶

Mean Albedo Values Around the World

Source: NASA Author: Crystal Schaaf Permission: Fair Use

The primary source of decreasing albedo is snow and ice melt. Rising temperatures melt ice, lowering planetary albedo, which in turn increases the amount of heat energy absorbed. Glacial melt across the globe contributes to lowering albedo, but the polar regions are the primary areas of concern. The Arctic ice cap, as it shrinks, creates more open ocean to absorb sunlight. This warms the Arctic Ocean, making seasonal ice recovery more difficult. The combination of decreasing peak ice coverage and warming waters will lead to ice-free Arctic during the summer by 2030.⁷ This is an example of a positive feedback in the climate system brought about by climate change. This loss of important sources of reflectivity will lead to an irreversible acceleration of global heating and is thus considered a tipping point in Earth's climate system.⁸ 

Geoengineering

Due to the consequences of a global reduction in albedo, many proposals have been put forward to increase the planet's reflectivity.  They vary widely in their complexity and potential side-effects to ecosystems.  Some examples include:

• Build white or otherwise lightly-colored roofs and roads, as advocated by Stephen Chu, the United States Secretary of Energy.⁹
• Increase the albedo of clouds through the mechanical generation of sea-salt spray into the atmosphere, which would potentially increase the concentration of clouds' condensation nuclei.¹⁰  This is commonly referred to as 'cloud seeding.'
• Cover expanses of desert with a highly reflective material, the top of which would be made of a white polyethylene.¹¹
• Emit increased amounts of sulfate aerosols, which are highly reflective.¹²
• Place so-called 'sunshades' in space as satellites that would reflect portions of incoming solar radiation before the energy in entered into the atomosphere.¹³

Footnotes

1. D. Budikova, M. Hall-Beyer and G.H.G. Hussein. "Albedo." Encyclopedia of Earth. Last revised 19 March 2008. Retrieved: 24 Sept 2009.

2. L.R. Kump, J.F. Kasting, and R.G. Crane.  The Earth System.  2nd Edition.  Prentice Hall, 2003.

3. L.R. Kump, J.F. Kasting, and R.G. Crane. The Earth System. 2nd Edition. Prentice Hall, 2003.

4. F.C. Moore and M.C. MacCracken. Lifetime-leveraging: An approach to achieving international agreement and effective climate protection using mitigation of short-lived greenhouse gases. International Journal of Climate Change Strategies and Management, Vol. 1 No. 1, 2009.

5. IPCC Fourth Assessment Report. Working Group 1.  Chapter 2: Changes in Atmospheric Constituents and in Radiative Forcing. 2007. p. 160.

6. L.R. Kump, J.F. Kasting, and R.G. Crane. The Earth System. 2nd Edition. Prentice Hall, 2003.

7. UNEP. Climate Change Science Compendium 2009.  p. 18-19.

8. T. Lenton, H. Held, E. Kreigler, J. W. Hall, W. Lucht, S. Rahmstorf and H. J. Schellnhuber. "Tipping elements in the Earth's climate system." Proc of the Nat'l Acad Sci USA 1786 -1793 (2008). p. 1788. Retrieved: 24 Sept 2009.

9. Gray, Louise. "Obama's green guru calls for white roofs." The Daily Telegraph. 27 May 2009. Retrieved: 24 Sept 2009.

10. T.M. Lenton and N.E. Vaughn. "The radiative forcing potential of different climate geoengineering options." Atmospheric Chemistry and Physics. Volume 9, January 2009, p. 2577. Retrieved: 24 Sept 2009.

11. T.M. Lenton and N.E. Vaughn. "The radiative forcing potential of different climate geoengineering options." Atmospheric Chemistry and Physics. Volume 9, January 2009, p. 2578. Retrieved: 24 Sept 2009.

12. T.M. Lenton and N.E. Vaughn. "The radiative forcing potential of different climate geoengineering options." Atmospheric Chemistry and Physics. Volume 9, January 2009, p. 2576. Retrieved: 24 Sept 2009.

13. T.M. Lenton and N.E. Vaughn. "The radiative forcing potential of different climate geoengineering options." Atmospheric Chemistry and Physics. Volume 9, January 2009, p. 2575. Retrieved: 24 Sept 2009.