Albedo

Albedo describes how much incoming radiation a surface or body reflects. It is commonly expressed as a fraction from 0 to 1 or as a percentage: a higher value means a larger share is reflected, while a lower value means more is absorbed. The term is used in astronomy for the reflectivity of celestial bodies and in Earth science for land, ocean, ice, clouds and the planet as a whole.

Albedo varies across surfaces and conditions rather than being one fixed value for every place. NASA Earthdata gives fresh snow as an example of a surface with albedo as high as 0.9, while open ocean tends to be below 0.1. Such contrasts help scientists study how sunlight is partitioned between reflected and absorbed energy.

Surface albedo refers to reflection from a particular part of Earth's surface. The planet's overall, or planetary, albedo also accounts for reflection by the atmosphere and clouds. UCAR's Center for Science Education describes planetary albedo as an average across Earth's land, ocean and ice, with clouds also reflecting sunlight; it gives an approximate global value of 31 percent. This planetary measure helps describe how much incoming solar energy returns to space rather than contributing to warming the Earth system.

Changes in albedo can influence climate by changing the amount of solar energy absorbed. Snow and ice are relatively reflective. If warming melts them, darker ocean or land can be exposed; those surfaces reflect less and absorb more sunlight, which can add to the warming. The reverse can occur during cooling when increased snow or ice raises reflection and reduces absorbed sunlight. This reinforcing process is called the ice–albedo feedback.

The feedback's strength depends on season and location, among other factors. In the Arctic, for example, little or no sunlight reaches some areas during winter, so the direct reflection effect is seasonal. Earlier spring melt and later autumn freeze-up can extend the period in which darker surfaces absorb sunlight. Albedo is one part of the climate system's energy balance, not a standalone explanation for climate change.

Researchers estimate albedo through measurements of incoming and reflected radiation and by using satellite observations of surface reflectance. NASA's Earth-observing instruments provide global measurements that help scientists examine differences among land cover types and track changes over time. These data support research into Earth's energy balance, land-surface change and interactions between the Sun and the planet. Albedo is therefore useful in both climate science and the study of planetary surfaces.