Albedo and Its Role in Climate Science

Albedo is a measure of the proportion of incoming sunlight that is reflected by a surface. It is a fundamental concept in climate science, influencing the Earth’s energy balance, surface temperatures, and feedback processes such as ice–albedo interactions. Variations in albedo occur naturally and through human activity, making it an important parameter in understanding and mitigating climate change.

Definition and Measurement

Albedo is expressed as a fraction between 0 and 1, where 0 represents a perfect absorber (a black body) and 1 represents a perfect reflector. Surface albedo is calculated as the ratio of radiosity (total reflected radiation) to irradiance (total incident radiation) received by a surface. It depends not only on surface properties but also on the spectral and angular distribution of solar radiation, which vary with atmospheric conditions, location, and time of day.
Unless specified as spectral albedo (measured for a particular wavelength), the term refers to reflectance over the full solar spectrum, typically between 0.3 μm and 3 μm. For example, fresh snow can have an albedo of around 0.9, while charcoal reflects as little as 0.04. Earth’s average albedo is approximately 0.3, with contributions from clouds, land, and ocean surfaces.

Types of Albedo

For land surfaces, albedo can be categorised as:

• Black-sky albedo: Reflectance under direct sunlight at a given solar zenith angle.
• White-sky albedo: Reflectance under diffuse light.
• Blue-sky albedo: A combination of the two, weighted by the proportion of direct and diffuse light.

The bidirectional reflectance distribution function (BRDF) is used to model how surface reflectance changes with viewing and illumination angles, enabling satellite-based albedo estimates.

Albedo and Climate Feedbacks

The ice–albedo feedback is a positive feedback mechanism in which melting ice reduces surface reflectivity, leading to further warming and additional ice loss. Arctic ice, which is highly reflective, is particularly important in this process. As snow and ice retreat, darker surfaces such as ocean water or bare ground absorb more heat, accelerating regional and global warming.
Geographic and seasonal variations influence albedo’s temperature effects. High-albedo polar regions remain cold due to low insolation, while deserts with similarly high albedo are hot due to intense sunlight. In tropical zones, changes in albedo can cause significant temperature fluctuations.

Human Influence on Albedo

Human activities such as deforestation, farming, and urbanisation alter local and regional albedo. Urban areas generally have lower albedo than surrounding croplands, contributing to the urban heat island effect. Increasing urban albedo by 0.1 globally has been estimated to produce a cooling effect equivalent to offsetting around 44 gigatonnes of CO₂ emissions.
Some large-scale land changes have produced measurable climatic effects. For example, the extensive greenhouses in Spain’s Almería province increase local albedo, leading to slight surface cooling. Strategies such as Passive Daytime Radiative Cooling (PDRC) aim to enhance surface albedo and thermal emittance to counter global warming.

Recent Observations

Satellite data from instruments such as NASA’s MODIS and CERES have enabled global albedo monitoring. Between 1998 and 2017, Earth’s reflectivity decreased by about 0.5%, partly due to reduced cloud cover over the eastern Pacific. This dimming may have contributed to a 1.7 W/m² warming effect since 2010, equivalent to a rise in atmospheric CO₂ concentration of about 138 ppm.