The measure of how much light a surface reflects rather than absorbs. Everything has one, from ice caps to open ocean, and it decides how much of the sunlight hitting an object actually stays there.
Albedo is expressed as a fraction or percentage of incoming light that gets reflected. A perfect mirror would have an albedo of 1 (or 100%), meaning it reflects all light, while a pitch-black surface would have an albedo of 0, absorbing everything. Snow and ice have some of the highest albedos, reflecting up to 90% of sunlight, which helps keep polar regions cool. In contrast, dark ocean water or dense forests absorb most of the energy they receive, leading to a lower albedo and a warmer surface.
On Earth, albedo is extremely important in climate regulation. When ice sheets grow, they reflect more sunlight, cooling the planet in a self-reinforcing cycle - a negative feedback. But when ice melts due to warming, darker surfaces like ocean water or land are exposed, absorbing more heat and accelerating climate change, this is known as the ice-albedo feedback loop, a positive feedback loop. Human activities, such as deforestation and urbanization, also alter Earth’s albedo, with cities and asphalt absorbing more heat than natural landscapes.
Beyond Earth, albedo helps astrophysicists study planets, moons, and asteroids. See posts like naked planet climate model for more details! Venus, with its thick, reflective clouds, has an albedo of about 0.75, making it one of the brightest objects in the night sky. Meanwhile, our Moon has a surprisingly low albedo of just 0.12, meaning it’s darker than you might expect, and its brightness comes from contrast rather than reflectivity.