How much sunlight a planet bounces back can swing its temperature by tens of degrees
Earth reflects about 30 percent of incoming sunlight, largely thanks to clouds. That share, called albedo, helps hold the average surface near 15 degrees Celsius. Freeze the whole planet solid and the extra brightness would plunge it below minus 40; cover it entirely in ocean and it would warm to almost 27.
Albedo, Latin for whiteness, runs from 0 for a surface that swallows all light to 1 for one that reflects everything. Fresh snow scores about 0.9 to 0.95, while charcoal, among the darkest materials, manages roughly 0.04, and deep shadowed hollows approach zero. Seen from afar, oceans and most forests look dark and deserts rank among the brightest landscapes; most land falls between 0.1 and 0.4. The figure usually covers the wavelengths carrying most solar energy, 0.3 to 3 micrometres, which includes visible light, so low-albedo trees look dark and high-albedo snow looks bright.
Measured albedo averages reflection over every sun angle across a period, from seconds to a year, and depends on the atmosphere, location and time as well as the surface. Satellite sensors like NASA's MODIS, aboard Terra and Aqua, see reflection from only one direction, so scientists use a model of how brightness varies with viewing and solar angles to estimate the full picture. Researchers also split it into black-sky albedo for direct sunlight and white-sky albedo for diffuse light, then blend them into blue-sky albedo for real conditions.
Ice drives a powerful feedback. Because it reflects far more energy than land or open water, shrinking ice lets a planet absorb more heat, which melts more ice. Glaciers covering just the continents would drag the global mean down to about freezing. In 2021 researchers reported, from careful measurements of earthshine on the Moon, that Earth dimmed by about half a percent between 1998 and 2017, though whether that is a lasting trend and how it ties to climate change remain open.
People alter albedo through deforestation, farming and building. Cities typically reflect 0.01 to 0.02 less than neighbouring cropland, adding to warming, while brightening them can ease urban heat islands. A 2022 estimate found that raising albedo by 0.1 across the world's urban areas would cool the planet as much as absorbing about 44 gigatonnes of carbon dioxide.
Source: Albedo
