Climate Number: An Albedo Difference of 0.1
Albedo describes how much of the Sun’s radiation an object reflects. New snow is very reflective, with an albedo as high as 0.9, meaning that 90 percent of the sunlight that hits it is reflected. Dark asphalt, on the other hand, has an albedo as low as 0.04, meaning it absorbs 96 percent of the sunlight it receives. This absorption warms the asphalt and the general tendency for urban spaces to have lower albedos than surrounding rural areas is one of the causes of the urban heat island effect. Changes in surface albedo have implications for local climates and if these changes are on a large enough scale, the implications can be global. One example of this is in the Arctic, where the average September sea ice minimum is approximately 1.3 million square miles smaller than it was at the beginning of the 1980s – 1.3 million square miles is enough ice to blanket most of Europe. Because seawater is less reflective than ice, this reduction means that larger areas of the Arctic Ocean are exposed to the sun’s energy for longer periods the year, allowing more solar energy to be absorbed and transmitted to the rest of the climate system. Also, not all Arctic ice is equally reflective, with older ice that has survived at least one melt season being more reflective than younger seasonal ice. The older ice has an albedo of 0.65 compared to an albedo of 0.55 for the seasonal ice, a difference of 0.1. In March 2011, only about 45 percent of the ice present in the Arctic was multiyear ice, compared with 75 percent in March 1980. While about 50 percent of the ice cover was particularly old ice (older than five years) in 1980, that number had declined to 10 percent by March 2011.
For comparison: The albedo difference of 0.1 between the seasonal and multiyear ice, combined with differences in how snow and melt ponds develop on the two different types of ice over the course of a melt season, mean that each square meter of seasonal ice absorbs about 342 more megajoules of energy each year than the older ice. This extra 342 megajoules is enough to thin a layer of ice by over three feet.
Seasons: Spring, Summer
Sources: Perovich, DK and Polashenski, C. “Albedo evolution of seasonal Arctic sea ice.” Geophysical Research Letters 39 (2012): L08501 and Maslanik, J et al. “Distribution and trends in Arctic sea ice age through spring 2011.” Geophysical Research Letters 38 (2011): L13502 and Francis, JA and Vavrus, SJ. “Evidence linking Arctic amplification to extreme weather in mid-latitudes.” Geophysical Research Letters 39 (2012): L06801.