Atmospheric gases scatter blue wavelengths of visible light more than other wavelengths, giving the Earth's visible edge a blue halo. At higher and higher altitudes, the atmosphere becomes so thin that it essentially ceases to exist. Gradually, the atmospheric halo fades into the blackness of space. This astronaut photograph captured on July 20, 2006, shows a nearly translucent moon emerging from behind the halo. Technically, there is no absolute dividing line between the Earth's atmosphere and space, but for scientists studying the balance of incoming and outgoing energy on the Earth, it is conceptually useful to think of the altitude at about 100 kilometers above the Earth as the ''top of the atmosphere.'' The top of the atmosphere is the bottom line of Earth's energy budget, the Grand Central Station of radiation. It is the place where solar energy (mostly visible light) enters the Earth system and where both reflected light and invisible, thermal radiation from the Sun-warmed Earth exit. The balance between incoming and outgoing energy at the top of the atmosphere determines the Earth's average temperature. The ability of greenhouses gases to change the balance by reducing how much thermal energy exits is what global warming is all about. Greenhouse gases aren't the only part of the Earth system that influence the energy balance. The percent of incoming sunlight the Earth system reflects (the Earth's ) is a key climate factor since whatever is reflected can't go on to warm the planet. Clouds, such as those blanketed the earth int he image above, snow, and ice have the biggest influence on how reflective Earth is. When any of these factors change, Earth'albedo can change. Since the late 1990s, NASA satellites have been observing the top of the atmosphere with sensors known as asd-www.larc.nasa.gov/ceres/ CERES, short for ''Clouds and the Earth's Radiant Energy System,'' and scientists have been using the data to look for signs of change in the amount of energy the Earth reflects or emits. Because /IOTD/view.php?id=7370 snow and ice are so reflective, scientists have long expected that melting of snow and ice in the polar regions will accelerate climate warming by reducing the Earth' albedo. Atmospheric scientist Seiji Kato of NASA's Langley Research Center and several teammates have used a suite of NASA and NOAA (National Oceanic and Atmospheric Administration) satellite observations to investigate whether this feedback is already underway. The team compared reflected sunlight, clouds, and sea ice and snow cover at polar latitudes from 2000-2004. What they found was a bit of a surprise: while snow and ice in the Arctic declined, the albedo didn't change. To read more about the team's investigation, read the Earth Observatory feature story earthobservatory.nasa.gov/Study/ArcticReflector/ Arctic Reflection: Clouds Replace Snow and Ice as Solar Reflector.
