“All day, every day the Sun hurls a steady stream of photons toward the Earth in the form of high-energy, shortwave radiation. Some photons are reflected by atmospheric particles, clouds, snow and ice, even ocean whitecaps. Other photons are absorbed by the Earth’s land and ocean surfaces, which heat up. These heated surfaces emit photons back to the atmosphere in the form of lower-energy, long-wavelength radiation. Some of these photons eventually make it all the way back out to space. The balance of incoming and outgoing photons over the whole planet, the net radiative flux, determines Earth’s temperature “set point.” The ability of greenhouse gases to absorb and emit photons is what global warming is all about.”
Image: Satellite data show the patterns of reflected shortwave solar energy (blue, left) and longwave thermal energy emitted from the Earth (red, right). Courtesy NASA.