SUNLIGHT IS NOT one single thing, but rather it is composed of a "spectrum of colors", where "color" is the response of the human eye to different wavelengths or energies of light particles ("photons"). Our eyes are only sensitive to a small portion of the entire "electromagnetic spectrum", namely the wavelength region called "visible" in the top panel. Heat, or "infrared" radiation in the figure, has a longer wavelength (or lower energy) than visible light, and "ultraviolet light" (UV) has a shorter wavelength (or higher energy) than visible light. So, light and heat (and microwaves and radio waves and x-rays and gamma rays) are all different wavelengths or energies of the same thing (electromagnetic radiation), but each wavelength interacts differently with the atmosphere or the surface or clouds or human beings.

THE RED CURVE in the top panel is the spectrum of electromagnetic radiation emitted by the sun, which peaks in the visible portion of the spectrum but also contains both infrared and UV radiation. The filled-in red area is the spectrum of sunlight that actually reaches the ground after traveling through the atmosphere, where the missing radiation is either absorbed by one of the specific molecules shown in the lower panels, or it is scattered in a different direction by molecules in the air ("Rayleigh scattering", which is why the sky is blue). The middle panel shows the total percentage of each wavelength of radiation that is absorbed or scattered by the atmosphere, so wherever there is an "absorption band" for one of the molecules in the lower panels, that radiation will not reach the surface and contribute to the red area in the top panel. The top and middle panels are two complementary ways of showing the same thing, namely how much solar radiation of a given wavelength reaches the surface. After sunlight reaches the surface it is either absorbed by the surface or reflected back toward space for a second trip through the atmosphere. The figure shows that much of the UV radiation emitted by the sun is either Rayleigh scattered or is absorbed by oxygen and especially ozone. Thankfully we now have regulations against emitting chemicals that destroy the ozone layer in the upper atmosphere that protects us from the most harmful "extreme UV" radiation from the sun. The other important molecule for absorption of solar radiation is water vapor, which absorbs a small amount of the visible light and a medium amount of the infrared light emitted by the sun. The chemical properties of a molecule determine which specific wavelengths of radiation it will absorb.

THE OTHER HALF of the Earth's radiation balance concerns the fate of infrared or thermal radiation (heat) that is emitted by the Earth's surface. All objects emit a spectrum of infrared radiation whose peak is at a wavelength that depends on its temperature. The blue curve is the spectrum of infrared radiation emitted by the Earth as a whole at an effective temperature of about 250 K (-23°C or -9°F), which is colder than the average surface temperature because some of the infrared emission comes from clouds at colder levels in the atmosphere. The figure shows that 70-85% of the emitted infrared radiation is absorbed by the atmosphere, and water vapor is the dominant greenhouse gas because it absorbs a large fraction of the infrared radiation. Carbon dioxide is the next most important greenhouse gas, followed by methane.

THE AVERAGE TEMPERATURE of the Earth is the result of a balance between energy that is added to the Earth System by the Sun, and infrared energy that is emitted by the Earth and escapes to space. Global warming is about increasing the concentration of greenhouse gases in the atmosphere, which absorb infrared radiation that would otherwise escape to space. This adds energy to the Earth System, which increases its temperature, which causes the Earth to emit more infrared radiation at the higher temperature, until a new balance with the energy input from the sun is established. Disclaimer: This is a super highly oversimplified description of one background aspect of the complex climate puzzle, and this description doesn't do justice to the current understanding of climate processes that is represented in modern climate models.

Figure Credit: This figure was created by Robert A. Rohde, a physicist and mathematician who documents climate science on the side at Global Warming Art. This is an immensely useful public service for a site like this that endeavors to be informative while referencing solid scientific data and analysis that is traceable to peer-reviewed publications.