Why do carbon dioxide and water cause the greenhouse effect whereas nitrogen and oxygen do not?

Answer 1

CO2 (carbon dioxide), H2O (water), CH4 (methane), and NO2 (nitrous oxide or "laughing gas") absorb infrared radiation, which causes these molecules to vibrate faster (heat up). O2 (oxygen) and N2 (nitrogen) directly absorb far less infrared energy. The radiation mostly passes on through these diatomic gases unimpeded.

Heat is light in the infrared range. This radiation is not visible to the human eye (optic nerves), but this long wavelength radiation can be detected by nerves that respond to temperature in our skin. Light is not simply the very narrow range of the electromagnetic spectrum to which our eyes are sensitive, but the entire range from very long radio waves through extremely short ultraviolet wave lengths.

Insulation traps heat, whereas conductors permit heat to pass more easily. Nitrogen and oxygen are not conductors, but these gases do not absorb infrared radiation with the ease of H2O, CH4, CO2 and NO2. Without these four "greenhouse" gases in our atmosphere, our world would be far colder.

The atmosphere of Venus is almost entirely CO2, and as a result it experiences what is known as the "runaway" greenhouse effect, making it the hottest planet in our solar system.

Mars too has a mostly CO2 atmosphere, but this atmosphere is so very thin the planet doesn't retain as much heat as it might.

When the concentration of CO2 was higher in our planet's past, so was the resulting temperature. Throughout most of the Mesozoic (the age of dinosaurs) atmospheric CO2 concentrations were higher than they are now, and our planet was, as a consequence, much warmer.

What changed? About fifty million years ago a freshwater fern, Azolla, proliferated over Arctic seas. When it died it sank to the sea floor, where it remained preserved in the cold water. After just a few hundred thousand years of atmospheric carbon sequestered (stored) on the sea floor the CO2 level plunged, causing Earth to cool. A long series of ice ages ensued. While ice ages are driven by orbital variations known as "Milankovitch Cycles," these same cycles existed throughout the Mesozoic. It was the reduction in CO2 that allowed Earth to cool enough for vast sheets of ice to form, in much the same way unzipping a sleeping bag on a cold night might cause a camper to freeze.

As Earth's CO2 rises due to its liberation (release) from the combustion of fossil fuel, so too has the planet's temperature increased. Some argue that the role of CO2 cannot be as important as that of H2O, but CO2 does not readily precipitate out of the atmosphere as H2O (water vapor) does. You have seen this gas literally rain right out of moisture laden air. In the early morning the ground is often coated with water condensed from the atmosphere--a phenomenon we call "dew."

CO2, in contrast, does not fall out so readily. As it increases, the amount of water vapor the air can hold increases as well, a sort of feedback loop further amplifying the temperature.

So while all gases contribute to Earth's "thermal blanket," CO2 and H2O are the chief insulating (greenhouse) gases.

Second Answer:

While Carbon dioxide and water absorb infrared, heat is transferred and absorbed in several ways. No substance is immune to being heated, which means that all gases absorb heat. In the earth's atmosphere, Nitrogen, Oxygen and Argon (>99% of atmospheric gases) absorb more heat than the tiny amount of CO2 and water vapor.

Heat consists of vibrating and colliding molecules. The motion of these molecules jostles their electrons around, and this emits light. Heat and light are thus strongly related, but they aren't the same. For instance, heat can't actually be radiated; only the light that heat brings about can. By the same token, light itself has no temperature because temperature is an index of molecular motion, and a beam of light isn't composed of molecules. In short, "heat" can be regarded as molecular excitement and light as electromagnetic excitement.

Heated masses always emit light (infrared). Always. That's a direct consequence of molecules in motion. And while it's true that some substances may be transparent to infrared light, it doesn't follow that they can't be heated or, if heated, might not emit infrared.

There are three ways for heat (better to say thermal energy) to move from one zone to another: by conduction, convection, and radiation. Conductive heat transfer involves direct contact, wherein vibrations spread from molecule to molecule. Convective transfer involves a mass in motion: expanded by heat, a fluid is pushed up and away by the denser fluid that surrounds it. Radiative transfer arises when molecules intercept the light that warmer molecules are emitting, which brings about a resonant molecular vibration -- i.e., heating.

By selective context and vagueness, then, NASA paints an impression that only light-absorbing substances can be heated. Thus, since nitrogen and oxygen don't respond to infrared, NASA feels justified to say that "only some gases have the unique property of being able to absorb heat."

The Climate Change movement fixates on the 1% of the atmosphere that can be heated by radiant transfer instead of the 99% and more that is heated by direct contact with the earth's surface and then by convection. If they made it clear that every species of atmospheric gas gets heated mainly by conductive transfer, and that all heated bodies radiate light, then even a child could connect the dots: "Oh. So the whole atmosphere radiates heat to the earth and makes it warmer. All of the atmosphere is a greenhouse gas."

Answer 2

Carbon dioxide and water vapor have a greater ability to absorb and re-emit infrared radiation, trapping heat in the atmosphere and causing the greenhouse effect. Nitrogen and oxygen, which make up the majority of the atmosphere, are mostly transparent to infrared radiation and do not have the same warming effect.