My roomie is watching “An Inconvenient Truth” and has a question for which I have absolutely no answer. She says she’s asked many people this and no one really has given her an answer.
Forgive me for my most basic of explanations below. I ain’t a scientist and my history major self never will be.
So the rays and such from the sun come through the atmosphere, then get trapped. Trapped in our atmosphere by all the junk we’ve pumped into our air. Got it.
But if the stuff can get through in the first place, why can’t it get back out again? I’m not sure if that’s phrased the best, but why can the rays/heat/whatever get back out?
The best I can figure relates back to when I took physics in the 10th grade. Is the bad stuff just bouncing around, reflecting and such upon all the junk in our atmosphere?
"I’m not sure if that’s phrased the best, but why can the rays/heat/whatever get back out? "
Should be:
I’m not sure if that’s phrased the best, but why can the rays/heat/whatever actually make it through our atmosphere to get trapped in the first place? Assuming things are so bad.
Not quite, AFAICT. See this GD thread where jshore explains:
In other words, the increase in CO2 is effectively making the atmosphere a less efficient radiator, i.e., a better insulator. The new CO2-richer atmosphere is less efficient than it used to be at sending the radiation back out into space, so it retains more of it, thus warming things up.
Kind of like replacing your flimsy cotton blanket with a thick fluffy wool one: it loses heat less rapidly, so it retains heat better, thus keeping you warmer under the blanket.
It just so happens that our atmosphere is composed of gases that are quite transparent to light, but relatively opaque to heat radiation. So incoming energy (in the form of light) makes it through the atmosphere and strikes the ground, heating it up. When the ground is warmed up, though, it doesn’t emit light again, but heat waves, which get “trapped” by the atmosphere because they can’t get back out, effectively heating up the planet. Water and carbon dioxide are particularly effective at trapping heat in this way, which is why they are very effective greenhouse gases. Just for fun, I’ve made a really simple ASCII model below, but you can find a much better picture here.
It’s a result of the fact that the earth is much cooler than the sun; the energy that penetrates our atmosphere from the sun is primarily visible light, which strikes the ground (or the ocean, or the ice caps, or whatever) and heats it up. The ground (or ocean or whatever), however, is not radiating visible light, because it’s not hot enough (fortunately for us). Instead, it radiates infrared, which the atmosphere (and carbon dioxide, water vapor, methane, and other greenhouse gases in particular) is not nearly as transparent to. As a result, the atmosphere retains more heat.
Good point, Dervorin and chorpler! I think you understood what Diosa was asking better than I did.
Another point about the different behavior of visible light versus infrared (heat) radiation: This is partly why the related climate-change factor of “albedo” or “whiteness” of the earth is important. Solar radiation that strikes white/shiny surfaces, such as ice and snow, is more likely to be reflected back outwards as light, which as you note has an easier time getting through the atmospheric gases back into space. But solar rays that strike dark surfaces, such as ground or sea, are more likely to be absorbed and then radiated outward as heat, which tends to get trapped in the atmosphere.
So if a significant amount of white/shiny surface on the earth gets changed into dark surface, e.g., by large areas of snow and glaciers melting, this could accelerate the greenhouse effect further by making the earth less reflective and more heat-trapping.
First, thank you guys for the great answers. That makes perfect sense now, but one more thing:
In regard to what you say above, why? Why is the water and CO2 more effective than the atmosphere in happier conditions? What specifically factors in to make the heat less able to exit?
Carbon dioxide and water absorb infrared radiation and O2 and N2 don’t. As the percentage of CO2 and water vapor increases in the atmosphere, the atmosphere as a whole absorbs more infrared radiation and heats up. Remember that some greenhouse effect is necessary for the atmosphere to be warm at all. Without any greenhouses gases, the atmosphere would likely be too cold for much of the life on Earth.
If you are curious as to why some molecules can absorb infrared radiation while others don’t, it mostly has to do with whether changing the a bond length changes the molecules dipole moment. Infrared radiation is not powerful enough to push electrons out of molecules, but it can stretch the bonds between a molecule’s constituent atoms. Oxygen and nitrogen gases have a single bond between the two atoms. No matter the length of that bond, the dipole moment doesn’t change: it is zero no matter what. In a water molecule, when each of a hyrdogen bonds are the same length, there is a slight negative charge around the oxygen and a slight positive charge in the space between the two hydrogen atoms. Stretching one of the hydrogen bonds shifts that dipole, moving the positive charge closer to the hydrogen atom with the shorter bond and moving negative charge closer to the hydrogen with a longer bond length. In carbon dioxide, we have a similar deal. CO2 is a linear molecule and when the bonds between the carbon and each oxygen atoms are equal it has no dipole. But if the bond between the carbon and one of the oxygen atoms is stretched, a dipole moment forms with a partial negative charge on the oxygen closer to the carbon.