Greenhouse Gases

I’m Scott Denning and I’m a professor of atmospheric science at Colorado State University. Today we’re going to talk about the greenhouse effect: how it works and why it’s important for climate change. All of the energy that comes into the earth has to get to the earth by electromagnetic radiation. There’s no air outside the Earth’s atmosphere so there’s no way for heat to get in and out of the earth. It’s just a big ball floating around in a vacuum. So the only way we get energy in and energy back out is through electromagnetic radiation. It’s actually waves of energy that come in. Short waves are visible to us as light and we can feel the Sun on us. That’s the short waves. And the long waves go back out. The long waves carry the energy back out to space, radiating energy. Just like you feel energy coming off a fire or you feel energy coming off a heat lamp, or you can feel heat energy coming off a hot stove. The Earth is radiating energy back out to space, cooling itself off all the time. The energy that comes in from the Sun is not enough to keep us warm here at the surface of the Earth. If that was the only energy we had believe it or not the whole earth would actually be so cold that all the oceans would freeze solid. Earth would be just this white ball of ice floating in space. It would be really cold. We wouldn’t even be here talking about it because Earth would not be a habitable planet. It’s really lucky for us that we have air. It actually turns out about two-thirds of the energy that warms us up at the surface of the earth is from the warm air that also radiates these waves. It all came from the Sun originally but some of it gets recycled. It goes up, gets caught by these all greenhouse molecules and sent back down. We call this the greenhouse effect. Energy comes in from the Sun, it goes back out to space but some of the energy gets trapped by the warm air and reradiated back down like a bunch a little heat lamps up in the sky. You may have heard that the greenhouse gas that we hear about a lot is co2, carbon dioxide. That’s actually just a small amount of the air. Ninety-nine percent of our air is not carbon dioxide it’s nitrogen and oxygen. N2 and O2. But N2 and O2 are not very good at absorbing energy. They’re both made of two atoms of the same element. So N2: two Ns glued together with the with a bond between them. Think of them like two tennis balls on a spring or tennis balls on a stick. These waves of energy can come up and hit that ball on a stick molecule and started it vibrating and the energy is actually then stored in the molecule. So the energy comes up from the earth hits the molecule and it starts vibrating. But then it can un-vibrate or de-excite and send that energy back out to the earth and then as it vibrating slower the energy comes back out. But there’s not a heckuva lot you can do with two balls on a stick there’s just not that many ways for them to move. CO2 is different. CO2 has three atoms so there’s a C-O2. As you can imagine the co2 molecule can vibrate just like N2 and O2, but it can also do other stuff. And all these different kinds of molecular dances that the co2 molecule can do will absorb different wavelengths of energy. So because it has all these different varieties of dances, or varieties of vibration that it could do up in the sky, it can absorb much more energy and reread it like heat lamps back to the earth, keeping us warm. So the co2 is a much more powerful greenhouse gas than O2 and N2 that make up most of our air. Water vapor is another one. Water vapors is H2O so again there’s three atoms in H2O, but unlike CO2 they’re not all in a straight line like this they’re actually bent like this. So the H2O molecule is even better at dancing and vibrating then the co2 molecule.

So what does all this have to do with climate change? As you know when we burn stuff; stuff that’s made of carbon like coal, oil and gas, we react it with oxygen in the atmosphere to make extra CO2 molecules. Each one of those little extra CO2 molecule captures that long wave energy going up, does its little molecular dance, sends it back down. So it’s like extra little heat lamps up in the sky that send energy to the surface of the earth. We calculated that if you double the number a CO2 molecules in the air it’s an equal amount of energy to putting a little kid’s four-watt night late in every square meter of the planet. So that’s not a lot these little four-watt night lights, but if you put one every square meter on the whole planet you’d expect that to warm things up. When we say we expect extra CO2 in the atmosphere to warm the surface of the earth we’re saying adding energy to something changes its temperature. We can argue about how much it’ll change or when it’s going to change, but the idea that putting extra CO2 into the air will warm the surface is pretty much a no-brainer. You’re adding energy to something it changes its temperature.