Greenhouse effect and greenhouse gases | High school biology | Khan Academy

In this video, we're going to talk about the greenhouse effect and also the greenhouse gases which cause the greenhouse effect. Now let's just start with a basic idea. Imagine if Earth had no atmosphere. What would happen? Well, you have the sun, which is on average 93 million miles away. It's sending electromagnetic radiation our way to the surface of the Earth.

We're actually getting a very, very small fraction of the total electromagnetic radiation of the sun, and then that would heat up the surface of the Earth. Now, what I have always found mind-blowing is anything with temperature will emit electromagnetic radiation, and so it's emitting some of that energy. It's losing some of that energy to electromagnetic radiation, so the surface would be releasing that, and it would go out into space.

But now let's introduce the idea of an atmosphere, and in particular, we're going to think about our lower atmosphere, which starts at the surface and goes up to about five to nine miles in altitude, often known as the troposphere. Now, the troposphere has molecules in it; it has gases in it like carbon dioxide, like water vapor, like methane. Others include nitrous oxide and chlorofluorocarbons.

You don't have to know the chemical formula of all of these things, but what's interesting about these gases that are in the lower atmosphere is that they can absorb some of those electromagnetic waves that the surface of the Earth is emitting. So some of that energy will make it out to space, but some of that energy will then be absorbed by these molecules, by these gases, and they will emit some of that back to Earth.

And so that's why it's called a greenhouse effect, because of all of that energy that might have been sent out to space, if you didn't have an atmosphere, not all of it is; some of it is reabsorbed by the atmosphere, which then sends it back down to the surface. That process can go on and on and on. You can imagine that it's trapping some of the energy.

Now, the greenhouse effect and greenhouse gases, the ones that I just listed, they're oftentimes associated with man-made climate change and global warming, and they are for good reason. But it's important to realize that we actually need some base level greenhouse effect just for Earth to be habitable in the way that it is. Without greenhouse gases, Earth's surface would be about negative 18 degrees Celsius, which is the same as zero degrees Fahrenheit, which I think most of y'all would recognize is very, very, very cold relative to what the actual averages are, which are 15 degrees Celsius or 59 degrees Fahrenheit.

And of course, these are average temperatures, but 59 is a nice refreshing brisk day, not a frigid day like zero degrees Fahrenheit. And of course, these are averages; it fluctuates around this depending on time of year and where you are. But this makes it clear that we do need these greenhouse gases to keep the Earth reasonably warm.

Now, the problem is if the concentration of these greenhouse gases goes out of equilibrium, becomes unusually high, and it does look like that is indeed happening. This right over here is a chart, and the way that we're able to figure that out is by taking ice samples and rock samples and looking into our past through the geological record of how much carbon dioxide there has been over the last 800,000 years.

And 800,000 years is a very long time period. Modern human beings have only been around for two or three hundred thousand years. What you can see is the concentration of carbon dioxide has roughly fluctuated between about 200 parts per million and about 300 parts per million, at least over the duration of this chart. But in recent times, we've gone well beyond that; we've almost gone double that average right over here, and this is actually the highest levels we've seen in 3 million years.

Now, it's important to realize that carbon dioxide makes up a small percentage of our atmosphere. In fact, all of these greenhouse gases make up a small percentage: 78 percent of the troposphere is nitrogen, 21 is oxygen, and the last one percent is things like argon, water vapor, carbon dioxide, and methane. So, even this small amount, when the concentration increases dramatically, can have a huge effect.

Now, you might say, "Hey, we were at these levels three million years ago." Roughly maybe this is just some type of cycle that we're seeing on Earth. And to recognize that this is indeed man-made, we just have to look at a chart like this. This tells us two things: this tells us annual emissions in this blue line, this blue curve, and then the total emission, or the total concentration in the atmosphere.

Because one other thing about one of the things about a greenhouse gas like carbon dioxide, when it's emitted, it doesn't just disappear. And we can see, if we go to the pre-industrial revolution or the early stages of the industrial revolution, CO2 emissions were pretty low, pretty close to zero, at least on this scale right over here. There might have been some basic CO2 emissions; people had fires and stoves and things like that.

But then, as the industrial revolution came into play and we started using fossil fuels more and more to fuel transportation and factories and other things, our emissions have gone up dramatically, and this coincides with the total concentration going well above that 800,000-year average. So it's important to keep in mind the greenhouse effect is needed to some degree, but the problem is when the concentration of greenhouse gases like carbon dioxide go well beyond their historic averages, which can over time warm the Earth.

And even a few degrees centigrade of warming the Earth can have huge consequences on our environment and on our weather. I'll throw out one last idea just for kicks, because even though it's called the greenhouse effect, it's actually not how actual greenhouses work. The greenhouse effect, as we just described it, is really based on this idea that things are getting the infrared radiation, the electromagnetic radiation is getting reabsorbed, which then gets re-emitted back to the surface and vice versa.

In a real greenhouse, you can imagine that it is made out of glass. What's happening is the sunlight can come in, and it's warming the surface and it's warming the air inside the greenhouse, and then that air is not allowed to circulate. So, if the greenhouse had a little hole at the top right over here, that hot air would be allowed to go out and circulate with the cool air up here.

But the air isn't allowed to mix, and so the air gets hotter and hotter and hotter. It actually turns out that the glass can let the electromagnetic radiation out unlike greenhouse gases, so it is a different actual physical process. But you can see where people tried to create the metaphor.