Climate change – the physics perspective

Phew! It's hot. The car is even hotter than outside. Hey, wondering why your car gets so warmed up when you leave it out in the Sun? Mmm...

yeah. The windows of the car let in the sunlight, which hits the interior of the car... all the seats and panels. A portion of the light is reflected and bounces off them. The rest of the sunlight energy is absorbed.

The seats and panels in the car are heated. The air inside the car also heats up, drawing warmth mostly from the heated objects. When everything inside the car gets warm, it starts to emit heat radiation, but this radiation has a different wavelength from the sunlight that entered. And while the sunlight can pass through the window glass, some of the heat radiation can not. This is reflected, bouncing back from the windows and heating the car even more.

The temperature rises, the car becomes hot. This is the greenhouse effect. Its name comes from greenhouses: glass houses used to create warm conditions for plants to grow in. But if more energy enters than exits, wouldn't the car get hotter and hotter until it starts to glow? No, the car does expel some of the heat radiation, and it expels heat faster and faster as it gets hotter.

When equal amounts of energy are being radiated in and out, it reaches equilibrium. At this point, the car stops getting warmer. The car gets hot, but not hot enough to glow. The car has a lot in common with the Earth. The sun shines on the earth, and a lot of the sunlight passes through the atmosphere and heats the earth.

The surface of the earth then emits heat radiation, and just like with the car this has a different wavelength from the light that entered. And as with the car's windows, some of the heat radiation bounces back down from the atmosphere. The greenhouse effect also works for our planet. The earth is heated. The warmer the earth is, the more energy is emitted from its surface.

And like the car, the earth reaches a state of equilibrium, at a temperature when the energy reaching the earth is equal to the energy released. This temperature is in a range that has enabled life to appear and evolve on earth. Without the greenhouse effect, the earth would be a lot colder, probably around thirty degrees celsius colder. Life would probably not exist on earth. Hmm...

I see. The greenhouse effect is important for our planet and for all life. But it is pretty annoying in my car. It feels like I am sitting in an oven! Yes.

And it is a serious problem for the planet as well. The greenhouse effect is important and natural, but excessive greenhouse effect is not. People have altered the earth's atmosphere, and thus the balance of energy entering and being released. The equilibrium has been changed. The average temperature on the earth has increased rapidly.

During the last hundred years, emissions from, for example, factories and cars have raised the levels of carbon dioxide and water vapor in the atmosphere. These are sometimes called greenhouse gases. More greenhouse gases means a smaller share of the heat radiation passing out through the atmosphere. The greenhouse effect becomes stronger, the earth becomes warmer. This is global warming.

What all this will mean for humans and animals in the future, we don't know, but the consequences might be serious unless the warming stops. - - Yes, in the car you can wind down the window, but the atmosphere of the earth doesn't have any windows to open. To stop global warming, we need to cut down the emission of greenhouse gases into the atmosphere. Yes, Philip. The choices we make in our lives do matter. It's hard to tell what's right and wrong, but it is important to think twice about what we do.