Greenhouse gases

Follow the energy

Tap each stage to open it up. Watch for the moment the wavelength changes: everything after it depends on that.

1. 1The Sun's radiation comes inRadiation from the Sun is short wavelength radiation. It travels through the atmosphere all the way to the ground.+
2. 2The ground absorbs itThe Earth's surface absorbs this incoming radiation.+
3. 3The ground gives out infrared, at a longer wavelengthThe surface gives out radiation of a longer wavelength than the radiation it absorbed. That longer wavelength radiation is infrared.+
4. 4Greenhouse gases absorb the infraredWater vapour, carbon dioxide and methane in the atmosphere absorb this long wavelength infrared on its way out.+
5. 5The gases send it out in every directionAfter absorbing it, the gases send the energy back out as radiation in every direction, and some of it goes back down to the surface.+
6. 6Less escapes, so the atmosphere stays warmerLess energy escapes into space, so the atmosphere is kept warmer.+

Predict, then check

One idea about energy explains why average temperatures change.

Energy is always flowing into the atmosphere, and energy is always flowing out of it. Suppose it starts flowing in faster than it flows out, while some is still escaping to space. What happens to average temperatures?

What you need to know

• Water vapour, carbon dioxide and methane are greenhouse gases.
• Greenhouse gases in the atmosphere keep temperatures on Earth high enough to support life.
• Short wavelength radiation from the Sun travels through the atmosphere to the ground. Greenhouse gases don't cut down how much of it comes in.
• The Earth's surface absorbs this radiation and gives out radiation of a longer wavelength: infrared.
• Greenhouse gases absorb the long wavelength infrared, then send the energy out in every direction, some back down to the surface, so less escapes into space.
• If energy is transferred into the atmosphere faster than it is transferred out, average temperatures rise.

The big picture

Water vapour, carbon dioxide and methane are greenhouse gases, and they keep temperatures on Earth high enough to support life. Short wavelength radiation from the Sun passes through the atmosphere to the ground. The surface absorbs it and gives out longer wavelength infrared: the Sun (around 6,000 °C) is far hotter, and the hotter a body is, the shorter the wavelength of its most intense radiation. Greenhouse gases absorb that infrared and send it out in every direction, some back down, so less escapes into space. If energy enters the atmosphere faster than it leaves, average temperatures rise.

⚠ Watch out

Writing that greenhouse gases 'block' or 'trap' the Sun's rays. They don't cut down the radiation coming in at all. What they absorb is the longer wavelength infrared the ground gives out, and they send that energy off in every direction, so less escapes, not none.

Memory hook

Short in, long out. The ground turns short into long, and the gases catch the long and throw it every which way, some back down.

Check yourself

Cover the page and talk through the energy's journey from the Sun to space. At which stage does the wavelength change, and why does that matter to the greenhouse gases?