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You've probably heard that

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carbon dioxide is warming the Earth,

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but how does it work?

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Is it like the glass of a greenhouse

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or like an insulating blanket?

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Well, not entirely.

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The answer involves a bit

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of quantum mechanics, but don't worry,

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we'll start with a rainbow.

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If you look closely at sunlight separated

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through a prism,

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you'll see dark gaps where bands of color went missing.

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Where did they go?

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Before reaching our eyes,

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different gases absorbed those

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specific parts of the spectrum.

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For example, oxygen gas snatched up

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some of the dark red light,

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and sodium grabbed two bands of yellow.

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But why do these gases absorb

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specific colors of light?

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This is where we enter the quantum realm.

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Every atom and molecule has a set number

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of possible energy levels for its electrons.

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To shift its electrons from the ground state

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to a higher level,

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a molecule needs to gain a certain amount of energy.

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No more, no less.

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It gets that energy from light,

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which comes in more energy levels than you could count.

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Light consists of tiny particles called photons

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and the amount of energy in each photon

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corresponds to its color.

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Red light has lower energy and longer wavelengths.

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Purple light has higher energy and shorter wavelengths.

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Sunlight offers all the photons of the rainbow,

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so a gas molecule can choose

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the photons that carry the exact amount of energy

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needed to shift the molecule to

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its next energy level.

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When this match is made,

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the photon disappers as the molecule

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gains its energy,

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and we get a small gap in our rainbow.

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If a photon carries too much or too little energy,

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the molecule has no choice but

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to let it fly past.

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This is why glass is transparent.

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The atoms in glass do not pair well

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with any of the energy levels in visible light,

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so the photons pass through.

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So, which photons does carbon dioxide prefer?

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Where is the black line in our rainbow

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that explains global warming?

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Well, it's not there.

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Carbon dioxide doesn't absorb light directly

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from the Sun.

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It absorbs light from a totally

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different celestial body.

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One that doesn't appear to be emitting light at all:

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Earth.

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If you're wondering why our planet

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doesn't seem to be glowing,

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it's because the Earth doesn't emit visible light.

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It emits infared light.

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The light that our eyes can see,

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including all of the colors of the rainbow,

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is just a small part of the larger spectrum

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of electromagnetic radiation,

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which includes radio waves, microwaves,

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infrared, ultraviolet, x-rays,

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and gamma rays.

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It may seem strange to think of these things as light,

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but there is no fundamental difference

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between visible light and other electromagnetic radiation.

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It's the same energy,

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but at a higher or lower level.

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In fact, it's a bit presumptuous to define

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the term visible light by our own limitations.

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After all, infrared light is visible to snakes,

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and ultraviolet light is visible to birds.

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If our eyes were adapted to see light of

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1900 megahertz, then a mobile phone

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would be a flashlight,

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and a cell phone tower

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would look like a huge lantern.

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Earth emits infrared radiation

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because every object with a temperature

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above absolute zero will emit light.

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This is called thermal radiation.

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The hotter an object gets,

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the higher frequency the light it emits.

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When you heat a piece of iron,

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it will emit more and more frequencies of infrared light,

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and then, at a temperature of around 450 degrees Celsius,

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its light will reach the visible spectrum.

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At first, it will look red hot.

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And with even more heat,

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it will glow white

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with all of the frequencies of visible light.

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This is how traditional light bulbs

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were designed to work

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and why they're so wasteful.

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95% of the light they emit is invisible to our eyes.

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It's wasted as heat.

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Earth's infrared radiation would escape to space

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if there weren't greenhouse gas molecules

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in our atmophere.

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Just as oxygen gas prefers the dark red photons,

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carbon dioxide and other greenhouse gases

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match with infrared photons.

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They provide the right amount of energy

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to shift the gas molecules into their higher energy level.

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Shortly after a carbon dioxide molecule

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absorbs an infrared photon,

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it will fall back to its previous energy level,

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and spit a photon back out in a random direction.

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Some of that energy then returns

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to Earth's surface,

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causing warming.

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The more carbon dioxide in the atmosphere,

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the more likely that infrared photons

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will land back on Earth

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and change our climate.