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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.