How quantum mechanics explains global warming - Lieven Scheire

You've probably heard that

carbon dioxide is warming the Earth,

but how does it work?

Is it like the glass of a greenhouse

or like an insulating blanket?

Well, not entirely.

The answer involves a bit

of quantum mechanics, but don't worry,

we'll start with a rainbow.

If you look closely at sunlight separated

through a prism,

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

Where did they go?

Before reaching our eyes,

different gases absorbed those

specific parts of the spectrum.

For example, oxygen gas snatched up

some of the dark red light,

and sodium grabbed two bands of yellow.

But why do these gases absorb

specific colors of light?

This is where we enter the quantum realm.

Every atom and molecule has a set number

of possible energy levels for its electrons.

To shift its electrons from the ground state

to a higher level,

a molecule needs to gain a certain amount of energy.

No more, no less.

It gets that energy from light,

which comes in more energy levels than you could count.

Light consists of tiny particles called photons

and the amount of energy in each photon

corresponds to its color.

Red light has lower energy and longer wavelengths.

Purple light has higher energy and shorter wavelengths.

Sunlight offers all the photons of the rainbow,

so a gas molecule can choose

the photons that carry the exact amount of energy

needed to shift the molecule to

its next energy level.

When this match is made,

the photon disappers as the molecule

gains its energy,

and we get a small gap in our rainbow.

If a photon carries too much or too little energy,

the molecule has no choice but

to let it fly past.

This is why glass is transparent.

The atoms in glass do not pair well

with any of the energy levels in visible light,

so the photons pass through.

So, which photons does carbon dioxide prefer?

Where is the black line in our rainbow

that explains global warming?

Well, it's not there.

Carbon dioxide doesn't absorb light directly

from the Sun.

It absorbs light from a totally

different celestial body.

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

Earth.

If you're wondering why our planet

doesn't seem to be glowing,

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

It emits infared light.

The light that our eyes can see,

including all of the colors of the rainbow,

is just a small part of the larger spectrum

of electromagnetic radiation,

which includes radio waves, microwaves,

infrared, ultraviolet, x-rays,

and gamma rays.

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

but there is no fundamental difference

between visible light and other electromagnetic radiation.

It's the same energy,

but at a higher or lower level.

In fact, it's a bit presumptuous to define

the term visible light by our own limitations.

After all, infrared light is visible to snakes,

and ultraviolet light is visible to birds.

If our eyes were adapted to see light of

1900 megahertz, then a mobile phone

would be a flashlight,

and a cell phone tower

would look like a huge lantern.

Earth emits infrared radiation

because every object with a temperature

above absolute zero will emit light.

This is called thermal radiation.

The hotter an object gets,

the higher frequency the light it emits.

When you heat a piece of iron,

it will emit more and more frequencies of infrared light,

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

its light will reach the visible spectrum.

At first, it will look red hot.

And with even more heat,

it will glow white

with all of the frequencies of visible light.

This is how traditional light bulbs

were designed to work

and why they're so wasteful.

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

It's wasted as heat.

Earth's infrared radiation would escape to space

if there weren't greenhouse gas molecules

in our atmophere.

Just as oxygen gas prefers the dark red photons,

carbon dioxide and other greenhouse gases

match with infrared photons.

They provide the right amount of energy

to shift the gas molecules into their higher energy level.

Shortly after a carbon dioxide molecule

absorbs an infrared photon,

it will fall back to its previous energy level,

and spit a photon back out in a random direction.

Some of that energy then returns

to Earth's surface,

causing warming.

The more carbon dioxide in the atmosphere,

the more likely that infrared photons

will land back on Earth

and change our climate.