carbon dioxide is warming the Earth,

二氧化碳正在暖化地球。

but how does it work?

但是怎麼做到的呢？

Is it like the glass of a greenhouse

它們是像溫室的玻璃，

or like an insulating blanket?

還是像隔熱毯？

Well, not entirely.

嗯，也不全然是。

The answer involves a bit

實際情況涉及一點點「量子力學」。

of quantum mechanics, but don't worry,

不過別緊張，

we'll start with a rainbow.

讓我們先從彩虹開始。

If you look closely at sunlight separated

如果仔細看太陽光 被三稜鏡折射出的光譜，

through a prism,

你會看見色塊之間， 有黑色的間隔。

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

那些顏色跑去哪了？

Where did they go?

在色光進入我們的眼睛前，

Before reaching our eyes,

空氣中的氣體分子

different gases absorbed those

就把特定的色光吸收了。

specific parts of the spectrum.

比如說，

For example, oxygen gas snatched up

氧氣會把暗紅光吃掉；

some of the dark red light,

鈉會吸掉一部分黃光。

and sodium grabbed two bands of yellow.

可是為什麼氣體要吸收色光呢？

But why do these gases absorb

這就要靠量子力學解釋了。

specific colors of light?

每個原子或分子，

This is where we enter the quantum realm.

都有固定的「能階」，

Every atom and molecule has a set number

容納一定的電子。

of possible energy levels for its electrons.

「基態」時的電子要從低能階

To shift its electrons from the ground state

「躍遷」到高能階時，

to a higher level,

分子必須要吸收特定的能量。

a molecule needs to gain a certain amount of energy.

不能太多也不能太少。

No more, no less.

它從光獲取能量，

It gets that energy from light,

這些能量可以帶來 你意想不到的能階改變。

which comes in more energy levels than you could count.

光中攜帶稱為「光子」的粒子，

Light consists of tiny particles called photons

不同色光的光子帶有不同能量。

and the amount of energy in each photon

紅光的能量低、波長長，

corresponds to its color.

紫光能量高、波長短。

Red light has lower energy and longer wavelengths.

陽光包含了所有色光的光子，

Purple light has higher energy and shorter wavelengths.

所以氣體可以自由選擇，

Sunlight offers all the photons of the rainbow,

分子要躍遷到更高能階時

so a gas molecule can choose

所需的光子和能量。

the photons that carry the exact amount of energy

當光子的能量被分子吸收，

needed to shift the molecule to

光子就會消失，

its next energy level.

我們就只看得到色塊間的黑線。

When this match is made,

如果光子攜帶太多或太少的能量，

the photon disappers as the molecule

分子就會放過它、隨它去。

gains its energy,

所以玻璃是透明的，

and we get a small gap in our rainbow.

因為玻璃裡的原子在可見光中，

If a photon carries too much or too little energy,

老找不到適合的光子，

the molecule has no choice but

只好讓它們都通過。

to let it fly past.

那二氧化碳喜歡哪種光子呢？

This is why glass is transparent.

彩虹光譜中的哪條黑線，

The atoms in glass do not pair well

代表全球暖化的進行呢？

with any of the energy levels in visible light,

嗯，一條也不是。

so the photons pass through.

因為二氧化碳不從日光 直接吸收光子，

So, which photons does carbon dioxide prefer?

而是吸收另一個天體的光，

Where is the black line in our rainbow

一個不太算是發光天體的星球：

that explains global warming?

地球。

Well, it's not there.

可曾想過為什麼

Carbon dioxide doesn't absorb light directly

地球不像恆星閃閃發光？

from the Sun.

那是因為地球不發出可見光，

It absorbs light from a totally

只射出紅外線。

different celestial body.

我們肉眼可見的光，

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

包括彩虹中的所有色光，

Earth.

只是廣大「電磁輻射」

If you're wondering why our planet

光譜中的一小部分。

doesn't seem to be glowing,

另外還有無線電波、微波、

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

紅外線、紫外線、X光和伽瑪射線。

It emits infared light.

你可能覺得這些不是光，

The light that our eyes can see,

但它們本質上並沒有不同；

including all of the colors of the rainbow,

可見光和其他電磁輻射，

is just a small part of the larger spectrum

同為能量的一種，

of electromagnetic radiation,

差別只在能量高低。

which includes radio waves, microwaves,

再說，

infrared, ultraviolet, x-rays,

用人類的眼光去定義 可見光也有失公允。

and gamma rays.

因為蛇其實看得見紅外線，

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

鳥類也看得到紫外線。

but there is no fundamental difference

如果我們的眼睛 能看見1900兆赫的光，

between visible light and other electromagnetic radiation.

那手機就會像一支支手電筒，

It's the same energy,

基地台就會像大燈籠。

but at a higher or lower level.

地球發出紅外線，

In fact, it's a bit presumptuous to define

因為任何有溫度的物體 都會「發光」，

the term visible light by our own limitations.

就是所謂的「熱輻射」。

After all, infrared light is visible to snakes,

當物體的溫度越高，

and ultraviolet light is visible to birds.

發出的光頻率就越高。

If our eyes were adapted to see light of

當你燒熱一塊鐵，

1900 megahertz, then a mobile phone

它會發出越來越高頻的紅外線。

would be a flashlight,

當溫度達攝氏450度左右，

and a cell phone tower

就會轉變為可見光。

would look like a huge lantern.

一開始我們會看到燒得通紅，

Earth emits infrared radiation

當溫度持續上升，

because every object with a temperature

開始發出白光，

above absolute zero will emit light.

也就是所有的可見光。

This is called thermal radiation.

這就是傳統燈泡的原理，

The hotter an object gets,

而且很耗能的原因。

the higher frequency the light it emits.

發出95%以上的光

When you heat a piece of iron,

是我們肉眼不可見的，

it will emit more and more frequencies of infrared light,

只會一直散發熱能。

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

如果大氣中沒有溫室氣體，

its light will reach the visible spectrum.

地球的紅外線就會跑到宇宙。

At first, it will look red hot.

就像氧氣喜歡暗紅光，

And with even more heat,

二氧化碳和其他溫室氣體

it will glow white

會和紅外線的光子結合。

with all of the frequencies of visible light.

它們提供剛剛好的能量

This is how traditional light bulbs

讓氣體分子轉換到更高能階。

were designed to work

當二氧化碳分子吸收紅外線光子，

and why they're so wasteful.

它會降回原來的能階，

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

再往隨機方向丟出一顆光子。

It's wasted as heat.

部分的能量又會回到地球，

Earth's infrared radiation would escape to space

造成暖化。

if there weren't greenhouse gas molecules

大氣中的二氧化碳越多，

in our atmophere.

紅外線光子就越會被留在地球，

Just as oxygen gas prefers the dark red photons,

並改變我們的氣候。

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.