You've probably heard that carbon dioxide is warming the Earth, but how does it work?
你可能听说过二氧化碳正在让地球变暖，但这是什么原理呢？
Is it like the glass of a greenhouse or like an insulating blanket? Well, not entirely.
二氧化碳像是温室的玻璃？或者像是隔热的毛毯？其实不全是这样。
The answer involves a bit of quantum mechanics, but don't worry, we'll start with a rainbow.
答案关系到一些量子力学的知识，但是不用担心，让我们从彩虹说起。
If you look closely at sunlight separated through a prism, you'll see dark gaps where bands of color went missing.
如果你透过三棱镜仔细观察被分散的阳光，你会看到光谱中有一些暗掉的缺口，一部分颜色的波段消失了。
Where did they go? Before reaching our eyes, different gases absorbed those specific parts of the spectrum.
它们去哪里了？在到达我们的眼睛之前，很多气体就已经吸收掉波谱中特定的一些频段。
For example, oxygen gas snatched up some of the dark red light, and sodium grabbed two bands of yellow.
例如，氧气夺走了一些深红色光线，钠夺取了黄色的两个波段。
But why do these gases absorb specific colors of light?
但为什么这些气体会吸收特定颜色的光呢？
This is where we enter the quantum realm.
我们现在就要进入量子学领域了。
Every atom and molecule has a set number of possible energy levels for its electrons.
每个原子和分子都有一定数量的电子能量等级。
To shift its electrons from the ground state to a higher level, a molecule needs to gain a certain amount of energy.
使电子从基态跃迁到一个更高的能级，分子需要获得一个特定量的能量。
No more, no less. It gets that energy from light, which comes in more energy levels than you could count.
不能多，也不能少。分子从光线中获得这种能量，光线里含有数不胜数的能量层级。
Light consists of tiny particles called photons and the amount of energy in each photon corresponds to its color.
光由被称为“光子”的微小粒子组成，每个光子里储存的能量与其颜色对应。
Red light has lower energy and longer wavelengths.
红色光线能量较低，波长较长。
Purple light has higher energy and shorter wavelengths.
紫色光线能量较高，波长较短。
Sunlight offers all the photons of the rainbow,
阳光提供了彩虹里所有颜色的光子，
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.
所以一个气体分子可以选择带有它们所需的特定能量值的光子，用以把分子提升到下一个能级。
When this match is made, the photon disappers as the molecule gains its energy, and we get a small gap in our rainbow.
匹配上之后，光子将消失，因为分子获取了它的能量，从而造成了光谱里的缺口。
If a photon carries too much or too little energy, the molecule has no choice but to let it fly past.
如果一个光子携带的能量太多或太少，分子就只好让它飞过。
This is why glass is transparent.
这就是玻璃透明的原因。
The atoms in glass do not pair well with any of the energy levels in visible light, so the photons pass through.
玻璃里的原子和可见光里的波段不匹配，所以这些光子全部直接通过。
So, which photons does carbon dioxide prefer?
那么，二氧化碳选择吸收哪些光子呢？
Where is the black line in our rainbow that explains global warming? Well, it's not there.
彩虹光谱中的哪一条黑线能解释全球变暖呢？其实，不在那儿。
Carbon dioxide doesn't absorb light directly from the Sun.
二氧化碳不直接从阳光中吸收光线。
It absorbs light from a totally different celestial body.
它吸收的光线来自一个完全不同的天体。
One that doesn't appear to be emitting light at all: Earth.
一个看起来根本不发光的天体：地球。

If you're wondering why our planet doesn't seem to be glowing, it's because the Earth doesn't emit visible light.
如果你在想为什么我们的星球看起来不发光，那是因为地球不发射可见光。
It emits infared light. The light that our eyes can see, including all of the colors of the rainbow,
它发射的是红外光。我们眼睛能看得见的光，包括彩虹里的所有颜色，
is just a small part of the larger spectrum of electromagnetic radiation,
都只是电磁辐射大光谱里的一小部分，
which includes radio waves, microwaves, infrared, ultraviolet, x-rays, and gamma rays.
大光谱里有无线电波、微波、红外线、紫外线、X射线和伽马射线。
It may seem strange to think of these things as light,
把这些都当成是光线可能有点奇怪，
but there is no fundamental difference between visible light and other electromagnetic radiation.
但是可见光和电磁射线之间没有根本的区别。
It's the same energy, but at a higher or lower level.
它们都是能量，只不过在能级上有高低之分。
In fact, it's a bit presumptuous to define the term visible light by our own limitations.
其实，我们用自身的局限来定义“可见光”是有点自大的。
After all, infrared light is visible to snakes, and ultraviolet light is visible to birds.
毕竟，蛇类看得见红外线，鸟类看得见紫外线。
If our eyes were adapted to see light of 1900 megahertz, then a mobile phone would be a flashlight,
如果我们的眼睛能看见1900兆赫的光线，那么一个移动电话将会变成手电，
and a cell phone tower would look like a huge lantern.
而一个手机信号塔将看起来像个大灯笼。
Earth emits infrared radiation because every object with a temperature above absolute zero will emit light.
地球发出红外射线，因为每个温度在绝对零度之上的物体都会发出光线。
This is called thermal radiation. The hotter an object gets, the higher frequency the light it emits.
这被称作“热辐射”。一个物体越热，它发出的光线频率越高。
When you heat a piece of iron, it will emit more and more frequencies of infrared light,
当你加热铁的时候，它将会发出越来越高频率的红外光，
and then, at a temperature of around 450 degrees Celsius, its light will reach the visible spectrum.
然后，当温度达到450摄氏度时，它发出的光将达到可见光范围。
At first, it will look red hot.
刚开始，它看起来是赤热的红色。
And with even more heat, it will glow white with all of the frequencies of visible light.
得到更多热量时，它发的光将变白，发出可见光波段里所有的光。
This is how traditional light bulbs were designed to work and why they're so wasteful.
这是传统灯泡的设计原理，也是为什么它们如此浪费能源。
95% of the light they emit is invisible to our eyes. It's wasted as heat.
它们发出的光中95%都是我们眼睛看不见的光。它以热量的形式浪费掉了。
Earth's infrared radiation would escape to space if there weren't greenhouse gas molecules in our atmophere.
如果我们的大气层中没有温室气体分子的话，地球的红外射线会逃到太空中。
Just as oxygen gas prefers the dark red photons,
就像氧气偏爱深红色光子一样，
carbon dioxide and other greenhouse gases match with infrared photons.
二氧化碳和其它温室气体和红外线光子匹配。
They provide the right amount of energy to shift the gas molecules into their higher energy level.
它们提供了正好合适的能量，用来使气体分子升到它们更高一层的能级。
Shortly after a carbon dioxide molecule absorbs an infrared photon,
当一个二氧化碳分子吸收了红外光子之后，
it will fall back to its previous energy level, and spit a photon back out in a random direction.
不久它便会回到之前的能级，并分离出一个光子，以随机的方向射出。
Some of that energy then returns to Earth's surface, causing warming.
这些能量的一部分就返回了地球表面，导致变暖。
The more carbon dioxide in the atmosphere,
大气层里的二氧化碳越多，
the more likely that infrared photons will land back on Earth and change our climate.
红外光子反回到地球表面的可能性就越大，使我们的气候随之改变。