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.
你可能有聽說, 二氧化碳正在暖化地球。 但是怎麼做到的呢? 它們是像溫室的玻璃, 還是像隔熱毯? 嗯,也不全然是。 實際情況涉及一點點「量子力學」。 不過別緊張, 讓我們先從彩虹開始。 如果仔細看太陽光 被三稜鏡折射出的光譜, 你會看見色塊之間, 有黑色的間隔。 那些顏色跑去哪了? 在色光進入我們的眼睛前, 空氣中的氣體分子 就把特定的色光吸收了。 比如說, 氧氣會把暗紅光吃掉; 鈉會吸掉一部分黃光。 可是為什麼氣體要吸收色光呢? 這就要靠量子力學解釋了。 每個原子或分子, 都有固定的「能階」, 容納一定的電子。 「基態」時的電子要從低能階 「躍遷」到高能階時, 分子必須要吸收特定的能量。 不能太多也不能太少。 它從光獲取能量, 這些能量可以帶來 你意想不到的能階改變。 光中攜帶稱為「光子」的粒子, 不同色光的光子帶有不同能量。 紅光的能量低、波長長, 紫光能量高、波長短。 陽光包含了所有色光的光子, 所以氣體可以自由選擇, 分子要躍遷到更高能階時 所需的光子和能量。 當光子的能量被分子吸收, 光子就會消失, 我們就只看得到色塊間的黑線。 如果光子攜帶太多或太少的能量, 分子就會放過它、隨它去。 所以玻璃是透明的, 因為玻璃裡的原子在可見光中, 老找不到適合的光子, 只好讓它們都通過。 那二氧化碳喜歡哪種光子呢? 彩虹光譜中的哪條黑線, 代表全球暖化的進行呢? 嗯,一條也不是。 因為二氧化碳不從日光 直接吸收光子, 而是吸收另一個天體的光, 一個不太算是發光天體的星球: 地球。 可曾想過為什麼 地球不像恆星閃閃發光? 那是因為地球不發出可見光, 只射出紅外線。 我們肉眼可見的光, 包括彩虹中的所有色光, 只是廣大「電磁輻射」 光譜中的一小部分。 另外還有無線電波、微波、 紅外線、紫外線、X光和伽瑪射線。 你可能覺得這些不是光, 但它們本質上並沒有不同; 可見光和其他電磁輻射, 同為能量的一種, 差別只在能量高低。 再說, 用人類的眼光去定義 可見光也有失公允。 因為蛇其實看得見紅外線, 鳥類也看得到紫外線。 如果我們的眼睛 能看見1900兆赫的光, 那手機就會像一支支手電筒, 基地台就會像大燈籠。 地球發出紅外線, 因為任何有溫度的物體 都會「發光」, 就是所謂的「熱輻射」。 當物體的溫度越高, 發出的光頻率就越高。 當你燒熱一塊鐵, 它會發出越來越高頻的紅外線。 當溫度達攝氏450度左右, 就會轉變為可見光。 一開始我們會看到燒得通紅, 當溫度持續上升, 開始發出白光, 也就是所有的可見光。 這就是傳統燈泡的原理, 而且很耗能的原因。 發出95%以上的光 是我們肉眼不可見的, 只會一直散發熱能。 如果大氣中沒有溫室氣體, 地球的紅外線就會跑到宇宙。 就像氧氣喜歡暗紅光, 二氧化碳和其他溫室氣體 會和紅外線的光子結合。 它們提供剛剛好的能量 讓氣體分子轉換到更高能階。 當二氧化碳分子吸收紅外線光子, 它會降回原來的能階, 再往隨機方向丟出一顆光子。 部分的能量又會回到地球, 造成暖化。 大氣中的二氧化碳越多, 紅外線光子就越會被留在地球, 並改變我們的氣候。