WEBVTT 00:00:06.687 --> 00:00:07.668 You've probably heard that 00:00:07.668 --> 00:00:10.452 carbon dioxide is warming the Earth, 00:00:10.452 --> 00:00:11.823 but how does it work? 00:00:11.823 --> 00:00:13.832 Is it like the glass of a greenhouse 00:00:13.832 --> 00:00:15.733 or like an insulating blanket? 00:00:15.733 --> 00:00:17.852 Well, not entirely. 00:00:17.852 --> 00:00:18.894 The answer involves a bit 00:00:19.272 --> 00:00:22.010 of quantum mechanics, but don't worry, 00:00:22.001 --> 00:00:23.950 we'll start with a rainbow. 00:00:23.959 --> 00:00:25.818 If you look closely at sunlight separated 00:00:25.818 --> 00:00:27.134 through a prism, 00:00:27.134 --> 00:00:30.365 you'll see dark gaps where bands of color went missing. 00:00:30.365 --> 00:00:31.753 Where did they go? 00:00:31.753 --> 00:00:33.168 Before reaching our eyes, 00:00:33.168 --> 00:00:35.080 different gases absorbed those 00:00:35.008 --> 00:00:37.616 specific parts of the spectrum. 00:00:37.688 --> 00:00:39.847 For example, oxygen gas snatched up 00:00:39.847 --> 00:00:41.695 some of the dark red light, 00:00:41.695 --> 00:00:44.914 and sodium grabbed two bands of yellow. 00:00:44.914 --> 00:00:46.308 But why do these gases absorb 00:00:46.308 --> 00:00:48.362 specific colors of light? 00:00:48.362 --> 00:00:51.279 This is where we enter the quantum realm. 00:00:51.279 --> 00:00:53.637 Every atom and molecule has a set number 00:00:53.637 --> 00:00:56.968 of possible energy levels for its electrons. 00:00:56.968 --> 00:00:58.979 To shift its electrons from the ground state 00:00:58.979 --> 00:01:00.297 to a higher level, 00:01:00.297 --> 00:01:03.701 a molecule needs to gain a certain amount of energy. 00:01:03.701 --> 00:01:06.119 No more, no less. 00:01:06.119 --> 00:01:08.123 It gets that energy from light, 00:01:08.123 --> 00:01:11.292 which comes in more energy levels than you could count. 00:01:11.292 --> 00:01:14.657 Light consists of tiny particles called photons 00:01:14.657 --> 00:01:16.761 and the amount of energy in each photon 00:01:16.761 --> 00:01:19.126 corresponds to its color. 00:01:19.126 --> 00:01:22.149 Red light has lower energy and longer wavelengths. 00:01:22.149 --> 00:01:25.823 Purple light has higher energy and shorter wavelengths. 00:01:25.823 --> 00:01:28.642 Sunlight offers all the photons of the rainbow, 00:01:28.642 --> 00:01:30.663 so a gas molecule can choose 00:01:30.663 --> 00:01:33.270 the photons that carry the exact amount of energy 00:01:33.027 --> 00:01:34.781 needed to shift the molecule to 00:01:35.024 --> 00:01:37.381 its next energy level. 00:01:37.381 --> 00:01:38.715 When this match is made, 00:01:38.715 --> 00:01:40.644 the photon disappers as the molecule 00:01:40.644 --> 00:01:41.965 gains its energy, 00:01:41.965 --> 00:01:43.973 and we get a small gap in our rainbow. 00:01:44.765 --> 00:01:47.887 If a photon carries too much or too little energy, 00:01:47.887 --> 00:01:49.366 the molecule has no choice but 00:01:49.366 --> 00:01:51.219 to let it fly past. 00:01:51.219 --> 00:01:53.261 This is why glass is transparent. 00:01:53.639 --> 00:01:55.711 The atoms in glass do not pair well 00:01:55.711 --> 00:01:58.384 with any of the energy levels in visible light, 00:01:58.384 --> 00:02:00.647 so the photons pass through. 00:02:00.647 --> 00:02:03.760 So, which photons does carbon dioxide prefer? 00:02:03.076 --> 00:02:04.901 Where is the black line in our rainbow 00:02:05.585 --> 00:02:07.960 that explains global warming? 00:02:07.096 --> 00:02:08.925 Well, it's not there. 00:02:09.789 --> 00:02:11.952 Carbon dioxide doesn't absorb light directly 00:02:11.952 --> 00:02:13.286 from the Sun. 00:02:13.286 --> 00:02:14.507 It absorbs light from a totally 00:02:14.507 --> 00:02:16.232 different celestial body. 00:02:16.232 --> 00:02:19.016 One that doesn't appear to be emitting light at all: 00:02:19.016 --> 00:02:20.739 Earth. 00:02:20.739 --> 00:02:22.175 If you're wondering why our planet 00:02:22.175 --> 00:02:23.876 doesn't seem to be glowing, 00:02:23.876 --> 00:02:27.138 it's because the Earth doesn't emit visible light. 00:02:27.138 --> 00:02:29.306 It emits infared light. 00:02:29.306 --> 00:02:30.920 The light that our eyes can see, 00:02:30.092 --> 00:02:32.164 including all of the colors of the rainbow, 00:02:32.992 --> 00:02:35.035 is just a small part of the larger spectrum 00:02:35.422 --> 00:02:37.825 of electromagnetic radiation, 00:02:37.825 --> 00:02:40.198 which includes radio waves, microwaves, 00:02:40.198 --> 00:02:43.286 infrared, ultraviolet, x-rays, 00:02:43.286 --> 00:02:45.103 and gamma rays. 00:02:45.103 --> 00:02:47.744 It may seem strange to think of these things as light, 00:02:47.744 --> 00:02:49.291 but there is no fundamental difference 00:02:49.291 --> 00:02:53.027 between visible light and other electromagnetic radiation. 00:02:53.027 --> 00:02:54.028 It's the same energy, 00:02:54.127 --> 00:02:56.394 but at a higher or lower level. 00:02:56.394 --> 00:02:57.492 In fact, it's a bit presumptuous to define 00:02:58.374 --> 00:03:01.969 the term visible light by our own limitations. 00:03:01.969 --> 00:03:04.865 After all, infrared light is visible to snakes, 00:03:04.865 --> 00:03:07.827 and ultraviolet light is visible to birds. 00:03:07.827 --> 00:03:09.835 If our eyes were adapted to see light of 00:03:09.907 --> 00:03:12.283 1900 megahertz, then a mobile phone 00:03:12.283 --> 00:03:13.297 would be a flashlight, 00:03:13.423 --> 00:03:14.425 and a cell phone tower 00:03:14.425 --> 00:03:17.157 would look like a huge lantern. 00:03:17.157 --> 00:03:19.315 Earth emits infrared radiation 00:03:19.315 --> 00:03:21.240 because every object with a temperature 00:03:21.024 --> 00:03:24.252 above absolute zero will emit light. 00:03:24.468 --> 00:03:26.726 This is called thermal radiation. 00:03:26.726 --> 00:03:28.158 The hotter an object gets, 00:03:28.158 --> 00:03:30.857 the higher frequency the light it emits. 00:03:30.857 --> 00:03:32.561 When you heat a piece of iron, 00:03:32.561 --> 00:03:35.956 it will emit more and more frequencies of infrared light, 00:03:35.956 --> 00:03:40.322 and then, at a temperature of around 450 degrees Celsius, 00:03:40.322 --> 00:03:42.398 its light will reach the visible spectrum. 00:03:43.082 --> 00:03:45.310 At first, it will look red hot. 00:03:45.031 --> 00:03:46.327 And with even more heat, 00:03:46.606 --> 00:03:48.112 it will glow white 00:03:48.112 --> 00:03:51.166 with all of the frequencies of visible light. 00:03:51.166 --> 00:03:52.857 This is how traditional light bulbs 00:03:52.857 --> 00:03:54.309 were designed to work 00:03:54.309 --> 00:03:56.187 and why they're so wasteful. 00:03:56.187 --> 00:04:00.265 95% of the light they emit is invisible to our eyes. 00:04:00.265 --> 00:04:02.174 It's wasted as heat. 00:04:02.174 --> 00:04:04.827 Earth's infrared radiation would escape to space 00:04:04.827 --> 00:04:07.358 if there weren't greenhouse gas molecules 00:04:07.358 --> 00:04:09.080 in our atmophere. 00:04:09.008 --> 00:04:11.762 Just as oxygen gas prefers the dark red photons, 00:04:11.834 --> 00:04:14.512 carbon dioxide and other greenhouse gases 00:04:14.512 --> 00:04:17.153 match with infrared photons. 00:04:17.153 --> 00:04:18.914 They provide the right amount of energy 00:04:18.914 --> 00:04:22.085 to shift the gas molecules into their higher energy level. 00:04:22.085 --> 00:04:24.410 Shortly after a carbon dioxide molecule 00:04:24.041 --> 00:04:26.358 absorbs an infrared photon, 00:04:26.727 --> 00:04:29.182 it will fall back to its previous energy level, 00:04:29.182 --> 00:04:32.714 and spit a photon back out in a random direction. 00:04:32.714 --> 00:04:34.619 Some of that energy then returns 00:04:34.619 --> 00:04:36.098 to Earth's surface, 00:04:36.098 --> 00:04:37.639 causing warming. 00:04:37.639 --> 00:04:39.447 The more carbon dioxide in the atmosphere, 00:04:39.447 --> 00:04:41.380 the more likely that infrared photons 00:04:41.038 --> 00:04:43.216 will land back on Earth 00:04:43.558 --> 00:04:45.124 and change our climate.