2 00:00:08,780 --> 00:00:13,442 In the greenhouse model that we did so far, we had a pane of glass 3 00:00:13,442 --> 00:00:16,402 and we had it absorb all of the infrared light, 4 00:00:16,402 --> 00:00:20,410 because it made it easier to understand. 5 00:00:20,410 --> 00:00:22,020 Gasses don't really do that. 6 00:00:22,020 --> 00:00:26,210 Gasses are very selective about what kinds of light they absorb. 7 00:00:26,210 --> 00:00:30,940 The situation kind of looks like this, where you have 8 00:00:30,940 --> 00:00:33,800 different frequencies of light that are shining up from the ground 9 00:00:33,800 --> 00:00:35,880 as part of the ground's black body spectrum. 10 00:00:35,880 --> 00:00:41,540 And some of the light is in the frequency range around 700 wave numbers. 11 00:00:41,540 --> 00:00:45,200 where the CO2 bending vibration absorbs light. 12 00:00:45,200 --> 00:00:48,490 The light has to fight its way out of the atmosphere. 13 00:00:48,490 --> 00:00:51,210 It gets absorbed by CO2 fairly quickly, 14 00:00:51,210 --> 00:00:54,960 and then some more gets re-emitted by that CO2 going up and down. 15 00:00:54,960 --> 00:00:59,140 And on and on and on. And then, finally, 16 00:00:59,140 --> 00:01:03,000 the light of that frequency that escapes to space has to come 17 00:01:03,000 --> 00:01:07,030 from some place fairly high up in the atmosphere, where it's fairly cold. 18 00:01:08,080 --> 00:01:12,800 Whereas, another frequency range around 900 wave numbers, 19 00:01:12,800 --> 00:01:16,340 we call that the atmospheric window 20 00:01:16,340 --> 00:01:19,740 because if there are no clouds in the sky or airplanes or birds 21 00:01:19,740 --> 00:01:22,212 or any other sort of condensed matter up there, 22 00:01:22,212 --> 00:01:27,840 there are no gases that really absorb light in the atmospheric window. 24 00:01:27,840 --> 00:01:32,220 So the light in the atmospheric window (around 900 centimeters wave numbers, or 25 00:01:32,220 --> 00:01:36,080 900 cycles per centimeter), goes right from the ground out to space. 26 00:01:37,560 --> 00:01:42,340 So, if we think about how this looks from space 27 00:01:43,760 --> 00:01:49,710 we can visualize this within the context of two black body curves. 28 00:01:49,710 --> 00:01:54,820 One that is fairly cold like the upper atmosphere, 29 00:01:54,820 --> 00:01:57,970 as if you had a black body that was cold at 30 00:01:57,970 --> 00:02:00,270 that temperature, and then another, as if you had a 31 00:02:00,270 --> 00:02:02,990 black body that's fairly warm, like the temperature of the ground. 32 00:02:02,990 --> 00:02:05,270 And because it's warmer, that means 33 00:02:05,270 --> 00:02:08,160 that curve is larger, because epsilon sigma T 34 00:02:08,160 --> 00:02:09,540 of the fourth is a larger number, 35 00:02:09,540 --> 00:02:13,080 because the temperature is higher, you get a bigger black body curve. 36 00:02:13,080 --> 00:02:14,710 If you're looking down from space, 37 00:02:14,710 --> 00:02:20,430 in the range around 700 wave numbers, what you will see, 38 00:02:20,430 --> 00:02:26,170 when you look down, is cold, a cold CO2 molecule up in the upper atmosphere. 39 00:02:26,170 --> 00:02:28,870 If this CO2 molecule was a black body 40 00:02:28,870 --> 00:02:31,940 and could shine at all frequencies, what you would see is 41 00:02:31,940 --> 00:02:34,270 a cold black body curve, but 42 00:02:34,270 --> 00:02:38,560 the CO2 is only really affecting the light in the 43 00:02:38,560 --> 00:02:40,070 CO2 bending vibration range. 44 00:02:40,070 --> 00:02:45,280 And so in this part of the spectrum what you see is following the 45 00:02:45,280 --> 00:02:50,640 cold black body curve, because the CO2 is emitting in that range and it's cold. 46 00:02:50,640 --> 00:02:54,220 And then if you look in the atmospheric window range, 47 00:02:54,220 --> 00:02:56,590 what you see is all the way to the ground. 48 00:02:56,590 --> 00:02:59,410 You see light that's coming from the warm ground. 49 00:02:59,410 --> 00:03:05,160 That light, following a black body curve, is at this intensity. 50 00:03:05,160 --> 00:03:10,230 You see a higher intensity there for the atmospheric window region. 51 00:03:10,230 --> 00:03:13,320 This is how the selectivity of greenhouse gases 52 00:03:13,320 --> 00:03:16,480 to the light kind of manifests itself 53 00:03:16,480 --> 00:03:20,400 in the intensity of the light that's leaving the planet. 54 00:03:20,400 --> 00:03:24,620 It works in the frequency ranges that it can cover, and in other frequency ranges, 55 00:03:24,620 --> 00:03:30,650 it just leaves the light alone. 56 00:03:30,650 --> 00:03:39,620 So let us look at the online model of infrared light leaving the atmosphere. 57 00:03:39,620 --> 00:03:45,590 I'm going to show you an effect called the band saturation effect. 58 00:03:45,590 --> 00:03:50,710 If we start out and we have no carbon dioxide in the atmosphere at all 59 00:03:50,710 --> 00:03:55,650 what we see is in that 700 wave number range 60 00:03:55,650 --> 00:04:00,660 the intensity follows the black body curve 61 00:04:00,660 --> 00:04:05,960 of the warm object because we can see light all the way from the ground. 62 00:04:05,960 --> 00:04:11,890 Now if we put just a little bit of CO2 into the atmosphere, I'm going to put one 63 00:04:11,890 --> 00:04:21,140 part per million in, What you see is the peak 64 00:04:21,140 --> 00:04:26,350 is very prominent. 65 00:04:26,350 --> 00:04:29,320 Just putting a little bit of CO2 goes a long way. 66 00:04:29,320 --> 00:04:33,560 The peak comes down quite noticeably from the warm 67 00:04:33,560 --> 00:04:37,310 black body curve, and then putting in more CO2. 68 00:04:37,310 --> 00:04:42,160 I'm going to do ten parts per million, and you can see now that the 70 00:04:45,280 --> 00:04:48,890 CO2 absorption peak, the bite that CO2 takes 71 00:04:48,890 --> 00:04:53,180 out of the the outgoing light from the planet, has gotten much deeper. 72 00:04:53,180 --> 00:04:55,320 And so it's changed quite a bit, 73 00:04:55,320 --> 00:04:57,710 the amount of energy that's leaving the planet. 74 00:04:57,710 --> 00:05:01,425 But as we add more CO2, so now I'm 75 00:05:01,425 --> 00:05:03,670 going to go up to 100 parts per million, 76 00:05:03,670 --> 00:05:05,800 what you see is that the peak has gotten 77 00:05:05,800 --> 00:05:11,420 broader, and a little bit deeper, 78 00:05:11,420 --> 00:05:18,750 but as the CO2 concentration rises even further, the peak can't get any deeper. 79 00:05:18,750 --> 00:05:22,690 It's absorbing everything that's coming up 80 00:05:22,690 --> 00:05:25,850 from the ground, and all you see is 81 00:05:29,040 --> 00:05:30,510 the cold black body curve. 82 00:05:32,450 --> 00:05:36,760 And then, going up still more to a 1000 parts per million, you 83 00:05:36,760 --> 00:05:41,190 see the peak is getting even fatter still, but it's not getting any deeper. 84 00:05:41,190 --> 00:05:44,210 This is called the band saturation effect. 85 00:05:44,210 --> 00:05:48,300 The impact of that is that, plotted 86 00:05:48,300 --> 00:05:51,070 I've plotted the data that I just took with the model, here. 87 00:05:52,880 --> 00:05:57,820 As we put just a little bit of CO2 in the atmosphere, we see that the 89 00:05:57,820 --> 00:06:02,770 intensity of the energy leaving the planet has decreased by quite a bit. 90 00:06:02,770 --> 00:06:08,850 From 318 watts per square meter down to 313 just per one part per million CO2. 91 00:06:08,850 --> 00:06:13,140 That's basically the difference between this point and this point. 92 00:06:13,140 --> 00:06:16,130 This axis here goes all the way up to 1000 ppm. 93 00:06:16,130 --> 00:06:19,160 So, one ppm is almost the same as zero on 94 00:06:19,160 --> 00:06:24,520 this plot, it's almost nothing. And then as you go up higher and higher in 95 00:06:24,520 --> 00:06:32,030 CO2 concentrations, it decreases the rate at which energy is being lost to space. 96 00:06:32,030 --> 00:06:34,109 You get less bang for your buck, 98 00:06:34,109 --> 00:06:37,620 the more CO2 you have in the atmosphere. 99 00:06:37,620 --> 00:06:40,710 It never becomes insensitive to CO2. 100 00:06:40,710 --> 00:06:46,230 But the sensitivity goes down the more of the gas you have in the atmosphere. 101 00:06:47,890 --> 00:06:54,070 It turns out that the impact of CO2 as a greenhouse gas, or most other 102 00:06:54,070 --> 00:07:00,930 greenhouse gases, tends to follow a kind of a logarithmic relationship. 103 00:07:00,930 --> 00:07:06,180 Where any doubling has about the same impact on the energy balance 104 00:07:06,180 --> 00:07:09,130 and on the climate of the earth as any other. 105 00:07:09,130 --> 00:07:13,340 So going from ten parts per million to 20 parts per million would give you just as 106 00:07:13,340 --> 00:07:15,690 much global warming, amazingly, as going 107 00:07:15,690 --> 00:07:18,580 from 1000 parts per million to 2000 parts per million. 109 00:07:22,550 --> 00:07:27,690 One thing that controls how this 110 00:07:27,690 --> 00:07:33,030 shape looks is the details of these absorption 111 00:07:33,030 --> 00:07:37,970 peaks. I said earlier that a 112 00:07:37,970 --> 00:07:43,020 gas can absorb light, if the frequency of the vibration of 113 00:07:43,020 --> 00:07:48,640 the gas is sort of the same as the frequency of the light that's coming in. 114 00:07:48,640 --> 00:07:51,460 That seems like it should be sort of an absolute thing, 115 00:07:51,460 --> 00:07:56,310 that the light has to be exactly the same frequency to really resonate and absorb. 116 00:07:56,310 --> 00:08:01,370 But it turns out that there is some slack. That makes this absorption peak. 117 00:08:01,370 --> 00:08:04,700 Imagine a very sharp-edged absorption peak, 118 00:08:04,700 --> 00:08:07,360 It'll absorb all of this light, 119 00:08:07,360 --> 00:08:10,610 but none of that light, is how it might be, ideally. 120 00:08:10,610 --> 00:08:14,650 But there's some, some sort of slope in that absorption peak, 121 00:08:14,650 --> 00:08:18,550 and that slop is very important to determining 122 00:08:18,550 --> 00:08:22,780 how much adding more CO2 changes the climate, if you've already got 123 00:08:22,780 --> 00:08:24,020 a lot there. 124 00:08:24,020 --> 00:08:28,250 Because it allows that bite out of the spectrum that you 125 00:08:28,250 --> 00:08:33,220 saw to get fatter as you add more of the greenhouse gas. 126 00:08:33,220 --> 00:08:37,480 The reasons why there is a slope in this, 127 00:08:37,480 --> 00:08:39,880 why it's not just on or off the way you might think it would be: 128 00:08:41,246 --> 00:08:45,090 One is due to what they call pressure broadening. 129 00:08:45,090 --> 00:08:50,820 Remember we said that if the carbon dioxide was frozen into 130 00:08:50,820 --> 00:08:54,560 dry ice and so it was condensed, it would be a black body. 131 00:08:54,560 --> 00:08:58,690 It could absorb pretty much everything probably, 132 00:08:58,690 --> 00:09:00,780 at least that's the way most condensed things are, 133 00:09:00,780 --> 00:09:03,710 and that's because you can 134 00:09:03,710 --> 00:09:06,560 vibrate at some characteristic frequency 135 00:09:06,560 --> 00:09:11,480 but If a light photon comes in and it's a little bit off, maybe you can adsorb it 136 00:09:11,480 --> 00:09:13,460 and kind of shovel off some of the 137 00:09:13,460 --> 00:09:16,600 excess energy to your neighbor who's just right there anyway, right? 138 00:09:17,608 --> 00:09:21,070 It turns out that making a gas 139 00:09:21,070 --> 00:09:26,170 more compressed makes it more like condensed matter. 140 00:09:26,170 --> 00:09:31,510 It spreads out how much of the light it can absorb. 141 00:09:31,510 --> 00:09:33,820 This is most important in the lower atmosphere. 142 00:09:33,820 --> 00:09:37,360 The other thing that's important is Doppler shifting. 143 00:09:37,360 --> 00:09:44,440 If the gas is moving, and the light is going up past it, 144 00:09:44,440 --> 00:09:49,690 and the frequency might not be exactly right if the gas molecule were steady, 145 00:09:49,690 --> 00:09:53,570 but if it's moving, that changes a little bit, the frequency that the molecule sees. 146 00:09:53,570 --> 00:09:56,608 It's like, when a train goes past, it makes this 147 00:09:56,608 --> 00:09:57,858 [professor's impression of a train whistle] 148 00:09:57,858 --> 00:09:59,640 kind of a sound. 149 00:09:59,640 --> 00:10:03,130 And say you had some sound absorption thing. 150 00:10:03,130 --> 00:10:05,580 And it couldn't absorb the sound of the 151 00:10:05,580 --> 00:10:08,260 train whistle if it was sitting there stationary. 152 00:10:08,260 --> 00:10:12,280 But maybe it's just exactly right for the train whistle as it's going away. 153 00:10:12,280 --> 00:10:15,020 So it would absorb from that train whistle. 154 00:10:15,020 --> 00:10:17,560 Because of the velocity of the train relative to you. 155 00:10:18,850 --> 00:10:23,200 It turns out that even if, 156 00:10:23,200 --> 00:10:26,130 the gas absorbed everything, 157 00:10:26,130 --> 00:10:30,220 adding more of the greenhouse gas would still effect the climate. 158 00:10:30,220 --> 00:10:33,580 We can see that by thinking back to our layer model. 159 00:10:33,580 --> 00:10:36,880 We had this layer model, where having no panes of glass left 160 00:10:36,880 --> 00:10:40,708 the atmosphere cold, and then putting a couple panes of glass made it 161 00:10:40,708 --> 00:10:44,140 warmer, and then more panes of glass make it warmer and warmer 162 00:10:44,140 --> 00:10:48,298 and warmer, and you saw that Venus, we couldn't, even with a single 163 00:10:48,298 --> 00:10:53,240 pane of glass, explain all the warming, the greenhouse effect of Venus. 164 00:10:53,240 --> 00:10:56,600 The more of these layers you add up 165 00:10:56,600 --> 00:11:00,760 the more warming you get at the ground. 166 00:11:00,760 --> 00:11:04,550 These panes of glass are completely saturated. 167 00:11:04,550 --> 00:11:05,590 They absorb everything. 168 00:11:05,590 --> 00:11:08,330 That's how we defined it, to make it simple. 169 00:11:08,330 --> 00:11:12,750 The band saturation effect is a very, very important effect 170 00:11:12,750 --> 00:11:17,260 for understanding how the climate of the earth responds to 172 00:11:17,260 --> 00:11:20,850 greenhouse gases like CO2, which is very abundant, 173 00:11:20,850 --> 00:11:24,500 or methane, which has a very low concentration. 174 00:11:24,500 --> 00:11:28,410 It's kind of up in this part of it's curve here. 175 00:11:29,530 --> 00:11:31,960 I mean the curve isn't exactly the same but it looks sort of like this 176 00:11:31,960 --> 00:11:33,450 and there's not much methane 177 00:11:33,450 --> 00:11:37,590 so methane is very sensitive, very strong greenhouse forcing. 178 00:11:37,590 --> 00:11:39,030 And then freons, 179 00:11:39,030 --> 00:11:44,260 the chlorofluorocarbon compounds that we use 180 00:11:44,260 --> 00:11:47,834 in refrigerators and air conditioners and things like that, 181 00:11:47,834 --> 00:11:52,650 they actually absorb in the atmospheric window region. 182 00:11:52,650 --> 00:11:56,330 And there were no freons on earth before we started making them. 183 00:11:56,330 --> 00:11:58,500 So they basically start from zero. 184 00:11:58,500 --> 00:12:01,660 And as a result of this band saturation effect, 185 00:12:01,660 --> 00:12:04,750 it turns out that one molecule of freon 186 00:12:04,750 --> 00:12:07,430 is worth something like 10 thousand molecules 187 00:12:07,430 --> 00:12:11,750 of CO2, in its climate impact. 188 00:12:11,750 --> 00:12:16,580 However, the fact that CO2 is so abundant, and is therefore band 189 00:12:16,580 --> 00:12:21,910 saturated, does not mean that the earth is insensitive to adding more CO2. 190 00:12:21,910 --> 00:12:27,400 It's a good thing we already had CO2 192 00:12:27,400 --> 00:12:30,150 in the atmosphere before the industrial revolution, 193 00:12:30,150 --> 00:12:34,500 because if there wasn't, and we started putting CO2 into 194 00:12:34,500 --> 00:12:39,820 a virgin atmosphere it would've just totally melted down the climate.