2 00:00:09,914 --> 00:00:13,834 Clouds represent the largest uncertainty in the climate forecast 3 00:00:13,834 --> 00:00:17,283 and they're really the hardest part of the whole thing. 4 00:00:17,283 --> 00:00:20,321 The biggest difference between different climate models, 5 00:00:20,321 --> 00:00:22,491 and how much they forecast a change in climate, 6 00:00:22,491 --> 00:00:24,537 is pretty much due to different ways 7 00:00:24,537 --> 00:00:28,260 in which different climate models treat clouds. 8 00:00:28,260 --> 00:00:35,900 Clouds have two possible ways of impacting the energy budget of the Earth. 9 00:00:35,900 --> 00:00:40,850 One is by visible light and the other is by infrared light. 10 00:00:40,850 --> 00:00:45,910 The way the clouds affect visible light is by a process known as scattering. 11 00:00:45,910 --> 00:00:49,570 We've talked before about light coming in 12 00:00:49,570 --> 00:00:53,170 and being absorbed by a droplet of water. 13 00:00:53,170 --> 00:00:55,770 Which means that the energy from the light has been 14 00:00:55,770 --> 00:01:00,340 translated into kinetic energy of the molecules in the water. 15 00:01:00,340 --> 00:01:01,320 Which is the temperature 16 00:01:01,320 --> 00:01:04,430 of the water. You absorb the light, you warm up. 17 00:01:04,430 --> 00:01:10,950 And then the warmer droplet will shine energy out in the infrared. 18 00:01:10,950 --> 00:01:13,880 We've seen this before, but another thing 19 00:01:13,880 --> 00:01:16,340 that can happen is that the visible light comes 20 00:01:16,340 --> 00:01:20,920 in and it's got this electrical dipole, 21 00:01:20,920 --> 00:01:25,950 and it can actually induce an electrical dipole in the droplet. 22 00:01:25,950 --> 00:01:28,500 The droplet of water can be a little bit plus on one 24 00:01:28,500 --> 00:01:31,020 side and a little bit minus on the other side, 25 00:01:31,020 --> 00:01:35,560 and flipping back and forth in resonance with the light that's coming in. 26 00:01:35,560 --> 00:01:40,820 This oscillating dipole, which is chained to the frequency 27 00:01:40,820 --> 00:01:46,810 of the light that comes in, can create light that then shines out. 28 00:01:46,810 --> 00:01:49,370 And, it's because the frequency is the same as 29 00:01:49,370 --> 00:01:52,160 the light that comes in, what leaves is visible light as well, 30 00:01:52,160 --> 00:01:55,220 so this is called scattering. 31 00:01:55,220 --> 00:02:01,490 And scattering works best if the size of the cloud droplet is comparable 32 00:02:01,490 --> 00:02:07,330 to the size of the wavelength, lambda, of the light. 33 00:02:07,330 --> 00:02:09,890 And it turns out that most cloud droplets are 34 00:02:09,890 --> 00:02:13,320 actually much bigger than the wavelength of visible light. 35 00:02:13,320 --> 00:02:18,540 The smaller the cloud droplet is, the better it can scatter. 37 00:02:18,540 --> 00:02:22,360 If cloud droplets are really big, that's when you see dark clouds. 38 00:02:22,360 --> 00:02:25,890 They are dark because they are absorbing the light, 39 00:02:25,890 --> 00:02:29,615 if you see dark clouds, you've got to take your umbrella, because it might rain. 40 00:02:29,615 --> 00:02:32,720 Whereas if the clouds are really bright, that means 41 00:02:32,720 --> 00:02:36,120 that the cloud droplets are small and that, you just 42 00:02:36,120 --> 00:02:39,050 know instinctively, based on your experience, that 43 00:02:39,050 --> 00:02:42,030 bright clouds don't rain as much, and this is why. 44 00:02:44,920 --> 00:02:48,980 There are basically three different kinds of 45 00:02:48,980 --> 00:02:51,410 clouds, I mean, there's about a bazillion kinds 46 00:02:51,410 --> 00:02:54,240 of clouds if you draw the 47 00:02:54,240 --> 00:02:57,420 distinctions fine enough, but basically there are three kinds. 48 00:02:57,420 --> 00:03:03,930 There are these really high altitude, very thin, wispy clouds called cirrus clouds. 49 00:03:03,930 --> 00:03:09,270 They tend to be found at about an altitude of maybe 10 kilometers or so. 50 00:03:09,270 --> 00:03:14,760 And then the other two main kinds are cumulus and stratus clouds, 52 00:03:14,760 --> 00:03:19,960 which are found somewhere between about zero and four kilometers altitude. 53 00:03:19,960 --> 00:03:23,969 And these are much thicker, much denser clouds. 54 00:03:25,240 --> 00:03:31,200 with lots more water in them, they are all formed, basically, 55 00:03:31,200 --> 00:03:36,960 when the air is rising, so cumulus clouds are formed by a focused upward 56 00:03:36,960 --> 00:03:42,200 motion of air, whereas stratus clouds, which are extremely 57 00:03:43,200 --> 00:03:50,768 wide and broad over large areas, are formed by broader scale upward motions. 58 00:03:50,768 --> 00:03:54,240 The distinctions between these clouds are that, 59 00:03:54,240 --> 00:03:58,940 the cirrus clouds, which are higher up, are in colder air. 60 00:03:58,940 --> 00:04:01,990 These cloud droplets are much colder than the ground. 61 00:04:03,880 --> 00:04:06,890 Cirrus clouds are also optically thin. 62 00:04:06,890 --> 00:04:13,430 cirrus clouds and maybe a few airplane contrails, you don't even 64 00:04:13,430 --> 00:04:15,960 really think of it as a cloudy day, because usually the sun 65 00:04:15,960 --> 00:04:19,460 can come right through cirrus clouds, they just don't absorb that much light. 66 00:04:19,460 --> 00:04:22,050 They's just not that much water up there. 67 00:04:22,050 --> 00:04:27,440 Whereas, these other clouds are lower altitude, where the air is warmer, 68 00:04:27,440 --> 00:04:29,350 and they also tend to be much 69 00:04:29,350 --> 00:04:30,680 more optically thick. 70 00:04:34,920 --> 00:04:36,720 Lucky I didn't have anything behind there. 71 00:04:36,720 --> 00:04:43,830 If we put these together these two different settings, 73 00:04:45,400 --> 00:04:50,430 We can deduce, or understand, the the impact that these 74 00:04:50,430 --> 00:04:54,390 different kinds of clouds have on the visible and infrared energy budgets. 77 00:04:57,600 --> 00:05:00,080 For the different kinds of clouds, we have 78 00:05:00,080 --> 00:05:03,900 the infrared effect, which is basically the greenhouse effect, 79 00:05:03,900 --> 00:05:07,540 and thinking about these clouds, the ones that are going to have the strongest 80 00:05:07,540 --> 00:05:09,680 infrared effect, are the ones that are 81 00:05:09,680 --> 00:05:13,350 high up in the atmosphere because they're colder. 82 00:05:13,350 --> 00:05:19,760 And so, they're absorbing the warm intense infrared light 83 00:05:19,760 --> 00:05:23,520 from the warm ground and replacing it with cold infrared 84 00:05:23,520 --> 00:05:26,580 light from up where they are, and so they 85 00:05:26,580 --> 00:05:31,770 have a very strong effect on the infrared energy balance. 86 00:05:31,770 --> 00:05:34,540 And any effect you have on the infrared, as it's affecting 87 00:05:34,540 --> 00:05:38,190 the light leaving the planet, tends to make the planet warmer. 88 00:05:38,190 --> 00:05:43,384 A strong impact on the the infrared is a strong warming 89 00:05:43,384 --> 00:05:48,780 effect on on earth's climate. Whereas low clouds, 90 00:05:48,780 --> 00:05:52,170 because they're lower in the atmosphere, they're fairly warm. 91 00:05:52,170 --> 00:05:54,350 They're not that much colder than the ground. 92 00:05:54,350 --> 00:05:58,380 If you have really bright infrared coming from the ground 93 00:05:58,380 --> 00:06:03,210 and then you absorb it and replace it with almost as bright 94 00:06:03,210 --> 00:06:06,990 infrared from not that high, much higher than the ground, you 95 00:06:06,990 --> 00:06:11,800 don't have a very strong effect on the infrared energy budget. 96 00:06:12,930 --> 00:06:14,200 This is a much weaker 97 00:06:14,200 --> 00:06:18,260 effect from the low clouds In the visible, 98 00:06:19,280 --> 00:06:23,120 the high clouds tend to be optically very thin. 99 00:06:23,120 --> 00:06:26,940 You can see through them and so they don't really scatter that much light. 100 00:06:26,940 --> 00:06:31,270 And so they have a weak effect on the visible energy budget. 101 00:06:31,270 --> 00:06:33,760 Now if you mess with the visible energy, what you're doing 102 00:06:33,760 --> 00:06:39,420 is you're reflecting incoming sunlight back out to space, you're increasing 103 00:06:39,420 --> 00:06:40,360 the albedo. 104 00:06:40,360 --> 00:06:44,910 That tends to always make the planet cooler. 105 00:06:46,120 --> 00:06:50,100 Lower clouds, because they are optically thick, have a stronger cooling 106 00:06:50,100 --> 00:06:54,190 effect on the Earth's planet, so, on the Earth's temperature. 107 00:06:54,190 --> 00:06:59,560 The different kinds of clouds have different impacts 108 00:06:59,560 --> 00:07:04,510 on on the Earth. The high clouds usually 109 00:07:04,510 --> 00:07:12,560 are warming because the infrared effect is strong, and the visible effect is weak. 110 00:07:12,560 --> 00:07:17,940 The infrared effect is to warming, and so high clouds tend to warm the planet. 111 00:07:17,940 --> 00:07:19,910 Low clouds, on the other hand, have a 112 00:07:19,910 --> 00:07:24,320 weak infrared effect, a weak warming effect, and they 113 00:07:24,320 --> 00:07:26,670 have a strong effect on the visible, because 114 00:07:26,670 --> 00:07:30,280 they're so optically thick, they scatter more light, and 115 00:07:30,280 --> 00:07:35,240 so they tend to cool the planet. Clouds can cut both ways. 116 00:07:35,240 --> 00:07:38,420 On balance, clouds 117 00:07:38,420 --> 00:07:42,825 lead to an overall cooling affect on Earth's climate 119 00:07:46,960 --> 00:07:49,320 The reason why it's so hard to simulate 120 00:07:49,320 --> 00:07:51,720 clouds in climate models, and why different climate 121 00:07:51,720 --> 00:07:55,700 models get different answers for, what they think 122 00:07:55,700 --> 00:07:58,040 the clouds would do in a changing climate, 123 00:07:58,040 --> 00:08:03,500 Is because in a grided climate model you don't really 124 00:08:03,500 --> 00:08:09,180 have enough grid points to simulate all of the processes that happen inside a cloud. 125 00:08:09,180 --> 00:08:12,000 Little gusts of wind bring the rain drops together. 126 00:08:12,000 --> 00:08:19,510 They coagulate, they fall out, they evaporate, 127 00:08:19,510 --> 00:08:24,580 all kinds of small-scale processes happen that determine how clouds work. 128 00:08:24,580 --> 00:08:29,250 And you just can't put all of that stuff into a climate 129 00:08:29,250 --> 00:08:33,660 model the way you would like to do it, from first principles. 130 00:08:33,660 --> 00:08:37,120 Instead, they, what we call, parameterize clouds. 131 00:08:37,120 --> 00:08:39,340 You know, if the temperature is such and such 132 00:08:39,340 --> 00:08:42,110 and you've got so much water vapor in those 133 00:08:42,110 --> 00:08:44,910 sorts of conditions you tend to find these kinds 134 00:08:44,910 --> 00:08:47,960 of clouds, or that kind of cloud, or no clouds. 135 00:08:47,960 --> 00:08:51,390 Clouds with certain cloud sizes of droplets. 136 00:08:51,390 --> 00:08:56,600 They can't base the cloud properties 137 00:08:56,600 --> 00:09:02,060 on first principles as they would like. 138 00:09:02,060 --> 00:09:03,750 They have to kind of fake it. 139 00:09:03,750 --> 00:09:06,760 And so, that's why modeling clouds is really 140 00:09:06,760 --> 00:09:09,949 the hardest part about the climate change forecast.