2 00:00:11,911 --> 00:00:15,389 The water vapor feedback is a positive feedback that has an important impact 3 00:00:15,389 --> 00:00:20,220 on Earth's climate with rising CO2 concentrations. 4 00:00:20,220 --> 00:00:23,360 In fact, if it weren't for the water vapor feedback, 5 00:00:23,360 --> 00:00:26,420 there really wouldn't be as much of a global warming problem at all. 6 00:00:26,420 --> 00:00:30,520 The water vapor feedback roughly doubles the sensitivity 7 00:00:30,520 --> 00:00:33,150 of the Earth to changes in greenhouse gas concentrations. 8 00:00:34,540 --> 00:00:36,950 At the heart of it, there's actually a negative 9 00:00:36,950 --> 00:00:41,650 feedback that controls the amount of water vapor in the atmosphere. 10 00:00:41,650 --> 00:00:45,830 The state variable here is the water vapor. 11 00:00:45,830 --> 00:00:49,000 And if the water vapor concentration is perturbed by 12 00:00:49,000 --> 00:00:51,830 some external thing, like if you run a garden sprinkler, 13 00:00:53,090 --> 00:00:57,805 it changes the rate of evaporation some place else, or else it just 14 00:00:57,805 --> 00:01:01,410 rains out some place else because you put more water in the air. 15 00:01:01,410 --> 00:01:05,780 The hydrological cycle leads to a negative feedback 17 00:01:05,780 --> 00:01:08,430 that stabilizes the amount of water vapor in the air. 18 00:01:08,430 --> 00:01:11,830 That's why running a garden sprinkler doesn't lead to global warming. 19 00:01:15,150 --> 00:01:19,570 But the vapor leads to a positive feedback 20 00:01:19,570 --> 00:01:23,860 operating on a state variable now of temperature. 21 00:01:23,860 --> 00:01:25,800 The way this works is that if you 22 00:01:25,800 --> 00:01:29,250 have some perturbation to the temperature, say by making the 23 00:01:29,250 --> 00:01:35,060 sun brighter or adding more greenhouse gases, it changes 24 00:01:35,060 --> 00:01:38,760 how much water vapor you want to hold in equilibrium. 25 00:01:38,760 --> 00:01:42,940 It redefines 100% relative humidity essentially. 27 00:01:42,940 --> 00:01:48,020 And so if you warm things up by some external means, that 28 00:01:48,020 --> 00:01:53,230 leads to an increase in the amount of vapor pressure 29 00:01:53,230 --> 00:01:56,660 in the air, and water vapor, as you know, is a greenhouse gas, 30 00:01:56,660 --> 00:02:00,230 because it's got three atoms, you know it's a greenhouse gas, and it's asymmetrical. 31 00:02:00,230 --> 00:02:06,720 Having more water vapor pressure means that there's more more greenhouse 32 00:02:06,720 --> 00:02:09,230 gas forcing to warm the Earth even further. 33 00:02:11,170 --> 00:02:16,410 The way this could go is you add some greenhouse gas 34 00:02:16,410 --> 00:02:21,800 or you make the sun warmer, and then that makes the temperature go up by itself, 35 00:02:21,800 --> 00:02:26,590 which causes the amount of water vapor in the air to go up, 36 00:02:26,590 --> 00:02:28,690 because the warmer air can hold more water vapor, 37 00:02:28,690 --> 00:02:32,160 moving in this direction on that pressure curve. 38 00:02:32,160 --> 00:02:35,990 and then that's a greenhouse gas so that causes the temperature to go up even more. 39 00:02:35,990 --> 00:02:38,050 It amplifies the temperature. 40 00:02:38,050 --> 00:02:40,310 You may wonder: you have this loop of 41 00:02:40,310 --> 00:02:43,760 cause and effect, you may wonder where does this end? 42 00:02:45,700 --> 00:02:51,880 Well, the answer can be seen on a diagram such as this one which is a 43 00:02:51,880 --> 00:02:58,820 phase diagram for water. It's got dimensions of temperature and 44 00:02:58,820 --> 00:03:05,110 pressure of water vapor here, and then it has these different regions on the graph. 45 00:03:05,110 --> 00:03:09,800 Up here where the temperature is fairly high and there's 46 00:03:09,800 --> 00:03:13,740 a lot of water vapor, you can have liquid plus vapor. 47 00:03:14,810 --> 00:03:17,410 If you have a lot of water vapor pressure, 48 00:03:17,410 --> 00:03:22,040 but it's very cold, you're going to have vapor plus ice. 49 00:03:22,040 --> 00:03:23,950 Or if the vapor pressure is very 50 00:03:23,950 --> 00:03:27,790 low, then you may not have any other phase other than vapor, you'll 51 00:03:27,790 --> 00:03:32,670 only have vapor in this region of the phase diagram. 52 00:03:33,990 --> 00:03:39,080 We're going to do a thought experiment where we start out a planet 53 00:03:39,080 --> 00:03:44,740 and we'll say, let's not put any water vapor in the atmosphere yet. 54 00:03:44,740 --> 00:03:49,210 And we'll just have it be dry, and then we'll calculate what the temperature that 55 00:03:49,210 --> 00:03:50,400 planet would be. 56 00:03:50,400 --> 00:03:56,180 And so, for Earth that would lead us to a point right here. 57 00:03:56,180 --> 00:04:02,520 If its got a liquid water, it's going to a evaporate, 58 00:04:02,520 --> 00:04:08,640 liquid water wants to evaporate until it starts to be along this line here. 59 00:04:08,640 --> 00:04:14,290 As the water vapor evaporates it causes the temperature 60 00:04:14,290 --> 00:04:19,000 of the planet to rise because water vapor is a greenhouse gas, but ultimately this 61 00:04:19,000 --> 00:04:25,460 trajectory hits the phase boundary to liquid and, at this point, it rains. 62 00:04:25,460 --> 00:04:31,030 And so the runaway greenhouse, the water vapor feedback, is limited 63 00:04:31,030 --> 00:04:36,450 on Earth by intersecting the phase boundary for a liquid water. 64 00:04:36,450 --> 00:04:39,400 The hydrological cycle is stabilizing, 65 00:04:39,400 --> 00:04:44,000 and it limits the the extent of the water vapor feedback. 66 00:04:45,110 --> 00:04:48,920 This is Mars. 67 00:04:48,920 --> 00:04:52,710 It starts out colder, and you add water vapor, and 68 00:04:52,710 --> 00:04:55,125 it goes up and maybe it hits the phase diagram. 69 00:04:55,125 --> 00:04:59,810 A phase boundary of ice plus vapor, so on Mars it would snow, 70 00:04:59,810 --> 00:05:04,750 but again the water vapor feedback would be limited by this phase boundary. 71 00:05:04,750 --> 00:05:09,060 Now Venus is a different story. Venus originally had water, 73 00:05:09,060 --> 00:05:13,970 it doesn't anymore because Venus suffered a runaway greenhouse effect. 74 00:05:13,970 --> 00:05:16,460 You start from a condition that's warmer and 75 00:05:16,460 --> 00:05:19,990 you raise the water vapor concentration, and raise 76 00:05:19,990 --> 00:05:23,500 the temperature, it can curve over and 77 00:05:23,500 --> 00:05:27,740 miss the phase boundary for liquid, or solid entirely. 78 00:05:27,740 --> 00:05:30,830 And this is called a runaway greenhouse effect. 79 00:05:30,830 --> 00:05:35,360 All of the water on the planet would go into the atmosphere. 80 00:05:35,360 --> 00:05:36,190 It would all boil. 81 00:05:39,480 --> 00:05:42,150 And, this is a one way street. 82 00:05:42,150 --> 00:05:45,100 If this ever happens on the planet it never can go back. 83 00:05:45,100 --> 00:05:48,830 Because, if you get too much water vapor up in the upper 84 00:05:48,830 --> 00:05:53,260 atmosphere where it's exposed to the ultra violet light from the sun, 85 00:05:53,260 --> 00:05:59,468 this ultraviolet light can split the water molecule into Hydrogens and Oxygens. 86 00:05:59,468 --> 00:06:01,440 And these Hydrogens are so small they 87 00:06:01,440 --> 00:06:04,430 travel quickly with their allotted kinetic energy, 88 00:06:04,430 --> 00:06:07,370 just put into that small mass they go very fast. 89 00:06:07,370 --> 00:06:09,700 And so they can be lost to space. 90 00:06:09,700 --> 00:06:17,320 A runaway greenhouse effect is the end of a biosphere for a planet. 91 00:06:17,320 --> 00:06:20,390 Looking at a picture of Earth from 92 00:06:20,390 --> 00:06:23,970 space is always astonishing to me because the atmosphere 93 00:06:23,970 --> 00:06:26,840 is so thin and, and the ocean is 94 00:06:26,840 --> 00:06:29,520 just right there spread out through the whole surface 95 00:06:29,520 --> 00:06:35,570 of the planet, and what keeps this from happening on Earth is a cold trap. 96 00:06:35,570 --> 00:06:38,800 The fact that it gets so much colder in the upper atmosphere, 97 00:06:38,800 --> 00:06:40,460 the water vapor can't get through it, 98 00:06:40,460 --> 00:06:45,100 it all rains or snows out. This is is very, very thin 99 00:06:45,100 --> 00:06:50,850 layer of cold air that has protected and guarded all of Earth's water... 100 00:06:50,850 --> 00:06:54,750 for these billions of years! It looks so fragile to me looking at a picture 101 00:06:54,750 --> 00:06:55,971 of Earth from space.