2 00:00:03,136 --> 00:00:10,454 CO2 is the greenhouse 3 00:00:10,454 --> 00:00:17,890 gas that we're most concerned about for climate change. 4 00:00:17,890 --> 00:00:21,090 But we're going to talk about methane first because it's easier, 5 00:00:21,090 --> 00:00:24,690 and makes it easier to understand CO2 which we'll do next. 6 00:00:24,690 --> 00:00:28,270 Methane is the second most powerful anthropogenic green house gas. 7 00:00:28,270 --> 00:00:30,280 It's not negligible anyway. 8 00:00:31,420 --> 00:00:34,680 Methane is a transient gas. 9 00:00:34,680 --> 00:00:40,400 It comes out of a gas burner and it can burn. 10 00:00:40,400 --> 00:00:44,400 It's very high energy and it 11 00:00:44,400 --> 00:00:50,370 burns in the atmosphere, too, naturally, just under the influence of sunlight. 12 00:00:50,370 --> 00:00:52,430 The chemical reaction looks something like this. 13 00:00:53,830 --> 00:01:00,630 The methane encounters this guy who is called a hydroxyl radical. 14 00:01:00,630 --> 00:01:06,440 Remember in seawater we had all these things that had charges, 15 00:01:06,440 --> 00:01:10,930 like the calcium that had two plus, and the carbonate ion that had two minus. 16 00:01:10,930 --> 00:01:12,220 Those are ions. 17 00:01:12,220 --> 00:01:14,290 Those can't really exist in gases. 18 00:01:14,290 --> 00:01:19,670 Gases have these things which have no electrical charge, but they have extra 19 00:01:19,670 --> 00:01:23,550 electrons that are hanging out there, and very reactive. 20 00:01:23,550 --> 00:01:25,700 It's an unstable molecule, 21 00:01:25,700 --> 00:01:30,780 and it's written as radical by putting this dot there. 22 00:01:30,780 --> 00:01:35,900 The OH radical is formed from water losing 23 00:01:35,900 --> 00:01:39,370 a hydrogen, and it really wants to get that hydrogen back. 24 00:01:39,370 --> 00:01:44,890 If it encounters a methane, it'll steal one of methane's hydrogens, 25 00:01:44,890 --> 00:01:48,960 leaving CH3, and the radical now on the carbon. 26 00:01:48,960 --> 00:01:54,260 And now the OH has satisfied itself and become a water. 27 00:01:54,260 --> 00:02:01,840 The average lifetime of methane in the atmosphere is ten years. 29 00:02:01,840 --> 00:02:15,820 In the dark, like in the bubbles of ice cores, 33 00:02:15,820 --> 00:02:19,450 you can measure the methane concentration from the atmosphere 34 00:02:19,450 --> 00:02:23,410 from 100,000 years ago, and it won't have reacted at all. 35 00:02:23,410 --> 00:02:28,660 The atmosphere is like a very slowly burning candle flame, which is 36 00:02:28,660 --> 00:02:30,900 only kept going by the sunlight 37 00:02:30,900 --> 00:02:34,190 that creates these radical compounds. 38 00:02:34,190 --> 00:02:36,460 And actually a candle flame 39 00:02:36,460 --> 00:02:42,755 works by the same radical chemistry as this. 41 00:02:43,080 --> 00:02:46,790 There are different ways that the concentration 42 00:02:46,790 --> 00:02:49,550 of methane in the atmosphere could be changed. 43 00:02:49,550 --> 00:02:54,270 You could either change how quickly you're 44 00:02:54,270 --> 00:02:57,780 releasing methane to the atmosphere, which we call the source of methane. 45 00:02:59,560 --> 00:03:02,600 or you could change the amount of OH radical 46 00:03:02,600 --> 00:03:05,560 that is around for the methane to react with. 47 00:03:05,560 --> 00:03:09,130 If we try the first case, we start out where 48 00:03:09,130 --> 00:03:15,500 the reaction of methane is balancing its production here initially. 49 00:03:15,500 --> 00:03:18,210 But now suddenly, we add a whole bunch of 50 00:03:18,210 --> 00:03:21,940 new methane to the atmosphere in an ongoing way, 51 00:03:21,940 --> 00:03:24,340 increasing the rate of method emission. 52 00:03:25,920 --> 00:03:28,680 At this point, you'd be putting more methane into the system 53 00:03:28,680 --> 00:03:32,560 than you're taking out and so the methane concentration would rise. 54 00:03:32,560 --> 00:03:34,470 And the rate 55 00:03:34,470 --> 00:03:39,710 that methane will react will go up as you have more methane. 56 00:03:39,710 --> 00:03:43,710 The reaction of methane 57 00:03:43,710 --> 00:03:46,539 will grow in until it balances the source again. 58 00:03:47,590 --> 00:03:52,680 And this takes about ten years that lifetime of methane in the atmosphere. 59 00:03:52,680 --> 00:03:55,830 Alternatively if you did something that suddenly 60 00:03:55,830 --> 00:03:59,920 depleted the the availability of this OH radical, 61 00:03:59,920 --> 00:04:01,900 you could start out with an 62 00:04:01,900 --> 00:04:06,010 equilibrium and then suddenly the reaction rate would drop, 63 00:04:06,010 --> 00:04:10,670 and the concentration would rise until 64 00:04:10,670 --> 00:04:14,740 the reaction rate again balanced the source. 65 00:04:14,740 --> 00:04:20,095 This should be looking a little bit familiar to you because it's 67 00:04:20,720 --> 00:04:23,190 basically our old friend the kitchen sink analogy 68 00:04:23,190 --> 00:04:25,800 again, applied to a totally different thing. 69 00:04:26,890 --> 00:04:31,230 Where the source of methane is the water coming out of the faucet, and 70 00:04:31,230 --> 00:04:35,690 the rate at which the water goes down the drain depends on the water level. 71 00:04:35,690 --> 00:04:40,780 The same way that the rate of methane reaction depends 72 00:04:40,780 --> 00:04:46,040 on the amount of methane that's there. If we assume 73 00:04:46,040 --> 00:04:51,400 that the amount of OH radical is staying the same, which it seems like it 74 00:04:51,400 --> 00:04:57,410 pretty much is, we can draw a equilibrium 75 00:04:57,410 --> 00:05:03,330 equation that says that the source should balance the reaction rate. 76 00:05:03,330 --> 00:05:05,150 Or the reaction rate has to be balancing the 77 00:05:05,150 --> 00:05:07,790 source, it's a slave to the source in equilibrium. 78 00:05:07,790 --> 00:05:11,440 And that could be given by the concentration divided by this lifetime 79 00:05:11,440 --> 00:05:12,290 of ten years. 80 00:05:14,270 --> 00:05:19,450 The main way that humans are altering the methane concentration 81 00:05:19,450 --> 00:05:24,600 of the atmosphere is by adding new sources of methane to the atmosphere. 82 00:05:24,600 --> 00:05:29,420 There are natural sources that existed before us and exist still today. 83 00:05:29,420 --> 00:05:32,200 And the main of these is swamps. 84 00:05:32,200 --> 00:05:36,530 We talked before when we were talking about coal formation that when you 85 00:05:36,530 --> 00:05:40,710 have water logged soil, it's hard for oxygen to get down in there. 86 00:05:40,710 --> 00:05:45,270 And in that case, bacteria can produce methane from the organic carbon. 87 00:05:45,270 --> 00:05:50,410 And so you hear methane called swamp gas for this reason. 88 00:05:50,410 --> 00:05:54,890 If you step in a swamp, you'll see bubbles coming out, and then 89 00:05:54,890 --> 00:05:59,100 maybe the bubbles can catch fire in the atmosphere and look like a UFO. 90 00:05:59,100 --> 00:06:01,640 They blame UFOs on swamp gas, which is probably 91 00:06:01,640 --> 00:06:02,160 kind of silly. 92 00:06:02,160 --> 00:06:06,170 But swamps are a main natural source of methane to the atmosphere. 93 00:06:06,170 --> 00:06:13,110 Termites and ruminant animals also produce methane in a natural way. 94 00:06:13,110 --> 00:06:19,510 Humans have impacted this story by growing rice which we do in flooded fields. 95 00:06:19,510 --> 00:06:23,250 It's not that the rice requires the flooding but it can 96 00:06:23,250 --> 00:06:27,280 tolerate it, and it makes it much easier to keep the other 97 00:06:27,280 --> 00:06:30,160 invasive plants out, if the field is flooded. 98 00:06:30,160 --> 00:06:32,020 A flooded field tends to 99 00:06:32,020 --> 00:06:36,060 make anaerobic conditions favorable to methane production, 100 00:06:36,060 --> 00:06:41,040 so rice agriculture is a major source of methane to the atmosphere every year. 101 00:06:41,040 --> 00:06:46,360 our domestic ruminant animals; cows also release methane. 102 00:06:46,360 --> 00:06:51,168 They exhale it actually, it's not a, it's not a, ahhh... 103 00:06:51,168 --> 00:06:53,030 The fossil fuel industry 104 00:06:53,030 --> 00:06:56,400 is also responsible for some leakage of methane to the atmosphere. 105 00:06:57,730 --> 00:07:02,880 Looking at the concentrations 106 00:07:02,880 --> 00:07:07,840 of methane through time it is about twice as 107 00:07:07,840 --> 00:07:13,270 high as the pre-anthroprogenic value which we can measure from bubbles in ice cores. 108 00:07:13,270 --> 00:07:18,070 And it was rising through the decades until about 1990 when it seemed 109 00:07:18,070 --> 00:07:20,940 to reach this plateau and stop changing. 110 00:07:20,940 --> 00:07:26,400 Everything else about the human impact on the planet is growing. 111 00:07:26,400 --> 00:07:31,540 And so its actually kind of a mystery why the methane concentration isn't going up. 112 00:07:31,540 --> 00:07:33,800 There are multiple theories which is what always 113 00:07:33,800 --> 00:07:35,919 happens when you don't really know for sure. 114 00:07:36,940 --> 00:07:40,540 One possibility is that the earth has been drier 115 00:07:40,540 --> 00:07:44,060 in the last few decades because of climate change, 116 00:07:44,060 --> 00:07:49,330 whick cuts down the emission of methane from wetlands. 117 00:07:49,330 --> 00:07:53,150 Another possibility is that the collapse of the eastern block 118 00:07:55,560 --> 00:07:58,930 capped a lot of leaky methane to the atmosphere. 119 00:07:58,930 --> 00:08:00,500 But it's not really very well known. 120 00:08:02,180 --> 00:08:04,730 Just in the last few years, there has been an 121 00:08:04,730 --> 00:08:09,610 uptick in the methane concentration, which is also not very well understood. 122 00:08:11,110 --> 00:08:14,950 The future of atmospheric methane is 123 00:08:14,950 --> 00:08:17,560 difficult to predict, given that we can't really tell you 124 00:08:20,350 --> 00:08:24,700 in detail why it has done what it has done in the recent past. 125 00:08:24,700 --> 00:08:32,090 However the the short lifetime of methane in the atmosphere means that if you want 126 00:08:32,090 --> 00:08:36,370 methane to go up, you have to keep adding more and more and more, all the time. 127 00:08:36,370 --> 00:08:39,475 And if you cut back on the emissions the concentration 128 00:08:39,475 --> 00:08:42,940 will fall within a fairly short time frame.