2 00:00:09,830 --> 00:00:15,680 In thinking about those temperature targets, it turns out that 3 00:00:15,680 --> 00:00:18,670 a lot of the complexity of the carbon cycle goes away. 4 00:00:19,730 --> 00:00:24,570 And that's illustrated by these schematic cases 5 00:00:24,570 --> 00:00:29,770 of releasing a slug of carbon either doing it very quickly, all 6 00:00:29,770 --> 00:00:34,970 at once, or doing it slowly, but for a longer time. 7 00:00:34,970 --> 00:00:40,170 The total amount of carbon is released is the same in either case. 8 00:00:40,170 --> 00:00:42,830 There's a fast slug of carbon 9 00:00:42,830 --> 00:00:45,520 released, and a slow slug of carbon released. 10 00:00:45,520 --> 00:00:48,908 If you release the carbon quickly, it reaches the 11 00:00:48,908 --> 00:00:51,526 CO2 peak at the end of 12 00:00:51,526 --> 00:00:54,606 the slug time, and then it slowly drifts down 13 00:00:54,606 --> 00:00:58,821 as it is neutralized by the oceans. 14 00:00:58,821 --> 00:01:00,261 If you release it more slowly, 15 00:01:00,261 --> 00:01:05,621 it reaches a peak that's later, and probably a lesser peak, because 16 00:01:05,621 --> 00:01:10,610 as you've been releasing it's had time to dissolve in the oceans. 17 00:01:10,610 --> 00:01:17,170 The temperature response goes up with rising CO2, but there's a lag time for the 18 00:01:17,170 --> 00:01:19,420 change in the temperature, of about 1,000 years, 19 00:01:19,420 --> 00:01:21,670 because you have to heat up the ocean. 20 00:01:21,670 --> 00:01:31,020 The temperature rises to a plateau. And then it basically stays there. 22 00:01:31,020 --> 00:01:34,370 As far as we're concerned, it stays there forever. 23 00:01:34,370 --> 00:01:38,560 But the crucial part to this is that the plateau temperature... 24 00:01:38,560 --> 00:01:43,120 that it reaches for these two scenarios is about the same. 25 00:01:43,120 --> 00:01:46,318 It turns out that the higher CO2 26 00:01:46,318 --> 00:01:51,540 is counter-balanced by the fact that it started dropping 27 00:01:51,540 --> 00:01:56,100 by the time the earth's temperature is catching up. 28 00:01:56,100 --> 00:02:00,380 And so, the time dependence of the absorption of CO2 is 29 00:02:00,380 --> 00:02:06,560 kind of compensated for by the time dependence of the warming. 30 00:02:06,560 --> 00:02:07,690 31 00:02:07,690 --> 00:02:11,090 This means that the temperature that you're going to reach 32 00:02:11,090 --> 00:02:15,780 doesn't depend on whether you release the CO2 quickly or slowly. 33 00:02:15,780 --> 00:02:16,834 You're going to reach 34 00:02:16,834 --> 00:02:19,160 the same temperature in either case. 35 00:02:19,160 --> 00:02:20,640 You'll reach it more quickly or more 36 00:02:20,640 --> 00:02:22,490 slowly depending on how quickly you release it. 37 00:02:22,490 --> 00:02:26,920 It's a much simpler picture now because we can just draw a one to one 38 00:02:26,920 --> 00:02:31,000 relationship between the amount of carbon ever burned 39 00:02:31,000 --> 00:02:33,020 and the temperature that the Earth will reach. 40 00:02:34,660 --> 00:02:42,040 It turns out that, about 1,000 gigatons of carbon would take the earth about 41 00:02:42,040 --> 00:02:45,180 to this two degree C threshold. 42 00:02:45,180 --> 00:02:50,930 500 gigatons of carbon would take it about to one degree C. 43 00:02:50,930 --> 00:02:54,120 For comparison, we've already burned and cut down about 44 00:02:54,120 --> 00:02:58,280 300 gigatons of fossil fuels and 200 gigatons of trees. 45 00:02:58,280 --> 00:03:03,040 We're kind of near the one degree C mark already, 46 00:03:03,040 --> 00:03:06,930 and we're about half way toward the two degree C mark. 47 00:03:12,030 --> 00:03:15,450 What would it look like to actually try to cut a emissions, and 48 00:03:15,450 --> 00:03:19,909 stick to a one-thousand gigatons of carbon target? 49 00:03:20,950 --> 00:03:26,360 Here is a plot of emissions of carbon per year. 50 00:03:26,360 --> 00:03:33,490 Business as usual is this line here; it's growing a rate of 3% per year. 51 00:03:33,490 --> 00:03:36,080 That's the trajectory that we continue to be on. 52 00:03:37,470 --> 00:03:44,520 And the total amount ever burned goes as the area under this curve. 53 00:03:44,520 --> 00:03:50,320 Let's say that in 2010 we had decided, instead of growing at 3% per year, 54 00:03:50,320 --> 00:03:55,470 we would cut emissions by about 2% per year. 55 00:03:55,470 --> 00:04:01,100 The emission flux would drop in this gradual way. 56 00:04:01,100 --> 00:04:03,470 And the total area under this curve 58 00:04:05,570 --> 00:04:09,510 would add up at about 1,000 gigatons of carbon. 59 00:04:09,510 --> 00:04:14,650 If we wait longer, we have to cut more quickly, in order for 60 00:04:14,650 --> 00:04:18,842 the total area under the curve to stay under a thousand gigatons. 61 00:04:19,860 --> 00:04:25,360 Here's a plot of the percent per year cuts that would be 62 00:04:25,360 --> 00:04:30,900 required in order to keep to 1000 gigatons of carbon for the total slug, 63 00:04:31,970 --> 00:04:34,430 as a function of the year that they start. 64 00:04:34,430 --> 00:04:40,490 Starting in 2010 cuts of about 2% per year would have done it. 65 00:04:40,490 --> 00:04:43,700 2020, we would have to cut by 4% per year. 66 00:04:43,700 --> 00:04:46,170 2030, about 10% per year. 67 00:04:46,170 --> 00:04:49,790 And pretty soon, by around 2040 or so, basically we'd have to 68 00:04:49,790 --> 00:04:54,850 quit cold turkey, in order to keep under a 1000 gigatons of carbon. 69 00:04:54,850 --> 00:04:55,630 It's kind of like, 70 00:04:58,260 --> 00:05:01,600 you know, you're in the army, and your sergeant, has caught 71 00:05:01,600 --> 00:05:05,400 you doing somethin wrong, he says, drop and give me 20 pushups! 72 00:05:05,400 --> 00:05:08,780 Or, if you don't want to do em today, you can do em tomorrow, but you gotta do 40. 73 00:05:08,780 --> 00:05:10,320 Or you can do them this weekend, but you've got to do 100. 74 00:05:10,320 --> 00:05:12,390 What are you going to do? 75 00:05:12,390 --> 00:05:18,760 It's much easier, cheaper to convert our energy infrastructure 76 00:05:18,760 --> 00:05:23,300 at a slow rate of a few percent per year, because that's kind of the rate 77 00:05:23,300 --> 00:05:28,150 at which power plants become obsolete and need to be replaced anyway. 78 00:05:28,150 --> 00:05:30,160 Whereas, if you wait until the end, you've 79 00:05:30,160 --> 00:05:33,180 got to basically build everything all from scratch, 80 00:05:33,180 --> 00:05:36,180 all at once, It's a much bigger and much more expensive proposition.