2 00:00:10,172 --> 00:00:12,372 How much CO2 is too much? 3 00:00:12,372 --> 00:00:16,452 The first question is why are we concerned in particular 4 00:00:16,452 --> 00:00:21,611 about CO2, rather than, say, methane, which is another greenhouse gas? 5 00:00:21,611 --> 00:00:27,536 The answer to that is that CO2, as you know, accumulates in the biosphere, 6 00:00:27,536 --> 00:00:31,802 whereas methane is a transient greenhouse gas 7 00:00:31,802 --> 00:00:34,818 that breaks down in about a decade. 8 00:00:34,818 --> 00:00:39,914 The long-term climate change that we're worried 10 00:00:39,914 --> 00:00:43,367 about is mostly driven by CO2. 11 00:00:43,367 --> 00:00:46,586 I've seen evolution of 12 00:00:46,586 --> 00:00:51,110 peoples' thinking on this over the decades. 13 00:00:51,110 --> 00:00:54,440 In the 1980s and 90s, 14 00:00:54,440 --> 00:01:00,980 people were talking in terms of stabilizing atmospheric CO2 concentration, 15 00:01:00,980 --> 00:01:05,290 trying to come up with how much CO2 we can release 16 00:01:05,290 --> 00:01:09,490 that can make the concentration rise, and then plateau at some level, 17 00:01:09,490 --> 00:01:12,160 like 750 parts per million, 18 00:01:12,160 --> 00:01:18,350 or 550 (doubling preanterpagenic value of about 280), 19 00:01:18,350 --> 00:01:20,450 or 450. 20 00:01:20,450 --> 00:01:27,180 When you release CO2 to the atmosphere, 21 00:01:27,180 --> 00:01:32,120 some of it dissolves in the ocean, and some of it goes into the land biosphere, 22 00:01:32,120 --> 00:01:37,700 and only some of it continues to reside in the atmosphere. 23 00:01:37,700 --> 00:01:46,400 h There were efforts to use carbon cycle models to figure out, 25 00:01:46,400 --> 00:01:50,990 given some set of emissions, what the concentration would be. 26 00:01:50,990 --> 00:01:52,530 Figuring out how to follow a 27 00:01:52,530 --> 00:01:57,590 stabilization scenarios was another 28 00:01:57,590 --> 00:02:01,500 inverse problem, like the bore hole temperatures, where you had to use the 29 00:02:01,500 --> 00:02:06,250 model to go backwards from the effect back to the cause. 30 00:02:07,460 --> 00:02:12,180 This was a little dodgy because slight changes in the 31 00:02:12,180 --> 00:02:17,660 trajectory could have very large changes in what they said we 32 00:02:17,660 --> 00:02:22,066 could release this year or next year. It's kind of a confusing situation.