2 00:00:09,734 --> 00:00:14,774 We can summarize the impacts of these feedbacks within the context of this 3 00:00:14,774 --> 00:00:18,086 parameter, this number that climate scientists talk 4 00:00:18,086 --> 00:00:21,250 about a lot called the climate sensitivity. 5 00:00:21,250 --> 00:00:23,408 If two climate scientists meet in a bar, 6 00:00:23,408 --> 00:00:26,270 and they want to discuss and compare their models, the 7 00:00:26,270 --> 00:00:28,320 first thing that they'll compare about the two models 8 00:00:28,320 --> 00:00:31,550 is what the climate sensitivities of the models are. 9 00:00:31,550 --> 00:00:35,510 The climate sensitivity is defined as the change in temperature 10 00:00:35,510 --> 00:00:37,060 from doubling CO2. 11 00:00:37,060 --> 00:00:40,850 It's written with this capital letter, Greek letter delta, 12 00:00:40,850 --> 00:00:45,000 which is often times used to mean a change in something. 13 00:00:45,000 --> 00:00:46,730 And the two x means doubled CO2. 14 00:00:46,730 --> 00:00:52,010 This is a convenient metric, because of the way that 15 00:00:52,010 --> 00:00:56,880 the temperature responds to CO2, because of the band saturation effect. 16 00:00:56,880 --> 00:00:59,070 You remember that a little bit of greenhouse gas 17 00:00:59,070 --> 00:01:00,750 goes a long way, but when you put 18 00:01:00,750 --> 00:01:06,208 more and more in, the, energy consequences of it get less and less. 19 00:01:06,208 --> 00:01:09,240 What actually, how it seems to work pretty well 20 00:01:09,240 --> 00:01:13,400 for CO2, is that any doubling of the CO2 concentration -- 21 00:01:13,400 --> 00:01:16,500 here's a doubling, here's a second doubling, 22 00:01:16,500 --> 00:01:18,090 here's a third doubling --- 23 00:01:18,090 --> 00:01:21,680 results in about the same amount of temperature change. 24 00:01:21,680 --> 00:01:25,440 Yyou can see this curve is kind of flattening out. 25 00:01:27,520 --> 00:01:32,730 You should be aware that it takes centuries for the Earth to 26 00:01:32,730 --> 00:01:36,670 fully equillibrate, to really change its temperature, when you change the CO2. 27 00:01:36,670 --> 00:01:41,140 But still, you can kind of use this climate sensitivity to give an order of 28 00:01:41,140 --> 00:01:44,840 magnitude, or a rough impression of what climate change will be. 29 00:01:47,710 --> 00:01:53,280 By multiplying the climate sensitivity by 30 00:01:53,280 --> 00:01:57,680 the number of doublings, which you can calculate here using logarithms. 31 00:01:57,680 --> 00:02:03,190 The logarithm of the ratio of the CO2 at the end of some time, 32 00:02:03,190 --> 00:02:07,800 versus the initial, so that is your change in CO2, divided by the logarithm of two. 33 00:02:07,800 --> 00:02:13,320 If you put in 560 over 280 you 34 00:02:13,320 --> 00:02:16,540 end up with one doubling. 35 00:02:16,540 --> 00:02:19,325 That would be one times the climate sensitivity. 37 00:02:22,340 --> 00:02:30,050 We can calculate what the climate sensitivity to a change in the energy 38 00:02:30,050 --> 00:02:35,530 flux should be without any feedbacks. Just using the Stefan Boltzmann equation. 39 00:02:35,530 --> 00:02:39,200 We have the the total energy flux 40 00:02:39,200 --> 00:02:42,530 here, in watts per square meter. Here is epsilon, 41 00:02:42,530 --> 00:02:46,630 the emissivity, which we've assumed is pretty much equal to one. 42 00:02:46,630 --> 00:02:48,040 Sigma, the Stefan Boltzmann 43 00:02:48,040 --> 00:02:50,400 constant is just a constant. 44 00:02:50,400 --> 00:02:52,850 And then the temperature raised to the fourth power. 45 00:02:52,850 --> 00:02:58,020 By running a model like the mod tran model, where you add more CO2, you 46 00:02:58,020 --> 00:03:01,860 can get that doubling CO2, changes the energy 47 00:03:01,860 --> 00:03:05,080 balance by about four Watts per square meter. 48 00:03:05,080 --> 00:03:12,578 If you run this twice, calculate two temperatures and change the 50 00:03:12,578 --> 00:03:15,860 forcing by four Watts per square meter, 51 00:03:17,060 --> 00:03:20,600 you end up calculating that the climate sensitivity of a bare rock 52 00:03:20,600 --> 00:03:25,286 with no greenhouse effect and no feedbacks would be about one degree centigrade. 53 00:03:27,190 --> 00:03:33,200 The climate sensitivity for CO2 alone would be about one degree centigrade. 54 00:03:33,200 --> 00:03:38,340 But then we've got the water vapor feedback, and the ice albedo feedback. 55 00:03:38,340 --> 00:03:42,480 And those together, just about double the climate sensitivity of the earth. 56 00:03:42,480 --> 00:03:45,910 If they didn't exist it would be much less of a problem. 57 00:03:45,910 --> 00:03:46,750 And then clouds. 58 00:03:47,860 --> 00:03:50,410 Clouds are the biggest uncertainty in climate models. 59 00:03:51,690 --> 00:03:57,950 Different models range from either having not much cloud feedback, 60 00:03:57,950 --> 00:03:59,560 and then they will have a fairly low climate 61 00:03:59,560 --> 00:04:03,100 sensitivity of maybe two degrees C for maybe doubling C02, 62 00:04:03,100 --> 00:04:07,540 or to having a higher feedback from clouds which 63 00:04:07,540 --> 00:04:11,600 results in an overall climate sensitivity that's much higher. 64 00:04:11,600 --> 00:04:17,200 It's really, unfortunate that the clouds are so difficult to model because 65 00:04:17,200 --> 00:04:19,590 They are formed by processes 66 00:04:19,590 --> 00:04:21,730 that happen on all different spatial scales. 67 00:04:21,730 --> 00:04:24,480 It's very difficult to do a whole 68 00:04:24,480 --> 00:04:27,630 atmosphere full of clouds, in a realistic way. 69 00:04:27,630 --> 00:04:28,148 And this 70 00:04:28,148 --> 00:04:33,338 is the main source of uncertainty in the climate models for the future prediction.