2 00:00:02,300 --> 00:00:07,497 There's the peak, and 3 00:00:07,497 --> 00:00:13,580 there's the long tail. 4 00:00:13,580 --> 00:00:19,178 And there's a couple of factors which tend to de-emphasize 5 00:00:19,178 --> 00:00:23,710 the peak, and emphasize the tail. 6 00:00:23,710 --> 00:00:27,540 One of them is the band saturation effect, which is the the fact 7 00:00:27,540 --> 00:00:33,420 that when you add CO2 to the air the temperature response 8 00:00:33,420 --> 00:00:37,670 Is not linear but it bends over like that. 9 00:00:37,670 --> 00:00:42,180 Actually the temperature goes up as the logarithm of the CO2, rather then 10 00:00:43,260 --> 00:00:48,120 as a direct multiplier times the CO2, which is what this shape looks like. 11 00:00:48,120 --> 00:00:52,560 That means that the really high CO2 of the peak doesn't translate 12 00:00:52,560 --> 00:00:57,700 into as really high a temperature as you would think, if it was just proportional. 13 00:00:57,700 --> 00:01:03,130 This function acts to fatten up this tail 14 00:01:03,130 --> 00:01:08,120 relative to what it looks like just in the CO2 concentration. 15 00:01:08,120 --> 00:01:10,990 Looking at CO2, you say, oh, there's a big peak and 16 00:01:10,990 --> 00:01:14,730 yeah, okay, kind of a long tail as an afterthought. 17 00:01:14,730 --> 00:01:18,450 But when you look at the temperature, it doesn't have such 18 00:01:18,450 --> 00:01:19,310 a pronounced peak. 19 00:01:19,310 --> 00:01:23,280 It looks like the temperature just changes and pretty much stays there. 20 00:01:23,280 --> 00:01:25,480 The other thing that tends to de-emphasize the peak 21 00:01:25,480 --> 00:01:32,020 is the slow ocean heat uptake time. 22 00:01:32,020 --> 00:01:35,370 To warm up the planet, you have to warm up the deep ocean. 23 00:01:35,370 --> 00:01:39,570 Today, as we'll learn in future lecture, the oceans are keeping 24 00:01:39,570 --> 00:01:43,820 us cool because a lot the excess heat from the greenhouse 25 00:01:43,820 --> 00:01:47,870 gases is slowly invading the ocean. 26 00:01:47,870 --> 00:01:53,950 And the time scale for heat invading the ocean is very similar to the timescale 27 00:01:53,950 --> 00:01:56,160 for CO2 invading the ocean, because they're both 28 00:01:56,160 --> 00:02:00,360 controlled by the ocean circulation, about a thousand years. 29 00:02:00,360 --> 00:02:05,570 As a result, the temperature evolution from 30 00:02:05,570 --> 00:02:09,000 the peak, over that thousand years there, is going to get chopped 31 00:02:09,000 --> 00:02:11,420 off by the slow ocean heat uptakes. 32 00:02:11,420 --> 00:02:15,440 That further de-emphasizes 33 00:02:15,440 --> 00:02:17,540 the peak relative to the long tail. 34 00:02:19,040 --> 00:02:24,550 There are parts of the climate system that respond 35 00:02:24,550 --> 00:02:29,640 most strongly to the long tail because they take a long time to respond. 36 00:02:29,640 --> 00:02:34,860 And the poster child for that is really the ice sheets 37 00:02:34,860 --> 00:02:43,030 and their impact on sea level. Here is a plot of reconstructed changes 38 00:02:43,030 --> 00:02:49,630 in sea level due to changes in the size of the ice sheets from the geologic past. 39 00:02:49,630 --> 00:02:55,710 Present day, we are here and sea level is by definition zero. 41 00:02:59,370 --> 00:03:04,210 This data point here is from the last glacial time, when it was maybe five 42 00:03:04,210 --> 00:03:06,560 or six degrees colder than today, and sea 43 00:03:06,560 --> 00:03:09,440 level was actually 120 meters lower than today. 44 00:03:10,910 --> 00:03:14,460 Looking further back in time, about 3 million years ago, time 45 00:03:14,460 --> 00:03:16,810 called the Pliocene when there wasn't much 46 00:03:16,810 --> 00:03:19,000 of any ice in the Northern hemisphere, 47 00:03:19,000 --> 00:03:21,800 and the temperature was was somewhat warmer then today. 48 00:03:21,800 --> 00:03:24,410 Its not totally clear exactly how much 49 00:03:24,410 --> 00:03:28,800 higher sea level was then or how much warmer it was; it's hard to put together 50 00:03:30,190 --> 00:03:31,710 information from the past like this. 51 00:03:31,710 --> 00:03:33,200 But here's the best guess. 52 00:03:33,200 --> 00:03:38,480 And then further back in time, this is from a time when 53 00:03:38,480 --> 00:03:41,870 when the Earth was in what's called a hothouse climate. 54 00:03:41,870 --> 00:03:45,690 There was no ice at all on Earth. 55 00:03:45,690 --> 00:03:49,500 The problem with understanding or figuring out 56 00:03:49,500 --> 00:03:53,420 the sea level during sometime this way, in the distant 57 00:03:53,420 --> 00:03:56,530 past like that, is if you find some fossil beach someplace, 58 00:03:56,530 --> 00:04:01,850 you don't really know if 59 00:04:01,850 --> 00:04:05,430 that's at the same elevation today as it was then. 60 00:04:05,430 --> 00:04:10,580 Because the land surface is moving up and down on geologic time scales. 61 00:04:10,580 --> 00:04:15,230 This 70 meters of sea level rise at 62 00:04:15,230 --> 00:04:19,480 that warmer temperature is based on how much ice there is today. 63 00:04:19,480 --> 00:04:23,020 If we wanted to make a hot house climate out of the earth today 64 00:04:23,020 --> 00:04:28,740 with the continental geography how it is, it would raise sea level by 70 meters. 65 00:04:28,740 --> 00:04:32,950 You know there's a whole lot of complexity in this line. 66 00:04:32,950 --> 00:04:36,950 But there is also just a fairly clear relationship. 67 00:04:36,950 --> 00:04:40,660 Especially when you compare it with the 68 00:04:40,660 --> 00:04:45,470 forecast for the year 2100, which calls for about 69 00:04:45,470 --> 00:04:49,010 2 to 4 or 5 degrees centigrade of warming. 70 00:04:49,010 --> 00:04:51,870 And a sea level change of maybe a meter or two. 71 00:04:51,870 --> 00:04:56,950 This is totally off this line from the geologic reconstructions. 72 00:04:56,950 --> 00:05:01,170 And the reason why is because it takes a long time, longer than 73 00:05:01,170 --> 00:05:06,390 just from now to the year 2100, hopefully, to melt the major ice sheets. 74 00:05:06,390 --> 00:05:11,170 And so the forecast for the year 2100 75 00:05:11,170 --> 00:05:16,730 doesn't really capture the time scale of the problem of the ice sheets. 76 00:05:16,730 --> 00:05:19,280 And that's why this forecast is a little bit deceptive. 77 00:05:19,280 --> 00:05:22,000 I mean, it's useful, because we're not, I mean, those of you 78 00:05:22,000 --> 00:05:25,740 who are watching this video probably are not going to live beyond the year 2100. 79 00:05:25,740 --> 00:05:28,520 and so this is an anthropogenic, a human time scale, 80 00:05:28,520 --> 00:05:31,250 but it's not the time scale for the ice sheets. 81 00:05:31,250 --> 00:05:38,270 But if the CO2 is going to continue to effect climate for, hundreds of thousands of 83 00:05:38,270 --> 00:05:42,430 years into the future, there's plenty of time for the ice sheets to respond fully. 84 00:05:42,430 --> 00:05:45,740 This is an example of a part of the climate system 85 00:05:45,740 --> 00:05:50,210 that is mostly sensitive to the long tail of the CO2. 86 00:05:50,210 --> 00:05:54,500 Other examples include a carbon cycle feedbacks, 87 00:05:54,500 --> 00:05:56,910 and warming the deep ocean and 88 00:05:56,910 --> 00:06:02,800 causing methane hydrate to decompose and release its carbon. 89 00:06:02,800 --> 00:06:06,080 This would happen on a very long time scale of thousands of years, because 90 00:06:06,080 --> 00:06:09,500 it takes a long time for heat to get into the deep ocean. 91 00:06:09,500 --> 00:06:14,870 Or melting permafrost soils on land and decomposing the carbon 92 00:06:14,870 --> 00:06:20,210 that's frozen there will also take centuries, presumably, to happen. 93 00:06:20,210 --> 00:06:22,090 These are all impacts 94 00:06:22,090 --> 00:06:25,850 that are more strongly tied to the long tail 95 00:06:25,850 --> 00:06:29,260 of the CO2 than to the actual peak itself.