2 00:00:10,657 --> 00:00:13,072 The land surface is the third component, 3 00:00:13,072 --> 00:00:16,410 the third reservoir, of Earth's carbon cycle. 4 00:00:16,410 --> 00:00:20,240 The difference between the land's surface and the solid earth 5 00:00:20,240 --> 00:00:24,780 is the land surface is just the carbon that's exposed 6 00:00:24,780 --> 00:00:28,040 at the surface which can respond quickly, as opposed to 7 00:00:28,040 --> 00:00:32,910 solid earth, I mean stuff that's buried for millions of years. 8 00:00:34,190 --> 00:00:39,770 The land carbon cycle is governed by plants mostly. 10 00:00:39,770 --> 00:00:42,600 Plants grow in the spring and summer and they 11 00:00:42,600 --> 00:00:45,680 take up CO2, and then in the fall and in 12 00:00:45,680 --> 00:00:48,530 the winter, they drop their leaves, which decompose, or 13 00:00:48,530 --> 00:00:52,610 turn back into CO2, and are released back to the atmosphere. 14 00:00:52,610 --> 00:00:56,380 There's a very strong seasonal cycle in the CO2 15 00:00:56,380 --> 00:01:00,240 concentration of the atmosphere which is driven by the land surface. 16 00:01:00,240 --> 00:01:00,890 It goes up in the winter 17 00:01:00,890 --> 00:01:06,400 and down in the summer. You can see that in 18 00:01:06,400 --> 00:01:11,900 the Southern hemisphere the seasonal cycle is much less intense 19 00:01:11,900 --> 00:01:16,600 in CO2 because there's less land in the southern hemisphere. 20 00:01:16,600 --> 00:01:19,400 You have much wider swings 21 00:01:19,400 --> 00:01:23,110 of atmospheric CO2 concentration in the Northern hemisphere. 22 00:01:23,110 --> 00:01:25,970 In the Southern hemisphere they're out of phase with the north because 23 00:01:27,160 --> 00:01:31,060 the seasons are out of phase. And then on top of the seasonal cycles 24 00:01:31,060 --> 00:01:34,800 from those two regions you can also see a 25 00:01:34,800 --> 00:01:38,660 secular trend. The rise in the CO2 concentration with 26 00:01:38,660 --> 00:01:45,640 time which is due to fossil fuels. 27 00:01:45,640 --> 00:01:52,160 There are a couple of ways that the land biosphere can affect the CO2 concentration 28 00:01:53,720 --> 00:01:58,190 with the fossil fuel CO2 and all. One is deforestation. 29 00:01:58,190 --> 00:02:01,540 If you go from a forested landscape and then 30 00:02:01,540 --> 00:02:05,170 you cut down the trees and make an agricultural landscape 31 00:02:05,170 --> 00:02:08,770 you have less carbon per square meter of area in 32 00:02:08,770 --> 00:02:11,860 this land after it's been cut than you had before. 33 00:02:11,860 --> 00:02:16,150 That actually provides a net source of carbon to the atmosphere. 34 00:02:16,150 --> 00:02:19,400 It's not as much carbon as our fossil fuel carbon 35 00:02:19,400 --> 00:02:23,460 but it's a significant amount of carbon released to the atmosphere. 36 00:02:23,460 --> 00:02:28,580 Something else that's going on that's sort of more mysterious is that in places where 37 00:02:30,970 --> 00:02:34,650 there's not active deforestation going on there appears to be a 38 00:02:34,650 --> 00:02:39,870 natural uptake of carbon by the terrestrial biosphere, they call greening. 39 00:02:39,870 --> 00:02:43,600 And it's not clear what's driving this. 40 00:02:43,600 --> 00:02:47,570 It could be that with the climate change we've experienced already, 41 00:02:47,570 --> 00:02:53,420 a longer growing season is allowing more plants to grow. 42 00:02:53,420 --> 00:02:56,350 Or maybe it's allowing area 43 00:02:56,350 --> 00:03:01,940 that use to be low-carbon tundra to convert into higher-carbon forests. 44 00:03:01,940 --> 00:03:06,540 It could be actually CO2 fertilization effect. 45 00:03:06,540 --> 00:03:09,720 Plants actually grow better if there's more CO2. 46 00:03:09,720 --> 00:03:11,230 That's one of the purposes of a 47 00:03:11,230 --> 00:03:13,840 greenhouse, is to have higher CO2 concentration. 48 00:03:13,840 --> 00:03:18,150 It makes the plants grow better. It could be fire suppression. 49 00:03:18,150 --> 00:03:21,350 Actually, it's not even known. Even though there's a 50 00:03:21,350 --> 00:03:24,430 fairly large amount of carbon going into the natural 51 00:03:24,430 --> 00:03:27,750 landscape someplace, we can't really find it for sure. 52 00:03:28,860 --> 00:03:30,580 Don't know where it is. 53 00:03:30,580 --> 00:03:33,310 The landscape is serving as a negative 54 00:03:33,310 --> 00:03:36,990 feedback today, because it's taking up carbon that we 55 00:03:36,990 --> 00:03:39,560 are releasing to the atmosphere, so it's stabilizing the 56 00:03:39,560 --> 00:03:43,410 climate and acting as a negative feedback. 57 00:03:44,980 --> 00:03:47,160 However, if you look at this map 58 00:03:47,160 --> 00:03:53,730 of the distribution of soil carbon on earth, what you see is that there's much 59 00:03:53,730 --> 00:03:58,930 more soil carbon in the high latitudes then there is in the low latitudes. 60 00:03:58,930 --> 00:04:03,320 Rain forests look like very carbon-rich, lush places, and there 61 00:04:03,320 --> 00:04:06,650 is a lot of carbon in rain forests but not in the soils. 62 00:04:06,650 --> 00:04:09,290 The soils in rain forests are bleached out. 63 00:04:09,290 --> 00:04:12,980 There's no carbon really there. It's because a leaf 64 00:04:12,980 --> 00:04:16,840 that falls to the ground in the tropics decomposes 65 00:04:16,840 --> 00:04:20,260 so quickly it doesn't accumulate in the soils. 66 00:04:20,260 --> 00:04:25,000 Whereas in cooler climates you have build-up of more carbon in the soils. 67 00:04:25,000 --> 00:04:29,320 And the end member case is in the very high latitudes in the permafrost 68 00:04:29,320 --> 00:04:31,820 regions, the soil is actually frozen, and 69 00:04:31,820 --> 00:04:34,430 so the carbon is very well preserved there. 70 00:04:34,430 --> 00:04:38,580 You can find intact mammoths 71 00:04:38,580 --> 00:04:42,250 from thousands of years ago that are still frozen and even 72 00:04:42,250 --> 00:04:47,010 approaching edible after all those years, because it's been preserved by freezing. 73 00:04:47,010 --> 00:04:51,490 If on a timescale of centuries we melt that 74 00:04:51,490 --> 00:04:56,370 permafrost and create a more tropical 75 00:04:56,370 --> 00:04:59,590 climate in the higher latitudes that could result 76 00:04:59,590 --> 00:05:03,820 in the net release of a considerable amount of carbon. 77 00:05:03,820 --> 00:05:07,862 Again, the land biosphere, like the ocean, has the 78 00:05:07,862 --> 00:05:13,108 potential to switch from being a negative feedback that's stabilizing 79 00:05:13,108 --> 00:05:17,924 and saving us from ourselves today, to being an amplifying 80 00:05:17,924 --> 00:05:22,114 positive feedback in the fullness of time in the future.