2 00:00:10,445 --> 00:00:14,360 The oceans are another major player in the earth's carbon cycle, 3 00:00:14,360 --> 00:00:19,050 and it also exchanges carbon with the atmosphere, but on a very 4 00:00:19,050 --> 00:00:22,480 different rhythm, a very different time scale than that of the solid earth. 5 00:00:24,030 --> 00:00:28,780 Basically there's a large amount of 6 00:00:28,780 --> 00:00:31,300 carbon dissolved in the ocean, much more than 7 00:00:31,300 --> 00:00:36,250 in the atmosphere, and some parts of the ocean are releasing CO2 to the atmosphere. 8 00:00:36,250 --> 00:00:41,860 In some places CO2 is degassing or evaporating from the ocean. 9 00:00:41,860 --> 00:00:45,790 In other places the ocean is taking up carbon dioxide. 10 00:00:45,790 --> 00:00:48,710 Say, where the water is colder so the CO2 dissolves 11 00:00:48,710 --> 00:00:52,620 more readily into the water than in warm water, for example. 12 00:00:52,620 --> 00:00:56,110 The back and forth fluxes of carbon 13 00:00:56,110 --> 00:00:59,330 between the atmosphere and the ocean are huge. 14 00:00:59,330 --> 00:01:01,912 They are something like 1,000 15 00:01:01,912 --> 00:01:05,700 times higher than the back and forth fluxes with 16 00:01:05,700 --> 00:01:09,370 the solid Earth. The unit that we use to talk 17 00:01:09,370 --> 00:01:15,060 about carbon, or one set of units that we use, is gigatons. 18 00:01:15,060 --> 00:01:21,600 Giga means billion, and ton in this case means metric ton which is 1,000 kilograms. 19 00:01:21,600 --> 00:01:27,680 A gigaton of carbon is also about 20 00:01:27,680 --> 00:01:31,480 ten to the 15th grams (1E15), if you find that helpful. 21 00:01:31,480 --> 00:01:34,690 Or another way to think of it is all of 22 00:01:34,690 --> 00:01:41,550 the people on Earth probably weigh less than a gigaton. 23 00:01:41,550 --> 00:01:47,210 There's much more carbon going back and forth between the atmosphere 24 00:01:47,210 --> 00:01:53,370 and the ocean every year, by a factor of 100 or more, 25 00:01:53,370 --> 00:01:56,120 than the total mass of all the people on earth. 26 00:01:57,300 --> 00:02:02,587 And this is much bigger than the flux to the solid earth. 28 00:02:04,510 --> 00:02:10,210 The oceans are thought to be responsible for changes in atmospheric 29 00:02:10,210 --> 00:02:15,600 CO2 through the glacial inter-glacial cycles, which 30 00:02:15,600 --> 00:02:21,630 are documented from records preserved in ice cores in Antarctica and Greenland. 31 00:02:21,630 --> 00:02:25,240 From little bubbles of air trapped in 32 00:02:25,240 --> 00:02:29,920 those ice cores we know that atmospheric CO2 concentration 33 00:02:29,920 --> 00:02:33,810 is generally about 280 parts per million 34 00:02:33,810 --> 00:02:37,880 during warm times like today, during interglacial periods. 35 00:02:37,880 --> 00:02:42,588 And then during cold times like the last glacial maximum 20,000 years ago, 36 00:02:42,588 --> 00:02:47,120 the CO2 concentration was lower, about 200 parts per million. 37 00:02:48,260 --> 00:02:52,200 And there's this astonishingly good correlation between the 38 00:02:52,200 --> 00:02:55,960 CO2 from the ice cores and temperature estimates 39 00:02:55,960 --> 00:02:57,500 from those same ice cores. 40 00:02:57,500 --> 00:03:02,800 It makes it very clear that CO2 plays a very strong role in Earth's climate. 41 00:03:04,380 --> 00:03:09,470 And the the timing of the ice ages is extremely 42 00:03:09,470 --> 00:03:14,750 well-correlated with fluctuations in Earth's orbit around the sun. 43 00:03:15,840 --> 00:03:21,160 And in particular what seems to matter is how bright the sunlight intensity 44 00:03:21,160 --> 00:03:24,360 is during the Northern Hemisphere's summer. 45 00:03:24,360 --> 00:03:26,870 The Northern Hemisphere is what's important, because that's 46 00:03:26,870 --> 00:03:29,250 where the ice sheets have room to form. 47 00:03:29,250 --> 00:03:30,500 There's more land in the Northern Hemisphere 48 00:03:30,500 --> 00:03:32,590 than in the Southern Hemisphere. 49 00:03:32,590 --> 00:03:36,660 And the summer is important, because that's when you decide whether to melt 50 00:03:36,660 --> 00:03:41,298 last winter's snow or keep it around and start to accumulate an ice sheet. 51 00:03:41,298 --> 00:03:46,370 There's a very good correlation between the orbital fluctuations and 52 00:03:46,370 --> 00:03:49,080 the amount of ice on earth. 53 00:03:49,080 --> 00:03:53,250 Twenty thirty years ago, that seemed like the whole story, but then 54 00:03:53,250 --> 00:03:57,370 they discovered that the CO2 concentration is also going up and down as the ice sheets 55 00:03:57,370 --> 00:04:01,390 are going up and down, so this seems like it must be a 56 00:04:01,390 --> 00:04:07,800 a positive feedback, that the orbit of the earth says "let there be ice sheets". 57 00:04:07,800 --> 00:04:09,980 And then the carbon cycle says "Okay", 58 00:04:09,980 --> 00:04:11,490 "if you're going to have ice sheets, I'm going to" 59 00:04:11,490 --> 00:04:16,400 "pull the CO2 down and amplify that cooling from the ice sheets." 60 00:04:16,400 --> 00:04:19,740 And so that's what makes the ice ages global instead 61 00:04:19,740 --> 00:04:23,490 of just from one hemisphere or the other, is the CO2 effect. 62 00:04:24,800 --> 00:04:31,990 On time scales of thousands of years and longer the ocean seemed to act like 63 00:04:31,990 --> 00:04:38,470 a positive feedback to the climate system. 64 00:04:38,470 --> 00:04:42,370 However, today we are releasing CO2 to the atmosphere. 65 00:04:42,370 --> 00:04:46,360 And so the oceans are taking up some of that CO2, 66 00:04:46,360 --> 00:04:50,990 so, on the timescale of decades today, the oceans are acting as a negative feedback. 67 00:04:50,990 --> 00:04:52,600 They're helping clean up, 68 00:04:52,600 --> 00:04:55,820 they're dampening, they're stabilizing Earth's climate. 69 00:04:55,820 --> 00:04:59,213 And so it's kind of an open question whether on 70 00:04:59,213 --> 00:05:03,476 time scales of 1,000 years or longer the oceans might 71 00:05:03,476 --> 00:05:09,358 revert to their positive feedback role, and amplify our climate excursion.