[MUSIC] The oceans are another major player in the Earth's carbon cycle and it also exchanges carbon with the atmosphere but on a very different rhythm, a very different time scale than that of the solid earth. So, basically, there's a large amount of carbon dissolved in the ocean, much more than in the atmosphere. And some parts of the ocean are releasing CO2 to the atmosphere. So the CO2 is sort of degassing, or evaporating even, from the ocean. Other places, the ocean is taking up carbon dioxide, say where the water is colder, so the CO2 dissolves more readily into the water than in warm water, for example. So, the back and forth fluxes of carbon between the atmosphere and the ocean are huge. They are something like a thousand times higher than the back and forth fluxes with the solid earth. So, the unit that we use to talk about carbon or one set of units that we use is gigatons. So, giga means billion and ton in this case means metric ton which is 1,000 kilograms. So a gigaton of carbon is also about ten to the 15th grams, if you find that helpful. Or another way to think of it is, all of the people on earth probably weight about less than a gigaton. So there's much more carbon going back and forth between the atmosphere and the ocean every year by factor of a hundred or more of the total mass of all the people on earth. And this is much bigger than the flux to the solid earth. So the oceans are thought to be responsible for changes in atmospheric CO2 through the glacial-interglacial cycles which are documented from records preserved in ice cores in Antarctica and Greenland. So from little bubbles of air trapped in those ice cores, we know that atmospheric CO2 concentration is generally about 280 parts per million during warm times like today, during interglacial periods. And then during cold times, like the last glacial maximum 20,000 years ago, the C02 concentration was lower, about 200 parts per million. And there's this astonishingly good correlation between the C02 from the ice cores and temperature estimates from those same ice cores. It makes it very clear that CO2 plays a very strong role in Earth's climate. And the timing of the ice ages is extremely well correlated with fluctuations in Earth's orbit around the sun. And, in particular, what seems to matter is how bright the sunlight intensity is during the Northern Hemisphere's summer. The Northern Hemisphere is what's important because that's where the ice sheets have room to form. There's more land in the Northern Hemisphere than in the Southern Hemisphere. And the summer is important because that's when you decide whether to melt last winter's snow or keep it around and start to accumulate an ice sheet. So this is very good correlation between the orbital fluctuations and the amount of ice on earth. And so, 20, 30 years ago that seemed like the whole story. But then they discovered that the CO2 is also going up and down as the ice sheets are going up and down. So, this seems like it must be a sort of a positive feedback that the orbit of the Earth says let there be ice sheets and then the carbon cycle says, okay, if you're gonna have ice sheets I'm gonna pull the CO2 down and amplify that cooling from the ice sheets. And so that's what makes the ice ages global, actually, instead of just from one hemisphere or the other is the CO2 effect. So on time scales of thousands of years and longer, the ocean seemed to act like a positive feedback to the climate system. However just today we are releasing CO2 to the atmosphere, and so the oceans are taking up some of that CO2. And so on a timescale of decades today, the oceans are acting as a negative feedback. They're helping clean up. They're dampening, they're stabilizing Earth's climate. And so, it's kind of an open question whether on time scales of a thousand years or longer, the oceans might revert to their positive feedback role and amplify our climate excursion. [MUSIC]
