Carbon cycle


House: The carbon cycle is, very simply, it’s about the cycling of carbon through natural systems – through plants, through soils, through the ocean – and back out into the atmosphere. Le Quéré: In the natural carbon cycle, there’s a lot of fluxes of carbon dioxide, so the carbon goes in and out of the ocean, in and out of the terrestrial biosphere every year. House: The carbon is constantly flowing between these different systems and large amounts of carbon moves all the time. Le Quéré: I mean in the terrestrial biosphere, in the trees and the forests, it’s very easy to see. If you live in a place that has a forest area with seasons, you see in the winter the trees they have no leaves, and the spring comes and the leaves build up. This is all good carbon dioxide that goes in the leaves. And in the fall and in the autumn when the leaves fall down then their carbon is emitted back in the atmosphere.

So you have a huge signal there of CO2 going in and out of the atmosphere. House: So the ocean will take up the CO2, it dissolves in the surface of the ocean and also when the ocean will release CO2 to the atmosphere and that depends on the concentration of CO2 in the atmosphere and the concentration of CO2 in the ocean. And they form a balance with each other. There’s a continuous massive exchange of carbon dioxide between the atmosphere on land and the atmosphere on the ocean. That is roughly in balance until we introduce human change. Osborn: The experiment that we’re inadvertently perhaps conducting with the climate system is to move huge volumes of carbon from these stores undergrounds in the form of fossil fuels and bringing them to the surface and burning them and adding this carbon to the atmosphere. Le Quéré: What we’re doing now is putting everything out of balance, so we’re adding carbon to the atmosphere. It’s new carbon. It’s not part of the natural cycle.

It’s one that we’ve dug out of the fossil reservoir where they were stored, and we’ve put them back in the atmosphere. This is new carbon, and it puts the system out of balance. House: Although the human emissions are much smaller than the natural fluxes, the natural fluxes approximately are in balance and so they’re not causing an increase of carbon dioxide in the atmosphere. The human emissions, however are very rapid, and the natural systems don’t have time to respond to them. And so you get a net imbalance of raised carbon dioxide concentrations in the atmosphere. Lunt: It’s unequivocal that the amount of carbon dioxide in the atmosphere is increasing and is increasing fast and is increasing faster than ever. House: Oh the rate of change now is incredibly rapid, and what’s more it’s pushed us outside the bounds of what we’ve seen in terms of atmospheric concentration throughout the Ice Ages. Thompson: We have not had levels of C02 at 400 parts per million by volume in 800,000 years of history. House: In the Earth’s past throughout in and out of the Ice Ages, the concentration of CO2 in the atmosphere ranged between about 180 parts per million to 280 parts per mission.

And it took thousands of year for it to change between those states. The difference is now it’s gone up to 350 and even topping 400 parts per million on a single day basis. And that’s happened over a period of a couple hundred years. Friedlingstein: Every single generation is emitting more than the previous generation because emission of CO2 increased exponentially. We emit it so far, if you start from the beginning, which is like the industrial revolution in 1750 or something, when we start to burn fossil fuel, from that time up until today we emitted something like 2000 gigaton of CO2. More than half of this has been emitted over the last 50 years. Thompson: And we know where that CO2 is coming from because we do the isotopes of the carbon. We know it’s coming from fossil fuels. Le Quéré: So carbon is increasing in the atmosphere, but it doesn’t entirely stay there, so about half of the emission and maybe a bit more than half of the emission that we put in the atmosphere ends up in the natural environment. It ends up in the ocean and in the forest. Friedlingstein: For the carbon cycle today absorbed about half of the emissions we put in the atmosphere, so we emit, as I said, 40 gigaton of CO2 per year, about half of it, 20 gigaton of CO2 are taken back from the atmosphere by the land and by the ocean.

House: There’s a multitude of different processes that remove carbon dioxide from the atmosphere. So for example, CO2 from the atmosphere dissolves in the surface of the ocean and then that’s turned over and taken into the deep ocean. Really for that amount of CO2 to be completely removed from the atmosphere it has to be completely dissolved and go down into the deep ocean. And then we’re talking about geological timescales – so hundreds and thousands of years. Le Quéré: So what happens when we put carbon emissions into the atmosphere, new carbon from burning fossil fuel or from different station, what happens is this takes a long time for this carbon to readjust in the land and ocean. Eventually if we’re prepared to wait long enough, so that’s thousands of years, a lot of this carbon, maybe 70 percent will end up in the ocean, and the reason this takes time is that you have different adjustment times, so the CO2 goes in the surface ocean, it takes about 1 year to dissolve. But how it is transported from the ocean’s surface to the intermediate and to the deep ocean depends on the ocean circulation.

The ocean circulation takes hundreds to a thousand years to mix the entire ocean. That’s the timescale that is really relevant here is taking a molecule of CO2, we’ve put it in the atmosphere, how long is it going to take before it ends in the deep ocean? House: So about 65 to 80 percent of the carbon dioxide pulse that’s put into the atmosphere will be removed within about 2 to 200 years. The rest of it, the remaining 35 percent, will take between 2 and 20 millennia to be completely removed from the atmosphere. So roughly you have to think whatever we’re doing today, whatever CO2 is being emitted, roughly a third of it is going to stick around essentially forever really when you consider it in our lifetime. Pelto: We can’t change the atmosphere, the chemistry, with one of the main constituents carbon dioxide by 25 percent and expect nothing to happen. You change your diet by 25 percent. You decide you’re going to start consuming 25 percent more calories, and you don’t change your exercise or anything else. You can’t realistically expect nothing to happen. And that’s what you have to understand.

If we change fundamentally our atmosphere chemistry, we can’t expect climate to stay the same..