Caroline Alden, BURN Contributor

Americans burn fossil fuels doing most of what we do every day – using electricity, driving to work, and buying food and goods. You probably know that the burning of fossil fuels results in the release of greenhouse gases, such as carbon dioxide (CO2), to the atmosphere. Maybe you’ve even calculated your carbon footprint.

But what about the big picture of carbon dioxide on Earth? How much CO2 is in the atmosphere already? Does it stay there forever? Leak out into outer space? (No.) Fall out of the sky as rain? (Also no.)

Carbon is everywhere, and the planet’s dynamic natural forces are continuously moving it from place to place. There are four major reservoirs, or stocks, of carbon on Earth: 1) in rocks (this includes fossil fuels), 2) dissolved in ocean water, 3) as plants, sticks, animals, and soil (which can be lumped together and called the land biosphere), and 4) as a climate-warming gas in the atmosphere.  (Check out the diagram below. Everyone loves dioramas, so it will henceforth be referred to as a diorama. You can do your best to envision it in 3D.)

carbonn cycle graphic

The Carbon Cycle (adapted from “Earth’s Climate: Past and Future,” by William F. Ruddiman)


In the carbon cycle diorama, the size of each reservoir  is expressed in GtC, and the transfer of carbon between reservoirs are written as GtC/yr. GtC stands for “Gigatons of Carbon”, which is the same as one billion tons of carbon.

One GtC/yr means one billion metric tons of carbon moved between reservoirs in one year. 

You can think of the carbon in each reservoir as a tiny building block – carbon is, after all, an atom. Under foot, carbon is a building block that helps create the structure of rocks and minerals. All around us, organic carbon forms the building blocks of life. In oceans and rivers, carbon is a building block of various molecules that exist together with H2O in all but the purest water. In the atmosphere, carbon is the central building block of several greenhouse gases, including carbon dioxide (CO2) and methane (CH4).


The biggest carbon reservoir on earth is in rocks, weighing in at some 66 billion metric tons of carbon. In very rare instances (as in roughly .004%), carbon in rock is in the form of coal, oil or natural gas. Most of the time it occurs as a chemical component of plain old granite, sandstone or limestone.

Carbon can leave rocks and enter the atmosphere. And, it can leave the atmosphere and go back into rocks.

Here’s how the first part works: rock-bound carbon enters the atmosphere via volcanoes, as shown by the yellow arrow in the diorama. (Apologies for not having drawn in a volcano; every good diorama should have one.)

For the second part: as wind and rain break down rocks over eons, CO2 is taken out of the atmosphere and put back into “rock” form as sediment (brown arrow).

The amount of carbon that enters and exits the atmosphere from volcanoes and into sediment each year is tiny compared to the amount that we emit by fossil fuel burning. Tiny, as in volcanoes typically emit less than 1/100th the amount of CO2 that humans emit every year.

For the record, human fossil fuel emissions may have  hit 9.7 billion metric tons of carbon in 2012 (emissions are still being tallied).

When fossil fuels are burned, the CO2 released enters far less stable reservoirs: first the atmosphere, and from there the trees and plants around us, and the ocean. Let’s look at how those reservoirs function, and what happens when they can’t handle any more carbon.


The next-biggest reservoir for carbon on Earth is the ocean. Scientists tend to split the ocean into two ‘pools,’ like a two-layer cake. The top layer goes from the surface to 100 meters down. Wind sloshes the water around, allowing CO2 gas to exchange with the atmosphere.

The bottom layer – or deep ocean – is bigger and less exposed to the atmosphere, and is therefore a good long-term storage place for large quantities of carbon.

Carbon moves between the ocean and atmosphere by diffusion. When the level of CO2 in the atmosphere increases, some of it dissolves into ocean water.

Now, back to our diorama. Notice that the value of the white “into-the-ocean” arrow is slightly bigger than that of the blue “out-of-the-ocean” arrow. This indicates that the ocean is sucking up excess carbon from the atmosphere.

It is fabulously useful that the ocean absorbs some of the excess CO2 in the atmosphere. Due to this imbalance, the oceans have been offering us a major (25%) global warming discount every year. In other words, 25% of fossil fuel carbon we emit gets drawn into the ocean for good. If the ocean weren’t such a sink for CO2, more would remain in the atmosphere, and more global warming would be happening.

There is a very bad downside to this discount, however. When ocean water absorbs carbon, it becomes more acidic. Hence the current degradation of the world’s coral reefs.

Furthermore, this discount won’t last for too much longer. The ocean’s chemistry will soon hit a threshold where it will stop absorbing CO2. When that day comes, we’ll have to reckon with much more global warming impact from each coal reserve and tank of gas that we burn.


The final reservoirs for carbon are the atmosphere and the terrestrial biosphere. As you can see in the diorama, they hold roughly equal amounts of carbon – a quantity close to that of the surface ocean.

Plants draw CO2 out of the atmosphere during photosynthesis. CO2 is plant food. During the night, some of that CO2 is returned to the atmosphere. When plants die and decompose, all of the rest of that CO2 is either returned to the atmosphere or turns to organic matter in soil.

Before the industrial revolution, the atmosphere contained roughly 600 GtC. As of March 2013, that number had risen to 843 GtC: a 40% increase. If the world’s oceans and plants hadn’t been sucking excess CO2 out of the atmosphere all these years, the increased burden of CO2 in the atmosphere could be something closer to 60 or 70%.

You’re rereading that sentence, aren’t you? Yes, that’s right. It’s not just the oceans; land plants are also sucking more CO2 out of the atmosphere than they are emitting back to the atmosphere. In fact, plants and the sea combined provide a 50% discount on emissions… as in, if these natural systems weren’t absorbing CO2, global warming would be twice as bad. This is an unbelievable stroke of good fortune for humans today.

The plant half of the discount is occurring because plants like a little bit of extra CO2 in the air – they use CO2 like we use food.

Sadly, though, we are close to reaching a level of atmospheric CO2 where plants will stop absorbing excess carbon from the atmosphere. Like a kid in a candy store, even plants hit the wall at some point and can eat no more.

As you can see, once carbon is unlocked from long-term storage as fossil fuels, that carbon goes into the atmosphere, land plants, and the surface ocean. One small forest fire, and all of the carbon stored in land plants returns to the atmosphere again to increase global warming. One Gigaton too many into the oceans and their waters will stop absorbing CO2.

The carbon cycle represents a vast and delicate balance. It seems clear that the safest option is for fossil fuels to stay deep underground where nature stored them millions of years ago.

Caroline Alden is a graduate student at the Institute of Arctic and Alpine Research in the Department of Geology at the University of Colorado at Boulder.