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Sunday, April 20, 2014

Where Has All the Carbon Gone?

By Scott Denning

Colorado State University
Most people know that burning carbon-based fuel produces CO2, and that all that extra CO2 in the air will warm Earth’s climate. But a lot of people don’t know that only half the CO2 from burning fossil fuel stays in the air, and that the other half “goes away.” Where does it go? How does it get there? How long will it stay there (wherever it goes), and what happens if it “comes back?” Most important, is there anything we can do to make more CO2 “go away” and to keep it gone?
People make over $300 billion per year buying and selling coal, oil, and natural gas, so they keep really good track of the stuff. We know from these sales that people burn about 10 billion tons of carbon each year. A billion tons is the mass of a cubic kilometer of water, and we call these gigatons (like gigabytes), abbreviated GtC. Measurements of CO2 all over the world show that only about 5 GtC/yr shows up in the air, and the growth rate of CO2 has stayed about half rate of fossil fuel burning for decades.
So where has all the carbon gone?
Some of it dissolves into the oceans. Dissolved CO2 in water is called carbonic acid, and it’s what gives soda or beer or wine a fizzy sparkle that complements cheese or chips. Carbonating the oceans is a bad idea, and ocean acidification could eventually prove deadly to plants and animals that make alkaline shells. More to the point, it’s really slow, because CO2 from the air can only dissolve into the thin layer of warmer water that floats on top of the ocean. More than 90% of the ocean water is extremely cold and dense and dark and sits quietly on the bottom where it doesn’t touch the air. Only near the poles in winter does the surface water get cold enough to sink carrying fossil CO2 into Davy Jones’ Locker.
Until 1963, nuclear weapons were tested in the atmosphere and produced a huge slug of radioactive CO2 that behaves exactly like all other kinds of CO2 and has been slowly dissolving into the oceans ever since. Radiocarbon (14CO2) from bomb tests is found throughout the surface oceans and thin plumes of it are slowly creeping along the bottom below the coldest parts of the Arctic and Antarctic seas. But the vast middle of the ocean has no bomb 14CO2, nor any other atmospheric pollutants. Most of the ocean hasn’t seen the air since the Middle Ages. It doesn’t know we’re here yet!
The rate of CO2 accumulation in the air accounts for about 50% of fossil fuel combustion. The rate of bomb 14CO2 uptake shows that the oceans take up another 3 GtC/yr, or 30% of fossil fuel emissions. So where’s the other 20%?
You’ve probably seen the green bumper sticker that reads, “Trees are the Answer.” It’s true that trees and all other plants eat CO2 for a living. Photosynthesis is the incredible process by which nonliving CO2 gas in the air is transmuted into living biomass using the energy of the sun.
Every year, photosynthesis transforms more than 1/7 of all the world’s CO2 in the atmosphere into leaves, stems, roots, wood, and crops. So how come we’re not worried that the biosphere will run out of CO2? Because unfortunately all living things eventually die, and that goes for plants too. When plants die or shed dead leaves they become food for animals or microbes which digest them to harvest all that stored solar energy, and then breathe the carbon back into the air as CO2. This planetary exhalation produces 1/7 of all the CO2 in the air each year, and if the rate of plant growth were balanced by the rate of death, the amount of carbon in the biosphere would stay constant.
Believe it or not, measurements show that plants and soils worldwide take up about 2 GtC/yr or 20% of fossil fuel emissions. Bluntly, the biosphere is growing faster than it’s dying! The total amount of biological carbon has been increasing for decades, despite massive tropical deforestation, clearing land for agriculture, suburban sprawl, and paving paradise to put up a parking lot. Despite what we see out the window, the planetary biosphere is expanding by billions of tons of carbon per year.
The simplest explanation for the net growth of land carbon stocks is that CO2 actually makes plants grow faster. Of course making extra plants by “CO2 fertilization” also provides more dead leaves and other microbe food, but the average lifetime of carbon in biological material is between 10 and 20 years (some kind of average between thousand year-old Bristlecone pines and the grass my dog dug up last week). As long as rising CO2 ramps plant growth up faster than decomposition, there’s a net uptake each year.
But most plants aren’t sitting around fat and happy waiting for additional CO2, as they might be in a greenhouse. Experiments in which whole outdoor ecosystems are exposed to high CO2 show that most plants are hungry for extra water nitrogen, light, warmth, space, or other things that limit their growth. For those plants, extra CO2 won’t help. Which is too bad, because CO2 fertilization is the gift that keeps on giving. It would be great if we could count on photosynthesis vacuuming up more and more CO2 as we burn more and more coal.
As it turns out, we’re also dumping huge amounts of nitrogen on the biosphere, both from fertilized crops and as a byproduct of urban and industrial air pollution. Hot combustion inadvertently burns some of the nitrogen in the air, making nitrogen oxides that stain the air yellow (“the brown cloud”). These nitrogen gases rain out downwind and act like dilute MiracleGro falling from the skies, and some of the growth of the biosphere is driven by this inadvertent fertilization.
When Europeans colonized the New World in the 18th and 19th Centuries, they converted huge areas from forests to farms, releasing vast amount of carbon in to the air. In the past century, much of that farm land has reverted to forest as people moved into towns and got office jobs. Every molecule of wood in New England was derived from atmospheric CO2, and we estimate that these regrowing forests sequester as much as 1 GtC/yr.
Ironically, our changing climate itself may be responsible for some of the terrestrial carbon sink. Growing seasons are longer in many parts of the world and have increased by 50% over the far North. In parts of the Arctic, woody shrubs that store a lot of carbon are invading landscapes that used to be tundra, storing lots of carbon.
The trouble with land storage of biological carbon is that it’s not nearly as reliable as the oceans. Fertilization only speeds up plant growth until nutrient demands are met, and can even harm plants if its overdone. Regrowing forests only suck CO2 out of the air until they mature and death catches up with annual growth. And a little Arctic warming may sequester carbon by growing shrubs, but too much could melt the permafrost, releasing more CO2 than decades of regrowth and fertilization could store.
It’s hard to predict what will happen to the Earth’s metabolism in the changing climate of the next few decades. Some ecologists think there’s still capacity for lots more growth of biomass. Others think the land is already about as full of carbon as it can get, and that climate change is more likely to release the fossil CO2 it’s already stored. It’s important to find out, because the fate of carbon stored on land has the potential to swing the balance of atmospheric CO2 over the next Century by as much as all the coal being burned in China!

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