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The Terrestrial Carbon Cycle

Terrestrial versus Oceanic Biosphere
The bulk of carbon on land is locked up in soils and in trees, with each source respectively representing the "soil carbon" and "biosphere" reservoirs. Of course, living things in the ocean are also part of the biosphere, but the amount of carbon they contain is small when compared with the mass of carbon in wood. In addition plants on land appear, for some reason, to be about twice as efficient in fixing carbon during photosynthesis than photosynthesizing organisms in the ocean.

That being said, it is not so easy to make a comparison between ocean and land carbon reservoirs in a meaningful way. For example, on land where so much of the carbon moves through wood, we can measure "productivity" fairly simply. It can be thought of as the mass of carbon in trees, divided by their average age (about 40 years). To this you have to include considerations of greening and decay, both of which can be seen in changes in the amount of carbon dioxide in the air over the course of a year, as we have mentioned previously. In contrast, ocean measurements of productivity are much more difficult. One reason is because many of the carbon-fixing organisms in the ocean are extremely short-lived.

So, is there even a purpose in comparing the fixation of carbon by photosynthesizing bacteria and other phytoplankton in the ocean with the fixation of carbon in wood on land? In the opinion of this instructor, there isn’t. What do you think?

Changing CO2 and the Terrestrial Response
There are two carbon cycles of interest on land: The cycle involving annual growth and decay, and the cycle involving long-term storage of carbon in wood and in the plant remains in soil and surface-near organic deposits. Both cycles have the atmosphere as intermediary. Although that is interesting, what we really want to know is this: How will the terrestrial biosphere and soil carbon, which has plenty of bacteria living in it, respond to global warming, and how will this feed back into the climate?

The curve of seasonal CO2 amplitude has increased over the last 45 years from about 5 ppm CO2 in the late 1950’s to over 6 ppm CO2 at the end of the 1990’s. Source: Keeling, Chin and Whorf in Nature 382: 146-149, 1996.
In the first cycle, whatever decays returns carbon dioxide to the atmosphere, a reservoir from which carbon dioxide can be extracted for renewed growth. This atmospheric sensitivity to land plant growth and decay is evident from one of the Keeling curves. Upon close inspection of the annual cycles in the figure below, you can see that the amplitude of the annual cycles increases with time. How should we interpret this? The favored interpretation is that the terrestrial biosphere is growing at an increasing rate; meaning there is more greening in spring and more decay in fall. This is particularly true of the northern hemisphere, which contains most of the world’s landmass. It is difficult to see how tropical forests could be expanding because we see them burning and disappearing at an enormous rate in the satellite surveys. Also, because they are in the tropics where there is not much seasonal variation, they would not show a very strong seasonal signal. Therefore, we must conclude that most of the observed biosphere expansion comes from temperate and northern forests.

What limits forest growth in temperate and high-latitudes? First and foremost is human interference. If you fly over North America or western Europe you can see the remaining patches of what were once great forests that covered vast regions. The most recent striking examples of deforestation can be seen on the razed mountain slopes of what were once forestlands in Oregon. In Europe serious deforestation has gone on for the last thousand years. (Recall that the Romans had problems getting anywhere north of the Alps, except along the Rhine, because they were stopped by impenetrable forests. Today, they could march to Lithuania in a month.)

So with all this tree cutting, why does the figure above show that the terrestrial biomass is expanding? Something must be disguising the observed trend of deforestation, or maybe there is some compensating process making it appear as though the biosphere is getting bigger. Maybe it is more vigorous growth (and decay) of annuals, deciduous trees, and bushes that are responsible for the increase in amplitude of the Keeling curve shown above.

Plant Growth Factors and Greening
Indeed, there is evidence that high carbon dioxide contents in air stimulate plant growth. This is because every plant has a bit of a problem balancing its need for letting carbon dioxide into its photosynthetic factories without letting water inside the plant escape as a result of the plant opening its pores (called “stomata”). Water is a limiting factor in plant growth over wide areas of most landmasses. So, if there is more carbon dioxide in the air, the pores, or stomata, on plant leaves do not need to open as much to get the same amount of carbon dioxide. That way, water can be retained a lot better in the plant. This allows the plant to grow more vigorously in places where water is a limiting factor.

Plot of the increase in green vegetation during the growing season (May through September) between 1982 and 1990 for latitudes north of 27.5°. Green refers to regions where the amount of vegetation increased by up to 5%; pink to a 5-25% increase; red to a 25-55% increase; and orange to a >55% increase. From: R.B. Myneni et al. Increased plant growth in the northern high latitudes from 1981-1991. Nature, 386:698-701, 1997.
Nutrients, like nitrogen, are also a limiting factor in plant growth. There is evidence now for increased concentrations of nitrogen compounds in rainwater due to human pollution. Possibly, therefore, there is large-scale nitrogen fertilization going on caused by the changing chemistry of rainwater. In addition, pollution can cause rain to be more acidic. Such acidic rain could conceivably mobilize nutrients in the soil that normally would be in short supply. Yet another possibility is that precipitation and evaporation patterns have changed due to global warming, favoring an overall increase in annual growth and decay.

Ultimately, the most likely explanation for the increased amplitude in carbon dioxide fluctuations in the figure above is that the growing season has lengthened due to global warming. Plants now just spend more time within the growth season in the latitudes where winter shuts things down until spring. This earlier greening of the northern hemisphere is already evident in satellite surveys. Earlier greening can be a good thing, especially if one is trying to grow wheat in cold places. However, there is also a downside. In temperate and high latitudes as the seasons change in response to warming, the programming of the trees (telling them “it is time to shed leaves when the days are short") will be out of synch with reality. Opportunistic shrubs that are not so programmed and tend to hang on until it gets too cold will be at an advantage. So will plants adapted to warmer climates. Trees do not spread across landmasses very fast. Thus, quite likely, we will see increased turmoil in the plant world, with weakened tree stands increasingly susceptible to infestation and fire.

The Soil Cycle
In the second cycle, that in which plant debris are deposited and buried in the soil, global warming is expected to increase the rate at which bacteria and fungi digest the deposited organic material. This is undoubtedly true for the portion of soil carbon that, until now, have always been frozen or close to freezing, like the vast areas of tundra and peat deposits of high northern latitudes. Thus, many scientists think that the response of soil carbon will be a positive feedback on warming, making our climate even warmer.

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