The Carbon Cycle and Atmospheric Carbon Dioxide

Besides the water cycle also the carbon cycle is fundamental for the Earth's climate. All Earth system components contain carbon and exchange it rapidly or slowly causing major changes of climate. The most active part of the carbon cycle is the exchange between the atmosphere and the biosphere, both on land and in the sea. As figure 2.3 shows the reservoir atmosphere with about 760 GtC looses about 120 ± 70 GtC per year because carbon dioxide (CO2) is

Global Cycle Carbon Transport
Figure 2.3: Carbon fluxes between the reservoirs in GtC and GtC/a, respectively (IPCC, 2007a)

taken up by terrestrial plants or enters the sea, where part of it is used by algae for biomass production. Nearly the same amount is going back to the atmosphere because of out-gassing from the ocean and respiration by plants, animals and humans. Thus, the undisturbed carbon cycle is nearly balanced, as only 0.2 GtC/a are buried in sea sediments and a still not well known small portion (0.4 GtC/a) enlarges peat bogs or is emanating from or buried in deep soils. From the above we learn that the lifetime of carbon dioxide in the atmosphere is rather short, about seven years, and the question arises "Why is there a problem with additional CO2 as "only" about 8 GtC/a, roughly one percent of the reservoir content, are emitted by human activities?" As Table 2.3 clarifies 4.1 ± 0.1 GtC/a of these emissions remain in the atmosphere, hence increase the atmospheric carbon reservoir by about half a percent per year. Only 2.2 Gt of the anthropogenic carbon enter the ocean per year and thus are taken away for hundreds of years. Would we stop emissions only about 15 percent of all former anthropogenic emissions will in the long-run stay in the atmosphere. Table 2.3 contains a further astonishing fact: In the 1990s the existing terrestrial biosphere grew, by about 2.6 ± 1.7 GtC/a, more than the emissions from deforestation and other land use change that reached 1.6 ± 1.1 GtC/a. Overall, the carbon stored in the biosphere increased in recent years.

How long will this favourable situation last? No answer can be given yet but coupled carbon cycle and climate models, forerunners of the emerging Earth system models, show at least that there is the possibility for a sign change in the latter part of the 21st century. Then climate change impacts are much stronger and the carbon dioxide fertilization effect draws down carbon dioxide levels in the atmosphere less as climate change puts more pressure on terrestrial and marine ecosystems, which then reduce the CO2-uptake. The lesson from this section: the net fluxes into the ocean interior count and not gross primary production on land or fluxes from the atmosphere across the ocean surface. While the carbon cycle was nearly balanced before industrialization, it is strongly imbalanced now with a "hangover" of more than 3 GtC/a for the atmosphere.

Table 2.3: Present anthropogenic carbon emissions and heat fluxes; all in GtC/a (IPCC, 2007a)

Period

1980s

1990s

2000 - 2005

Atmospheric increase

3.3 ± 0.1

3.2 ± 0.1

4.1 ± 0.1

Emissions (fossil + cement)

+5.4 ± 0.3

+6.4 ± 0.4

+7.2 ± 0.3

Net flux ocean/atmosphere

-1.8 ± 0.8

-2.2 ± 0.4

-2.2 ± 0.5

Net flux land/atmosphere

-0.3 ± 0.9

-1.0 ± 0.6

-0.9 ± 0.6

Land use change flux

+1.4 (0.4 to 2.3)

+1.6 (0.5 to 2.7)

n.a.

Residual terrestrial sink

-1.7 (-3.4 to 0.2)

-2.6 (-4.3 to -0.9)

n.a.

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