Extension 17.7: Planting Trees

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Energy, Ch. 17, extension 7 Planting trees 2 Fig. E17.7.1 The carbon cycle. (Adapted from Department of Energy data and Refs. 202, 203) Another, pictorial, view of the carbon cycle is shown in Fig. E17.7.1. (202,203) The net exchange with the oceans is 2.2 Gt and with plants on land is 1.7 Gt. While gross exchanges are much larger than human emissions, human emissions are greater than net exchanges. It is estimated by some that forests contain 1 teratonne of carbon (a bit higher than the estimates above). (41) The yearly fluxes are large, too. Carbon exchange between land and atmosphere is 120 Pg (120 billion tonnes, or 120 Gt), going both directions; exchanges between ocean and atmosphere total 90 Pg (90 Gt). (721) Only about half the CO 2 from
Energy, Ch. 17, extension 7 Planting trees 3 burning of fossil fuels, about 6.3 Pg (6.3 Gt), appears in the atmosphere, which means that the other half is going somewhere else. As the amount of carbon in the atmosphere increases, though, the amount that is exchanged will have to increase in step. By 2100, the exchanges are expected to be larger, and the ocean sink is expected to increase from 1.7 ± 0.5 Gt/yr (Reference 721) or 2.0 ± 0.6 Gt/yr (Reference 722) to 5 Gt/yr. Land, which is thought to have been a net carbon source at mid-twentieth century, is now estimated to be a sink (including a source term from deforestation of about 1.6 Gt/yr) of 1.4 ± 0.7 Gt/yr (Reference 721) or 1.4 ± 0.8 Gt/yr (Reference 722), and this total is expected to increase as well to around 5 Gt/yr. (721,722) The carbon will be there permanently, at least as measured in terms of human lifetimes. (85) How much carbon is stored or emitted by forests? There are several reasons for disagreements and uncertainties (such as indicated above and in the next section). However, in the context of the Kyoto Protocol, with possible offsets for carbon storage in forests, the situation requires clarification. (723) An experiment that combined satellite monitoring with a sophisticated ecosystem model (NASA-CASA) suggested that forests lose 1.2 to 1.3 Gt of carbon every year. (724) The highest losses are sustained by countries close to the Equator that are losing tropical forest such as Brazil, the Congo, and Indonesia. (26-29,31,42) The greatest gains in carbon storage are in the Northern Hemisphere, especially in the Unites States and Canada. (724) According to this report “the potential for ... carbon gains from forest area regrowth or expansion are dwarfed by the continuing losses from deforestation and fires.” (724) Work on Pacific Northwest forests that similarly combines satellite mapping and modeling showed that the forests used in the pilot study were a source of 1.13 tonnes of carbon per hectare per year, rather than acting as a sink. (725) A study at the Harvard
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Extension 17.7: Planting Trees

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