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530 pptd

102 years

15,800

a Within the troposphere (Fig. 1.1b) estimated on a global scale.

b Relative capacity of a ''greenhouse gas'' to absorb outgoing infrared radiation within the next century, as determined by the Intergovernmental Panel on Climate Change (Houghton et al., 1996).

d Not applicable to gases and aerosols that have nonuniform distributions in the global atmosphere.

dppt (parts per trillion: 0.000000000000001).

Data Sources: Intergovernmental Panel on Climate Change (Houghton etal., 1996) and NASA Goddard Institute for Space Studies (http://www.giss.nasa.gov/data/si99/ghgases).

a Within the troposphere (Fig. 1.1b) estimated on a global scale.

b Relative capacity of a ''greenhouse gas'' to absorb outgoing infrared radiation within the next century, as determined by the Intergovernmental Panel on Climate Change (Houghton et al., 1996).

d Not applicable to gases and aerosols that have nonuniform distributions in the global atmosphere.

dppt (parts per trillion: 0.000000000000001).

Data Sources: Intergovernmental Panel on Climate Change (Houghton etal., 1996) and NASA Goddard Institute for Space Studies (http://www.giss.nasa.gov/data/si99/ghgases).

dioxide contributes most to the greenhouse effect because of its relatively high concentrations, long atmospheric lifetime, and broad absorption of infrared radiation (Table 8.1 and Fig. 8.8b).

Like other greenhouse gases that are naturally produced (as opposed to chlo-rofluorocarbons, which are human engineered), carbon dioxide is coupled directly with biological processes [Eq. (1.1)]. For carbon dioxide, this biological connection is well illustrated by the seasonal photosynthesis of leaves on deciduous trees that blossom in spring and drop onto the ground in autumn, tangibly converting carbon dioxide into biomass. This seasonal cycle of photosynthesis has been recorded continuously from both hemispheres—inhaling and exhaling throughout the Earth system—since the mid-20th century (Fig. 8.9).

Atmospheric measurements from Mauna Loa, Hawaii, also reveal that the global concentration of carbon dioxide has been increasing during the past five decades (Fig. 8.9). Telescoping backward in time, ice-core records from Antarctica further indicate that global carbon dioxide concentrations have been increasing exponentially (Fig. 8.10) along with the human population since the mid-19th century (Fig. II). Moreover, the current concentrations of carbon dioxide (around 365 parts per million by volume) are the highest at any time during the past 420,000 years (Fig. 7.4).

These recent atmospheric changes in carbon dioxide are largely related to the industrial burning of fossil fuels, which has added previously buried carbon dioxide to the atmosphere. Carbon dioxide increases have been further compounded by the removal of forests, grasslands, and other plant communities that a

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