other components that can influence the energy flow, with the amount
determined by human activity. Carbon dioxide accounts for over half of
human-induced warming. Two components that are relatively minor in
amount but significant due to their ability to absorb heat, are nitrous oxide
and sulphur hexafluoride. Some contributors occur exclusively due to
human production, such as chloroflurocarbons and halocarbons, which
were once used primarily as refrigerants. An international treaty banned
these chemicals in 1987 as their presence in the atmosphere led to a
decrease in the amount of ozone.
Numerous processes involving the soil, atmosphere, and living organ-
isms regulate the concentrations of these gases, with the flow of carbon
dioxide termed the carbon cycle (Figure 2.10). In the cycle there is a bal-
ance between two processes. Respiration results in the uptake of oxygen
and output of carbon dioxide. Photosynthesis results in the uptake of car-
bon dioxide and production of oxygen. These natural processes absorb
large amounts of carbon dioxide but the additional contributions from the
burning of fossil fuels result in a net increase in the amount of carbon
dioxide in the atmosphere; hence the amounts of greenhouse gases in the
atmosphere increase. Since the Industrial Revolution, the atmospheric con-
centration of carbon dioxide is estimated to have increased by about 30%,
methane concentrations have doubled, and nitrous oxide concentrations
have gone up by 15%. Sulfur aerosols result in a cooling of the atmosphere
due to their ability to reflect sunlight back into space but these gases are
short-lived in the atmosphere.
Figure 2.10A schematic representation of the carbon cycle. Units: billion
metric tons measured in carbon equivalent terms. Modified from
International Panel on Climate Change (2001).
42 PARTI THERMODYNAMICS AND KINETICS
Natural flux Anthropogenic flux
Vegetation
and soils
2000
Ocean
38,000
Atmosphere 730
Fossil fuel
combustion and
industrial
processes
Global gross
primary production
and respiration
Carbon flux indicated by arrows:
119
12 0
88
90
6.3
Changing
land use
1.7
1.9
