Climate Simulation and Prediction - Climate Dynamics - page 184

Table 12.1 . Some estimated global methane sources and sinks

Natural sources Anthropogenic sources Sinks

(TgCH 4 /yr) (TgCH 4 /yr) (TgCH 4 /yr)

Wetlands 174 Fossil fuels 172 Tropospheric OH 467

Termites 22 Rice agriculture 54 Stratosphere 39

Ocean 10 Biomass burning 47 Soils 30

Geologic 9 Ranching/ruminants 84

Forests 5 Landfills/waste 54

Total 220 Total 411 Total 536

Source : Based on Table 7.6 in IPCC, 2007.

OH concentrations. (The OH radical is very difficult to observe because it reacts
quickly in the atmosphere. Its levels are inferred by comparing emission levels and
atmospheric levels of gases for which reactions with OH are the primary sink,
such as methyl chloroform. A trend in OH has not been identified.) Other sinks
of atmospheric CH 4 are oxidation in soils and migration into the stratosphere.

THE NITROGEN CYCLE

For the climate change problem, global cycling of nitrogen (N) is modeled to
understand how the concentration of atmospheric N 2 O is determined. The ni-
trogen cycle must also be studied to understand the formation of NO x com-
pounds for pollution studies. The nitrogen cycle consists of myriad paths and
processes as nitrogen in various compounds passes among plants, animals, soils,
atmosphere, and water. A brief overview is provided here.
The atmosphere is 78% molecular nitrogen (N 2 ), which is largely inert.
This molecular nitrogen enters the nitrogen cycle when it is deposited into
the soil by rain and, to a lesser extent, in dry deposition. Legumes (e.g., peas,
beans, alfalfa, clovers, vetch), blue-green algae, and lichens convert molecular
nitrogen into forms that plants can use, primarily ammonia (NH 3 ), to produce
proteins and amino acids. This nitrogen fixation is accomplished by bacteria,
for example, those that live in legume root nodules or freely in the soil. Other
nitrifying bacteria in the soil convert ammonia into nitrites and nitrates, forms
of nitrogen that plants can use to produce proteins and amino acids. The ni-
trogen cycles through animals when they eat plants, and their residues (excre-
ment and bodies) are decomposed by bacteria and fungi. Nitrogen is lost from
this soil-based cycle in denitrification, , the process by which bacteria convert
nitrogen to nitrous oxide (N 2 O) and others forms of nitrogen for emission to
the atmosphere; in ammonia volatilization , which is essentially the evaporation
of ammonia gas from urea on the surface; and in leaching, when nitrates are
washed below the root zone.
A similar nitrogen cycle occurs in the ocean. Soils covered by natural vegeta-
tion are estimated to produce 6.6 TgN (teragrams, or 10 12 g, of nitrogen) each
year, and the oceans add approximately 3.8 TgN/yr. These surface emissions