that screens most of the ultraviolet energy form the lower atmosphere, and (2) the
buildup of breathable oxygen (O 2 ) as a chemical byproduct of photosynthesis
that was allowed once the ozone layer was established. Additionally, oxygen
enters the atmosphere as part of water vapor during evapotranspiration and via
the weathering of rocks (oxygen is the most prominent component of rock at the
planetary surface). Oxygen leaves the atmosphere through the precipitation of water
and inhalation by animals. At present, Earth’s atmospheric oxygen supply is stable
and there are no worries that it will become scarce in the foreseeable future.
The nitrogen cycle is interesting in the way it provides useful nitrogen into
plants and animals. Nitrogen forms the bulk of the atmospheric gases and is, yet,
not usually useful to life until it is fixed into one of a series of compounds called
nitrates. Plants can directly take up these compounds to supply life processes.
Only a tiny fraction of gaseous nitrogen is fixed. On land, most nitrates are produced
by bacteria in the root nodules of certain plants and in the soil itself. Exotically,
cosmic rays and lightning both produce small amounts of nitrates. Also, minor
amounts are fixed by marine life. Fixed nitrogen can return to the atmosphere by
denitrification caused by bacteria. Humans manufacture and concentrate nitrates
as fertilizer for agricultural purposes. Runoff from fields and other fertilized surfa-
ces has had unintentional, deleterious effects on life in streams and small water
bodies because increased nitrate concentrations decrease dissolved oxygen.
The carbon cycle is significant to life and has had major interference from
human activities. Carbon, hydrogen, and oxygen atoms in combination are build-
ing blocks for myriad organic molecules. Carbon enters plants via photosynthesis
and animals via the consumption of plants. Carbon-based carbohydrates include
chemical energy derived from solar radiation. As animals consume plants this car-
bon is passed through the web of life. Animals exhale this carbon dioxide. Organ-
ism death and decay result in the release of carbon to the atmosphere. Inorganic
sources such as the outgassing of volcanoes and the weathering of certain rocks
also provide carbon back to the atmosphere. Every burning process, natural or
human-related, combines oxygen and carbon that becomes carbon dioxide.
Of great concern are the agricultural, industrial, and transportation processes
now releasing carbon dioxide into the atmosphere in amounts that surpass natural
sources. Since the start of the Industrial Revolution in the middle 1800s, the car-
bon dioxide content of the atmosphere has risen by over a third. Although intake
by photosynthesis and solution into oceans buffer the planetary atmosphere from
containing all of this carbon dioxide, it is unknown how much capacity these
“sinks” possess. Such an increase in carbon dioxide is not toxic nor will it decrease
breathable oxygen in the atmosphere but carbon dioxide is one of the primary
greenhouse gases such that humans may be causing unintended impacts on Earth’s
energy budget.30 Biogeochemical Cycles