Geography Reference
In-Depth Information
most common element in the universe and is essential for life.
Carbon moves through and across the Earth in several forms and
via a number of processes in what is known as the carbon cycle
(Figure 2.4). The carbon in your body has been recycled millions
and billions of times forming parts of lots of other organisms, rocks,
water bodies and the atmosphere. The natural carbon cycle
involves the transfer of carbon from the atmosphere to the land and
ocean where it remains in forms such as living matter, rock, soil or
dissolved compounds, before eventually returning to the atmo-
sphere. The total amount of carbon on Earth remains the same
through time, but the same carbon exists in different forms. The
carbon cycle operates over both long (millions of years) and short
(hours/days) timescales. There is much variability in how long
carbon will be stored in particular locations but on average a
carbon atom will stay in the atmosphere for five years and in the
oceans for 400 years.
One of the main mechanisms for the transfer of carbon out of
the atmosphere is plant photosynthesis whereby the Sun's energy
helps plants transform carbon dioxide into carbon and oxygen. The
carbon helps form the structure of the plant material while the
oxygen is immediately released into the atmosphere. Most biologi-
cal matter is about 40 to 60 per cent carbon when it is dried and
the constituent parts are weighed. Carbon is transferred when
animals eat plants (see Chapter 5). Respiration uses oxygen to
break down organic matter into carbon dioxide and water. Unlike
photosynthesis (which only occurs in plants) respiration occurs in
plants and animals. For animals, respiration is the breakdown of
food by oxygen to release the energy stored by the food and this
produces carbon dioxide. Death of plants and animals means that
carbon can be returned to the ground. Some of this is very quickly
returned to the atmosphere by fire or decayed by bacteria which
form carbon dioxide, whereas some may be stored in the soil for
longer periods. Temperature is an important factor controlling the
carbon cycle. Rates of decay are much faster in tropical environ-
ments and slower in cold environments. The amount of carbon
that is lost from the atmosphere through photosynthesis and then
released back to the atmosphere is very large. This means that the
annual carbon dioxide concentration in the atmosphere fluctuates
during each year according to peak periods of growth and decay.
