An introduction to climate change - Climate Change: Biological and Human Aspects - page 11

(Kuo et al., 1990). This confirmed that carbon dioxide and global temperature over
that period were significantly statistically correlated to over 99.99%. This is to say
that were 10 000 alternative copies of the Earth similarly measured, only one would
give similar results due to sheer chance and 9999 would give results because there is
a link between carbon dioxide concentrations and global temperature. But before we
look at how the human addition of carbon dioxide to the atmosphere affects climate
we need a better understanding of the natural sources and sinks of atmospheric carbon
dioxide. Fundamental to this is the carbon cycle.

1.3 Thecarboncycle

Carbon is one of the fundamental elements necessary for life. It is found in virtually
all molecules (but not quite every molecule) associated with life. These include all
carbohydrates, all proteins and all nucleic acids. As such, carbon is fundamental
to biological structures, of both micro- and macro-organisms, including plants and
animals; for example, lignin in plants and cartilage and bone in animals. Indeed
biomolecules, as we shall see in Chapter 2, can be of great use to palaeoclimatologists
because some of them (and hence the remains of species in which they are found)
can be used as climatic indicators.
The carbon cycle itself refers to the circulation of carbon in the biosphere. The
circulation is driven primarily (but not solely) by biological processes. A planet
that does not have any biological processes would have carbon flow through its
geosphere 3 driven solely by geophysical processes. On Earth carbon, in the form
of carbon dioxide, is fixed by photosynthesis into organic compounds in plants and
photosynthetic algae and returned to the atmosphere mainly by the respiration of
plants, animals and micro-organisms in the form of carbon dioxide, but also by the
decay of organic material in the form of both methane and carbon dioxide. Abiotic
drivers include the burning of organic material, be it natural (e.g. forest fires) or
through human action (e.g. the burning of firewood or fossil fuels). Another abiotic
driver is that of plate tectonic movement. This contributes to the so-called deep
carbon cycle operating on a scale of millions of years. Tectonic plates in the process
of subduction carry with them organic sediments down into the Earth's mantle.
This plate movement results in volcanic activity that in turn converts the organic
sediments to carbon-containing gases (again mainly carbon dioxide but also other
volatile compounds) that come to the surface via volcanic and related activity. There
is a variety of other abiotic processes, including the chemical oxidation of methane
in the atmosphere to carbon dioxide as well as processes (which frequently also
accompany biotic ones) in organic sediments. An estimate of the principal carbon
movements in the carbon cycle is given in Figure 1.3.
The carbon cycle is at the centre of biology's relationship with the global climate
(and hence global climate change). More than this, it demonstrates the importance of
both the molecular biological sciences and the whole-organism approach to biology.

3

On a lifeless world there is no biosphere, only a geosphere.