An introduction to climate change - Climate Change: Biological and Human Aspects - page 18

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Fig. 1.5 Thegrowthinatmosphericcarbondioxide.Thegraphshowsmeasurementsofairtrappedinice(icecore

measurement,),andmorerecentdirectatmosphericmeasurementstakeninHawaii(solidblacklineonthe
right).

20th century) there has been direct measurement of atmospheric carbon dioxide.
Second, there has been historic measurement of the bubbles of air trapped in ice caps
(mainly from either Greenland or Antarctica) as snow has fallen and then turned to
ice. Together these show a continuous growth in the atmospheric concentration from
around the time of the Industrial Revolution to the present (see Figure 1.5).
Much (but, remember, not all) of this growth in atmospheric carbon dioxide is
due to the burning of fossil fuels. In essence this represents a short-circuiting of the
aforementioned deep carbon cycle: the part of the carbon cycle that takes millions of
years to complete. For while, as discussed above, much of the annual movement of
carbon dioxide is due to respiratory and photosynthetic processes and contributes to
the fast carbon cycle, there is also a slower accumulation of about 1 billion t of carbon
a year in marine sediments and more being trapped terrestrially in soils and wetlands.
After millions of years much of this ultimately ends up as coal and oil, the larger
deposits of which we have only recently (in geological terms) mined and burned as
sources of energy. We are burning these fossil fuels at a far faster rate, thousands
of times faster, than they are currently being formed, for the process of fossil fuel
formation continues today. It is as if there are two carbon cycles within the overall
carbon cycle. One is driven by photosynthesis, respiration and forest fires and operates
over a short period of time. The other is the deep carbon cycle operating in so-called
deep time of many millions of years and is driven by the geological formation and
entrapment of fossil fuels and the tectonic subduction of carbon-rich sediments at the
edge of some plates followed by the emission of carbon from volcanoes.
There is currently much research being done to elucidate the carbon cycle, and
to measure or infer carbon sinks (Houghton, 2002). This work might be grouped as
follows.