Geoscience Reference
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Fig. 5.10
Theworldpopulation1950-2002.
the official scientific consensus, it is worth summarising their conclusions with regards
to the principal greenhouse gases in the current century.
5.4.1 Carbondioxide
As noted in Chapter 1, the human contribution to atmospheric carbon dioxide is
increasing. The breakdown of the principal sources of this carbon dioxide is given in
Table 1.3.
The IPCC's 2001 assessment concludes that carbon dioxide emissions from fossil
fuel burning are 'virtually certain' to be the dominant influence on changes in atmo-
spheric concentrations for the 21st century. Further to the greenhouse gas information
given in Chapter 1, the reason for this dominance is 2-fold. First, our lifestyles are
becoming more energy-intense: people are consuming more energy on a per-capita
basis. Second, the global population is growing (see Figure 5.10). Both these factors
are multiplicative rather than additive. We will return to global population when
looking at the likely future population as part of human ecology in the next chapter.
With reference to the carbon cycle (Figure 1.3), the IPCC (2001b and affirmed
in 2007) concludes that as atmospheric carbon dioxide increases the ocean and land
will absorb a smaller fraction of subsequent releases. This means that burning a
litre of petrol in 10 years time will result in more carbon dioxide residing in the
atmosphere for a century or so than a litre burnt 10 years ago. This in turn means
that the warming effect from burning a single litre will be greater. (In other words
the release of a unit of carbon dioxide will have a greater warming ability, even if
its global warming potential [GWP] remains 1 by definition.) With regards to Table
1.1 and the 2010 concentration of carbon dioxide, the IPCC (2001b) conclude that
whereas pre-industrial levels of carbon dioxide were around 280 ppm, and the 2006
concentration was around 382 ppm, or a 36% rise (and it was 392 ppm, a 40%
increase, in 2011), the projected 2100 concentration is likely to be 540-970 ppm (or
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