INTRODUCTION: SOCIOECONOMIC ASPECTS OF BIOMASS CONVERSION - Biomass as a Sustainable Energy Source for the Future - page 10

Another difference is that the principal anthropogenic sources of methane-bacterial
fermentation in rice paddies and in the intestines of cattle are related to food produc-
tion and, hence, are roughly proportional to the number of people on the planet.
Because CH 4 has such a short atmospheric lifetime, the amount that is in the air is
a good indicator of how much is being added with time. Should the global population
double over the next half century, the concentration of CH 4 could also double, but it is
not likely to rise by much more than that. This would add, at most, a few tenths of a
degree to the mean temperature of the Earth. Future CO 2 increases could, in contrast,
warm the climate by 10 C or more.
Nitrous oxide (N 2 O) and CFCs are in some ways more like CO 2 in that once
released they remain in the atmosphere for a century or more. The production of
N 2 O, however, is only indirectly dependent on human activities. Its principal source
is a natural one, the bacterial removal of nitrogen from soils, and although the world
population swells in coming years, the amount in the air should increase only slowly.
The outlook for many CFCs is even more promising. Today, the most abundant of
these man-made compounds, freon-11 and freon-12, are being phased out of produc-
tion altogether by international agreements because of their damaging effects on strat-
ospheric ozone. Indeed, the concentration of one of these gases, freon-11, peaked in
1994 and is now in a slow decline that should continue for the next century or so. The
freon-12 concentration has not yet leveled off, but is expected to do so within the next
few years. In terms of climatic effects, the main threat from CFCs comes from other
long-lived compounds that may be used to replace the ones that have been phased out
and that could also act as GHG. Since these possibly harmful replacement gases are as
yet present in only small amounts and since, as noted earlier, projected increases in
CH 4 and N 2 O are so much less severe, we shall for the rest of this discussion focus
solely on the most important anthropogenic GHG: CO 2 .
Some experts have estimated that the Earth s average global temperature has already

increased by more than 0.5 C since the mid-1900s due to this human-enhanced green-
house effect; also impacts on sea level (rising) and snow coverage (tending to decrease)
have been investigated, the results of which are summarized in Figure 1.4.
Like most other planets and planetoids in the universe, the Earth contains a great
deal of carbon, which is slowly and continually transported from the mantle to the
crust and back again, in the course of volcanic eruption and subduction phenomena.
The portion that finds itself near the surface is continually exchanged and recycled
among plants, animals, soil, air, and oceans. In some of these temporary stocks, car-
bon is more securely held, while in others it more readily combines with oxygen in the
air to form CO 2 . In order to predict how atmospheric CO 2 levels and climate may
change in the future, it is important to understand where carbon is stored and what
its dynamic cycling behavior looks like. The carbon reservoirs that are most relevant
to global warming are listed in Table 1.1, with the total amount of carbon that they
contained in 2000.
The atmosphere contains approximately 720 Gt C in the form of CO 2 ; current
measured atmospheric CO 2 concentrations are nearly 400 ppm v . The rate of change
in this carbon stock not only depends on human activities but also on biogeochemical
and climatological processes and their interactions with the global carbon cycle.

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