The human ecology of climate change - Climate Change: Biological and Human Aspects - page 436

was noted in the previous subsection is in inorganic (non-biotic) forms. The other
big carbon difference between the terrestrial and marine biotic reservoirs is that,
whereas terrestrial ecosystems have an annual carbon turnover of roughly 20% of the
terrestrial biotic stock, the carbon turnover of the marine carbon stock is in excess of
1000%.
In other words, the terrestrial biota represent a large mass of biotic carbon with more
modest carbon flows to and from this volume. Conversely, marine biota represent
a small mass of carbon with a comparatively very high carbon turnover whereby
the carbon ends up in the large inorganic carbon ocean reservoir. Managing the
marine carbon cycle to offset atmospheric increases in carbon dioxide from fossil
fuel burning depends on this naturally high turnover and indeed this is the attraction
of such management.
As noted in section 1.3, there have been attempts to encourage algal blooms with
the IronEx experiments. However, while successful in forming blooms, the energy
required to make, transport and distribute the iron used to fertilise the ocean more or
less balanced the fossil energy that was saved in carbon sequestration. Furthermore,
the ecological implications should an IronEx-type scheme be implemented on a large
scale are unknown. With regard to marine ecosystem manipulation, the IPCC (2001a)
conclude that much remains 'unresolved' and that such schemes 'are not, therefore,
ready for near-term application'.
7.5.3 Biofuels
Biofuels arise from living species and are greenhouse-neutral in that they sequester
atmospheric carbon dioxide when grown and return it when burnt. However, if the
products of biofuel consumption are captured (for example, from the flue of a biofuel
power station) and stored, then there is a net transference of carbon from the fast to
the deep carbon cycle (see section 8.2.4). This is one way in which carbon dioxide
can be usefully sequestered from the atmosphere. Biofuels can also be considered
to have a positive effect offsetting fossil carbon emissions when biofuels displace
oil, gas and coal consumption. Here the transference of carbon from the deep carbon
cycle to the atmosphere is reduced.
There is a continuum of agricultural biological resources from which biofuels may
be generated. This continuum is essentially founded within the agriculture sector
but also includes forestry. At one end of this spectrum the biofuel resources are
derived almost by happenstance from agricultural wastes such as animal slurry (from
piggeries, poultry wastes, etc.) and vegetative wastes (such as straw from cereal
production). At the continuum's other end, biofuels can be derived from existing
agricultural products such as sugars and oils. In between these there are both ded-
icated and dependent biofuel sources. Dedicated biofuel sources are those whose
planting solely provides biofuels and nothing else of economic value. The tropical
grass Miscanthus sinensis , for example, has no other real value as a crop in the USA
or Europe (although it is used as an ornamental plant in the European domestic sec-
tor). Conversely, dependent biofuel sources are those that are dependent on another
economic gain (commonly food-product-related) from growing the crop. However,
the reality is that few biofuel sources fall strictly into either category, even though