The human ecology of climate change - Climate Change: Biological and Human Aspects - page 430

carbon with fast-cycle carbon (or biofuels). However, care would be needed to ensure
that we did not build up ecological reservoirs of carbon that would subsequently be
released through climate change.
In terms of tapping into the deep carbon cycle, it is broadly estimated that the total
carbon released by human action between the onset of the Industrial Revolution in the
18th century and the middle of the 21st century's first decade is around 500 GtC (Allen
et al., 2009) and of this broadly two-thirds (over 300 GtC) comes from fossil fuel. This
compares with a broad upper estimate of conventional fossil carbon resources (past
and the next two centuries) of approximately 1700 GtC. Alternatively, if one includes
unconventional fossil reserves (such as tar sands and shale oils), then this increases to
a potentially exploitable total fossil carbon inventory of almost 5000 GtC. (And this
last excludes carbon fuels that we have yet considered as unconventional resources,
such as marine hydrates [clathrates], which the IPCC estimate at 12 000 GtC.) This
estimate of approximately 300 GtC for historic fossil releases compares to the forecast
total historic and future releases (1880-2100), which the IPCC (2001a) use based
on their Special Report on Emission Scenarios (SRES), of between 1000 and 2150
GtC for the 21st century. In other words, the IPCC think it likely - depending on
various economic futures and compared to the 12 decades up to the end of the 20th
century - that 21st-century carbon releases may be double to over six times that
released historically.
The aforementioned data give us a rough upper estimate for average annual fossil
carbon emissions for the 21st century of around 18 GtC year − 1 for a high Business-
as-Usual (B-a-U) scenario (although more is most likely to be released annually at
the end of the century than the beginning). For biological sequestration to have a
significant impact it is going to have to be extremely effective. So what is its likely
potential?
7.5.1 Terrestrialphotosynthesisandsoilcarbon
The B-a-U annual fossil release of 18 GtC for the 21st-century is small compared to
the roughly 50 GtC photosynthetically captured by the Earth's terrestrial ecosystems
(i.e. excluding ocean productivity) each year after allowing for respiration. In theory,
therefore, it should be possible to harvest 18 GtC of terrestrial vegetation (say, as
wood) each year and bury it (effectively removing carbon from the atmosphere) or
burn it as fuel (effectively part-offsetting fossil carbon emissions). This would negate
human average annual 21st-century carbon emissions. However, to do this we would
need to harvest the equivalent of all new tree growth globally each year and this is
clearly impractical. (Note: notwithstanding in addition to the approximately 50 GtC
photosynthetically captured there is also a carbon stock from previous years' growth
and this last forms the bulk of the carbon stocks depicted in Figure 7.15.)
However, the theoretical possibility of photosynthetically drawing down such a
large amount of carbon can perhaps be better understood by looking at terrestrial-
atmospheric carbon exchange. Such is the present rate of carbon removal from the
atmosphere that at current rates the entire atmospheric volume of carbon in theory
might be exchanged within about a decade. Of course, there are mixing problems
and just as carbon is drawn down in the form of carbon dioxide it is also respired
back as well as returned in other ways and places (such as the oceans), not to mention