Geoscience Reference
In-Depth Information
communities) tend to be greenhouse-neutral. ('Broadly speaking' because a forest's
carbon balance is climate-dependent and will change with temperature and water
availability; see Chapters 1, 5 and 6.) It is the establishment of new forests that has a
net, short-term effect sequestering carbon from the atmosphere. If you like, this is the
opposite of deforestation, which is the second anthropogenic factor contributing to
the build-up of atmospheric carbon dioxide (see Table 1.3). This new forest necessity
places a severe constraint on forestation as a form of carbon sequestration. Not
only is land itself finite, it is also required for other uses. Furthermore, not all the
Earth's land is suitable for forestation. The IPCC's second assessment report (IPCC,
1995) estimated that by 2050 some 60-87 GtC could be conserved or sequestered
in forests. Compared to the above-estimated mean annual 21st-century B-a-U fossil
fuel emissions of 18 GtC, this represents an annual saving of around 1.2-1.7 GtC, or
around 6-9% of annual emissions.
Soils can also be managed to increase their carbon content (although we have a
lot to learn about soil-carbon mobility and fixation). The potential here might at first
appear quite considerable, especially as roughly three times the amount of carbon
is stored in soils than as vegetation above ground. Indeed, as noted previously, sub-
Arctic soils already hold a considerable volume of carbon (albeit a small proportion
of terrestrial carbon in both vegetation and soils). Of the various soil types, peatlands
represent a huge store of carbon (see Chapter 4). High-latitude peatlands (which
include parts of tundra, boreal and semi-desert soils and wetlands), with between
180 and 455 GtC, represent up to about a third of the global soil carbon pool. Some
70 GtC has been sequestered since the LGM but the LGM was more than 15 000 years
ago and the Holocene interglacial is only 11 700 years old, so this represents a small
annual sequestration rate, which would be difficult to enhance to meaningful levels.
So, despite being a large carbon pool, high-latitude soils do not have a commensurate
potential for further sequestration. If anything, there is concern for the opposite. As
with forests these high-latitude peatlands and permafrost soils are climate-sensitive.
It is quite likely that they will release their store of carbon if warmed. In short, with
global warming they could become a carbon source.
One added problem is that, while some high-latitude soil carbon has been
sequestered since the end of the last glacial (15 000 years ago), some is very old
carbon. In 2008 Canadian geologists led by Duane Froese and John Westgate repor-
ted that some ground ice in sub-Arctic Canada is more than 700 000 years old. The
implication is that this ice and so indeed the carbon in these soils has survived past
interglacials that were warmer than our own to date. If so, then our current warming,
which is set to take us to global temperatures above that of even these warmer past
interglacials, will release carbon that effectively has been out of the carbon cycle for
700 000 years or more.
Conversely, agricultural soils do have more potential. Soils that are currently used
for agriculture are regularly ploughed, so bringing organic carbon to the surface and
destroying plant root networks that physically trap carbon compounds. Ploughing
also greatly aerates soils, so facilitating oxidation of carbon compounds. On the other
hand natural and semi-natural grassland and forest systems are not subject to regular
ploughing but additionally provide the soil with carbon compound inputs. The IPCC
(2001a) notes that some agricultural land, such as set-aside (which represents some
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