The human ecology of climate change - Climate Change: Biological and Human Aspects - page 433

10% of agricultural land in the EU), could be managed to enhance soil carbon. In
total the IPCC's second assessment report (1995) estimated that up to 2050 some
23-44 GtC could be sequestered by agricultural soils, or around 2.6-5% of estimated
mean annual 21st-century B-a-U emissions. This excludes the potential saving of
fossil fuel emissions (mitigation) from biofuels.
To take one example of management options, an estimate of annual change in soil
carbon stocks for Scotland between 2000 and 2009 is 89 kt year − 1 , equivalent − 810 ±

0.004% year − 1 . It is thought that increasing the area of land-use change
from arable to grass has the greatest potential to sequester soil carbon, and reducing
the area of change from grass to arable has the greatest potential to reduce losses of
soil carbon. Across Scotland, model-simulated changes in soil carbon from carbon-
rich soils (C content

to 0.037 ±

35 Mt
from non-carbon-rich mineral soils; losses from carbon-rich soils between 2000 and
2009 make up 64% of the total soil carbon losses. One mitigation option that could
be used in upland soils to achieve zero net loss of carbon from Scottish soils is to stop
conversion of semi-natural land to grassland and increase conversion of grassland to
semi-natural land by 125% relative to the present rate (Smith et al., 2010).
The IPCC's 2001 estimate for carbon mitigation from both vegetation and soil
management up to the year 2050 was 100 GtC. This is equivalent to 10-20% of fossil
fuel emissions estimated for that period. The IPCC (2001a) also notes that 'hypothet-
ically', if all the carbon released due to historic land-use change (forest clearance and
such) could be reversed, then projected 2100 atmospheric concentrations (approxim-
ately 800 parts per million (ppm) under B-a-U scenarios) could be reduced by 40-70
ppm. This last would be a one-off gain from the finite land area historically affected
by land use to release carbon dioxide (see Chapter 5), for once a new forest is fully
established it ceases to be a net absorber of carbon.
Managing the terrestrial short-term carbon cycle carbon can offset a small, albeit
significant, proportion of carbon likely to be emitted from 21st-century fossil fuel
burning. However, it is not risk-free. The more carbon built up in an ecosystem the
more the likelihood of possible carbon leakage. Clearly, if an ecosystem is devoid of
carbon then there is none to leak. On the other hand, a system rich in carbon, be it
through peaty soils or woodland, has plenty of carbon that can escape. For instance, a
forest fire can release nearly all the above-ground carbon extremely quickly, whereas
ploughing grassland managed to maximise soil carbon can undo much of the benefit
in a single season. Leakage can also take place in soils whose carbon pools have
adjusted to the comparatively stable Holocene climate of the past 10 000 years or
so, but which could well release their carbon with warming anticipated for the 21st
century that takes them beyond temperatures seen in this past time (see Chapter 1).
Some research suggests that the state of the Earth system is such that soils' ability
to be greenhouse gas sinks will be limited. Carbon dioxide promotes plant growth
and plant roots help incorporate carbon into soils. This is all well and good, but there
are other greenhouse gases in addition to carbon dioxide. Methane and nitrous oxide
emissions from soils are anticipated to increase in a warmer world. In 2011, Kees
van Groenigen, Craig Osenberg and Bruce Hungate reported that increased carbon
dioxide (ranging from 463 to 780 ppmv) stimulates both nitrous oxide emissions
from upland soils and methane emissions from rice paddies and natural wetlands.
These emissions are expected to negate some (at least 16.6%) of the climate change

> 6%) between 1950 and 2009 is − 63 Mt, compared with −