The human ecology of climate change - Climate Change: Biological and Human Aspects - page 434

mitigation potential previously predicted from an increase in the terrestrial carbon
sink under increased atmospheric carbon dioxide concentrations due to increased
plant growth. Their argument is that increased carbon dioxide leads to reduced plant
transpiration (the evaporation of water from plant surfaces, and leaves in particular
due to fewer and/or more closed stomata; see section 6.1.3), which in turn increases
soil water content and so promotes the existence of anaerobic microsites in soils. This,
together with increasing biological activity, probably stimulated denitrification and
consequently nitrous oxide production. Also, the carbon dioxide-induced increase in
root biomass may have contributed by increasing the availability of labile carbon, a
crucial energy source for denitrification. The carbon dioxide induced stimulation of
methane emissions from wetlands and rice paddies was probably the result of higher
net plant production, leading to increasing carbon availability for substrate-limited
methanogenic microorganisms. The problem is that both methane and nitrous oxide
are more powerful greenhouse gases compared to carbon dioxide (see Table 1.2).
(See also Knohl and Veldkamp, 2011.) This could be the making of yet another
future critical transition resulting in crossing a climate threshold (section 6.6.8) in
addition to that due to warming alone.
The potential for ecological carbon leakage should be of concern to policy-makers
concerned with climate change. As we shall see in the next chapter, some countries
(typically nations that use a lot of fossil fuels) have insisted that it be possible
to trade, using permits, the right to emit carbon dioxide for carbon-sequestering
ecosystem-management schemes. Trading in such greenhouse permits at best can
have a short-term mitigating effect. However, it comes with a risk, and there is the
question as to what would happen in this permit game if after a number of decades
a forest - created, say, by funding from a fossil fuel station - burned down? Or if its
soil carbon was released with global warming? How would the trading permit be paid
back and the permit's environmental impact nullified? These questions have not been
resolved. Indeed, the IPCC (2001a) warn that 'larger carbon stocks [in ecosystems]
may pose a risk for higher CO 2 emissions in the future'. It says that 'if biological
mitigation activities are modest, leakage is likely to be small. However, the amount
of leakage could rise if biological activities became large and widespread'.
Boreal and tundra soils hold considerable carbon stocks, and of these, as noted in
Chapters 1 and 4, high-latitude soils currently have up to a third of the global soil
carbon pool. These, as per the IPCC warning, pose a risk for higher carbon dioxide
emissions with warming. Already there is some experimental evidence suggesting
that warming will result in carbon loss from these high-latitude soils and indeed
carbon loss from temperate and tropical soils (see Chapter 1). Yet soil carbon loss
may also increase another way as the climate warms.
The amount of carbon held in a soil should really be viewed as a balance between the
rate of carbon entering the soil and the rate at which it leaves. Such a consideration
should include the living plant dimension, as it is the photosynthetically driven
primary productivity (plant growth) that largely draws down carbon, with plants
creating roots within the soil and plant remains lying on the soil surface, from which
carbon compounds can leach into the soil beneath. Therefore, if something were to
hinder primary productivity then the balance of carbon flows would shift. In 2005
it was reported (Ciais et al., 2005) that this indeed happened in Europe during the
2003 heatwave. Then the July temperature in places was up to 6 ◦ C above long-term