An introduction to climate change - Climate Change: Biological and Human Aspects - page 26

growth). Nonetheless, their results are consistent with an acceleration of the terrestrial
carbon cycle in response to global climate change.
The fear is that as, globally, soils hold 1500 GtC (compared with some 750 GtC in
the atmosphere) and that as about a third of soil carbon is in arctic and boreal soils,
warming of these soils might make a significant contribution to atmospheric carbon.
Should this occur, this carbon contribution would be beyond what is being added
to the atmosphere through human actions such as fossil fuel burning and land-use
change. It would therefore exacerbate current global warming.
Returning to the aforementioned variable results of carbon-release change with
temperature in temperate and tropical soils, matters may be explained by considering
soil carbon to be in various forms with different turnover rates. A three-carbon-
pool model, with each pool having differing turnover times, was proposed in 2005
(Knorr et al., 2005). This was then applied to data from 13 previously published
soil-warming experiments covering tropical and temperate soils that lasted between
about 100 days and 2 years. It gave somewhat varying results, but importantly this
model was compatible with earlier work. What appears to be happening is that the
pools of carbon with a faster turnover mask the effect of pools with slower and larger
rates of turnover. This model also suggests that higher carbon release from warmed
soils might continue over a number of decades. This last has yet to be tested, but we
may get the chance to find out as the Earth continues to warm up.
To put the Alaskan experiment into context of current increases in atmospheric
carbon dioxide and existing carbon pools, high-latitude warming could at some stage
further increase the current rate of (largely fossil fuel-driven) increase in atmospheric
carbon dioxide by between half as much again and double, for a period of two decades
or more. However, the exact global effect of climate change on soil carbon is uncertain
but (as we shall shortly see) progress is being made. Nonetheless, the key point here
is that warming would itself enable the release of more tundra soil carbon, which
would fuel further warming, and so on (this is another positive-feedback cycle).
The three-pool carbon soil model, applied to real experimental results, suggests that
increases in soil carbon release with temperature rises also apply to temperate and
tropical soils and not just to high-latitude (and high-carbon) boreal soils. As we shall
see in Chapter 7, this could undermine the policy proposals of temperate nations to
use soils as carbon sinks to offset atmospheric carbon dioxide increases from fossil
fuel burning. For now it is worth noting that soil carbon has a feedback to atmospheric
carbon dioxide mediated by temperature change, even if the precise strengths of this
feedback have yet to be discerned.
In addition to the issue of whether, with current global warming, the Earth is now
shifting towards a new feedback system, the second key question is how much more
carbon will be released into the atmosphere with warming? In other words, what is
the magnitude of the climate sensitivity of the global carbon cycle?
In 2010 a team of researchers led by David Frank, based in Swiss and German
institutes, looked at the global climate together with atmospheric carbon dioxide
concentrations between the 11th and 18th centuries. They chose this time period
because it was before the Industrial Revolution and so humans were not adding
carbon dioxide to the atmosphere through intensive fossil fuel burning, and wholesale