40 The greenhousegases
Feedbacks in the biosphere
As the greenhouse gases carbon dioxide and methane are added to the
atmosphere because of human activities, biological or other feedback
processes occurring in the biosphere (such as those that arise from the
climate change that has been induced) influence the rate of increase
of the atmospheric concentration of these gases. These processes will
either tend to add to the anthropogenic increase (positive feedbacks) or
to subtract from it (negative feedbacks).
Two feedbacks, one positive (the plankton multiplier in the ocean)
and one negative (carbon dioxide fertilisation), have already been men-
tioned in the text. Threeother positive feedbacks are potentially impor-
tant, although our knowledge is currently insufficient to quantify them
precisely.
One is the effect of higher temperatures on respiration, especially
through microbes in soils, leading to increased carbon dioxide emissions.
Evidence regarding the magnitude of this effect has come from studies
of the short-term variations of atmospheric carbon dioxide that have
occurred during El Ni ̃no events and during the cooler period following
the Pinatubo volcanic eruption in 1991. These studies, which covered
variations over a few years, indicate a relation such that a change of
5 ◦C in average temperature leads to a forty per cent change in global
averagerespiration rate^7 – a substantial effect. A question that needs to
be resolved is whether this relation still holds over longer-term changes
of the order of several decades to a century.
A second positive feedback is the reduction of growth or the die-
back especially in forests because of thestress caused by climate change,
which may be particularly severe in Amazonia^8 (see box in Chapter 7
on page 173). As with the last effect, this will increase as the amount of
climate change becomes larger. A number of carbon cycle models show
that, through these two effects, during the second half of the twenty-first
century the residual terrestrial sink (Table 3.1) could change sign and
become a substantial net source (Figure 3.5).
The third positive feedback is the release of methane, as tempera-
tures increase – from wetlands and from verylarge reservoirs of methane
trapped in sediments in a hydrate form (tied to water molecules when
under pressure) – mostly at high latitudes. Methane has been generated
from the decomposition of organic matter present in these sediments
over many millions of years. Because of the depth of the sediments this
latter feedback is unlikely to become operative to a significant extent
during the twenty-first century. However, were global warming to con-
tinue to increase substantially for more than a hundred years, releases
from hydrates could make a large contribution to methane emissions into
the atmosphere and act as a large positive feedback on the climate.