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wetlands display clear seasonal patterns in their
methane and carbon dioxide emissions, whereas
tropical wetlands are much less seasonal (Mitsch
and Gosselink 2007). Thus, the balance of gas
emissions has climatic implications and partly
depends, in turn, on climatic conditions (see
chapter 8.4).
In a general way, peatlands exert a negative
feedback inl uence on climate (Charman 2002).
In warm interglacial times, as during the
Holocene, peatlands grow upward and spread
outward, thus storing carbon removed from
the atmosphere and reducing the greenhouse
effect. Much of the 500 Gt or so of carbon
held in peatlands has accumulated during the
past few millennia. This drawdown of green-
house gases, in fact, may have limited poten-
tial global warming and led to late Holocene
cooling (see chapter 9.4). Franzén (1994) pro-
posed that this mechanism was responsible for
initiating glacial cycles during the Pleistocene
(Fig. 10-8). Conversely, during glacial episodes
peatlands were much reduced both by cold
climate and expansion of ice sheets, so carbon
was released into the atmosphere, strength-
ened the greenhouse effect, and limited poten-
tial global cooling. Glaciation sculpted vast,
poorly drained landscapes in which peatlands
could develop again during each subsequent
warm interval (Fig. 10-9). “In this respect,
comparisons between the global distribution
of wetlands with areas subjected to glaciation
during ice ages show a remarkable corre-
spondence” (Franzén 1994, p. 300).
This theory remains controversial. Certainly
links exist between continental glaciation, dis-
tribution of peatlands, carbon storage and
greenhouse gases, and these connections result
in a negative feedback inl uence for climate
change. However, this cannot be viewed as the
only or primary driving force for Pleistocene
climatic/glacial cycles. Much larger carbon res-
ervoirs (ocean, permafrost), stronger climatic
factors (solar energy, albedo), and ocean-land-
glacier interactions were equally, if not more,
important for modulating Pleistocene climate.
Nevertheless, “the size and functioning of any
large carbon pool are of importance to past and
future climate change” (Charman 2002, p. 192).
Figure 10-8.
Organic-rich peaty soil exposed in the
Lancer moraine, southwestern Saskatchewan, Canada.
The peat has yielded an uncorrected radiocarbon date
of 31,300
1400 years old. This material was
presumably derived from mid-Wisconsin interstadial
deposits that were deformed and overrun by late
Wisconsin glaciation. Photo by J.S. Aber.
±
10.3 Fossil fuels
10.3.1 Fossil-fuel consumption
The large-scale extraction and burning of coal
ushered in the Industrial Revolution and contin-
ues to be a primary energy source for modern
industry and society in many parts of the world
(Figs. 10-10 and 10-11). Coal represents ancient,
so-called dead carbon that accumulated in peat-
forming wetlands and then was effectively
removed from the carbon cycle and stored for
tens to hundreds of millions of years. Mega-
mining and massive burning of fossil fuels (coal,
oil and natural gas) have multiple direct and
indirect impacts on modern wetlands both
locally and globally. In addition, natural gas is
the feed stock for most inorganic ammonia-
based fertilizers (see section 10.1.2).
Burning fossil fuels, forest clearance, and
other human activities are causing atmospheric
carbon to increase by about 3 Gt per year (Adams
1999). In comparison, current carbon burial rate
in all northern peatlands is only 0.07 Gt per year
(Charman 2002). As a result, atmospheric carbon
dioxide has grown from
<
320 ppm in the mid-
twentieth century to
390 ppm in the early
twenty-i rst century, an increase of
>
20% in
only half a century (Tans 2010). The mean
>
