Environmental Engineering Reference
In-Depth Information
Atmosphere
750
3.0
Photosynthesis
and respiration
Physical and
biological processes
Deforestation
Combustion
62
60
92
90
6.8
1.5 ± 1
Land, biota and
soil litter 2000
Fossil fuel
5000-10,000
Ocean 40,000
Figure 10.7
Rates of carbon exchange between biosphere and atmosphere and between ocean and
atmosphere. Also shown are emissions from fossil fuel combustion and forest burning (Gt y
−
1
). Carbon
reservoirs in soil, ocean, fossil fuel, and atmosphere (Gt).
return into the atmosphere about 90 Gt y
−
1
by respiration and outgassing. Thus, the oceans are also
net absorbers of CO
2
. We note that the net increase of atmospheric carbon, about 3 Gt y
−
1
, is quite
small compared to the annual cycling of 150 Gt y
−
1
via land and water. It is the extremely slow
secular change in the latter that has accompanied glaciation periods.
Currently, about 6.8 Gt y
−
1
of carbon (25 Gt y
−
1
CO
2
) are emitted into the atmosphere by
1Gty
−
1
are emitted due to deforestation
and land use changes, mainly artificial burning of rain forests in the tropics and logging of mature
trees, which disrupts photosynthesis. The atmospheric carbon content is increasing by about 3 Gt
y
−
1
. Because the biosphere and oceans absorb about 2 Gt y
−
1
each, about 1.3
fossil fuel combustion (see Chapter 2). Another 1.5
±
1Gty
−
1
of carbon
are unaccounted for. Most likely, the biosphere and oceans absorb more CO
2
than is indicated in
Figure 10.7.
Figure 10.8 shows the growth of CO
2
concentrations in the global atmosphere from 1000 to
2000. The atmospheric concentrations before 1958 were estimated from ice cores. During descent in
the atmosphere, snow flakes equilibrate with the atmospheric concentration of CO
2
. Subsequently,
the snow fall is compressed into ice, with its CO
2
content preserved. Knowing the age of the ice
layer, it is possible to reconstruct the prevailing atmospheric concentration of CO
2
when the snow
flakes were falling. In 1958, Charles Keating established an accurate atmospheric CO
2
concentration
measurement device using infrared absorption on top of Mauna Loa, Hawaii. This instrument is
still operating, and the measurements thereof provide an accurate record of global average CO
2
concentrations ever since.
The historic record indicates that before the twentieth century the atmospheric concentration of
CO
2
hovered around 280 ppmV, with a dip in the sixteenth and seventeenth centuries, corresponding
to the “little ice age.” Starting around 1900, when the use of fossil fuels accelerated, the CO
2
concentration began a steady increase of about 0.4%/y, reaching close to 370 ppmV in 2000. If
that rate of increase were to continue into the future, a doubling of CO
2
concentrations would
occur in about 175 years. However, if no measures are taken to reduce CO
2
emissions, then due to
the population increase and the concomitant enhancement of fossil fuel use, the rate of growth of
CO
2
concentrations will increase more than 0.4% per year, and the doubling time will be achieved
sooner.
±
