Environmental Engineering Reference
In-Depth Information
a result of deforestation and the destruction of
other vegetation by fire (Stuiver 1978). The
burning of fossil fuels produced only half that
much CO
2
over the same time period. Current
estimates indicate that the atmospheric CO
2
increase resulting from reduced photosynthesis
and the clearing of vegetation is equivalent to
about 1 billion tonnes per year (Moore and Bolin
1986), down slightly from the earlier value.
However, the annual contribution from the
burning of fossil fuels is almost ten times what it
was in the years between 1850 and 1950.
Although the total annual input of CO
2
to the
atmosphere is of the order of 6 billion tonnes,
the atmospheric CO
2
level increases by only
about 2.5 billion tonnes per year. The difference
is distributed to the oceans, to terrestrial biota
and to other sinks as yet unknown (Moore and
Bolin 1986). Although the oceans are commonly
considered to absorb 2.5 billion tonnes of CO
2
per year, recent studies suggest that the actual
total may be only half that amount (Taylor 1992).
The destination of the remainder has important
implications for the study of the greenhouse
effect, and continues to be investigated. The
oceans absorb the CO
2
in a variety of ways—
some as a result of photosynthesis in
phytoplankton, some through nutritional
processes which allow marine organisms to grow
calcium carbonate shells or skeletons, and some
by direct diffusion at the air/ocean interface
(McCarthey
et al.
1986). The mixing of the ocean
waters causes the redistribution of the absorbed
CO
2
. In polar latitudes, for example, the added
carbon sinks along with the cold surface waters
in that region, whereas in warmer latitudes
carbon-rich waters well up towards the surface
allowing the CO
2
to escape again. The turnover
of the deep ocean waters is relatively slow,
however, and carbon carried there in the sinking
water or in the skeletons of dead marine
organisms remains in storage for hundreds of
years. More rapid mixing takes place through
surface ocean currents such as the Gulf Stream,
but in general the sea responds only slowly to
changes in atmospheric CO
2
levels. This may
explain the apparent inability of the oceans to
absorb more than 40-50 per cent of the CO
2
added to the atmosphere by human activities,
although it has the capacity to absorb all of the
additional carbon (Moore and Bolin 1986).
The oceans constitute the largest active
reservoir of carbon in the earth/atmosphere
system, and their ability to absorb CO
2
is not in
doubt. However, the specific mechanisms
involved are now recognized as extremely
complex, requiring more research into the
interactions between the atmosphere, ocean and
biosphere if they are to be better understood
(Crane and Liss 1985).
The changing greenhouse effect
Palaeoenvironmental evidence suggests that the
greenhouse effect fluctuated quite considerably
in the past. In the Quaternary era, for example,
it was less intense during glacial periods than
during the interglacials (Bach 1976; Pisias and
Imbrie 1986). Present concern is with its
increasing intensity and the associated global
warming. The rising concentration of
atmospheric CO
2
is usually identified as the main
culprit, although it is not the most powerful of
the greenhouse gases. It is the most abundant,
however, and its concentration is increasing
rapidly. As a result, it is considered likely to give
a good indication of the trend of the climatic
impact of the greenhouse effect, if not its exact
magnitude.
Svante Arrhenius, a Swedish chemist, is usually
credited with being the first to recognize that an
increase in CO
2
would lead to global warming
(Bolin 1972; Bach 1976; Crane and Liss 1985).
Other scientists, including John Tyndall in Britain
and T.C.Chamberlin in America (Jones and
Henderson-Sellers 1990), also investigated the link,
but Arrhenius provided the first quantitative
predictions of the rise in temperature (Idso 1981;
Crane and Liss 1985). He published his findings at
the beginning of this century, at a time when the
environmental implications of the Industrial
Revolution were just beginning to be appreciated.
Little attention was paid to the potential impact of
increased levels of CO
2
on the earth's radiation
