Geography Reference
In-Depth Information
Figure 2.1
Trend line of carbon dioxide concentrations
over the past 300 years based on (a) measurements from
Antarctic ice core records prior to 1958 (dashed line), (b)
direct measurements from the Mauna Loa observatory in
Hawaii (solid line).
biosphereatmosphere cycle has an annual
throughput of about 100 Gt. Both show a small
but significant total net loss to the atmosphere of
3.7 Gt. The total annual throughput of carbon,
chiefly as carbon dioxide, in the atmosphere is
therefore about 25 per cent of the atmospheric
reservoir of 750 Gt. The sensitivity to change in
the throughput of carbon in the atmosphere needs
to be considered in any discussion of global
warming, since the climatic effect of relatively
small changes in atmospheric carbon could be
amplified within the system.
Radiative forcing and equilibrium
Radiative forcing may be defined as a change in
average net radiation at the tropopause which is
the upper boundary of the troposphere, due to
changes in incoming solar or outgoing infrared
radiation. Such a change 'perturbs' the Earth's
radiation balance, as shown in Figure 2.2, altering
the balance between incoming and outgoing
radiation. It also alters the nature of the
greenhouse effect, which is usually expressed in
degrees C (IPCC 1995). Under present climatic
conditions and carbon emissions, the natural
greenhouse effect amounts to about 33°C if
clouds, which act as radiation 'blankets', are
Source:
After Houghton 1997 and IPCC WGI 1996.
ppmv adds 3.8 Gt (gigatonnes) to the atmospheric
carbon reservoir of 750 Gt (Houghton 1997). This
is about half of the calculated amount of 7.5 Gt
being emitted into the atmosphere from forest
fires and fossil fuel burning. It is generally accepted
that the remainder—about 3.7 Gt. —is absorbed
by the oceans.
The atmospheric carbon reservoir forms part
of the global carbon cycle in which the natural
atmosphere-ocean component has an annual
throughput of about 90 Gt, while the
Figure 2.2
Simplified
diagram showing the
mean annual radiative
balance of the
atmosphere. Radiative
equilibrium is maintained
since net incoming solar
radiation is balanced by
net outgoing infrared
radiation (240 Wm-2).
Total earth-atmosphere
albedo is about 30 per
cent (103/343×100). The
natural greenhouse effect
(33°C), shown
qualitatively as infrared
radiation, is absorbed and
partially re-radiated back
to the surface.
