Geoscience Reference
In-Depth Information
an overall decrease and all other factors remain constant, there will be a net cooling
of the earth's surface. Conversely, if there is an overall increase, there will be a net
warming. This brings us to the principle of 'radiative forcing'. We have seen that the
earth receives energy from the sun and loses energy to space mainly through infrared
radiation. On average, there is a balance between energy inputs and outputs. Any
factor that disturbs this balance is called 'radiative forcing'. The climate responds to
such disturbance by changing (i.e., becoming warmer or cooler) until a new balance is
achieved. A change towards a warmer state is popularly (and inaccurately) called the
'enhanced greenhouse effect'. (I say 'inaccurately' because the glass in a greenhouse
allows the sun's heat to penetrate and warm the air inside the greenhouse, which slows
the loss of outgoing long-wave radiation, but it does not itself absorb and store the
heat in the same way that our atmosphere does.)
Let us now consider the geologically recent changes in certain atmospheric gases,
such as carbon dioxide, nitrous oxide and methane (
Figure 25.3
). Trapped as air
bubbles in the ice of Greenland and Antarctica is an archive of the mean atmospheric
composition at the time the air within snow was finally sealed away from contact with
the atmosphere as the snow turned to ice, a process that can take several centuries. The
longest records come from ice cores collected by combined Russian and French teams
from the site of Vostok in central Antarctica (Jouzel et al.,
1997
; Petit et al.,
1981
; Petit
et al.,
1990
; Petit et al.,
1999
; Jouzel et al., 2007) and more recently by the EPICA
team from EPICA Dome C (EPICA Community Members,
2004
; EPICA Community
Members,
2006
;Luthi et al.,
2008
). These ice cores span the past 800 ka. During that
long interval of time, the atmospheric concentration of carbon dioxide (pCO
2
) ranged
between 280-300 parts per million by volume (ppmv) during interglacial maxima
and 180-200 ppmv during glacial maxima (Jouzel et al., 2007;L¨uthi et al.,
2008
).
Corresponding methane values were about 800 ppbv during interglacial maxima
and 400 ppbv during glacial maxima (Petit et al.,
1999
). As might be expected,
temperatures were lowest on the ice caps during glacial maxima and highest during
interglacial maxima. We also saw in
Chapter 9
that concentrations of wind-blown
dust from Patagonia and Australia were also highest during glacial times.
At the start of the Industrial Revolution (about1750 AD), the carbon dioxide con-
centration during our present interglacial was about 280 ppmv (
Table 25.1
). From
then on, it rose steadily and at an accelerating rate, reaching 379 ppmv in 2005 (
Tabl e
25.1
) and 386 ppmv in 2009. By April 2013, the global pCO
2
level had reached
400 ppmv for the first time in nearly 1 million years, and it is continuing to rise at a
rate of 2 ppmv/year (Raupach and Fraser,
2011
). The same holds true for both meth-
ane and nitrous oxide concentrations, both of which are rising steadily (
Table 25.1
).
Over the past 250 years, carbon dioxide has been the main contributor to radiative
forcing (equivalent to about
2.5 watts/m
2
). Methane has an impact on radiative for-
cing that is about one-third that of carbon dioxide, and nitrous oxide (N
2
O)'s impact
+
