Geoscience Reference
In-Depth Information
adjustments in the system. Perhaps that is why the climate
sometimes changes abruptly without any evidence of a
clear change in external conditions.
Figure 9.17
shows
such an effect which has been proposed as a cause of the
Ice Ages. A quite small cooling of temperature at the poles
of only 1-2
which of these circulation types is typical of Earth, but
they could account for the known sudden changes. We do
not necessarily have to look to external or internal forcings
for rapid change.
Finally we must not forget that different parts of the
climatic system respond at different rates. In general the
atmosphere responds rapidly to any forced change.
However, ice sheets and the oceans normally respond
very slowly to change, so that there is a considerable lag
time between the initial forcing and the final equilibrium
in these areas. Even here, recent work suggests that some
changes can be rapid when ocean currents can change
suddenly or ice sheets dramatically break up. These lag
factors make it more difficult to predict what changes will
happen and when.
C delays the summer melting of the Arctic ice
cap. Because the ice survives for longer, the albedo of the
surface stays high longer. More incoming short-wave
radiation is reflected back to space. Reduced heating of the
surface therefore occurs, allowing the ice caps to survive
even longer, which increases reflection further, which
lowers temperatures further . . . and so on. The cycle is
self-perpetuating. Once they have been initiated, positive
feedback processes magnify the effect of the initial change
and cause major adjustments in the system - possibly even
an ice age.
Another factor which may affect the state of the climate
system results from the complex non-linear behaviour of
the atmospheric circulation. In a transitive system there
would be one normal state of circulation, and any
disturbances in the circulation would be expected to revert
to the norm. In an intransitive system there are two equally
acceptable outcomes, depending upon the initial state.
However, mathematicians have found that some systems
can be almost intransitive, i.e. the circulation resembles a
transitive state for an indeterminate length of time and
then suddenly switches to an alternative resultant state.
With such a circulation it is impossible to know which is
the normal state and when a switch may take place.
Attempts to model such a system with any confidence
would be very difficult. Unfortunately geological and
historical data are insufficiently detailed to determine
From this information can we say what the future climates
of Earth may be like? Numerous predictions have been
made. Climatic models have been used to investigate the
effects of known or highly likely changes in the near
future. They would include the effects of an atmosphere
with more greenhouse gases in its composition, together
with the orbital variations that we know will take place
over the next 100,000 years (
Figure 9.18
).
Output from the
orbital models indicates that climates as warm as those of
today are relatively rare and suggests that, other things
being equal, the global climate should start to change
more rapidly. Unfortunately because of the very different
time scales of operation it is difficult to incorporate
astronomic, oceanic and atmospheric effects into the same
model. The Inter-governmental Panel (IPCC) in its fourth
Assessment Report in 2007 concentrated only on what
climatic conditions might prevail for the next century. At
present any discussion of the climatic impact relies on
informed judgement and speculation. Impact will also be
influenced by the level of economic and social develop-
ment of a country. Sea-level rise could have different
consequences for a country like the Netherlands than for
the Maldive Islands. We shall consider this in more detail
in
Chapter 28.
Our uncertainty about the future climate is based upon
the many different forcing factors, some linear and some
non-linear, which operate over many different time scales,
all superimposed on each other and each operating over
a different time cycle.
Figure 9.16
shows the latest figures
from the IPCC about the radiative forcing components
that are affected by human activities. Clearly carbon
dioxide is the most important, though other long-lived
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decrease in insolation and lower surface temperatures may
generate further cooling and perhaps even an ice age.
