Environmental Engineering Reference
In-Depth Information
Figure 9.15
A positive feedback loop, demonstating how a
decrease in insolation and lower surface temperatures may
generate further cooling and perhaps even an Ice Age.
adjustments in the system. Perhaps that is why the climate sometimes changes abruptly
without any evidence of a clear change in external conditions. Figure 9.15 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° 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 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
