Climate surprises - Climate Change: A Very Short Introduction - page 106

(b) Muted or limited response ( Figure 28b ). In this case, the GHG forcing may be
strong, but the part of the climate system is in some way buffered and therefore gives
very little response. An example of this is the East Antarctic ice sheet that has been
stable at much warmer temperatures than today. This is the 'pushing the car up the
hill' analogy: you can spend as much energy as you like trying to push the car, but it
will not move very far.
(c) Delayed and non-linear response ( Figure 28c ). In this case, the climate system may
have an initial slow response to the GHG forcing but then responds in a non-linear
way. This is a real possibility when it comes to climate change if we have underes-
timated the positive feedback in the system. This scenario can be equated to the car
on the top of a hill: it takes some effort and thus time to push the car to the edge of
the hill; this is the buffering effect. Once the car has reached the edge, it takes very
little effort to push the car over, and then it accelerates down the hill with or without
help. Once it reaches the bottom, the car then continues for some time, which is the
overshoot, and then slows down of its own accord and settles into a new state.
(d) Threshold response ( Figure 28d ). In this case, initially, there is very little response in
that part of the climate system to the GHG forcing; however, all the response takes
place in a very short period of time in one large step or threshold. In many cases, the
response may be much larger than one would expect from the size of the forcing and
this can be referred to as a 'response overshoot'. An example of this could be the
Greenland ice sheet that has started to melt but that could sudden accelerate causing
a catastrophic collapse. This scenario equates to the bus hanging off the cliff at the
end of the original film The Italian Job ; as long as there are only very small changes,
nothing happens at all. However, a critical point (in this case weight) is reached and
the bus (and the gold) plunges off the cliff into the ravine below.
Though these are purely theoretical models of how the global climate system can respond,
they are important to keep in mind when reviewing the possible scenarios for future cli-
mate change. An added complication when assessing climate change is the possibility that
climate thresholds contain bifurcations. This means the forcing required to push the sys-
tem one way across the threshold is different from the reverse. This implies that once a
climate threshold has occurred, it is a lot more difficult to reverse it and in some cases it
may be irreversible.
The term 'tipping points' is used a lot in climate change research and discussions.
However care must be taken as there are two usage of this word. First are the climate tip-
ping points, which are the large scale shifts in the climate system, such as irreversible