Threshold response (Figure 30d). In this case, initially, there is very little response in that part of the climate system to the GHG forcing. However, on reaching a threshold, all the response takes place in a very short period of time, in a single, large step. In many cases, the response may be much greater than one would expect from the size of the forcing, and this can be referred to as a ‘response overshoot’. 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,
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