The zener and the regular diode attempt to simulate a Diode in real life (with forward bias voltage). This forward bias voltage changes with temperature (on the order of raising by 25mv per degree K lowered). Raise the breakdown voltage on the zener diode to simulate a cold environment (and see how the pull up resistor becomes ineffective (raises the resistance)). The point at which the pull-up resistor becomes ineffective is when the LED starts blinking, and this is when the circuit fails.
