This table from the Hadley Centre for Climate Change summarises how climate change models are designed. Note how potential feedback systems are predicted throughout the model.
Feedback within the climate system
Feedback systems currently concerning scientists modelling the climate system are "open loop". This also known as "positive feedback" and occurs where the outcome of a change exaggerates the change itself. This is not the same as "negative feedback" where a system settles about a set point. An example of negative feedback is a central heating thermostat.
Scientists are less certain about these events than the outcomes predicted in the IPCC report. The things excluded by the IPCC are physical collapse of the Greenland Ice sheet, rapid melting in Antarctica, a shut-down of the Atlantic Conveyor ocean current, the release of methane from soil, the ocean bed and melting permafrost.
What could send the global climate spiralling out of control? One suggested example is a temperature rise high enough to melt the white snow and ice covering the north and south poles. This could decrease the Earth's ability to reflect back incoming sunlight and further increase global temperatures.
Other examples of open loop feedback on PEEP: